OSIRIS catches activity in the act

Rosetta’s scientific imaging system OSIRIS has witnessed a new jet of dust emerging from the surface of Comet 67P/Churyumov-Gerasimenko. The image was presented during the European Geosciences Union General Assembly, EGU, in Vienna last week.

Sometimes it is all a matter of being in the right place at the right time. Or, as in the case of ESA’s space probe Rosetta, of opening your eyes at the exactly right moment. In mid-March, the OSIRIS camera was able to catch the elusive moment when a new dust jet from comet 67P/C-G burst into life.

Rosetta’s OSIRIS wide-angle camera captures the moment a jet bursts into action. The first image was captured at 07:13 CET on 12 March 2015, the second two minutes later. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta’s OSIRIS wide-angle camera captures the moment a jet bursts into action. The first image was captured at 07:13 CET on 12 March 2015, the second two minutes later. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The comet’s activity has been significantly increasing over the last weeks and months. As the comet moves closer to the Sun along its orbit, its nucleus gets warmer and warmer. Frozen gases sublimate from its surface, carrying dust particles with it and enshrouding the nucleus in a dense coma. With only four months to go until perihelion – the closest point to the Sun – this process is well underway, with pronounced dust jets seen at all times on the comet’s day side.

The scene at 07:13 CEST on 12 March. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The scene at 07:13 CET on 12 March. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The two images released today show the remarkable onset of such a jet for the first time. They were taken on 12 March from a distance of 75 kilometres. In the first image, obtained at 07:13 CET, several rays of dust jets frame the upper, illuminated side of the comet. The dark underside shows no such features. Two minutes later, the picture has changed: a spectacular new jet has emerged on the dark side, hurtling dust into space and displaying a clearly discernable fine structure.

“This was a chance discovery,” says OSIRIS principal investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. “No one has ever witnessed the wake-up of a dust jet before. It is impossible to plan such an image.”

Tracking variations in brightness along the jet, the researchers estimated the velocity of the dust grains to be at least eight metres per second. This is compatible with measurements made with Rosetta’s GIADA instrument on other occasions, for dust particles emitted from the comet’s surface.

Surprisingly, the new-born jet arises from a shadowed area on the underside of the comet, close to the centre of the Imhotep region. So far cometary activity has only been observed on the comet’s day-side.

The scene at 07:15 CEST on 12 March. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The scene at 07:15 CET on 12 March. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“In these images we see Imhotep on the brink of dawn,” OSIRIS scientist Jean-Baptiste Vincent from the MPS explains. “It is possible that the first rays of sunlight hit some cliffs or outcrops that remained hidden to Rosetta due to the orbital position at the time.”

The onset of activity could also be the result of a different type of more explosive activity. That is, the outburst could have been triggered by a wave of heat reaching ices trapped in a deeper layer beneath the surface.

OSIRIS could not continue to observe the new feature after 07:17 CET because Imhotep was soon fully illuminated, making it impossible to discern individual jets in the overexposed coma. It is therefore not clear whether Rosetta witnessed the birth of a continuous jet or a short-lived outburst.

“Usually, 67P’s dust-jets are rather long-lived,” says Vincent, who has been monitoring the comet’s activity over the past months. “Most of them last for a full dayside rotation of approximately six hours and even reappear in the next rotation.”

However, scientists believe that eruptive outbursts can also occur. This style of outburst might have triggered the observed increase in brightness in the comet’s coma on 30 April 2014. At that time the coma expanded over 1800 kilometres – but faded again a few weeks later.

Last month’s unique observation, along with the continued monitoring of 67P/C-G’s global activity patterns, will give the scientists the chance to test different models of activity.

About OSIRIS: The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d'Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.



  • Marco says:

    At least this clears it up for this jet. Thermal inertia cannot explain it, and an interior source of energy appears more plausible in this case..
    The cliff theory sounds outlandish, given we have seen a closeup of imhotep...

    • Dale says:

      If you download the solid model of the comet from http://www.thingiverse.com/thing:625767 and display it in something like the free version of 123D, you will see that there is some large relief with cliff like structures that could be illuminated that the camera can not see.
      So I don't find that improbable.

      • Marco says:

        It needs more than a cliff at that spot, it needs an Eiffel Tower high spire to reach the sun at that point in the rotation.
        This is also not an isolated case. Comet Hartley was also seen to have jets emanating from the dark side. Just due to that flyby observation, it has not been unexpected so much as still defying a "simple" explanation.

        What is more, lack of visible erosion commensurate to the observed outgassing demands the question of where the mass of outgassing is coming from.?

      • Marco says:

        I've got a thought. Just to play devils advocate here. The jet origin could be two different places on the edge of Imhotep already in sunlight. The Jets could just be sending material through the shaded region and are seen intersecting in perspective but only when they hit sunlight, thus only appearing to emanate from the shaded Region?

        • Ramcomet says:

          Marco, I can see that possibly being the case. I can also almost see pitting just inside the terminator where the shaded jets would originate... although the image is very low res, so probably just my "wishful seeing".

          Not that jets always need to form from the sunlit portion, so many other physical possibilites other than "EU".

          Thermal tides... or even mechanical material tide effects from recent spin down could be recompressing some of the neck material back down into the body. If it has to go somewhere, seems the most likely place to swell outward would be the the flat bottom. Likely it would refracture all around the perimeter of Imhotep's great flat plain as it swells back out.

          Put another, more tactile way, I am feeling that during spin up, when the head fractured and stretched away from the body (refer to A. Coopers well documented images), it was as if someone slowly tore off the top off of a half baked muffin, with some of the stretchy semi-raw dough pulling out of the body behind it, (Hapi valley and all).

          This would cause the bottom of the muffin to suck in and become concave, (which it is). It would likely create fracturing of the harder, brittle "icing", all around concave Imhotep. (Which also seems to have lost a huge dome of material blown away. I have to check A. Coopers images again. Anyway, so flat, easier to flex!)

          Now if 67P is recompressing due to spin down approaching the sun, and the bottom of the muffin, Imhotep, is re-swelling, mechanically heating from inside, fracturing around the perimeter, well...jets in the dark could be easily possible!

          I would love to see Mattias or someone try to animate the whole process, however they may envision it happening, themselves.

          Just a flight of fancy, but now I'm hungry - My muffins are almost done! 😛

          • Sovereign Slave says:

            Now that the conversation seems to be moving away from jets via sublimation through direct heat from the sun and towards internal forces generating the necessary heat, still wondering about a few things. Broken down to its basics, for jets via sublimation (regardless of how it’s done) you need the following, right?
            Dust more or less evenly mixed in with the dust
            A heat source
            Funneling formations of some sort
            Pressure buildup to explosive yet sustained release
            Compared to the EU model, which only needs rock and electricity, this already seems like a complicated model with a lot of (as of yet) unsubstantiated assumptions and moving parts. Actually, none of the above have been substantiated, but let’s join the crowd and assume it’s true anyway. Gas easily escapes through the tiniest of openings and materials. Just one hairline fracture in a propane tank will do it. In fact, it’s quite difficult to actually contain gas. Just think of all it requires to do it – man made non-porous materials, man-made seals and valves, etc etc. In nature, gas can build up, but off the top of my head, seems this usually happens deep in the earth. And for this and many other reasons, as hard as it is to contain gas, seems even harder to contain it enough to build up gas pressure. Any slight fracture in the encasing material will slowly release it, or porousity of the encasing material will slowly release it before much or any pressure is built up. So it seems that if some cometary process that’s generating the heat required for sublimation also creates countless fractures, there’d be no way to contain the sublimated gas long enough for it to build to explosive pressures. And if the comet is mainly composed of ice and dust, and it’s density is indeed about half that of water, how can this relatively light, porous material contain gas at all anyway, much less gas that’s pressurized to levels similar to a man-made gas tank? And no, I’m not expecting real answers here, as there have not been real answers to my other questions along these lines, nor indeed can there be.

          • Sovereign Slave says:

            Oh, and while its very very difficult to contain gas and build its pressure (actually, the following would require a sealed container and a valve, but then perhaps these are being provided by the comet mining fairies), its astronomically more difficult to explosively release the gas in a manner that at the same time maintains an equalized chamber pressure that then maintains a continuous, completely steady gas release.

          • Gerald says:

            Sovereign Slave, we don't know the actual gas pressure in the ground. The speed of gas relaxing to the vacuum is about its speed of sound, almost independent of the actual pressure in the gas deposit.
            The size and distribution of the pores in the deposit, the size of the deposit, the viscosity of the gas, the permeabilty of the covering layers determine the release of the gas, together with the undergroud sublimation rate.
            The underground sublimation rate depends on the mean temperature on the surface, and thermal conductivity properties of the layers with depth.

            Observations of the jets can constrain these parameters, but not nail them down exactly.

          • Gerald says:

            ... Don't think of kilobars. A few Pascals or even less may be sufficient to explain the jets.
            From the Rosetta flybys and the drag on Rosetta, together with the supersonic speed of the gas, you may estimate the density of the gas, and track it back to the pressure at the surface of the comet. The pressures need to be low, otherwise Rosetta would easily been blown away from the nucleus.
            The pressure should be at most somewhere in the micro Pascal range some 10s kilometers away from the nucleus, to allow Rosetta to define reasonable trajectories (compare it to gravity, which was needed for bound orbits, and consider the large solar panels of Rosetta). Say 1 micro Pascal at 10 km as a very rough model estimate. Applying an inverse square law, we get about 25 micro Pascal at the surface. Assume 1% of the surface as active, the pressure in those regions would be 2.5 milli Pascal.
            Any pressure in the ground beyond this would be a pressure build-up.
            Conditions on the comets are rather different from those on Earth.

          • Gerald says:

            ... it's an ultra-high vacuum, micro-gravity environment.

          • Marco says:

            Sovereign Slave,
            I don't know where to start. Rock and electricity might be simple, but it is incoherent. You may as well say that the rock fairies and the electricity fairies did it. That's simple. I don't see any evidence of electricity or rock.

            Broken down to its simplest you don't necessarily need all or any of what you are talking about for internal forces being the source of the outgassing. Solar energy could still be all, part or not the initial energy source. There could theoretically be liquid water in the interior, so we wouldn't need ice or sublimation. Internal heat, if verified, would set the cat among the pigeons in terms of comet models. I think at the moment, it is simpler to assume there is no internal heat unless discovered.

          • Gerald says:

            ... For a better understanding, kinetic gas theory is probably the best way to start with:

            Then continue with diffusion and effusion:
            That's processes to be considered, when gas travels through porous paterial.

            Next is relaxation of a gas into vacuum. I didn't find an easy introduction.
            Bur this sentence may help:
            "The face of the gas moves forward into the vacuum with a velocity 3a, where a is the speed of sound in the gas cloud in its initial state."

          • Sovereign Slave says:

            All good posts, Gerald, thanks for the information. Gas dynamics is obviously a detailed and highly involved subject matter, and while I’m obviously no expert on gas and sublimation, the one thing that’s pretty apparent is that the jets-via-sublimation theory continues to totally defy detailed modeling. If a theory is sound, new data should generally reinforce the theory’s models and provide enlightening information as to the details of how things actually work. Things should generally make more and more sense and keep falling into place to the point of the model becoming predictive. Regarding P67, this has not been the trend with this theory, and in fact it remains a theory without an intelligent working model, and is currently being held up by little more than a pervasive belief that it’s true. And that’s fine, without belief in a theory it would never be pursued further, but at some point it needs to actually be able to provide reasonable, detailed explanations as to causes and effects. And if the jets-via-sublimation theory were correct, seems after this many months of testing there would be much more certainty about it and the model, many less puzzles, and not so many things that seem to directly contradict it.

          • Gerald says:

            Sovereign Slave, I don't see the doubt - you are talking of - about the jets via sublimation, among scientists.
            To me it's obvious and immediate. Another mechanism would be nice, but I don't see any hint for doubt, thus far.
            I see other people (mainly non-scientists) who don't understand the mechanism, and I think I can duplicate why they don't understand, but that doesn't affect my - at least basic - understanding.
            Everyday's experience is clearly misleading in this case. You need to go down to molecular level to understand, or do vacuum experiments.

          • Sovereign Slave says:

            Great, then please provide what the detailed, step-by-step, jets via sublimation model is. How do they begin, at what pressures, sun's heat vs surface depth, how the chambers grow, size of nozzles or openings, percentage mix of hard and fluffy dust with ice, how the hard and fluffy dust mixed to begin with, or if they didn't, how they both are being propelled, how the individual jets are merging into larger jets, length of time jets via sublimation last, rate of sublimation, how the sublimated gas is contained then released, how they jets are being produced in areas that have been in shadows for hours, the large width of the jets at their base, on and on and on. Maybe I missed something, but if there is a DETAILED, STEP-BY-STEP sublimation model specific to P67 that clearly explains how they work that backs up what you say above that's been presented, I've missed it.

          • Gerald says:

            Two approaches:
            - "the" traditional approach (abstract) without need for cavity/nozzels: http://www.researchgate.net/publication/223094523_Collimation_of_cometary_dust_jets_and_filaments

            - Your favored cavity/nozzle approach:

            In my opinion both approaches sound reasonable, and I wouldn't rule out, that further mechanisms may be found in future.
            I'm sure, that for 67P there will be written similar and more detailed papers.

          • Sovereign Slave says:

            Hadn't seen the first paper, only shows the abstract, which has no details, and apparently the full text isn't available. However, at this point it could be little better than educated guessing dressed up in "scienteze" language, much like the second paper. Already read it, and discussed long ago and find it highly improbable. I know no model can fully represent reality, and will ultimately contain failures of explanation. But a model should at least be useful, and for the life of me, the ONLY thing I see the sublimation model being useful for is explaining why there is water in the coma. Otherwise, the only other usefulness I see for it is providing the backdrop for the ongoing list of surprises P67 is providing.

          • Gerald says:

            The Rosetta mission adds such a huge amount of new data, that I'm sure the models will be refined by orders of magnitude.
            But this will take time. The scientists now are very busy with collecting as much science data as possible. After that they'll work on "data reduction", and write simulation software. Then they'll write their final papers.
            So all you can expect right now, are preliminary assessments and opinions.
            The older papers have been based on much less data, and can thus only represent a rough approximation to 67P.
            But the fundamental solar heating and sublimation/decomposition principles are very, if not exceedingly, unlikely to be challenged.

          • Sovereign Slave says:

            This all makes sense, and as you say, coming to some kind of understanding about all the things going on here will be a process, and there is a lot the models will have to account for. But with the primary mechanism of sublimation already being an unquestioned, foregone conclusion, I'm afraid much of the scientific process will simply be shoving square shaped findings into round sublimation holes.

          • Gerald says:

            @Sovereign Slave
            The findings will be almost "round". That's the main reason. If they would be squres, it would have been known before. Comets aren't that totally unknown, that just some unbased hypotheses have been repeated until everyone believes.

            Scientists will look for "small buldges" with eagle eyes.
            But it's also true, that any deviation from established "knowledge" and convictions needs particularly high evidence to be accepted, since this would mean, that older, also well-founded work would be at least partly incorrect.
            Likely are (lots of) new findings within the generally accepted constraints. Metaphorically the interesting findings to expect are the paintings filling the blank circle.

  • Lucas says:

    Where there any predictions made before rosetta reached 67P?

    Good theory should confirm predictions, if not, theory is false...

    • Attila Stersky says:


    • Ross says:

      A good theory should also be falsifiable. There is no possible observation which would single-handedly disprove the dirty snowball model as they can always be explained away using indirect observations and assumptions.
      The probability of a jet originating on a dark side and light side are equal because the thermal properties have little or nothing to do with the phenomena we observe.

  • 0,14 ppb says:

    rosetta catches tschuri in the act of outgassing... forza rosetta!

  • Phil Stooke says:

    We don't know enough to predict something like this. Also, it is observation which confirms predictions, rather than theory. The theory is what leads to the prediction, so if a theory does not match a prediction, something must be very wrong!

  • Bill Harris says:

    Although the a activity on 67P has been quite subtle thus far, it is my suspicion that near or after perihelion as the insolation and interior warmth increase, the nature of the activity may become somewhat cataclysmic.

    We don't know much about the energy wrapped up in these cryo-clathrates with involved gases but I have seen suggestions that at critical temperatures the clathrate-gas lattice will undergo a structural change resulting in a volume change or energy release, much like with decomposition explosives.

    There are too many large boulders, aeolian dust flows and Maar-like craters scattered about the surface to be from "subtle" processes. There has to be something less subtle that goes BANG...


  • Jim Robertson says:

    Like switching on an engine.

    If there is any net offset among the jets and the center of gravity of 67P, that will change its rate of rotation. What a show!

  • Gerald says:

    What a rare luck! Most of new-born jets certainly appear on the dayside, where they escape observation due to the bright background.

    Thermal stress close to the terminator might have been another factor opening the protective surface, e.g. by forming a fracture, or by removing dust during a "comet quake".
    Now we've to wait half a year or so for a flyby to test hypotheses.

    • harvey says:

      Bit like a Champagne cork maybe; you take the wire off, push gently, & just when you least expected it, BANG 🙂
      67P is good at fun surprises.

    • Kamal Lodaya says:

      Gerald: Not clear why this jet should be "new-born". Can't a jet be like a sun-heated geyser: shut down for the night and rise again at dawn (in two minutes) ? The white rocks we see in the long evening shadows of 67P might be evidence of a shutdown process.

      • Gerald says:

        Of course. There are several reasonable options.

      • Ross says:

        No because the jets are assumed to be pockets of ice and volatiles withith the nucleus. TEmperatures and pressures can't just shut down and restart, there must be an observable weakening in the stream as the temperatures cool and gradual increase as they're forced out of the assumed nozzle-shaped hole. I'm skeptical that any hole will be found, the origin of the jet is likely a cliff of sharp edge being eroded.

  • Sovereign Slave says:

    Mysteries piled upon mysteries. As the article states, there is a two minute time span between no jet and jet, and the event seems to be focused at one location. And yet, there are more than one jet, at least two. How does the sublimation model account for at least two jets coming from one event/source? For there to be jets by sublimation, the dust and gas has to be funneled through an opening, so accordingly there would have to be multiple openings for this chamber. But then, how do you maintain pressure? Also, one jet is quite large, one much smaller, from the same event. In fact, the large one looks huge at its base, certainly larger than one nozzle. So either many many nozzles all pointing in the same direction, or a very big opening, neither of which is not plausible for this model. Also curious that the jet lengths are more or less even in distance. And of course, all this in addition to the jets not only coming from a shaded surface, but a shaded surface that will soon be rotating into sunlight. That means it’s been shaded for many many hours, and again, if shaded craters on Mercury hold ice that isn’t sublimating, no way these jets were produced from underground by anything having to do with the sun’s heat. I can’t think of anything that falsifies the jets being caused by sublimation more than this event, not to mention all the other problems for this model I asked about in my 15/04/2015 post.

    • THOMAS says:

      Many thanks, Sovereign Slave, for this suitably succinct list of observation-based questions which indeed severely challenge the mechanisms invoked in the standard theory account of "outgassing".

      I would simply add one more question of my own:
      - how comes it that the TWO observed jets (and maybe more...) are leaving the same apparent source at an angle of *at least* 15% to each other (as seen from a simple line-of-sight perspective)? A hypothetical "explosive" event from deep below the surface would surely tend to preclude this, for obvious reasons.

      + One further comment: this is actually far from being the first time that jets have been observed emanating from "shaded regions" of the comet, the most famous example to date being the "plumber's pipes" coming out of the deeply shaded Hathor cliff-face (http://blogs.esa.int/rosetta/2015/03/20/cometwatch-14-march-6-hours-later/). There are many more, all totally inexplicable by the standard "ice-being-gently-warmed-by-the-sun-causing-outgassing" hypothesis.

      These are all just "smoking-gun" type observations which first need to be accounted for. The equations can wait until we've finally understood the basic processes at work.

      • Gerald says:

        THOMAS, so you are calling more than 100 Kelvins "gently"?

        • THOMAS says:

          Sorry, Gerald, but apparently you're not picking up on my no doubt typically English irony. I was actually suggesting that the alleged "gentle warming", coming as it does at the very end of several hours in deep shade, at a temperature which is considerably below that recorded in the coldest place on Earth (in Antarctica), is patently too pathetic to be the cause of ANY form of sudden sublimation, let alone the alleged “explosive” outburst which has apparently become the most recent ad hoc explanation for this observation.

          Sorry again, but what did you mean exactly with your “100 Kelvins"?

          • Gerald says:

            Your "gentle" warming applies to e.g. a perfectly smooth sphere.
            At least to my "untrained" eye the shape of the comet is "slightly" different from a perfectly smooth sphere.
            Since human vision is subjective to some degree, electric universe people might not have noticed this, yet. So I state the obvious, especially for you.
            Now, on a concave surface rotated in the sunshine, it might happen, that a previously shadowed area is suddenly exposed to more than "gentle" heating.

            I don't know, whether the sun ever shines to the location you're living. But if so, walk out of a shadow looking roughly towards the direction of the sun. You'll then immediately experience what I'm talking of.

          • THOMAS says:

            Sorry yet again, Gerald, but your comparison is as inappropriate as it could be, for obvious reasons, but perhaps I need to spell them out for you: as the comet rotates, the regions which have just spent 6 hours in night-time conditions wake up to a new day, just like on Earth. I don’t know where you live on Earth, but where I live and, as far as I know in most other places, the coldest time of the whole 24-hour cycle is immediately before and after dawn. And I don’t see any reason why the same should not be true of any other rotating object in the Solar System. Now this new jet on 67P exploded into life at precisely that instant, just before dawn.

            Now let’s consider the nature of the sunlight experienced by the Imhotep region as it emerges from nighttime conditions into the dawn of a new day. Again, if it’s anything like what happens on Earth, the first, near-horizontal rays bring absolutely no warming effect and the following few minutes virtually nothing either, as the Sun slowly rises above the horizon. But in any case, there is not even the slightest sign of any sunlight striking any part of Imhotep in the image: the sudden outburst started up BEFORE dawn. Any surmising to the contrary, despite the ocular evidence of the OSIRIS image, is simply the product of a desperate search for a new ad hoc hypothesis to keep the sublimation theory alive.

          • THOMAS says:

            BTW, Gerald, I find your idea that EU proponents had not noticed that 67P is not as round as a dirty snowball rather amusing, to say the least… We were the ones who expected to discover a lump of more or less irregular rock, much like the other comet nuclei which have been imaged!

          • Gerald says:

            THOMAS: "Again, if it’s anything like what happens on Earth, the first, near-horizontal rays bring absolutely no warming effect and the following few minutes virtually nothing either, as the Sun slowly rises above the horizon."

            Unfortunatels, despite your perfectly accomplished prediction of the comet's shape you didn't understand the implications.
            The condition "if it’s anything like what happens on Earth" doesn't apply.
            The Earth is almost spherical, and has a relevant atmosphere. This is the cause for the "absolutely no warming effect" of the "near-horizontal rays".
            Both conditions don't apply to the comet. Hence your incorrect conclusions.
            You'll find a better Earth analog in the mountains (concave and less atmosphere) in the (late) morning during summer. As the sun appears behind a neighbouring mountain you'll feel the rise in temperature. It's much more intense in the vaccum without filtering atmosphere.
            Add the close to black surface, and low thermal inertia.

            On the comet even such strange things like temporary shadowing can happen due to the very concave shape.
            You may even take photographs two minutes apart without catching solar illumination in-between.

            That's the straigntforward "cliff" scenario.
            If it holds, it could be used to make more reproducible observations of such fresh jets.
            If not, loads of other hypotheses are waiting.
            The next-simple option would be opening of a fracture due to thermal stress near the terminator, releasing super-volatiles.

      • originalJohn says:

        Another point THOMAS is that despite being in the shaded region with no incident (reflected) sunlight the new jet was illuminated. The light emitted must therefore have been intrinsic. It was in fact glowing, a plasma jet in glow mode. As the jet extended further into the open, where some incident light might have been expected, its brightness faded.

        • Gerald says:

          Originaljohn, did you consider, that the region wasn't as shaded as you suggest?

          • THOMAS says:

            Gerald, what is obvious in this new image is the brighter nature of the jets at their point of origin despite being in shade; this source-brightness has already been observed in other images, most typically:

            Even more interestingly, this sort of intrinsic brightness has also been observed, spectacularly, at the base of Martian “dust-devils”, as in this image from the NASA Spirit rover: http://www.nasa.gov/images/content/123624main_dust_devil_mars_web.jpg.
            or in these images from the AGU Blogosphere:
            (Note too the black scorch-marks on the Martian sand independently of the shadow being cast, a sure sign of extreme heat due to the electric discharge nature of the phenomenon: it is the same phenomenon which explains the blackened, low-albedo appearance of the comet nuclei we have imaged, starting with 67P).

          • Gerald says:

            Sorry, that's no intrinsic brightness, neither the jets near the comet nor on Mars.
            Although dust devils on Mars may actually charge by triboelectricity. Even discharges on Mars have been proposed. But that's not the cause for the dark streaks. Mars is covered by a thin orangish dust layer (orangish due to iron-iii, e.g. in hematite). Dust devils remove this dust layer and expose the dark basaltic material below.
            Extreme heat on Mars occured due to volcanism billions of years ago, and by impacts. Extreme heat doesn't occur near dust devils. The Opportunity rover has frequently been cleaned by dust devils, without any issues due to heat nor discharges.
            The Curiosity rover also experienced some gusts. The voltage is monitored all the time. Fluctuations of the voltage are mostly due to the rover itself, e.g. stepper motors, easily reproducible, or due to interaction of the rover with the terrain. Those fluctuations are well below 10 Volts.
            Temperature and wind are measured once per second over significant periods of time, look for the "*RNV*.TAB" files in this PDS directory: http://atmos.nmsu.edu/PDS/data/mslrem_1001/DATA/

            Frankly speaking, your interpretation of the dust devils as a discharge phenomenon is horribly ridiculous.

          • harvey says:

            "(Note too the black scorch-marks on the Martian sand independently of the shadow being cast, a sure sign of extreme heat due to the electric discharge nature of the phenomenon: it is the same phenomenon which explains the blackened, low-albedo appearance of the comet nuclei we have imaged, starting with 67P)."

            Black 'scorch marks' are generally a feature of scorching organic materials like wood, leaving carbon.
            If I 'scorch' sand, needing a rather high temperature, you can get all sorts of colored marks depending on the impurities present. Essentially you form an impure, 'doped' glass. Black is a possibility (a mess of different impurities) but I could make any color you like!
            Iron, the most obvious impurity, tends to lead to greens & browns, depending on how oxidizing the environment is.

            The cometary low albedo is perfectly well explained by organic UV induced photochemistry, & the IR spectra are consistent with that.

          • THOMAS says:

            @ Gerald

            "Frankly speaking, your interpretation of the dust devils as a discharge phenomenon is horribly ridiculous."


            Regarding the electro-magnetic properties discovered in dust-devils observed both on Mars and on Earth, please read the impeccably peer-reviewed article “Electric and magnetic signatures of dust devils from the 2000–2001 MATADOR desert tests“ at http://onlinelibrary.wiley.com/wol1/doi/10.1029/2003JE002088/full

            Here’s the extract:
            “Dust devils are significant meteorological phenomena on Mars: They are ubiquitous, continually gardening the Martian surface, and may be the primary atmospheric dust-loading mechanism in nonstorm seasons. Further, dust grains in the swirling dust devils may become electrically charged via triboelectric effects. Electrical effects associated with terrestrial dust devils have been reported previously, but these were isolated measurements (electric fields only) with no corroborating measurements. To study the fluid and electrical forces associated with dust devils, NASA's Human Exploration and Development of Space (HEDS) enterprise sponsored a set of desert field tests with a suite of mutually compatible and complementary instruments in order to determine the relationship between electric, magnetic, and fluid forces. The project (originally a selected flight project) was entitled “Martian ATmosphere And Dust in the Optical and Radio” (MATADOR). In this work, we present a number of interesting examples of the electromagnetic nature of the dust devil. We also describe potential hazards of the dust devil and how similar devil- and storm-related forces on Mars might affect any human occupation."

            “Interesting”, indeed. No further comment needed, I presume. (But please feel free, by all means, if you have more recent findings to counter both the article’s findings and its conclusions).

          • Gerald says:

            THOMAS, thanks for the link!
            The paper is perfectly consistent with my post above, it's maybe even a little more sceptical about possible electricity on Mars than me.

            "It is believed that the first few centimeters of soil are lifted by the dust devil, revealing the darker subsurface soil. " That's almost exactly the same I've written above.

            Or: "...the electrostatic energy in the dust devil,...is approximately 10^−4 J/m³ ...".
            How should this tiny energy heat rock? The 10^−4 J/m³ have been measured on Earth; Martian dust devils are even much less energetic.
            Compare it to the 1 million-fold higher thermal energy: "the dust devil's thermal energy ... is ∼10^2 J/m³." Still much too low to heat rock noticibly.

          • THOMAS says:

            I note that you carefully refrain from commenting on what is the very heart of the abstract:

            "In this work, we present a number of interesting examples of THE ELECTROMAGNETIC NATURE OF THE DUST DEVIL." (My capitals for emphasis).

            When, in your earlier post, you wrote: "Frankly speaking, your interpretation of the dust devils as a discharge phenomenon is horribly ridiculous", I had understood you to be denying, in the strongest (and rather unscientific…) terms, even the slightest possibility of dust devils possessing anything resembling electromagnetic properties.

            So could you now kindly give your view on this “electromagnetic nature of the dust devil” being presented in this impeccably peer-reviewed article?

            (For those who missed the beginning of this discussion, here is the link I provided earlier, showing a dust devil on Mars with a very bright base: http://www.nasa.gov/images/content/123624main_dust_devil_mars_web.jpg. Here’s another link I’ve found since, showing the same phenomenon in even more spectacular fashion, with several dust devils crossing the Martian landscape, all of them brightly illuminated at their base: http://blogs.agu.org/martianchronicles/files/2012/03/mars_a2.gif).

            “Horribly ridiculous” in the light of the conclusions of the peer-reviewed article I’ve referenced? Could you please explain your objections to these conclusions in slightly more scientific terms?

          • Gerald says:

            THOMAS, all matter made of atoms has electromagnetic properties, including dust devils, skateboards and rivers.
            So stating dust devils as owning electromagnetic properties is a null-statement. Large-scale discharges are a very particular electromagnetic phenomemenon.

            The paper investigated these electromagnetic properties for some dust devils on Earth, and assessed any discharge probability on Mars as low, despite one of its purposes has been to look for potential hazards on Mars.

            And as I've told before, the paper is based on assumptions in favor of a potential hazard on Mars due to discharges. Those - despite overestimated - assessed as very low hazards due to discharges triggered by dust devils haven't even been confirmed by the actual rover missions.

            Read the paper once again, count the "assume"s, look at the provided numerical measurements, particularly those I've cited in my previous post.
            And if you still believe in discharges on Mars, look at the actual rover and orbiter data.

            The bright base of the dust devils is just fine dust illuminated by the sun.
            The dust devil removes the this dust layer and leaves the dark basaltic rock below.
            Dust devils on Mars are not electromagnetic discharges. There will occur some triboelectric charging, as always in those dynamic situations; but the involved energy is tiny in comparision with the involved thermal or kinetic energy, and beyond this by far insufficient to cause any relevant discharges or communication issues.

            The idea of dust devils being electric discharges or causing discharges strong enough to cause noticible heating is "horribly ridiculous", since all in-situ data - and there are millions - show the contrary.

          • Gerald says:

            Here is btw the explanation of the dust devil images:

          • THOMAS says:

            @ Gerald

            "The bright base of the dust devils is just fine dust illuminated by the sun."

            Whereas the dust further up in the columns is a DIFFERENT sort of dust (less fine?) and is NOT illuminated by the sun? Please explain the reason for the difference in that case. (I'm referring in particular to the several dust devils in the animated gif: http://blogs.agu.org/martianchronicles/files/2012/03/mars_a2.gif))

          • Gerald says:

            THOMAS, there is less dust, and in average finer dust, further up, since the dilute Martian atmosphere isn't able to keep dust particles much larger than 3 microns for longer timespans.
            Most of the coarser dust leaves the central axis of the dust devil near the basis and sinks back to the ground, visible as a dust cloud surrounding the basis of the dust devil.

            For much higher-energy tornado on Earth you may observe a similar effect of grain sorting with height:

          • Gerald says:

            THOMAS, be also aware of the images being processed (motion enhanced) :
            "Note that the image is motion enhanced (that is, the contrast has been stretched on the pixels that change between frames); this enhancement makes the dust devil more noticeable", from this paper:

            Here is an example of a raw, unprocessed version of one of those images:

            And as a special service for you, THOMAS, here an animated gif of the Spirit Sol 1120 dust devil observation in unprocessed EDR format:

          • Gerald says:

            The direct link to the gif doesn't show the credit line, so credi t for the raw Sprit EDRs: NASA / JPL

          • Gerald says:

            Ok, THOMAS, and here a quick and dirty example of motion enhancement:
            You'll note the additional brightening of the base of the dust devil, as well as the darkening of the upper part above the horizon.

    • Gerald says:

      A plasma discharge in the (virtual) "electric universe" with 8 m/s would be rather slow. In the physical universe - the universe we are living in - electrical discharges progress much faster, wheras dust particles near the comet have been measured up to about 10 m/s velocity, very consistent with the newly published OSIRIS images.

      • THOMAS says:

        Sorry for stating the obvious, but the alleged " 8 m/s" figure for the speed of the two (at least) "900m" jets is based entirely on the assumption that they exploded from the surface IMMEDIATELY AFTER the first jet-less image was acquired and thus had a full two minutes to deploy. In all "logic", we might just as well assume that onset occurred immediately BEFORE the second image was acquired, in which case the speed needs to be multiplied by around 100, thus broadly agreeing with the 700m/s figure initially announced several months ago.

        In all probability, the truth lies somewhere between the two extremes and the speed of the jets is thus at least several times higher than that announced. But whatever the true speed, what worries me most is that the 8m/s figure is being presented as a scientifically observed fact, whereas it is actually only mere, improbable speculation. Unless someone can find a flaw in my logic?

        • Gerald says:

          I agree, that it is more like a lower bound. But the speed can't bee too high due to the exposure time.
          I would feel more comfortable with increasing dust velocity as the comet gets closer to the Sun, since the overall gas density should increase and hence improve drag.

          Electric discharges nevertheless would even be faster than the speed of sound. The OSIRIS camera wouldn't be able to capture a plasma discharge during its formation.

          "The corona discharge front moves as a wave with the velocity of the order of ion drift..."

          • THOMAS says:

            @ Gerald

            “Electric discharges nevertheless would even be faster than the speed of sound. The OSIRIS camera wouldn't be able to capture a plasma discharge during its formation.”

            1) The dust-jets may perfectly well be travelling “faster than the speed of sound” as I pointed out, since you find no fault with the logic of my argument.
            2) The onset of a plasma discharge is, for all intents and purposes, instantaneous and may last for an indefinite period of time at possibly varying degrees of intensity. Why shouldn’t the OSIRIS camera be able to capture it?

          • Gerald says:

            ad 1) Dust velocities above about 10 m/s aren't compatible with GIADA measurements thus far.
            ad 2) Do some of your proposed plasma discharges end a few hundreds of meters away from the comet?

        • Marco says:

          I agree with you THOMAS, the 8m/s is presumptuous and probably represents a minimum. Also presumptuous is the possibility of a cliff being in sunlight there. The shape models and lighting angle of the sun are well known enough that it should be an easy calculation to make. My reckoning is that it is unlikely to have been in sunlight at all, let alone got warm enough, quickly enough to do that.

        • Rob says:

          The flaw in your argument is this: you have misread the article's words "the researchers estimated the velocity of the dust grains to be at least eight metres per second" as if they had meant that the velocity WAS eight metres per second.

          As you rightly surmise, the researchers have taken the conservative view that, if the jets had started immediately after the first snapshot had been taken, then an estimate of "at least eight metres per second" would be about right. Your "worry" that "the 8m/s figure is being presented as a scientifically observed fact" is unfounded.

          • THOMAS says:

            @ Rob

            I think you've missed the point I was making, so I'll say it again for you:

            In what way is the ASSUMPTION that the jets started immediately after the first image was taken justified from a scientific and/or statistical point of view?

            In addition, your use of the word "conservative" to describe the "researchers' view" is meaningless in a scientific context when talking about quantification. Science should not be about "views" being either "conservative" or, for example "radical"; it should be about OBJECTIVE DATA. If by "conservative", you actually mean "what is likely to conserve the status quo", this would be even more worrying than I thought, since mission scientists should not be interpreting data simply to prop up the standard theory and I sincerely hope they are not.

            I have "misread" absolutely nothing and maintain, in all logic, that the speed of the jets could just as well be 800m/s as 8m/s. But that it's probably somewhere between the two.

        • sjastro says:


          "Unless someone can find a flaw in my logic?"

          Given the exposure time for these images is typically 1-2 minutes and the sublimation event is at time scales way beyond this, it is totally irrelevant to consider when the image was taken.

          The image is a snapshot of the event.
          Apart from the aesthetic value, photometry can be performed on the image and since the image scale is known, the variation in surface brightness of the jet can calculated as the jet fans out from the comets. From this a rough calculation of the dust velocity can be obtained.

          If you bothered to read the article you would have noted the rough calculation of 8 m/s is compatible with DIRECT measurements taken by GIADA..

          • THOMAS says:

            @ sjastro

            I hardly know where to start...

            So just let's take things in order:

            1) “Given the exposure time for these images is typically 1-2 minutes…”
            Wrong: if you look at the two images again, you will quickly notice that each of them shows very sharp outlines, both of the comet nucleus – especially the black night-shrouded contours of the Imhotep region -and of the radial jets, with two totally distinct images at two minutes interval. Exposure time is therefore necessarily limited to a handful of seconds, rather than “1-2 minutes”. Otherwise all outlines would be completely blurred and it would be virtually impossible to distinguish between the first and second images, apart from the outburst of the new jet. This is not at all the case, as you will see for yourself if you take a couple of seconds to look at the images again.

            2) “…and the sublimation event is at time scales way beyond this…”
            Not even wrong, just irrelevant to the argument: the whole point of my comment was to point out the implausibility of a “sublimation event” as a possible cause of the jets, given the statistical improbability of the outburst having started up in the 2 - 3 “seconds immediately following the first image, as required by the 8m/s speed for the jets. Your hypothetical “sublimation event” is simply part of your assumptions which are being very precisely and very severely challenged by this sort of actual OBSERVATION.

            3) “…it is totally irrelevant to consider when the image was taken…” No further comment needed. (See above).

            4) “…the variation in surface brightness of the jet can calculated as the jet fans out from the comets [sic]….” If you look at the image again, you will see that this is not ONE jet “fanning out”, but (at least) TWO jets shooting out at an angle of between 15 and 30° from each other: just one other enormous problem for the theory of the “explosive” outburst from a hypothetical sub-surface chamber. As for the “calculation” of the “surface brightness” of the jets, this is not even what is claimed above in Emily’s text, which is: “Tracking VARIATIONS IN BRIGHTNESS along the jet, the researchers ESTIMATED the velocity of the dust grains to be AT LEAST eight metres per second.” And here again, the “estimated” velocity of the dust grains of “at least” 8m/s is based simply on two assumptions: that the “brightness” of the jets is 1) actually measurable in such a fleeting snapshot (the article does not even mention “surface brightness” BTW, as you will notice if you read it more carefully) and 2) that the very notion of “SURFACE brightness” is simply irrelevant, given that these jets emerging from deep shade clearly possess INTRINSIC brightness, strongly suggestive of electric discharge activity.

            5) “If you bothered to read the article you would have noted the rough calculation of 8 m/s is compatible with DIRECT measurements taken by GIADA.” What the article actually says is “This is compatible with measurements made with Rosetta’s GIADA instrument ON OTHER OCCASIONS, for dust particles emitted from the comet’s surface. The “dust particles emitted from the comet’s surface” “on other occasions” clearly do not concern the major jets from the neck region which were measured by Rosetta scientists soon after Rosetta’s arrival at the comet as having a speed of “700m/s”. The “8 m/s” velocity observed “on other occasions” presumably comes from the more modest “sputtering” described by mission scientists around six months ago on the main lobe, which is presumably the cause of the several metre high halo increasingly enveloping most regions of 67P. The “900 metre” jets observed fleetingly in the OSIRIS image above clearly belong more in the category of the spectacular “700m/s” neck-region jets than in that of the feeble “sputtering” emanations.

            I think it is YOU who should read ALL the texts more carefully, sjastro.

          • Gerald says:

            Here an image with a known exposure time of 330 seconds = 5.5 minutes, as a comparison:

            So Sjastro's estimate of 1-2 minutes exposure time sounds reasonable.
            Take a 10 seconds break between two images, and we are at 110 s exposure time.

        • Kamal Lodaya says:

          Thomas: When someone announces a measurement, your first reaction is to mistrust it!

          • THOMAS says:

            @ Kamal Lodaya

            But this is NOT a "measurement", it's an extremely improbable (in the mathematical sense) interpretation (see my argument above). I always mistrust interpretations if they are based on what I consider to be faulty assumptions. Sorry, that's what I was trained to do and I can't seem to break the habit. If you prefer to take an interpretation as a measurement, you're perfectly free to do so. Just don't claim it's science.

          • Gerald says:

            THOMAS: "I always mistrust interpretations if they are based on what I consider to be faulty assumptions. "

            Why don't you start with the obviously "faulty assumption" of electric discharge activity at 67P/C-G, e.g. based on months of measurements of the magnetic field?

        • sjastro says:

          Where do you come up with the multiplication factor of 100X leading to a 700m/s value Thomas?
          Your derivation is a work of pure fiction rather than based on logic.

          Using the exposure times one can calculate what the maximum multiplication factor actually is.
          For simplicity let's assume the image download time and the dark and flat field subtractions (if performed) are included in the total exposure time and the second exposure instantaneously follows the first.

          The first exposure starts at t=0 and ends at t=120s, the second begins at t=120s and ends at t=240s.
          Since the sublimation jet occurs during the second exposure, the time T for this event is
          T = 240-(1-n)*120 where n is in the range [0, 1].
          If n=0, T=120s and the event starts at the beginning of the exposure.
          If n=1 T=240s and the event starts at the very end of the exposure.

          If the dust is travelling at a constant velocity then
          V=S/T where S is the distance travelled by the particle at time T and is calculated by measuring the angular size of the jet in the image.
          Taking the extreme values of T at 120 and 240s, the particle velocity is v1 and v2 respectively, and the maximum factor is.

          In reality however no photons are recorded in the image if the sublimation event occurs at T=240s, hence
          v2> 8m/s is the lower limit and V1<16 m/s is the upper limit.

          So much for claiming that this agrees with your 700m/s value.

          • THOMAS says:

            @ sjastro
            The "fiction" is on your side, with your presumed exposure times of 2 min, whereas the two images are perfectly distinct from each other (see my earlier comment above). Once your fiction has been replaced by the reality of mere seconds-long exposure, your basic assumptions are shown to be groundless and the resulting mathematical demonstration irrelevant. In all probability (in the mathematical sense of the term), the new jets are travelling at several hundred metres/second, like the jets from the neck region have been shown to do.

          • Gerald says:

            THOMAS, the image looks over-exposed. This may be either due to long shutter-times, or due to post-processing. All we know from the blog, is that exposure times are below 2 minutes.
            Even if we assume just a 2 second exposure, "in all probability (in the mathematical sense of the term)" is faulty in two ways:
            - "all probability" isn't a mathemetical notion,
            - if we would run a Monte-Carlo simulation to find out probabilities, the probability converges towards zero for the visible dust portion of the jets travelling with 700 m/s.

            The probability for the start of the jet is a-priori the same for any instant between the two takes. By constraining the interval to shortly before the second exposure, the probability for this scenario is proportional to the length of the constraint interval, zero (impossible in a physical sense) for interval length zero, and one (shure in the physical sense) for interval length two minutes.
            The probability for the jet to start within two seconds before the second take is hence 2s/120s = 1/60 = 0.0167, even if assuming an infinitisimal short shutter time.

            For any longer shutter times the a-priori probability for your scenario, THOMAS, is further reduced.

          • sjastro says:


            It is quite apparent you do not comprehend the mathematical arguments in which case I wouldn't expect you to adopt the same standard in showing why the new jet is travelling at several hundred metres per second. It is therefore nothing more than an ad hoc value pulled out of thin air.

            As far as the camera exposure time "presumed" to be 2 minutes, you should read the caption under the first image.
            You seem to have a fundamental difficulty in separating camera exposure time from the appearance of the jet in the second image. The camera exposure time is 2 minutes irrespective of when the jet appeared.

            The science of photometry or the measurement of surface or stellar brightness in an image doesn't require one to assume that the jet has to occur at the start of the camera exposure or at any other time for that matter.

            The camera or CCD does more than take pretty pictures, it also serves as a photon counter that allows information such as brightness to be extracted from the image.

            The most important requirement for photometry is that the signal to noise ratio of the jet in the image is high for accurate photometric data to be extracted and is based on an EXPOSURE LATITUDE rather than a specific exposure time.
            If the exposure time is too low, the jet is too noisy and the photometric data is not accurate.
            Conversely if the exposure time is too long, the CCD response becomes non linear, the image becomes saturated, making photometry impossible.
            The exposure latitude is the range within these extremes.

            Since the signal to noise ratio of the new jet is high enough, the camera exposure time minus the time in which the jet appears is within the necessary exposure latitude.

            Hence it doesn't matter whether the jet appeared at the start of the camera exposure, near the end, or any time in between provided the signal to noise ratio is high enough.

            The surface brightness in the region of interest is calculated by measuring the pixel values and "comparing" to the boundary with a low pixel value region corresponding to "dark" areas in the image.

            From this information a rough value of the dust velocity can be obtained.

    • ianw16 says:

      Well, what is the alternative? Given the fact that electric woo can be ruled out due to numerous findings, such as the remarkably non-magnetised state of the comet, the enormous voltage required for the comet to have an apparent density of ~470kg/m^3, whilst really being > 2000kg/m^3 (see Harvey's post in the GIADA thread), not to mention the detection of water and numerous other gases which sublimate from ices, plus the spectral analysis of the comet which also suggest ices, particularly in the neck region, then what are we left with? Underground comet mining fairies, who shift stuff around causing it to vent from collapsed surface material?
      Actually, after "mainstream" theories, this is my second favourite, as it's more likely than anything the EU crowd have come up with.

      • Kamal Lodaya says:

        ianw16: Caves in the interior semi-full of ices which slosh around as the comet rotates! (Assuming that Consert analysis doesn't work for the bigger lobe.)

        • THOMAS says:

          So, special pleading for the bigger lobe with regard to the published CONSERT findings (hypothesized "caves in the interior")? And ice which is allowed to "slosh around" under simple, slow rotational effects, at temperatures well below the coldest recorded on Earth? There must surely be a better explanation....

          • Kamal Lodaya says:

            I agree it is a weak point. Also "slosh" hides the phase, is it solid or liquid or some combination that one is talking about? "Semi-full" is also ambiguous, is there a vacuum or some gas in the non-full part? Taking the idea further, can Philae be in some cave like structure of which the roof has collapsed (since the sky is visible)? If so, does the Consert analysis rule out more such cave like structures in the neighbourhood?

        • Kamal Lodaya says:

          A side question which I had not thought about: will ices sloshing around generate a detectable magnetic field>

          • Gerald says:

            Only indirectly by sublimation and subsequent ionization.
            Very closely you could maybe determine some diamagnetism for large fragments; that's a magnetic effect of "non-magnetic" materials, a modification of an existing exterior magnetic field.

      • Sovereign Slave says:

        Hi Ian. Made no mention of the electric hypothesis in my post, and these questions have nothing to do with electric theory, and everything to do with jets via sublimation theory, which either needs to be intelligently defended by its proponents as new data and observations appear, or set aside if data and observations obviously falsify it. Contending that an opposing theory is nonsense is a poor defense for your own theory, or defending it by saying the equivalent of “what else could it be?” But that’s the human tendency and danger, as I’ve stated many times. People get locked down into a theory so much that they stop honestly looking for alternative, and when needed fundamental, explanations. When this happens, all old and new data is either interpreted as supporting their theory and models, or the data is not considered to seriously challenge it, or the data is simply ignored if it’s too challenging, or rationalized away, or not considered and questioned in enough detail, or the ad hoc add-ons start happening to save the theory. And yes, this can be said of EU proponents as well. And even though to me EU fits better as an overall cosmology, the devil is in the details as they say, and it obviously is also directly challenged by some of the specific data about P67 at this point. But I think if P67 is proving anything, it’s that we all know less about comets and their activities (and no doubt their origins) than we thought we knew, and probably much less. So at some point, it seems advisable (and there may be no other choice) to take a few steps back and honestly ask some pretty fundamental questions about comets and P67 without assuming our models and theories are right, look at it with fresh eyes, and simply creatively play with the data and observations, as well as the possibilities. It’s hard to step outside of compartmentalized pre-programmed thinking and honestly reconsider long held theories, or consider new ones, but even considering underground comet mining fairies helps break up rigid thinking habits. Would also be refreshing to hear, “We’re not exactly sure what it is, that’s why we’re here to investigate.”

        • Gerald says:

          The last sentence is certainly self-evident.
          But it doesn't invalidate results of prior investigations, e.g. sublimation due to solar illumination as the main cause for the dust jets, including the significant presence of ices (or clathrates) on comets.
          The mechanisms aren't known in all detail.
          Particularly interesting is the organic chemistry; there are lots of unknowns in this field to be investigated.
          Detailed measurements of the evolving magnetosphere are a "first". Collecting cometary dust with very low relative velocity, and investigating individual grains is a "first", too.
          This mission will certainly more than double the knowledge about comets.
          Restricting the view to the - silly - question about jets interpreted as electric discharges, is a vast underappreciation of the actual science done by Rosetta and Philae.

      • originalJohn says:

        As you are clearly easily persuaded ianw16 you can rule out an electrical explanation, based on less than a handful of incomplete measurements and ad hoc assertions as well as calculations filled with biased assumptions and lack of knowledge of the environment Perhaps you will be ruling it back in when a reasonable and scientifically valid amount of data is available and published.
        You note the non magnetised state of the comet (nucleus) but a magnetic field has been detected in a limited region of the coma, in keeping with those of other solar system bodies, considering the comparatively minute size of the nucleus.
        Also the water and other compounds identified in the coma cannot be exclusively linked to sublimation. As i have previously pointed out they are also products of hydrocarbon combustion, including some specific to incomplete combustion.
        There are far more plausible explanations than the dying sublimation story and no measurements have yet been made to discount them.

        • ianw16 says:

          No, I'm not easily persuaded. That's why I'm not an acolyte of the EU nonsense. When I was a child, and into my early teens, I was quite enraptured by Velikovsky and Von Daniken. Then I grew up, and got an education.
          As for electric explanations, well it looks like you are consigning Coulomb to the EU dustbin of history as well, yes? For this comet to be rock, having been blasted off a planet by some sort of discharge, then it would have the density of rock. 470kg/m^3 isn't a typical density of rock, funnily enough. Were it to be the true density of rock, say > 2000kg/m^3, then all of Rosetta's measurements are wrong, and it would have crashed into the comet long since. The only (desperate) explanation I've seen of this apparent show stopper for the EU lot, is that it was electrostatic repulsion. Now, don't forget, this repulsion must be acting with the same strength at all times, in order to mimic gravity, otherwise plotting an orbit in an ever changing "gravity" field would be impossible. Therefore it must scale as 1/r^2. It will be dependent on the charge on the spacecraft and comet not changing. We now know the charge on the spacecraft. Harvey has made an assumption on the capacitance of the spacecraft and the comet, both ballpark figures, but well within an order of magnitude. A very similar estimate, before Rosetta's charge was known, is made here: http://dealingwithcreationisminastronomy.blogspot.co.uk/search/label/Electric%20Comets
          I ran Harvey's figures again, and there is no error. You are either saying this is rock, and carries a charge of 10^4 Coulombs, and a voltage of ~150GV, or it isn't rock. Or you dismiss Coulomb completely, as his pesky equations don't fit with EU expectations.
          I'm sure there are some very interesting observations and discoveries to be made at 67P, a number of which may give some significant tweaks to the standard theory. That, after all, is why it was sent. Do I believe they will turn out to be caused by electric discharges or glows or whatever else the EU lot are invoking?
          Not a dirty snowballs chance in sunlight.

          • Harvey says:

            Thanks for the link, interesting, they reached the same figures independently, I'd not seen it.
            The other amusing implication of course is that even with the insane voltages, they isn't enough charge to run any sizeable discharge for more than seconds!
            So there was a sort of half baked retreat from the 'charged comet' to some version where it's is somehow picking up electrons at the same time, it's magically an anode and a cathode simultaneously, with electric fields pointing in opposite directions and a mysterious low impedance connection between them.........
            I couldn't agree more; lots to discover, things in the standard view will topple for sure.

            The comet will pass perihelion, Rosetta will sadly eventually 'die', we will understand things better.
            And the EU will continue to scream 'discharge, you didn't make the right measurements, it's all a conspiracy of people supporting the standard model, you are hiding the data that proves we are right............' They won't of course provide any numbers, formulae, computer models, comparisons to the data; just incoherent arguments. But they will be utterly sure they are right, whatever the data says.

      • Kamal Lodaya says:

        Sovereign slave: There is no "sublimation theory". Sublimation is a well understood process and many current comet models use it in their explanation of the processes going on on a comet.

        The "electric theory" is an idea. There is no comet model which uses electricity to explain what goes on on a comet. The basic idea proposed is that a comet is nothing but an asteroid, and for asteroids one can use whatever models people have developed (so if asteroids are rocks, so are comets). The only special thing about a comet is that it has tails. For the ion tail one uses whatever models people have already developed (so if a Nobel laureate developed one, use his name wherever you can rightly or wrongly), so that is not special. The dust tail is explained by talking about electrical machining, and here there is a major lacuna: setting up a convincing model of the process explaining the observed data. Where the current comet models score is that most of whatever little we know of comets they can explain.

        You can feel that the EU ideas tie in better with your intuition and will revolutionize all knowledge of comets. But I can feel differently. Our sitting in armchairs and postulating doesn't settle an argument.

        • Sovereign Slave says:

          Hi Kamal, all good points, especially the last paragraph, although since Emily and ESA have graciously afforded us the armchairs, it's certainly fun to take advantage and pipe in with postulates.

          Yes, sublimation is of course well established. I think it's clear the theory I'm referring to is jets via sublimation. I'm not saying this discounts there possibly being ice on P67, or sublimation, just that the sublimation jet hypothesis specifically is in dire jeopardy for obvious reasons. And this latest occurrence is even raising questions in the scientific community if comments are being made to the effect that other mechanisms for producing the jets may need to be considered. So, it either needs to be logically remodeled in a way that better fits observations and data, or set aside altogether for better constructed models.

          More importantly though, thanks for introducing me to yet another great new word, lacuna! Is no doubt the underlying theme for most of the discussions we’re all having here.

    • Sovereign Slave says:

      In follow up, good article about this event here:


      I guess the current sublimation jet model is hanging on a cliff by its fingernails, literally. Not sure how they’re going to determine whether there were any cliffs in daylight at that precise moment, or where the jet was specifically coming from in relation to the cliffs even if so. Perhaps it will just be ultimately assumed that the sunlit cliffs were there (they had to be, right?) so we can kick the sublimation can down the road a bit further. But even if there were sunlit cliffs, they could not have been in sunlight very long. And to think that in a matter of minutes the incredibly weak source of heat from the sun at this distance could penetrate through at least several centimeters of surface thickness to warm the theoretical dust and frozen volatiles enough for them to explosively sublimate is simply not rational. Nor is thinking that nature has created all these perfectly sized nozzles that maintain just the right internal chamber pressure (with no other cracks or openings at all) while shooting all the dust into jets that all travel at pretty much the exact same speed. But anyway, if it is confirmed that there were for sure no sunlit cliffs found, as Carsten Güttler says in the BBC article, “we have to find another mechanism.” He has a flair for understatement. But my guess is that, in the quest to save the dirty snowball theory, there will be many more rationalized mental contortions that ascribe to the comet what can only be called magical properties even less rational than explosive jets via sublimation.

      • Gerald says:

        Sovereign Slave, "your're misrepresenting the theory/argument, then poking holes at the misrepresentation." (Sovereign Slave)

        "I guess", "Not sure", "Perhaps", "my guess".

        Wait for the papers of the scientists, before drawing wrong conclusions.

        • Sovereign Slave says:

          Good one, Gerald!

          OK, perhaps if you could again explain, step by step and in detail, the jets via sublimation theory/argument, you could enlighten me as to the error of my representations and conclusions.

          • Gerald says:

            Sovereign Slave: "And to think that in a matter of minutes the incredibly weak source of heat from the sun at this distance could penetrate through at least several centimeters of surface thickness".
            Wrong or unbased presumptions: - The sun isn't incredibly weak, it's more than 100 W / m² at the current distance of the comet to the sun.
            We don't know the thickness of the dust layer at the base of the jet. At the Philae landing site there is no dust layer.
            We don't know the composition of the emitted gas. CO2 and CO sublimate at much lower temperature.

            Sovereign Slave: "Nor is thinking that nature has created all these perfectly sized nozzles that maintain just the right internal chamber pressure ..."
            Simple exposure of ices to the vaccum above a temperature specific for the type of ice/clathrate is sufficient to produce a gas flow with the respective speed of sound, since the velocity of the individual molecules makes the velocity of the gas in absence of significant collisions. This gas is able to drag dust if present.

            Besides this the scientists are hoping to find other mechanisms, like a phase change of ice. This would be something they could write a paper like "First In-Situ Observation of a Phase Change of Cometary Ice". That would sound good, and I'm looking forward to that kind of discoveries.

          • Gerald says:

            I should have added a link to phases of water ice:
            Somewhat similar to carbon, which can exist as graphite or diamond (and other modifications), water ice can exist in different phases.
            Recently phase number 17, called ice XVI has been suggested:
            (I think logan has pointed to this before.)

          • Sovereign Slave says:

            Well, saving jets via sublimation is obviously a work in progress, and no doubt being undertaken with all due imaginative diligence. I will offer this thought regarding EU theory. The 09/04/2015 article post stated an unexplained surprise (and prompted probably the hottest and heaviest debates yet) - negatively charged fluffy dust. As far as I can tell amidst all the back and forth posts flying around, this was indeed an unexplained surprise that has not been resolved, besides vague non-explanations such as the plasma environment.

            Here are my thoughts… Prior to ESA announcing this occurrence, I’d be willing to bet that if an EU advocate had speculated that the dust coming off the comet was going to be found to be negatively charged, you would have again rolled your eyes and provided all kinds of arguments and equations pointing out how ridiculous that was. Yet ESA announces it, and sure it’s a head scratcher, but no counter arguments or equations from you protesting it. So currently, this is a confirmed phenomenon (slowed dust mostly likely due to its being negatively charged) that has no worked-up standard theory, model, math, or real explanation. AND, it’s an electrically based phenomenon that is causing an unexpected effect. Could it be possible, or even probable, that there are other unexpected and unexplainable electrical phenomena to be discovered that are causing effects not currently accountable by standard theory beliefs, theories, models, equations, etc etc?

          • Gerald says:

            Sovereign Slave: "Could it be possible, or even probable, that there are other unexpected and unexplainable electrical phenomena to be discovered...?"
            I hope so (replace "unexplainable" by "not yet explained"). But please based on data and physics.
            Within all this "electric universe" spam it's hard to focus on actually interesting phenomena, including electric ones.

            Actually I've suspected charged dust months ago when analysing curved streaks caused by dust due to long shutter times.
            But the streak wasn't perfectly parablic as I would have expected for charged dust. So I wasn't quite sure. Partial disruption of the fluffy grains might explain the discrepancy; I didn't consider the latter as likely, hence was unsure about the electric repulsion approach.
            Surprising to me, that part of the dust is so extremely fluffy. To me charged dust wasn't a surprise; I just missed the definitive measurement. An electric charge of more than about 10 V would have been not quite as straightforward. Although besides photoelectricity by uv I'd consider triboelectricity and charge separation for yet to detect fast dust jets. This mechanism might allow for higher voltages. We'll see whether this will ever occur.

          • Sovereign Slave says:

            Fine, let's remove electrical forces as possible explanations for the cometary jets. Also, according to Mattias Malmer regarding the jet in this post's article,
            "The sun did not hit the "underbelly" of the duck until one hour 15 minutes later. And the area had been in shadow for five 1/2 hours or so." So we can rule out jets by sublimation due to the sun's direct heating right now. The sun did not cause this jet, at least not by it's sunlight. It HAS to be caused by something else, no if's and's or but's, and as Carsten Güttler says, another mechanism needs to be found. So, Gerald, we're all ears, what is the other mechanism?

          • Gerald says:

            Sovereign Slave, the first thing to do would be to double-check the SPICE data, to be sure that they contain the actual, and not the planned trajectory.
            If we can then assume, that no short-term solar illumination is involved, an approach could be varying assumptions about the composition of the gasses, better would be measurements. The composition of the sublimating ices determinates the temperature (as a function of pressure) of their sublimation/decomposition.
            A second ingedient is the velocity, heat (by solar illumination) penetrates the outer layers of the comet.
            Due to seasonal (not just diurnal) increase of solar heating, the overall temperature of deeper layers of the comet is increasing. Those deeper layers may not be much affected by diurnal changes of solar illumination, but by longer-term seasonal temperature changes.
            That way volatiles in deeper layers of the comet get warmed above the respective sublimation point, not necessarily directly related to local time-of-day (like melting permafrost on Earth responsible e.g. for some landslides/rockfalls in mountains).
            So we get some pressure build-up by sublimated volatiles in those deeper layers (with or without forming pockets).
            Several mechanisms could release these volatiles.

            - A simple one is just disruption of surface by the pressure of the volatiles.

            - Thermal contraction of the outermost layer due to the low temperatures at night could open a fissure in the top-most layer and allow release of gasses from layers below.

            - A more complex mechanism would be fatigue due to thermal cycling, which is going to become stronger due to increasing day-night temperature differences. That way the top-most layer would get more permeable for volatiles.

            - Portions of the ices may change their phase or just soften, no longer able to withstand gas pressure. Hardness of water ice reduces with increasing temperature.

            If equally likely option exist, a mix of them may actually be present.

            Depending on the composition of the ices, differently deep layers may be affected.
            Layers rich in carbon monoxide may start to decompose at a few tens of Kelvins. Layers rich in water ice may persist up to a little above 200 Kelvins.

  • steve says:

    Does these outbursts cause concerns for trajectory change? For example, put it on an orbit course for us the next time it comes round? Or will it be 1000s of miles out?

    • Gerald says:

      Thus far the momenta of the outbursts are tiny in comparison with the momentum of the comet (seen from the common center of mass).
      No problem for Rosetta to adjust to the tiny trajectory changes.

  • originalJohn says:

    Hmm,, an explosive outburst deep inside the nucleus producing a cylindrical hole at the surface as though a shaped charge had been ignited. Interesting new ad hoc explanation, nothing to do with the sublimation story.
    A new point of electrical discharge is a much better explanation. Dark side or not is irrelevant then. If you start logging and mapping these discharges Holger and Jean-Baptiste perhaps you will find them stopping and later starting again from the same point. Would explosive discharge explain that if the hole was already there. Whereas the charge concentration at the surface is probably constantly changing.

    And Emily, may I remind you that the ice sublimation explanation is an unproven hypothesis yet you are presenting it as fact.

    • Gerald says:

      Emily presents sublimation (including possible clathrate decomposition) as a fact, because it is a physical fact.

      • dave says:


        What exactly are the facts of your sublimation model, forget EU for a second, where is the proof for sublimation that you are so sure of?

        I don't think its any way as cut and dried as you think

        • Gerald says:

          It's not "my" sublimation model. It's since decades the only widely-accepted family of models.
          Estimates about dust/ice ratios, trapped gasses, and the presence of a partially organic surface crust may have changed over time.

          The presence of ices has been inferred spectroscopically from the the coma gasses decades ago.
          Nothing fundamentally has changed since then. Additional evidence has been found by more observations and flyby missions, including Deep Impact.
          More details are expected to be (and some already have been) pinned down by the Rosetta mission.

      • originalJohn says:

        Sublimation can occur Gerald. We all know that. The question is is it occurring with this comet or has it with any other. Who knows. There is no supporting evidence. It is a hypothesis. What you call evidence is speculative interpretation ie further hypotheses. There is nothing scientific to permit the presentation of the occurrence of sublimating ice on or in this comet nucleus as fact.

        • Gerald says:

          We don't know exactly how the ices on the comet decompose, whether it's clathrate decomposition, sublimation or decomposition of some other hydrated state, or some mix of these mechanisms.

          But without going that deep into detail, it's valid to subsume it as sublimation.

    • Kamal Lodaya says:

      OriginalJohn: You are now supposing a cylindrical hole, no one is suggesting such a thing has been seen on Imhotep.

      How does the electric theory explain a new discharge? Or if you are suggesting that this discharge point already existed, what caused the discharge to restart? If all one can say is that this depends on the charge distribution on the comet, which places are more negative than others, then I don't see how one would go about testing this.

      An increase in activity happens in all comets as they approach the Sun. In the electric theory this has to be explained by moving towards a more positively charged region of space. Why should this give rise to more jets?

  • THOMAS says:

    What I also find remarkable about these new jets is their precise location: slap bang in the very middle of Imhotep. In other words, apparently exactly on the comet's longitudinal axis. I wonder if this can be a random occurrence? Especially since the vast majority of the jets observed since Rosetta's arrival at the comet have been radiating from the neck region, i.e. in a plane which is orthogonal to the direction of the new jets.

    I believe these are the first major jets to have been observed emerging in the longitudinal axis.

    It will be extremely interesting to observe whether, form now on, as 67P approaches perihelion, more jets do the same. This was very much the case, for example, in the case of Comet Hartley 2 (http://www.jpl.nasa.gov/news/news.php?release=2010-371). This preferred localisation of jets in either the waist/neck region or in the longitudinal axis, if confirmed, would be an extremely interesting general characteristic of comets. The coming weeks and months will tell.

    • A.Cooper says:


      This may be an answer for the longitudinal axis location of the new jet, though I'll admit the marginal force changes described below are small. For the purposes of this conjecture we'd need to bear in mind that the longitudinal axis (Y-axis) is on the rotation plane. Indeed, as it rotates, it sweeps out the rotation plane.

      The jet seems to emanate from the edge of the anomalous red area noted by ROSINA in this post:


      This red area is thought to be the source of anomalously large amounts of CO and CO2 sublimation and is centred on the shallow scree slope that rises from the Imhotep plain to a very smooth top stratum layer (to the right in this photo).


      I know you're not a fan of sublimation but I'll scrub round that. The main point I want to make is that this stratum layer appears to have broken away. There's what appears to be another broken stratum further down the slope as well. So there are exposed fracture planes, between strata, in the vicinity of the new jet.

      If we consider this in the light of the comet's rotation speeding up, these fracture planes, which are towards the extremity of the long axis, are being subjected to increasing 'centrifugal' force (though not much more admittedly). This could be opening the fracture plane just enough to cause seismological disturbances in the interior. Moreover, these could be sudden slippages after weeks of rotational speeding up and sudden release of tensile forces. All other things being equal, this potentially sudden, rotation induced disturbance is just as likely to happen at night as in the day but it may release gases that were on their way through the fracture plane during the last sunset but were choked off by thermal contraction of the stratum.

      You may be able to work this rotation speed-up/ fracture opening into EU theory. The main point I'm making is that spin-up may be seismologically affecting the strata in the ROSINA red areas. This may explain not only this new jet but also the anomalous CO and CO2 readings.

      Although the marginal force changes on the strata are small, as I say, they could have built up and all been released at once. And any sudden new appearance of a jet is going to be due to a marginal change between the conditions just before the event which weren't conducive, to conditions at or just after the event that were conducive.

      • A.Cooper says:

        I meant to say spin-down, not spin-up. The rotation period did speed up by 20 minutes from 2009 to 2014 but now it's slowing. This just means the same conjecture applies in reverse: the strata are more likely to collapse together under increasing influence of gravity with spin-down. So compressive resistance that has been resisting collapse for weeks suddenly gives way and chambers or fissures collapse to a smaller volume, expelling gases.

      • Marco says:

        The change in force due to diurnal tidal effects (and possibly thermal expansion and contraction) is far greater than the gradual increase or decrease due to rotational velocity changes. Additionally, this close to Equinox, the diurnal effect is nearing a maximum.
        At Equatorial surface midday and midnight, the tidal effect is greatest and the surface gravity is at a minimum. The opposite at dawn and dusk. The fracture on Imhotep does appear to be in the right spot for a fracture related slippage, and the "spring" tidal effect a likely candidate to precipitate movement there.

        • Gerald says:

          Thermal tides are an option.
          Tides by gravitational tidal forces are probably too tiny; I tried to estimate an upper bound of the energy release here:

          • Marco says:

            Hi Gerald, as I explained in a comment to that comment: As on Earth, on 67P the energy is not coming from the tidal force itself, so your calculation is irrelevant to the postulated process at hand. The energy is from the rotational kinetic energy.

            That is what the calculations should be aimed at. Our Earths moon can move further away due to tidal friction, and a similar process can stretch the comet due to tidal friction, and the heat generated by the friction is dependent on the evolution of the relative motion, not on the tidal energy.

          • Gerald says:

            Hi Marco, either there is a flaw in your mechanism, or you're referring to something I'm ignorant of. There is a lot of literature about tidal heating.
            Could you link to a paper, I'm possibly ignorant of, which uses other mechanisms than tidal energy (including tidal waves for low-viscosity objects) to explain tidal friction and tidal heating?

          • Gerald says:

            ... I've tried more accurate calculations and got something like 10 pico Watts (1e-11 W), but with lots of estimates e.g. about the viscosity of the interior of the comet.

          • Gerald says:

            ... An example of my attempts; maybe you can help me overcoming my flaws, or sourcing the imaginary part of the second degree complex Love number of the comet:

            G = 6.67e-11 N m² / kg²

            Eccentricity e = 0.64102
            Mass m = 1.0e13 kg
            Mean density rho = 0.47 g/cm³
            Semi-major axis a = 3.4630 AU (518,060,000 km)
            r < 2050 m

            Mass M = 1.99e30 kg

            n = sqrt(G (M+m) / a³)

            n = sqrt(6.67e-11 N m² / kg² (1.99e30 kg + 1.0e13 kg) / (5.18e11 m)³)
            = sqrt(9.55e-16 N m² / kg² kg / m³) = sqrt(9.55e-16 N / (m kg)) = sqrt(9.55e-16/s²) = 3.09e-8/s

            k_2 the second degree complex Love number

            http://arxiv.org/pdf/1402.2378.pdf, equation (29)
            Q_tide = (-21/2) ((R_p n)^5 e² / G) Im(k_2)
            = (-21/2) ((2050 m 3.09e-8/s)^5 0.64102² / (6.67e-11 N m² / kg²)) Im(k_2)
            = 6.6e-11 ((m^5 / s^5) / (N m² / kg²)) Im(k_2)
            = 6.6e-11 (kg m² / s³) Im(k_2)
            = 6.6e-11 W Im(k_2)

          • Marco says:

            Hi Gerald,

            This following excerpt from Wikipedia explains the process of angular momentum exchange between Earths rotation and the moon's orbit: note that even though there is an increase in gravitational potential energy with the moon further away, even added to the net kinetic energy in this exchange, the net energy is reduced somewhat, . Where is the excess energy converted to? Heat in the Earths oceans through friction. Note that none of the tidal energy that you are calculating is used up, Just the rotational kinetic energy of the Earth.

            "The notional tidal bulges are carried ahead of the Earth–Moon axis by the continents as a result of Earth's rotation. The eccentric mass of each bulge exerts a small amount of gravitational attraction on the Moon, with the bulge on the side of Earth closest to the Moon pulling in a direction slightly forward along the Moon's orbit (because Earth's rotation has carried the bulge forward). The bulge on the side furthest from the Moon has the opposite effect, but because the gravitational attraction varies inversely with the square of distance, the effect is stronger for the near-side bulge. As a result, some of Earth's angular (or rotational) momentum is gradually being transferred to the rotation of the Earth–Moon pair around their mutual centre of mass, called the barycentre. This slightly faster rotation causes the Earth–Moon distance to increase at approximately 38 millimetres per year. Conservation of angular momentum means that Earth's axial rotation is gradually slowing, and because of this its day lengthens by approximately 23 microseconds every year (excluding glacial rebound). Both figures are valid only for the current configuration of the continents."

            With regards to 67P, If the comet is stretching, the same kind of angular momentum exchange is happening between the lobes of the comet. The gravitational potential energy increase due to the lobes being further apart is less than the decrease in the kinetic energy due to the conservation of AM. The excess energy will be converted to heat in the neck where it would be stretching.

          • Gerald says:

            Thanks Marco! This helps following your line of reasoning.
            I've shown here, that the angular energy (with respect to its baryenter) of the comet is very low, insufficient for relevant heating over more than a few days, even at a small surface area, and with rapid spin-down:
            The key issue is the low equatorial velocity of just 28 cm/s. The kinetic energy is proportional to the square of the velocity. Earth rotates with 465.1 m/s at the surface on the equator. That's 1661-times faster than the comet, hence 2,759,158-times (with about two significant digits) more energy per mass.
            Even this doesn't heat Earth very much; spinning down takes Earth billions of years.

          • Marco says:

            Hi Gerald,
            Firstly, thanks for the figures and calculations, as they are certainly relevant for what I am talking about, and I will go back to them again, when mission scientists measure how much current stretch is happening.

            However, you have been expert at disproving something that I don't believe in myself. I have checked my comments to make sure that I didn't say that I believed that the tidal friction is the primary thermal input that causes the outgassing. The friction generated via 67P stretching is concentrated in the weakest point, being an internal point (rather than a volume) well below the surface. Thus a more important calculation would be calculating the total energy release for a slowdown of one second per rotation, and the volume of water (or mix of volatiles) this could liquefy. On every rotation, you can see this mechanism could generate local internal liquid water hot spots, this would open up other possibilities such as serpentisation - a very exothermic reaction you mentioned that requires liquid water.

            Internal point heat sources such as these, knowing some of the minerals inside comets, could cause a mini chain reaction of plausible exothermic reactions, multiplying the initial heat source. The remainder of the comet may act as a kind of igloo, keeping a lot of the heat inside, rather than insulating from the sun outside. Due to the low pressure, and lack of good seal of any internal compartments, gases would push through to the surface as they built up pressure, creating geysers or tunnels of sorts. Gases would recondense in these tunnels, allowing heat exchange between surface heat sources and internal heat sinks, and visa versa depending on different chemistry, insolation and the relative importance of different heat sources at different times.

            Clearly, all of this is moot if the comet is found not to be currently stretching at all. Evidence of past stretching is very compelling, and a current slowdown is very indicative, given that last perihelion it was speeding up.

            Conversely, a de-stretch speeding rotation up would imply negative friction, -ie a non-passive source of torque performing work on the comet to both keep it together and speed it up. This would be akin to saying the Moon would speed up if it could get closer to the Earth - but obviously it can't, because friction cannot be negative.

          • Gerald says:

            Hi Marco,

            a one second spin-down corresponds to an energy loss ratio of 1 - (W - delta_W) / W = 1 - (v - delta v)² / v² =1 - (28 cm/s - 28 cm/s * 1s/12.4h)² / (28 cm/s)² = 1 - (1 - (1s/12.4h))² = 1 - (1 - 2.24e-5)² = 1.6e-6.

            As previously estimated, the rotational energy content of the comet is less than W/m = v²/2 = 0.0784 J / kg. (http://blogs.esa.int/rosetta/2015/04/15/cometwatch-12-april/#comment-445141)
            The spin-down of 1s per 12.4 h is therefore at most
            1.6e-6 * 0.0784 J / kg = 1.254e-7 J / kg.
            For an estimated mass of the comet of 1e13 kg the spin-down energy is 1e13 kg * 1.254e-7 J / kg = 1.254e6 J.

            The energy needed to melt 1 kg water ice at 0°C to water at 0°C is 334775 J.

            The spin-down of the comet of 1s/12.4h hence releases energy to melt 1.254e6 J /kg / 334775 J = 3.747 kg of water ice at 0°C.

            The difficult thing is focusing the energy to one small point. The second law of thermodynamics will be a high hurdle:
            Whether the mineralogy of the comet would allows serpentinization provided liquid water would be available is an open question, imho.

          • Marco says:

            Thanks Gerald,
            I will have another look at the numbers as it doesn't quite ring true to me that such a big mass can be slowed by so little energy.

            I was wondering as a second check on order of magnitude, how much weight of outgassing at how much speed would give the same amount of thrust slowing down the same amount. The amount of energy would be considerably more due to the faster speed of outgassing required.

            It is hard to imagine that such a small amount of energy can be associated with the change in movement of such a large mass.

            I don't think the second law of thermodynamics is a barrier to the concentration of the energy. It is the truss that gives way that takes the brunt of the energy when a structure collapses, or the axle of a poorly designed wheel that heats up.

          • Gerald says:

            Hi Marco, here the second check:
            For a given mass of gas, the thrust is between zero and an upper bound depending on the symmetry of the outgassing.
            With 700 m/s for the gas, and 0.28 m/s for the comet, the upper bound for the kinetic energy is a factor of 700² / 0.28² = 6,250,000. Dividing the mass of the comet by this factor returns 1e13 kg / 6.25e6 = 1.6e6 kg.
            This amount is released within 1.6e6 kg / (1 kg/s) = 18.5 days for a low activity of 1 kg/s. From an energy perspective the comet could be stopped within days by outgassing.
            For the momentum the upper bound factor is 700 / 0.28 = 2500. Applied to the mass, you need 1e13 kg / 2500 = 4e9 kg outgassing to stop the rotation.
            That's 4e9 s / 86,400 = 46,296 days or 46,296/365.25 = 126.75 years.
            A slow-down of 1s / 12.4 h = 1 / 44,640 needs at least 4e9 s / 44,640 = 89,606 s = 1.04 days at a low outgassing rate of 1 kg /s. (I've neglected the mass-loss of the nucleus during spin-down; that's negligible for an order of magnitude estimate in this case.)
            Since the spin-down has been less, we can conclude, that the outgassing has been overall rather symmetrical.
            We can also conclude, that only a small fraction of the energy is consumed for the spin-down; most of the kinetic energy from outgassing remains in the gas.

      • A.Cooper says:

        Mattias Malmer has just released a picture of where he thinks this jet emerged from, using his shape model. It's exactly at the place I suggested in the last comment which is the rim of the scree slope on Imhotep:


        This is where the suspected end-on stratum layer is located. It can be seen in the a following link, 3rd photo down. The top of the stratum is the flat area to the right of the orange line and the scree slope is to the left of it. The jet is emerging from the area between the 3rd and 4th orange dots. That's where I suggested the end-on fracture plane is located. I therefore think this is evidence for the strata fracture planes acting as conduits for gases deep within the layers.


  • Bouke says:

    Is it just me or does some of the dust surrounding the comet seem to move around slowly? It would be great to see that in a longer animation.

  • Graham Hall says:

    Its recognised that in a vacuum substances such as H2O, CO2 and CO sublimate within the temperature ranges found near the comet's surface. The mechanism for the formation of jets hasn't been fully nailed down though. We've seen other close up images where it appears that large sections (several sq metres) of crust material have been pushed aside from below, and an explosive event seems to be the likely cause. While such an event is more likely in a sunlit area, that doesn't preclude the pressure building up in the light and continue to build up in the dark as the heat soaks into the sub surface. So I think nothing is disproved by these images, and sublimating material remains the overwhelming candidate for the root cause of the jets.

    And if this event had taken place in a sunlit area it wouldn't have been detected. So we have to be grateful that it happened in a dark area and was witnessed.

    But sublimation as a cause of the jets isn't proven, although I've yet to hear of a peer reviewed paper proposing an alternative mechanism to sublimation for the formation of the jets.

    And there are some that don't believe man went to the moon and some that believe they've been (unfortunately unsuccessfully!) abducted by aliens. I don't have to listen to those people or to combat their wacky theories, as they're not open to reason.

    • dave says:

      That was great Graham until the last paragraph, I was pondering what you were writing until then.


    • originalJohn says:

      An alternative hypothesis does not need to be peer reviewed to be reasonable and valid Graham Hall. Peer approval has nothing to do with reason. It would be a good way however of filtering out an awkward or irrefutable hypothesis in favour of a peer consensus one unsupported by any evidence.

      • ianw16 says:

        470 kg/m^3. Evidence. Refute it.

      • Harvey says:

        No, an alternative hypothesis does not need to be peer reviewed to be reasonable and valid. However it does need to agree with the basic laws of physics, material properties and the observations. The EU satisfies none of those.

        'Peer approval has nothing to do with reason'; really; I wonder how many peer reviews you read a week? As a Journal editor I probably average 15 or so. Any reviewers lacking 'reason' would be off the list and their review discounted. Ditto any reviewer saying 'must be wrong, doesn't fit the party line.'
        Maybe 50 years ago there was a problem with that; long gone. Easy to get yourself heard now. Yes, you might get a bit of resistance, but if your evidence, arguments, data, stack up, you will win.
        This argument is repeatedly used to rubbish the 'establishment view'; it's a kind of Luddite approach, the more you actually know, the more you can run the numbers, understand the formulae, the computer models, the better your formal qualifications, the less you should be believed.
        In contrast we should apparently believe a few basically failed academics, who never expose themselves to real criticism, never publish, never provide quantitative models, numbers or formulae. It must be right just because it's anti the establishment view.
        Complete nonsense.

      • Kamal Lodaya says:

        Here is some peer review for the Osiris team: This blog says the pictures were taken at 7:13 and 7:15, and by 7:17 Osiris couldn't see the jet, presumably because of sunlight. Mattias Malmer says that his comet model shows that sunlight would have reached that Imhotep location more than an hour later.


    • Kamal Lodaya says:

      OriginalJohn: Peer review is not a conspiracy. I am not saying that everything is hunky-dory, there are aberrations. There are times when I have had my paper rejected when I wondered whether the reviewers "had it in" for me. But since I review papers too and sometimes reject those I find poorly argued, I can see how things look from the other side. The way I see it is that if you get your paper through a peer review you have managed to convince at least a couple of people who are professional, yet skeptical, and not swayed by your ideas just because they sound so brilliant to you. (You will not be surprised to know that my ideas always sound brilliant to me.)

      • ianw16 says:

        All good points Kamal. It's not conspiracy that keeps EU hypotheses out of respectable peer reviewed journals. Any decent hypotheses should be testable and falsifiable, and should be able to show numerical arguments.
        Anthony Peratt, for example, proposed a galaxy model back in the 80s.. I won't go into details, as it's rather off-topic, but he got it published, despite no evidence for it (in IEEE; not sure it would have got such an easy ride at Ap. J, for instance, but hey ho). However, he made predictions, based on calculations, for the observations by future surveys that would verify or falsify the model. Subsequent surveys (COBE, WMAP, Planck) did indeed rule it out, and he hasn't bothered with it in decades, as far as I can see. But at least he had the scientific honesty and integrity to provide a testable hypothesis which would live or die due to future data.
        EU hypotheses do none of this, and would never pass peer review due to sloppy scientific work and lack of prediction and falsifiability, and basic errors of understanding.

      • harvey says:


        I'd agree with that.

        Of course normally there are at least two independent referees.
        There is also a 'referee' in this process, the Editor.
        Any editor worth his salt weeds out the (fairly uncommon) obviously biased referee, & look into reasons for big disagreements between referees. The referee has to present *evidence based* arguments, not mere 'opinions.'

        In the worst case, you submit to another Journal, different editor & very probably get different referees; if you still have a problem, it's probably real.

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