Sunset jets

Activity on Comet 67P/Churyumov-Gerasimenko continues to rise, with new images from OSIRIS showing that some regions remain active even after nightfall. This report is provided by the OSIRIS team at the Max Planck Institute for Solar System Research (MPS) in Germany.

Rosetta’s scientific camera OSIRIS captured the sunset jets in the Ma’at region of the comet’s small lobe in late April. Images were taken approximately half an hour after the Sun had set over the region and show clearly defined jets of dust escaping into space.

Comet 67P/C-G on 25 April 2015, from a distance of approximately 93 kilometres, seen through the narrow-angle OSIRIS camera. The image shows jets emanating from the comet’s small lobe after nightfall. The image exposure time was 0.096s.  Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Comet 67P/C-G on 25 April 2015, from a distance of approximately 93 kilometres, seen through the narrow-angle OSIRIS camera. The image shows jets emanating from the comet’s small lobe after nightfall. The  exposure time was 0.096s. The image is presented to show the details of the jets, rather than the nucleus. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“Only recently have we begun to observe dust jets persisting even after sunset,” says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany.

Until recently, the comet’s activity originated from illuminated areas on the day side. As soon as the Sun set, these jets subsided and did not re-awaken until after sunrise the following day. One recent exception was the event captured on 12 March, which caught the onset of a dust jet at the brink of dawn, in the Imhotep region on the comet’s large lobe.

Close-up of the dust jets seen on 25 April 2015 on the comet’s small lobe. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Close-up of the dust jets seen on 25 April 2015 on the comet’s small lobe. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

According to the OSIRIS scientists, the sunset jets are another sign of the comet’s increasing activity.

“Currently, 67P is rapidly approaching perihelion in mid-August,” says Sierks. “The solar irradiation is getting more and more intense, the illuminated surface warmer and warmer.”

At the time the image was taken, the comet was 270 million kilometres from the Sun. By the time the comet reaches perihelion on 13 August, the separation will be just 186 million kilometres.

The OSIRIS team think that the comet can store the incoming heat for some time beneath its surface, resulting in sustained activity from these regions even after nightfall.

“While the dust covering the comet’s surface cools rapidly after sunset, deeper layers remain warm for a longer period of time,” says OSIRIS scientist Xian Shi from the MPS, who is studying the sunset jets.

The scientists suspect that the comet’s supply of frozen gases that fuel the comet’s activity exists in these deeper layers.

Previous comet missions, such as Stardust’s flyby of Comet 81P/Wild 2 and Deep Impact’s mission to Comet 9P/Tempel 1, also found evidence of jets sustained on the night side.

“But only the high-resolution images of OSIRIS now allow us to study this phenomenon in detail,” adds Sierks.

 

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.

Comments

70 Comments

  • Phil Stooke says:

    Very spectacular! It will be very interesting to compare those source areas with illuminated views before and after this.

  • Bill Harris says:

    Interesting. This is (may be??) off on the "same general area" (quadrant??) where Philae landed.

    --Bill

    • Kamal Lodaya says:

      Bill: Very close. Can a picture like this at some particular wavelength catch Philae as a spot of reflectance?

      Kamal

  • Ross says:

    If the ices being sublimated are in the deep interior, then why have I not seen a single vent or passage to any sub-surface dwelling? Can ESA or someone please make a prediction of the null hypothesis, which should state that 67P is not a chunk of ice? This theory is not in the least bit falsifiable, so any observation can be conformed to the first hypothesis on the subject.

    THERE IS NO SUB-SURFACE ICE

    • Marco says:

      Yes. All the volatiles are liquid.

    • originalJohn says:

      Good observation Ross. The presence of ice already appears to be being taken for granted and the idea reinforced by repetition of the sublimation theory.

      There are however plasma and temperature measurements that could be made that could refute the sublimation theory.

      • Gerald says:

        The presence of ices/clathrates has never been challenged.

      • ianw16 says:

        Or they could smash an impactor into it and see what comes out:: http://www.planetary.brown.edu/pdfs/3546.pdf
        Turned out to be water ice at Tempel 1, not sure why you'd expect anything other than H2O or CO2, to be honest.

        • originalJohn says:

          Turned out to be an electric discharge at Tempel 1 and a flash thousands of time more intense than expected, and a crater of imperceptibly small depth. The only material identified as water ice was a few minute frosty patches in one area of the surface, hundreds of times too little to account for the water volume in the coma but extremely likely to be material condensed from the coma.

          • ianw16 says:

            Sorry, that is total nonsense.

            http://www.planetary.brown.edu/pdfs/3546.pdf
            Where do we see anything of the nonsense you posted in the paper?

            "Two multi-spectral imagers and a near-infrared
            (IR) spectrometer observed the event from the flyby spacecraft (Hampton et al., 2005; Klaasen et al., 2005). The IR spectrometer was specifically designed to capture the signatures of pre-impact volatiles on the surface and in the ambient coma, as well as in the post-impact material ejected from the interior (Sunshine et al., 2005)."

            The IR signatures seen are those of H2O.

            "The IR spectra of this vapor plume include emission
            from H2O, CO2 and organics, which are consistent with gases at temperatures of ∼1000–2000 K. The quantity of hot water detected in the vapor plume is considerably greater than the water in the ambient coma. Thus, the vapor plume must have sources that include water ice."
            And..............
            "This ejecta cloud represents very low-speed,
            high-angle ejecta that originates from the greatest penetration depths of the DI impactor (Schultz et al., 2005, 2007a). Thus, water ice is present in the interior of Tempel 1 at least to the maximum depth ejected by the impact, ∼10 to 20 m."

            The observed crater was ~ 150 m diameter. Good luck getting a crater that size with an impactor of the size and mass of this one at ~ 10 km s^1 in silicate rock!

  • dave says:

    Well, I suppose there is no longer any argument about intrinsically illuminated jets at source!
    There is a mass of detail to fill in though, The OSIRIS team only THINK that the comet can store the incoming heat for some time beneath its surface. No mention of how deep, so we still have a mystery.
    Still at least we have an OSIRIS picture, that's a cause for celebration.

    I wait with baited breath for further information

    • Gerald says:

      This should now really be a small step to find out from which surface feature these jets emanate.
      Looking beneath, maybe with microwaves.

      • THOMAS says:

        Those intrinsically bright white bulbs at the base of each of the "dust-jets" in the image measure several metres in diameter. “Nozzles” are thus ruled out because of the pressure problem (the ensuing jets are dead straight and are presumably travelling extremely fast). What precise standard theory mechanism is at work here (explaining the intense luminosity at the point of origin, the large diameter of the “jets” and their energetic, collimated appearance)?

        • originalJohn says:

          If the OSIRIS team are right and residual heat ( some relative temperature figures would be interesting) causes the jets by sublimating water vapour, carrying dust (assumed), from ice ( undetected) beneath the nucleus surface, perhaps they could hypothesise also how the sunlight that illuminates the jets by reflecting off the dust particles bends around the edge of the nucleus to illuminate the brighter base of the jet. And if this were possible how the shadow is retained on the surface of the nucleus.
          Clearly we are seeing further evidence of the intrinsic glow( or possibly arc in this case) of plasma discharge. No residual heat required. No sunlight required.
          And residual heat or not according to the sublimation hypothesis the the jets would be colder than the surface temperature of the nucleus. So measure the temperature of the jets. I predict they will be hotter. Much hotter.

          • Sovereign Slave says:

            Hate to play devils advocate here, but I'm not sure myself about intrinsic glow though. Regardless, since none of the sublimation crowd has piped up about this (perhaps they need to sharpen their observation based critical thinking processes), and it's not mentioned in the article, I'll ask a question for them. Perhaps it's due to imaging somehow, if so tell me, but my question is why is there no evidence of jet activity in the rest of the much bigger, older shadows below these jets if the jets are due to electrical causes? Can't imagine there's not one jet in that whole area, and if there is, why isn't it lit up? So unless its purely an imaging phenomena, seems this aspect of the photo would support it being the last remnants of jetting activity due to residual sunlight heat.

          • Gerald says:

            Originaljohn: "perhaps they could hypothesise also how the sunlight that illuminates the jets by reflecting off the dust particles bends around the edge of the nucleus to illuminate the brighter base of the jet."
            Nonsensical presumption of necessity of non-linear rays of light. Wrong presumption, that the base of the jets are bright. Actually the bases of the jets are in the shadow.

            "And if this were possible how the shadow is retained on the surface of the nucleus." Conclusion already off the road.

            "Clearly we are seeing further evidence of the intrinsic glow": Clearly wrong.

            "...of plasma discharge": buzz-wrong as nonsense can.

            "according to the sublimation hypothesis the the jets would be colder than the surface temperature of the nucleus." Why? The emitted dust is heated by the Sun.
            By albedo and distance from the Sun the planetary equilibrium temperature can be calculated; that temperature should give a 0th estimate for the mean temperature of the illuminated dust portion of the jets. Dust in the shadow may well be colder due to thermal radiation, but "self-heating" from the illuminated surface of the nucleus needs to be considered.
            The gas should be warmer than the ices, since the "hottest" molecules sublimate. Dilute gas doesn't undergo adiabatic cooling due to lack of collisions.

            "So measure the temperature of the jets. I predict they will be hotter. Much hotter."
            Numbers please. Provide your expected temperature range. 5K? 50K? 500K? 10,000K? What does mean "Much hotter"? For a white glow you would need about 5000K. Is that your "much hotter"?

          • Gerald says:

            Sovereign Slave, it is as you say.

            The thing I'm going to get a little unsure is, whether your favored nozzle model might be applicable here to some degree, since the delayed jet activity points towards subsurface sublimation below the top-most layers.
            Immediate stop of activity after sunset, as previously observed, pointed towards sublimation from very close to the surface.
            But nozzles are only one option. It's also possible, that the dust layer is a little thicker at the delayed jets, such that cooling after sunset takes some time to get to the ices below the dust.

        • Gerald says:

          THOMAS:
          "intrinsically bright white bulbs"
          Wrong. The brightness is scattered sunlight.

          "“Nozzles” are thus ruled out"
          Wrong. There may be a series of smaller nozzles. And which pressure problem?

          "the ensuing jets are presumably travelling extremely fast"
          Wrong, not the dust. And what does mean extremely fast in terms of m/s?

          "intense luminosity at the point of origin"
          Wrong, not at the origin, but only in the sun-illuminated zone.

          • originalJohn says:

            Ah Gerald now you are modifying physics to suit your argument. No adiabatic cooling in the jets at the point where they would exit the "nozzle" and in the jets near the nucleus surface. I think you would find there would be collisions in the nozzle area and a lack of further into the jet. You have to think of some way of getting heat into the jets eh.
            And alright you want a figure for jet temperature. Impossible of course to know for sure in a completely new and previously untested situation but I will humour you and make it easy for you and give you a studied guess, A tentative prediction. i would say combustion hot adjacent to the nucleus surface ie 900 - 1200 deg K. Much hotter than the surroundings and much hotter than your hopeful mechanism could achieve. So, you have a figure. Get the measurements done. Anything above 300 deg K would be impossible for your mechanism. Anything in between and I would say combustion with unconsidered losses.

          • Gerald says:

            Originaljohn, thanks for defining "much hotter"!

            I'm far from modifying physics, at least on this benign level, that's the domain of the "electric universe".
            Under high vacuum conditions as in outer space we are in the regime of free molecular flow:
            https://en.wikipedia.org/wiki/Free_molecular_flow
            To which degree adiabatic cooling is applicable therefore is't straightforward.

            300 K on the surface of the comet are not at all impossible.
            The surface of our moon is up to 390 K:
            https://en.wikipedia.org/?title=Moon
            The albedo of the comet is lower at many places, and the dust appears to be very fluffy at some locations. So, surface temperatures could be even higher than on the moon at the same distance from the Sun. But since the comet is more distant to the Sun, my guess would be, that surface temperatures above 400 K appear unlikely, since thermal radiation increases with T^4. On temperatures above that level I would calculate, where exactly the theoretical temperature limit for the surface of the comet is.

            Above (hypothetical) cometary surface temperatures of 500 K at more than 1 AU, I'd say something must be completely wrong with the modeled processes.
            Well, the theoretical limit is the surface temperature of the sun under extreme conditions (large paraboloc mirrors focusing onto a surface patch of the comet), but that's not to be expected.

            At 900 K surface temperature the "standard" models would be at least as wrong the electric universe models, and we would need to start thinking from zero.
            But this doesn't happen. We would already see the comet glowing red-hot on the night-side.

            High mean temperatures (well above planetary equilibrium temperture) for free-floating dust grains are unlikely, since they radiate their excess heat in all directions. Out of the self-heating zone of the comet, I would expect a temperature of 252 K for black free-floating dust at 67P perihelion. Since the dust isn't perfectly black, the equilibrium temperature may be a little less, e.g. 250 K. The temperature might be a little higher, if the nearby surface temperature of the comet is higher, but less than the maximum surface temperature of the comet.
            For the comet, those 250 K are just some mean temperature, not particularly predictive for noon maximum temperatures.

            There are several definitions of temperature for dilute gas and plasma. So I think, we won't get a clear discrimination between the approaches by looking at plasma or gas temperatures.

          • Gerald says:

            Originaljohn, in the meanwhile the published data say, you're roughly 900 K off:
            http://blogs.esa.int/rosetta/2015/06/19/miro-maps-water-in-comets-coma/

          • Gerald says:

            Regarding adiabatic cooling: MIRO data seem to be consistent with adiabatic cooling:
            http://www.aanda.org/articles/aa/pdf/forth/aa26094-15.pdf, subsection 3.2.2

            Finally some clues to learn something substantially.

    • Guili says:

      "Still at least we have an OSIRIS picture, that's a cause for celebration."

      Agreed !
      But note the irony that this picture is overexposed (on purpose) and the extra definition provided by the Osiris instrument vs the Navcam is voided by the overexposure.
      *sigh*

    • Gerald says:

      CometWatch 5 June
      http://blogs.esa.int/rosetta/2015/06/12/cometwatch-5-june/
      clearly shows, that only the sun-illumiated parts of the jets are bright.
      It also shows, that the electric universe proponents again and again try to trick people with their obvious mis-interpretations.

    • originalJohn says:

      w, this is simply a poor interpretation of what actually happened. Study the photographs yourself and see if you can see the imagined crater. And how was the depth measured. It was not. It is an estimate. Or to be realistic a guess. Remember that before the mission NASA were expecting a deep conical penetration into soft agglomerated material. Watch their video of the impact. The intense double flash was a complete surprise to them although forecast by Wallace Thornhill. The water detected was a product of the violent electrical discharge reaction at the surface, which also vaporised the copper impactor. It very probably never touched the surface. The presence of water in the discharge is no evidence that ice existed in the nucleus. The intensity of the discharge reaction was way beyond NASA expectation. The controllers were concerned that they would see the impact at all. And the dust cloud and the energy intensity completely
      obliterated the mothercraft's instrumentation, so that they had to return years later with another redeployed craft to image the effects. No ice was detected. Ice was inferred. The expectation was for agglomerated ice and the interpretation was fitted to that. As I said and as NASA reported a few small frost patches were detected on the
      surface, from memory a few percent of what would have been required to account for the water quantity in the coma.
      You should question the information you are presented with more carefully w. In fact the effects observed were exactly those that would be expected from a charged metal object approaching at high speed and interacting with a charged rock
      body. A violent electrical discharge between the two affecting only the surface material, which as with all comets included hydrocarbons. Thornhill also predicted that it was likely that the discharge jets from the nucleus would be modified by disturbance to the charge distribution. Images taken after the impact showed that the jets had changed in number, position and intensity..

      • ianw16 says:

        Sorry, but yet again that proves that you either didn't read the paper, or are incapable of understanding it. The impact was observed for several minutes prior to, and after impact. Nothing was affected by the imagined electrical discharge.
        The only links to a "double flash" I can find link to pseudoscience sites, so I ignore them.
        Additionally, the craft was turned to gather look-back data 45 minutes after impact. Not years later. All the data reported in that paper were collected contemporaneously with the impact and its aftermath.
        As I actually quoted for you in the previous post, the observations *ARE* of H20. They are not inferred. I realise Thornhill has been leading you all down the garden path about this, but at 2.7 microns they are emission features of a vibrational state of H2O, those at 3 microns from absorption
        They were even able to identify the organics at 3.3 - 3.5 microns.

        You said "and the energy intensity completely
        obliterated the mothercraft's instrumentation, so that they had to return years later with another redeployed craft to image the effects."

        Again, that is pure fiction. The instrumentation was absolutely fine, as evidenced by the data gathered before, during and after the impact. Added to which, the supposedly fried craft was then renamed EPOXI, and carried on to make observations of extrasolar planets, as well as observations at comet Hartley 2.
        The mission from a couple of years later, was by Stardust, which had been to Wild 2, and then renamed NExT, which imaged the crater left behind. And yes, it was 150 m diameter.
        All this is in the public domain. Look it up.

  • Bill Harris says:

    Matches up nicely with the 18 Mar 15 Navcam. Maybe more, but this is a "visible surface detail" image to start with,

    More later...

    --Bill

    • Kamal Lodaya says:

      Surprised by the number of jets in such a small area. Did we see so many in the Hapi region which was our most prolific source earlier? It could also be because we have a sharper precision Osiris image.

    • Daniel says:

      This is an alternative view of the area where the jets originate from:
      http://i.imgur.com/SpSU18G.png

      I used an image from the image archive (20141008T140825, thanks for those once again ESA!). It's quite detailed as it's from a period when Rosetta was still doing close orbits.

      Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0 ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA https://creativecommons.org/licenses/by-sa/3.0/igo/

  • jingles says:

    The jets look like the white spots on Ceres.

  • masanori says:

    So my question is: Is there really no possibility that by these jets Philae would get off the surface and Rosetta would never be able to get a call from Philae as long as Rosetta is listening to something from Abydos?? if NO, why?? Philae is still too heavy??

    • Gerald says:

      No one can know for sure. But it's not to be expected, that Philae can be blown away by emanating gas. Compare the size of the ejected dust grains with Philae. The density of the grains may be comparable to Philae in some cases. But by far the most ejected grains are much smaller than Philae.
      Besides this, most of Philae is in the shadow, not the best condition for high activity beneath. And Philae is probably locked to the ground, at least to some degree.
      In contrast to Rosetta, Philae has no large solar panels to be dragged by gas.
      Add some optimism, that things will go well.

  • bob says:

    Looks like the white spots on Ceres

  • Graham Hall says:

    Ross

    You say "there is no ice". So where does the water emitted by 67P come from?

    As to vents, it depends on how large you think these need to be. Osiris couldn't find Philae because the camera's resolution (excellent though it is) still isn't sufficient to pick it out. So vents a metre across wouldn't be visible either. I suggest you stop predefining the answers and await proper evidence.

    • bob says:

      H from sun, O2 from 67P.

      • Harvey says:

        That simply does not work. There are many, many orders of magnitude too few protons from the sun; the sputter yield is very low; there is no way to form molecular water; and what happens to the silicon etc.
        It is not feasible.

        • originalJohn says:

          Not feasible according only to your assumptions Harvey, which are unsubstantiated, unless you too, like other claimants on here, know the solar proton density at the comet nucleus surface. What is it ? or even anywhere in the coma. But anyway as discussed before the oxygen /proton reaction is not necessary. The oxygen/ hydrocarbon reaction fulfils the water requirement as well as that of other identified products.

          The oxygen comes from the rock where it exists in abundance and is dissociated by the high energy solar protons. The silicon also produced is discharged in the jets as dust, in amounts directly proportional to the reaction rate at each discharge site.

          Now that the lander looks as though it is active again close up measurements of these surface reactions will be possible as well as measurements of ion density at the surface.

    • dave says:

      Graham
      'So where does all the water emitted come from emitted by 67p

      Surely This is the 64 thousand dollar question Graham?

      Even P Feldman et al are curiously non committal about it
      'these far ultraviolet observations COULD BE (my capitals) mapping the spatial distribution of water plumes gushing from the surface of the nucleus'

      Add to that, the required ice is still remarkably well hidden, how many months have we been alongside this comet, yet the ice continues to hide.

      • Gerald says:

        "So where does all the water emitted come from emitted by 67p?"
        That's a rhetoric question. The trivial answer is of course: From ice/clathrate deposits in the subsurface of the nucleus.

  • Ramcomet says:

    These are not intrinsically lit jets. They only are lit past the terminator shadow, plus some "Dustlight", as in Earthlight seen on the Moon's shaded quadrant.

    • THOMAS says:

      @Ramcomet

      "These are not intrinsically lit jets. They only are lit past the terminator shadow".

      This might be true if the jets were uniform in both diameter and luminosity throughout their visible length, whereas there are brilliantly-lit bulbous formations clearly visible at the base of the jets in this image. The phenomenon is strictly identical to that which had already been observed at the base of the night-side jets on Comet Hartley 2: http://apod.nasa.gov/apod/image/1011/hartley2jets2_epoxi_big.png.

      In both cases, this intense luminosity can only be intrinsic. Your unsupported assertion is simply the fruit of your prior assumptions and is invalidated by the evidence of the images.

      • ianw16 says:

        And, as I posted below, the jets on 103P/Hartley 2 were shown spectrally to be from CO2 sublimation. So I imagine that invalidates any assumptions of electrical processes for the jets. Not sure how you'd get CO2 from silicate rock.

        • originalJohn says:

          Yeah, easy ianw16. You get it from the reaction between oxygen, released from the rock by the energy of the impacting solar protons, and the hydrocarbons which coat the nucleus surface. Look up the hydrocarbon combustion equations. CO potentially too as an incomplete combustion product.

          The proton energy supplied is an electric current and the reaction products are discharged in the plasma jet emanating at high velocity from each reaction site.
          A significant potential gradient should exist between the nucleus surface and the surrounding environment. No need to speculate on that though. It would be easily measurable by the instrumentation available on the lander and the orbiter.

          • ianw16 says:

            How many times do you need to be told that the proton flux is many, many orders of magnitude too small to account for the observed gases? At the level you require them, assuming every single proton somehow combines with an atom of O or C (which it won't), then you need at least 10^7 times more. That would mean we wouldn't even have these lovely pictures, because the spacecraft would be toast.
            It is a non-mechanism, dreamed up by EU proponents to get themselves out of a corner they've been painted into by following the unscientific proposals of someone who isn't very good at science, and knows little to nothing about comets, or much else for that matter.

      • Marco says:

        Hi Thomas,
        When first looking at the Hartley pictures several years ago, my first thought was that it was evidence of intelligent life, in the same way an alien race might look at the dark side of Earth and see lit cities and come to that conclusion as other planets are dark on their night side.
        However, seeing that this NAVCAM image in particular is heavily overexposed, and Hartleys image is processed also to enhance the view of the jets, I wouldn't jump to such conclusions.

        All dark areas that have had jets looking bright at what looks like their base have these features in common. Also, due to the angle of sunlight, the higher parts of the jet are in bright sunlight, but have the very dark background of the shadowed comet. There is bound to be a little back scatter to lower parts of the jet and the combination of over exposure (or processing) and the dark shadowed background makes them look bright at their "base" even though it is very hard to tell if you are looking at the actual base. I think the same photo with normal exposure would demonstrate what I am talking about.

        • originalJohn says:

          And how many times do you have to be told w16 that you have no idea what the proton density is at the surface of the comet nucleus, and as you do not know of any mechanism for naturally increasing current density in plasma you assume, unscientifically, that the proton density is the same as the background level in the heliosphere. Ridiculous. Until those proton density measurements are made w you have no argument. And they will have to be made close to the nucleus. The known boundary effects in plasma must also be acknowledged. With a Debye length in the plasma there likely to be a few centimetres the double layer ( acceleration zone) could be only a few tens of centemetres in extent. The distribution of proton density throughout the coma is also unknown.

          The current conditions surrounding the orbiter are likely to be completely different from those of the nucleus and no reason at all to assume that it would be automatically toast. It could be though, under some conditions yet to be encountered.

          Now, w, as far as your judgements go do they make you feel better. They do nothing for your argument. The electrical explanation of comets is a rational, plausible and consistent one supported by many observations on this and other missions, soundly based on plasma physics. Continual confirmatory revelations. No painting . No corner. My own combustion hypothesis is also completely rational and scientifically based. Once again it relies on proton density and no argument has yet been advanced to disprove its likelihood, other than baseless assumptions.
          The ice sublimation hypothesis on the other hand is completely ad hoc, completely unscientific and completely lacking in supporting evidence. A definite corner approaching there.

      • Gerald says:

        THOMAS: "Your unsupported assertion is simply the fruit of your prior assumptions and is invalidated by the evidence of the images."
        This is exactly what applies to your statment "this intense luminosity can only be intrinsic"

        In contrast, Ramcomet is exactly right.

        • THOMAS says:

          Gerald, your assertions would be more convincing if they were backed up by evidence/argumentation/references. But it's true that you're allowed to express your own *opinion*, for what it's worth.

          • ianw16 says:

            @Thomas
            That's a bit rich, coming from a proponent of EU nonsense.
            Where is the evidence of electrical activity on this or any other comet? Where are the magnetic fields that should be present?
            Numerous links have been supplied to show that H20 and CO2 have been observed *DIRECTLY* at this and many other comets. I provided one for the observation that the jets on 103P/Hartley 2 were composed of mainly CO2.
            Here's another: http://www.astronomynow.com/news/n1011/11hartley/
            As for "opinion"; that is all that EU proponents have, as there is certainly no evidence for this electric comet nonsense, and I can't find any papers on the subject. All the evidence, for at least 30 years, is that it is nonsense, with nothing in the academic literature to suggest otherwise.
            You have no sensible mechanism to explain the amount of H2O and CO2 *directly observed*, nor the measured density of numerous comets.
            It is a busted flush, and has been for a long time.

          • Gerald says:

            Thanks, THOMAS!
            Look at the 3D shape of the shadows (cast by the comet), and intersect them with the jets, the visible part extrapolated back to their shadowed base. The base is near the intersection of the extrapolated jets with the 3D shape model. But it's sufficient to know, that the base is within the shadow, since otherwise the cometary surface would be sunlit at the base of the jets.
            The jets aren't perfectly collimated, hence the denser the closer to their base (in the shadow). The denser the dust (dust particles per volume weighted by scattering area of each particle), the more scattered sunlight, the brighter-looking the jets.
            Actually the jets are rather faint as you can see by the over-exposure / brightness-stretch of the image; look to the sunlit areas of the surface.
            Straightforward, isn't it, THOMAS?

            More on light scattering by particles:
            https://en.wikipedia.org/wiki/Light_scattering_by_particles

      • daposter says:

        well, no: this is exactly what we see on earth when anything crosses a terminator shadow
        I would say the density of the jets decreases rapidly from the originating point on so that this 'looks like' a bulbous flame like thing. The high contrast of the disk that is seen at the terminator line shows it's a fairly round jet being cut by the light.
        so, no aliens or electrowhatever, but since we speak of bad sciene: so where's the water now?

        I saw the very unscientific pub in science mag, where in a graph the presence of other things than waters was rasterized over the surface, stating that it is this way in tracks because the probe scanning is rasterized, while the 'water content' was shown is shown as an all over smoothed out color. I am amazed that this made it through the review (well, science mag is a mag....).

        so, these jets should be pure water; why are the 'gases' not mentioned....

        all unscientific maintenance of early theories, ego holds up science: curious when this will make room for real science.

  • Ruben says:

    Have Rosetta photographed changes in the surface related with jets or it´s too premature make this correlation?

    • Gerald says:

      It has, but it's very subtle in the published images. Probably too premature to withstand all questioning.

      As soon as close flybys will be possible again, probably near the end of this year, changes should be much easier to identify. Changes should then be at least in the order of 10s of centimeters, locally maybe meters. If the equipment will be in good health, this should turn out to be an easy task.

  • ianw16 says:

    Once they can get some spectral results from this area, then we should see what is causing them; H2O or CO2. On 103P/Hartley 2 it turned out to be CO2.
    http://phys.org/news/2010-11-primord...omet-jets.html

  • ianw16 says:

    Sorry, link got cut short in the previous comment:
    http://phys.org/news/2010-11-primordial-ice-fuels-comet-jets.html

    "Images from the flyby show spectacular jets of gas and particles bursting from many distinct spots on the surface of the comet. This is the first time images of a comet have been sharp enough to allow scientists to link jets of dust and gas with specific surface features. Analysis of the spectral signatures of the materials coming from the jets shows primarily CO2 gas (carbon dioxide) and particles of dust and ice."

    "We now have unambiguous evidence that solar heating of subsurface frozen carbon dioxide (dry ice), directly to a gas, a process known as sublimation, is powering the many jets of material coming from the comet. This is a finding that only could have been made by traveling to a comet, because ground based telescopes can't detect CO2 and current space telescopes aren't tuned to look for this gas."

  • Cesare Guaita says:

    The big surprisee is not the presence of jets
    in the night of the comet, but the fact
    that OSIRIS team released a picture !!
    I agree with Dave that a public OSIRIS
    picture has to be considered one of a biggest
    event of the entire Rosetta mission.....

  • Jon says:

    Back in November ESA scientists reported that the surface was unexpectedly warm. Too warm for surface ice, so they speculated that there could be areas of mixed ice and dust. If the surface is too warm for ice and space is colder than cold, does that mean that the surface is being warmed from within? And perhaps too warm internally for volatiles to sublimate or even exist in the nucleus?
    Thanks!

    • Harvey says:

      Jon.
      There have been various suggestions for significant internal heat sources, but none of them appear to stack up.
      People think of space as 'cold', but it's really rather misleading.
      In interplanetary space, the pressure is extremely low, and the temperature of an object is mainly determined by radiation, not to any significant extent by the conduction and convection we are familiar with.
      A 'thermometer' in space can give wildly different results depending on its interaction with solar radiation and how much it radiates; ie, what its surface is coated with.
      It's the balance of how much power the object absorbs and how much it radiates that determines its temperature.
      In turn, these are determined by the intensity and spectrum of the source and the object, which depend on its temperature, but also on a property of the surface called its emissivity - which also depends on the wavelength! Broadly, shiny, metallic reflecting surfaces have low emissivity, black objects high - in the visible at least, but might be 'shiny' in the infrared for example (black anodised aluminium are like that.)
      So if you have a large, cold object like 67P sailing into the inner solar system, with a black surface that absorbs sunlight well, the surface will warm. Heat will be conducted down into the core and warm the upper layers, causing sublimation. The details of that remain obscure, we don't know enough about the comet materials and structure. Sublimation itself removes some of that downwards propagating heat via the latent heat of sublimation. The warming surface will also radiate more power to space - depending on its emissivity at a fairly long infrared wavelength, around 15um will dominate (not its black colour in the visible.)
      So the whole story is rather complex, but that summarises some of the issues.

      • Jon says:

        Thanks Harvey, Gerald and Ian. I suppose my curiosity then becomes why was the seasonally unlit side warm in Novemeber and why do we see powerful jetting from unlit areas?
        Thanks again.

        Hopefully Philae can help us better understand.

        • Gerald says:

          The seasonally unlit side wasn't warm in November. The diurnal illumination however was short at some temporarily "warm" (-50°C) locations, just lasting a few hours per comet day. But direct, unfiltered solar illumination can warm up the surface of black fluffy dust rather rapidly (low thermal inertia). This is, what mainly happened. The temperature measurements have been taken near local noon.
          A second effect is so-called "self-heating": An illuminated surface is warmed up, and therefore itself starts to radiate infrared (heat) radiation. This radiation adds warmth to areas in the line of sight of these illuminated areas. This can include shadowed areas, if geometry allows, like in the neck region.

          One mechanism of powerful jetting in the shadow are waves of heating. By the rotation of the nucleus sunlit areas are rotated into the shadow, similar to sunset. The warmth of the top surface is then partially radiated back to space, but another part of the heat continues to warm subsurface layers for some time. If those subsurface layers contain ices, they may start to sublime and cause the jets. The dry top surface layer has been warmed well beyond the sublimation point of the subsurface ices.

          The dark background of the jets in the image allows a strong brightness stretch (or overexposure) to enhance the actual brightness of the jets. This makes them looking brighter than they atually are.

          Some physical background:
          https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
          https://en.wikipedia.org/wiki/Thermal_conduction
          https://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia

          About self-heating at the base of the neck:
          " If self-heating were not included, the base of the neck would receive ~30% less total energy."
          http://www.sciencemag.org/content/347/6220/aaa1044.figures-only, Fig. 5

          About the time of VIRTIS temperature measurements:
          " The maps, ..., show the temperature for local time between 12h and 14h."
          http://blogs.esa.int/rosetta/2014/09/08/virtis-maps-comet-hot-spots/

    • Gerald says:

      The surface of the nucleus is warmed by the Sun.
      The top layer of the nucleus is exposed to the vacuum. Any ice on the surface would rapidly sublimate in the sunshine (above about -70°C). Hence the sunlit surface of the comet is mostly free of ice.
      But below the protective layer of dry dust, ice can survive until heat migrates from the surface through the dust layer into the subsurface.
      The subsurface largely consists of an ice/dust mix. The ice starts to sublimate as the heat reaches the ice/dust mix. The deep interior of the nucleus is very cold, probably well below -200°C.
      There are several types of ices, e.g. water ice, dry ice (frozen carbon dioxide), frozen carbon monoxide. Each of these types of ices start to sublimate at a different temperature. Besides these pure ices there may exist kind of "alloys", called clathrates, composed of more than one type of ice, e.g. of water ice and methane to form the famous methane hydrate:
      https://en.wikipedia.org/wiki/Methane_clathrate

    • ianw16 says:

      What is happening here Jon, is that the temperature will vary due to the reflectance, or albedo, of the object. They knew from Earth based measurements that 67P had a low reflectance, so this reading was no great surprise, as this low albedo has been seen in other comets, including Halley.
      The warmth is coming purely from sunlight hitting a dark surface rather than a light one. Same as happens on an asphalt road. So no warming from within.
      The volatiles exist, but you need the heat from the surface to penetrate down through the dust and any organic gunk, until it warms the volatiles above the sublimation temperature. Once it does they will start to sublimate through that dust and gunk, taking it along for the ride.

  • Sol88 says:

    Well done OSIRIS!! Keep'em coming

  • Stephen Kramer says:

    Could someone please explain more of our current understanding of the jets? What are they composed of, what is the mass flow, the velocity, the temperature the propelling energy source?

    thanks!

    • Gerald says:

      The jets are composed of gas and dust. Only the sunlit dust is visible in the NavCam images.
      Gas is partially ionized by uv and decomposed by freed electrons, such that gas is visible in uv (spectrometers) due to hydrogen (mostly Lyman-alpha) and oxygen emission lines.

      There are several gasses, all a result of sublimation of subsurface or in some cases surface ices.
      The ices are mainly ices of water, carbon dioxide, and carbon monoxide. But ther are several additional volatile species.
      Besides ices there may also be more complex substances present. They are known as clathrates. Those are related to ices, but trap other gasses within their crystal lattice.
      The composition of the gasses in the jets varies with location and time. One reason is the varying temperature, since different ices sublime at different temperatures.

      The composition of the dust varies, too. GIADA is able to analyse the composition of individual dust grains.
      Silicates, iron sulfides, organic material is to be expected, similar to the composition of interstellar dust.
      I'm not aware of pubished quantitative compositional data of the dust.

      http://blogs.esa.int/rosetta/2015/01/22/giadas-dust-measurements-3-7-3-4-au/

      Velocity of the gas is about 700 m/s, velocity of dust up to about 10 m/s, as far as published.

      Sublimation has been about 1 litre / second at low activity, should now be near 100 litre / second as order of magnitude, but recent date aren't published yet AFAIK.

      Propelling energy source is illumination by the Sun, transformed to kinetic energy of gas molecules by sublimation or clathrate decomposition.

      Temperature is variable. Sublimation of water ice becomes relevant above about 200 K (-70°C).
      During approach last year the surface temperature has been up to 230K at noon. The lowest temperatures are below the range of the instruments, probably a few 10s of Kelvin.
      Dry dusty black surfaces can rapidly heat up in solar illumination to above 0°C at the present location of the comet, but I'm not aware of recently published surface temperature measurements.

  • logan says:

    Very interesting here how a flash of over-exposing occur at the point where the jets 'touch' the sun.

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