The colour-changing comet

This story is mirrored from the ESA web portal,


Click for higher res and more caption info

Rosetta’s comet has been seen changing colour and brightness in front of the ESA orbiter’s eyes, as the Sun’s heat strips away the older surface to reveal fresher material.

Rosetta’s Visible and InfraRed Thermal Imaging Spectrometer, VIRTIS, began to detect these changes in the sunlit parts of Comet 67P/Churyumov–Gerasimenko – mostly the northern hemisphere and equatorial regions – in the months immediately following the spacecraft’s arrival in August 2014.

A new paper, published in the journal Icarus, reports on the early findings of this study, up to November 2014, during which time Rosetta was operating between 100 km to within 10 km of the comet nucleus. At the same time, the comet itself moved along its orbit closer to the Sun, from about 542 million km to 438 million km.

Four image mosaic of comet 67P/C-G, using images taken on 19 September 2014 (rotated and cropped). Credit: ESA/Rosetta/NAVCAM

Four image mosaic of comet 67P/C-G, using images taken on 19 September 2014. Credit: ESA/Rosetta/NAVCAM

VIRTIS monitored the changes in light reflected from the surface over a wide range of visible and infrared wavelengths, as an indicator of subtle changes in the composition of the comet’s outermost layer.

When it arrived, Rosetta found an extremely dark body, reflecting about 6% of the visible light falling on it. This is because the majority of the surface is covered with a layer of dark, dry, dust made out of mixture of minerals and organics.

Some surfaces are slightly brighter, some slightly darker, indicating differences in composition. Most of the surface is slightly reddened by organic-rich material, while the occasional ice-rich material shows up as somewhat bluer.

Credits: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; G. Filacchione et al (2016)

Credits: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; G. Filacchione et al (2016)

Even when Rosetta first rendezvoused with the comet far from the Sun, ices hidden below the surface were being gently warmed, sublimating into gas, and escaping, lifting some of the surface dust away and contributing to the comet’s coma and tail.

VIRTIS shows that as the ‘old’ dust layers were slowly ejected, fresher material was gradually exposed. This new surface was both more reflective, making the comet brighter, and richer in ice, resulting in bluer measurements.

On average, the comet’s brightness changed by about 34%. In the Imhotep region, it increased from 6.4% to 9.7% over the three months of observations.

 Mosaic of six OSIRIS narrow-angle camera images of the geologically diverse Imhotep region on Comet 67P/Churyumov–Gerasimenko. The mosaic comprises images taken on 3 August, 25 August and 5 September 2014 from distances of 272 km, 52 km and 43 km from the comet centre, respectively. As such, the image scale varies from 5 m/pixel to 0.8 m/pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Mosaic of six OSIRIS narrow-angle camera images of Imhotep region on Comet 67P/Churyumov–Gerasimenko. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“The overall trend seems to be that there is an increasing water-ice abundance in the comet’s surface layers that results in a change in the observed spectral signatures. In that respect, it’s like the comet is changing colour in front of our eyes,” says Gianrico Filacchione, lead author of the study.

“This evolution is a direct consequence of the activity occurring on and immediately beneath the comet’s surface. The partial removal of the dust layer caused by the start of gaseous activity is the probable cause of the increasing abundance of water ice at the surface.”

“The surface properties are really dynamic, changing with the distance from the Sun and with the levels of comet activity,” adds Fabrizio Capaccioni, VIRTIS principal investigator.

“We’ve started analysing the subsequent datasets and can already see that the trend continues in the observations made beyond November 2014.”

“The evolution of surface properties with activity has never been observed by a cometary mission before and is a major science objective of the Rosetta mission,” says Matt Taylor, ESA’s Rosetta Project Scientist.

“It is great to see science papers being published directly addressing this topic and we’re looking forward to seeing how things have changed over the entire mission.”

“The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. 1) Pre-landing phase,” by G. Filacchione et al. is published in Icarus. doi:10.1016/j.icarus.2016.02.055

A follow-up paper is in preparation covering the period November 2014 to May 2015.

Related stories:
Exposed ice on Rosetta’s comet confirmed as water
Rosetta reveals comet’s water-ice cycle

Exposed water-ice detected on comet’s surface





  • ianw16 says:

    Very interesting. Proof, if it were needed, that the layers just below the surface are relatively ice rich.
    It also shows that changes can occur due to sublimation that aren’t immediately obvious to the naked eye. Looking forward to the follow-up paper.

    • Dave says:


      ‘Proof looks a bit strong Ian, certainly its an indication, but we also know there is a water/ice cycle that deposits ice back on the surface during the shaded periods.

      Are you sure Proof is the right word?


      • logan says:

        “On average, the comet’s brightness changed by about 34%”…

        Well, here we have a little object that has gone 34% more reflective from 3.62 to 2.93 AU.

        Because the document is fully Open: “I/F spectra have been corrected for thermal emission removal in the 3.5–5.1 µm range and for surface’s photometric response.”

        Also: “..along with a decrease of the 0.5–0.8 and 1.0–2.5 µm spectral slopes”

        [Just a quick look at the abstract]

        Totally mind receptive to alternative scripts…

    • Sovereign Slave says:

      Am curious, Ian, what do you mean by “proof?”

      • ianw16 says:

        @SS & Dave’
        You mean apart from the fact that MIRO already detected ice below the surface in the southern hemisphere? That sort of proof? If the surface layers are relatively ice-free (not in every area, as we have already seen), then where is the vapour coming from to remove that surface layer to reveal the more ice enriched layers beneath? Is it coming from below, as would be inferred by all the evidence so far; or is the surface layer being removed by some other hitherto unknown mechanism, which then reveals the icier layers below? Either way, there is icier material below the surface. How is it getting to the surface to be seen in the first place? Any ice ‘frozen’ at the surface after daytime outgassing, will only last until sunrise. This study was over a long period, and wasn’t looking merely at areas that are moving from shadow to sunlight.
        Probably best not to forget that an impact into Tempel 1 excavated ice grains as well. I’m sure they didn’t just spontaneously form within the impact ejecta.

        • Sovereign Slave says:

          Ian, your post demonstrates I think a fundamental flaw in your approach to scientific information, which has been reflected in many of your other posts where you insist that so-and-so evidence proves this and that, or in and of itself disproves something else. You apparently believe that evidence leads to “proof,” to something being objectively true, but it does not, and proof, as you are using the term, has little to do with science. Evidence may support a theory, but it does not scientifically prove a theory as a fact of reality, or show that you are somehow “right.” Nor should evidence be used in this way as it inevitably leads to a sort of crusading that leaves no room for doubt, or objectivity, or alternative ways of interpreting the evidence, or recognizing and accepting contradictory new evidence. In a word, it leads to becoming set in your beliefs, and viewing and interpreting all the world or universe through that set of beliefs, an easy trap for anyone.

          • Sovereign Slave says:

            Did a quick lookup, here are a couple of links that address this some (the second from an unusual source), though I know volumes and volumes have been written about this:



          • ianw16 says:

            Thanks for the lecture. Unneeded, thank you. So what would you suggest is the reason for the bluer spectral slope? Given that the team have been using areas of increased blue spectral slope to identify areas of surface ice, and have been successful in finding it (including ~1500 m^2 patches in the SH, and CO2 ice in the same hemisphere). Also given the ice detected by MIRO beneath the SH? And the ice excavated from below the surface of Tempel 1? And the ice entrained in the jets at Hartley 2?
            My interpretation is that it means that there is ice (or ice enriched material) below the surface. I’m willing to bet that when they look(ed) at it, that is what they found

          • ianw16 says:

            If one is going to quote Wikipedia, try this:

          • Kamal says:

            SS: Your post demonstrates a lack of knowledge of science of the last couple of centuries. There are hundreds of particles in modern physics and you can ask what is the proof that they exist. Modern quantum theory is an edifice based on this supposition. A scientist assumes this view is true and builds semiconductors, or superconductors, and finds that indeed they work. That goes to strengthen one’s belief in the theory. Now we have so many devices based on this technology that it is hard to come up with an alternate explanation.

            Your idea of proof goes back to the philosophers, that you consider all possible worlds and validate the truth of your hypothesis in all of them. But enumerating the possible worlds is no longer feasible. Science works with consistency, and if you think there is something wrong with a theory you have to propose an alternate hypothesis which explains all the observations. Einstein did that and he is seen as a great physicist. The “laziness” that Ramcomet mentions is justified because nature is the hardest taskmaster around, if you leave out something in your theory the chances are that nature will catch you with a counterexample. To justify his theory Einstein proposed a counterexample and nature obliged.

            In fact this physics logic is what we use every day in our lives. Do I stop using my phone because the theory behind it does not have an impeccable logical proof? No, I assume that the existing theory is okay, and the people who made the phone knew what they were doing. If someone reports some strange behaviour of phones, then I check that this kind of thing does not apply to my device. I do not demand that scientists have to meet some ideal standards I impose because of my own philosophy, when I do not have the same expectations from other people.

          • Harvey says:

            Sovereign Slave.
            You seem to have a very strange conception of how science is done. Having worked in it for my entire career, I don’t recognise it.
            Most scientists view our current model as just the best current approximation. There are no ‘laws’; just what stands up at the moment. ‘Proof’ is not used in some absolute sense; it just means yup, it supports the. Current model; until something better comes along – frequently a better approximation, the old one having been tested in some way which reveals its limitations.
            We do not have ‘beliefs’; that is a term that belongs in religion, not science. And most of us are only too pleased when something comes along that breaks the old paradigm; it’s fun!
            Now sure, occasionally people, groups, do get too attached to some theory, and suffer confirmation bias in their thinking. I’d not deny it happens. But it’s more the exception than the rule – and generally someone bows them out of the water easily enough.

            Your picture of ‘no room for doubt….’ Etc is exactly the opposite of what I experience. Everything is always doubted!

          • Sovereign Slave says:

            Harvey, you say my concept of science is strange, which may be, but then you basically just sort of do a restatement of what my own post says, so not sure how to respond to that. Kamal, obviously there is very well established science that has produced technology, etc., but my post is specific to using the word “proof” as applied to there being absolute certainty “that the layers just below the surface are relatively ice rich,” as Ian did, and has regularly done with other presented information and theories related to gathered data from missions of scientific discovery. Dave also took exception to this, but Ian, and I guess both of you seem to think that it’s fine and normal science to claim such evidence as proof, though this in its own turn seems exceedingly strange to me. But since I’m not the scientist, I’ll just have to take your word on it that it’s standard procedure to claim that such things as a bluer spectral slope is absolutely, inarguably, beyond any doubt “proof” of ice. As a non-scientist, I guess I also didn’t know that it was standard procedure for scientists writing papers on cosmology to claim so many things as facts that in my little non-science brain don’t seem like established facts at all, as has happened time and again. So my misconception was that scientific investigation into cosmological discovery presented evidence that may or may not support theories, but as you say, no, evidence is proof as declared by the edifice of modern physics. And of course, as scientists, belief has absolutely nothing to do with what you, uh, believe as scientists.

          • sjastro says:

            SS wrote,.
            ” As a non-scientist, I guess I also didn’t know that it was standard procedure for scientists writing papers on cosmology to claim so many things as facts that in my little non-science brain don’t seem like established facts at all, as has happened time and again. So my misconception was that scientific investigation into cosmological discovery presented evidence that may or may not support theories, but as you say, no, evidence is proof as declared by the edifice of modern physics.”

            Why does the subject of cosmology frequently crop up in your posts?
            Are you aware of the BICEP2 fiasco which is the very antithesis of how you think Cosmology operates.

            Not to be confused with the recent LIGO discoveries of gravitational waves, these primordial gravitational waves are thought to originate during the inflationary stage of the Universe.


            This is how science (including Cosmology) operates.

        • logan says:

          Hi, SovereignSlave: From all documents seen around here, just one was openly conclusive. [Remembering I took exception -on that spirit-].

          Just signaling that -in Science- being conclusive is indicative of a personal of Team intention of keeping on that same track.

  • originalJohn says:

    Interesting interpretation, applying the ice hypothesis.
    Interesting too that Matt claims:

    “The evolution of surface properties with activity has never been observed by a cometary mission before…”

    Have a look at the comet Tempel 1. Tens of metres of cliff erosion. An example from the Planetary Society website comparing an image from July 4th 2005 with a subsequent one from Feb 15th 2011 and spanning one orbit of the comet around the Sun.

    • emily says:

      In the context of this story, ‘surface properties’ is referring to the spectral/compositional/albedo etc changes, not the physical changes observed in the surface appearance.

      • originalJohn says:

        Thanks for that clarification Emily. Matt’s statement seemed unequivocal. Surface properties would normally be understood to include obvious physical properties first, particularly with regard to evolution.

    • Ramcomet says:

      Original John,

      Interesting, and 67P’s perehelion is 1.3 AU from the sun while Tempel 1 is farther, 1.5 AU. One might expect more erosion from 67P compared to Temple 1, right?

      67P’s perihelion was 4 AU from the sun with no visible coma viewable from Earth until a close pass with Jupiter started nudging it closer after 1840. All things equal, one might expect Tempel 1 to be more “ashen”, or freeze-dried, which might reduce erosion, while 67P should have retained more ices, although we don’t know either’s complete Perihelion history over eons.

      NASA’s site says that in 1881 Tempel 1’s Perhelion actually increased from 1.8 to 2.1 AU due to a Jupiter pass of .55 AU. So I just learned close Jupiter passes can decrease or increase perhelion distances.

      So without the full history, we cannot know, without measuring how dried up or icey Tempel 1 really is compared to 67P, and how that relates to erosion or…

      Lightening Bolts?

      • originalJohn says:

        Good points Ramcomet and its good that you have reminded other commentators on this blog that a comet’s orbit can change from orbit to orbit and by definition this perturbation occurs mid orbit so the duration of the in leg can indeed differ from that of the out leg.

        Certainly the intensity of the perihelion passage could be significant in surface changes but probably also the integration of all the activity throughout the active phase of the orbit. 67P has on more than one previous orbit from Earth observations been seen to increase in activity many weeks after perihelion. These observations come up in a straightforward online search.

        Lightning bolts? I don’t know but I think it is obvious looking at the the progressive erosion of Tempel 1 that something has been tracking on the surface, ie an electrical discharge.
        There have been a handful of pictures on this mission pointing out surface changes with time but nothing compared to the thousands of images processed. Closer examination would probably reveal some interesting things, if it was considered a priority.

        • ianw16 says:

          There are no electrical discharges. Rosetta has been observing this comet from close quarters for ~20 months. It has seen nothing regarding any electrical discharges, and they would be very obvious across the whole EM spectrum, as has been explained numerous times.
          The evolution of the surface of Tempel 1 is *obviously* caused by the processes that have been observed at this comet. That is, mass wasting of icy material due to heat from the Sun. There is also the possibility of outgassing due to phase changes of the ice. No need to invent impossible, unobserved mechanisms to explain it
          And yes, orbits can be perturbed due to close encounters with planets (mainly Jupiter). That is how the JFC comets are thought to be in their present orbits. There can also be non-gravitational changes due to the outgassing. A simple check of the ‘Where is Rosetta’ page, however, will show that the distances from the Sun are the same for x days prior to perihelion as they are for x days after perihelion.

  • A. Cooper says:

    Some time back, I commented on the idea that it would be a good idea to study Apis in depth before the mission ends because it was the only piece of unprocessed crust that I could identify according to the tenets of stretch theory.

    “unprocessed” means relatively intact since any Oort Cloud processes that may have formed or affected the crust before the comet entered the inner solar system. “Intact” really means not stretched or ripped to shreds, via spin-up, like almost every other square inch of the comet (according to stretch theory; not entertained by the mainstream).

    Apis can be identified on Filacchione’s figure as being just to the right of the the very dark red patch centred on 7° E, 3° S in the first frame. If you know the shape of Apis, it will leap out at you- that straight bottom edge is a giveaway. If not, it straddles the equator (as predicted by stretch theory) from 8° S to 10.5° N and its West-East perimeters are bounded by 12° E and 26°E. It’s like an almost vertical rectangle (in this view) but with a slant across its top-left corner. Above that slant is a very shallow scallop, which is almost flush with Apis (in terms of its height), towards us. But it looks scraped in the close-up photos. I’ve always considered this scallop to be part of the same chunk of crust as Apis but a section that did have its very top layer flaked off.

    Apart from that curious dark blob next to it, Apis-plus-scallop is one of the two large, dark red regions on the comet. The other dark region is on the opposite side of Imhotep. Both regions, together, look as if they constitute a fairly large area of the comet’s surface in this view. But since it’s a cylindrical projection it exaggerates areas on or near the equator as these two areas are. In fact, they are both relatively small areas when you look at them on the shape model.

    On the opposite side of Imhotep there lies a newly identified piece of crust that would also be unprocessed if stretch theory is correct. It’s the ‘finger’, described a few weeks ago on the stretch blog. In fact, Apis and the finger are, so far, the only two identifiable pieces of unprocessed crust according to stretch theory. And both are shown in Filacchione’s figure as being the obvious dark red areas. The only apparently confounding issues are:

    1) the darker red blob next to Apis. However, the close-ups show this lump to have broken away from Apis, as well as another perfectly fitting semi-circular chunk right next to it. So that’s why the lump is also red.

    2) the triangular shape of the red area opposite Apis is much bigger than the finger itself. However, this exact same triangle was already described in the stretch blog as the crust that pulled the finger back. It’s almost pristine crust but not thought to be quite as pristine as the finger.

    3) a small dark red area along the Hatmehit cliff. I can’t explain this fully. However, it’s at the opposite end of the long axis to Apis and also straddles the equator. It should indeed exhibit the same characteristics as Apis (both are crust sections riding at 90° to the centrifugal force vector while other crust had to slide (tensile resistance held for small crust thicknesses while shear resistance didn’t). But I would expect that top scallop of Bastet behind the cliff to be dark red as well. Bastet is one of very few regions that hasn’t been fully explained (yet) by stretch theory. Hence the Bastet scallop may be unprocessed but may have slid.

    So the only significant areas of dark red in Filacchione’s figure correspond pretty well exactly to what has already been identified as unprocessed crust in the stretch blog. The sole method for identifying these areas was by ‘rewinding the stretch movie’, not by knowing they were redder and less active or even by observing that they have similarly ‘sealed-looking’ surface textures. That was noticed only in the artefact. No other surfaces exhibit this look except, perhaps, the Bastet scallop.

    I reiterate that studying Apis as closely as possible (and comparing it with the finger) will possibly prove to be a very fruitful exercise.

    • A. Cooper says:

      Whoops, I got the cylindrical projection/area thing the wrong way round. Maybe they appear big in relation to the area with data? Thinking they appear big is what made me jump to the “cylindrical” argument. But that actually makes them look smaller. They do look quite small in the normal photos.

    • A. Cooper says:

      And my refs to 7° 12° and 26° E should have said “east of 180°. Great. #tunnelvision.

    • Janice Moore says:

      Hi, Andrew Cooper,

      I hope that other commenters will excuse my related, but a bit tangential, comment, here. Not sure where else to leave it!

      Your detailed analysis (both here and on your blog) is far beyond what my knowledge and education in this area can meaningfully evaluate, but, I can make a few observations about your obvious scientific integrity and commitment to upholding best practice scientific methodology.

      1. “rewinding the stretch movie” — stood out to me: you are making your observations FIRST, then, applying the “stretch theory.” That is, you properly followed: observation –> hypothesis –> model –> theory.

      2. My relatively quick study of the 67P photos brought to mind the mud flow caused by the eruption of Mt. St. Helens (May 18, 1980). 67P’s “mud flow,” looked like a major flow from north –> south, with a weaker, second flow from about 30 deg toward 210 deg. This appearance reminded me of “Inference to the Best Explanation.” The best explanation appears to be a liquid-but-heavy material pushed (or pulled) by wind (or other force) velocity and or gravity.

      3. The “stretch theory” may not be conventional, but, it has no less evidence for it and is just as good an explanation as other ideas. The conventional theory assumes almost all its “evidence.” How do we KNOW that H2O exists only as ice in a comet’s interior? Why cannot CO be coincident (i.e., not negating the presence of H2O — it could be a sort of “shell” or just nestled in side-by-side) with H2O in a comet? Why couldn’t the heat (friction?) of travel through space penetrate the interior, but, then, be balanced by the cooling done by supervolatiles inside, i.e., why must it be either-or?

      Conventional theory may be correct, but its inferences are NO MORE valid than those of “stretch theory.” They have just been around a long time and people have a mental groove for them, now. Good for you and Marco and others to stop — step away — and keep your mind from following a rut. The rut may be correct, but it most certainly is not a clearly better solution to this conundrum.

      4. Re: Ockham’s Razor type reasoning — the simplest solution being the likely one is ONLY to be assumed until evidence makes that improbable. Stretch Theory is supported by much probative evidence; that it is not the most “elegant” (or simplest) solution is irrelevant (as Einstein — I think he really did say this one, lol — said, “… leave elegance to [the] tailor.”) Further, as the evidence accumulates, it is appearing more and more that Richard Feynman’s “if the data contradicts the theory, that theory is wrong” (quoted from memory, only) is coming to pass, here.

      To all: JUST BE OPEN to this observation (run the movie backward…) –supported, highly plausible, theory, O Scientist of Integrity. 🙂

      Thanks, so much, for letting me share, here.

      An admirer from afar…. another galaxy away from you, I think (smile),


      • A. Cooper says:


        Thank you for reminding us all that it would be propitious to keep stretch theory in mind. My reply to Harvey on the ‘Cometwatch 10th April’ thread is a case in point.

  • ianw16 says:

    An interesting recent paper on the possible location of formation 67P:
    “The presence of clathrates in comet 67P/Churyumov-Gerasimenko.”
    Luspay-Kuti, et al.

    From the abstract (edited):
    “Recent measurements of the time variation of major and minor volatile species in the coma of 67P by the ROSINA instrument provide insight into the possible origin of this comet. The observed outgassing pattern indicates that the nucleus of 67P contains crystalline ice, clathrates, and other ices. If the building blocks of 67P were formed from crystalline ices and clathrates, then 67P would have agglomerated from ices that were condensed and altered in the protosolar nebula closer to the Sun instead of more pristine ices originating from the interstellar medium or the outskirts of the disc, where amorphous ice may dominate.”

    This folows another paper I read recently, suggesting the same thing based on N2/CO and Ar/CO ratios:

    “A protosolar nebula origin for the ices agglomerated by Comet67P/Churyumov-Gerasimenko.”
    Mousis, et al.
    Published in Ap. J, but available free at:

    • A. Cooper says:


      (Second try. Captcha ate it)

      I saw this too. The most interesting excerpt would perhaps be the following:

      “Dynamical model results suggest that both JFCs and Oort cloud comets may have formed in the same environment extending over heliocentric distances of tens of astronomical units (35). If the nucleus of 67P agglomerated from crystalline ices and clathrates, then it likely formed closer to the Sun than previously considered for JFCs.”

      This would suggest a formation zone within less than “tens” of AU which, strictly interpreted, would be less than 20AU. Conversely, the “collisional problem” presented by 67P’s contact binary status was prompting the team to suggest that both of the two primordial constituents were formed much further out from the sun than originally thought rather than closer. That way, they could coalesce into 67P and thereafter avoid the argy-bargy of the collisions going on at lower radii. That is, the collisions that brought about the ‘collisionally relaxed’ population of TNO’s at lower radii.

      No solid numbers (that I know of) to within 10AU have been mooted either for the contact binary scenario or the clathrate scenario and I’m sure it’s quite difficult to do so. However, seeing as the Kuiper Belt stops at around 50AU and CB theory was suggesting a much more distant formation, I would infer from all the statements so far made that CB theory requires a formation zone above 50AU and clathrate presence requires a formation zone of sub 20AU. 67P has been established as being both a contact binary and rich in clathrates. So the comet would appear to embody a paradox in the form of a 30AU anomaly. That’s the distance between the formation zone locations it needed to inhabit in order for it to exhibit both CB status and clathrate status.

      So do we now have a “compositional problem” as well as a “collisional problem”?

      • ianw16 says:

        @A. Cooper,
        TBH, I think that we are in desperate need of one of two things to answer these questions definitively; 1) a time machine to allow us to investigate what happened in our own solar system, or 2) a faster than light spacecraft to have a look in the other solar systems that we can see in the process of formation.
        A sample return mission may go some way towards answering some questions, but from how deep would you need the sample to be, to be sure that it is unaltered, i.e. primordial?
        I’m glad it isn’t my problem!

        • Marco Parigi says:

          Hi Ianw16,
          I believe the issue is that a comet cannot answer the question at all unless it is actually primordial in some way, which is not proven by any stretch of the imagination. If it was primordial, the data we get in a long term study such as Rosetta would be expected to constrain the possibilities. It is doing virtually the opposite, with data often giving counter indications in terms of formation zone, giving a confusing array of contradictory possibilities. One set of data may indicate accretion relatively close to the sun, while another set of data indicates distant accretion. Even a deeply excavated sample return may give further difficult to reconcile data. Although I hope for a mission to do this, my expectations given budgets are quite low. Surely there is something interesting enough that a follow-up mission to 67P or orbiting a different comet is high on the priority list of funding.

          • Marco Parigi says:

            Hi Gerald,

            I’m hoping that Bennu is found to be a dormant comet, or with activity below thresholds visible from Earth. Then we would get our comet sample return without planning as such.

          • Gerald says:

            Hi Marco,
            this would be big fun, indeed.
            I guess, that there are more “silent” comets than active ones, depleted of volatiles. They may still contain some volatiles in their interior, protected by a thick crust.
            But since Bennu is about as close to the Sun as Earth, not too much of volatiles will be left over.
            So, if it’s of cometary origin, only refractory material is to be expected to be found, at least near the surface.
            Bennu seems to be of low density, near that of water, hence either very porous, with a porosity of about 70%, or additionally made of light minerals.

            Since on Earth there are more, and more sensitive instruments than can be provided for space probes, even small traces or remnants of “the initial” composition are easier to detect.

      • logan says:

        “… Conversely, the ‘collisional problem’ presented by 67P’s contact binary status was prompting the team to suggest that both of the two primordial constituents were formed much further out from the sun than originally thought rather than closer.”.

        Thanks for bringing that back, Cooper 🙂 Never quite bought into binary scripts. But admitting former incredulity about nurseries being illustrated at proto-disk emplacements. Recent [last months] papers have brought some plausibility to this issue.

        On fitting all incoming data, admitting actual preference for a multi steeped GROWING (in this scenario) environment for Comets.

    • Ramcomet says:

      Hi Ian,

      Thanks so much for the links!

      This is in no way aimed at you, but is a complaint to everyone writing papers and their peer reviewers, especially: (And a call to anyone who can shed light on my broad complaint, including moderators, please do!)

      As a non scientist, but avid science reader/explorer, I am becoming increasingly frustrated, equally baffled and annoyed when papers claim a “FACT”, based on a “MAYBE”.

      Just like the last sentence in your quotation marks above…
      That is one BIG MAY-be!

      This just in:

      Raises huge questions about the basis of dust outside the solar system, since folks have been leaving out the million degree blasts of supernova bubble fronts of their ASSUMPTIONS!

      And the Contact Binary paper, who’s authors needed over 100 simulations to get one to look like 67P, but that video showed half of the comet getting pulverized. And yet without breaking stride, they outlined gravity vectors of the onion skin Strata, which would not EXIST, had the pulverizing occurred!

      But still, it was ” UNAMBIGUOUS” proof, that 67P was accretionary, a contact binary.

      And others recently presented here.

      This trend seems awful for science. Is it laziness? Overstating things for what? Funding? They are sending a precedential message that it is OK to exaggerate, and that is damaging in many ways.

      Really the trend is doing a huge disservice to not only science itself, but all the young aspiring scientists out there. Somehow it has got to stop, (and peer reviews don’t seem to be helping!).

      Thanks, everyone for listening. I hope you too are concerned.

      • ianw16 says:

        I’m not quite sure what you are getting at here, if I’m honest. I just had a quick read through the second paper (which I assume is the one you are referring to). Is the objection based on the title? Maybe they should have used a question mark, or thrown in the word “probable” or “possible”. Other than that, their language is far from stating that this is a certainty. They use the word “if” on a number of occasions, and state that “further laboratory experiments are needed…..”.
        Even the abstract, which is as far as some people will read, makes it clear that “If these measurements are representative of the bulk…..;”. So no chance of anybody being led astray into thinking that they are saying that this is a done deal, merely that current evidence favours it. I’m sure the peer reviewers would have had something to say had the authors concluded that all other possibilities were ruled out.

        • Ramcomet says:

          Hi Ian,
          Very fair and respectful position, and I want to spend some time carefully answering back with my own excerpts soon.
          I regret the capital letters too, which always appear like shouting, even though that was not my intention. My own laziness, on a tablet.
          I may, or may not have a point in general about slight hyperbole of some papers, or even specifically to the new changing findings on the predisposed pristine conditions of the Oort cloud (per my link). But I am here to learn, be corrected, or even hopefully to advance a point or two from a different perspective.
          The last sentence in your linked paper seemed quite a bit of a convoluted tangle. “If-Then”, but then, even before a period comes along we have ” because-may”.
          And you identified good corrections, possible, probable, even sometimes question marks are necessary in titles and body copy as often seen in journals.
          I am probably over sensitive about this, so sorry if I offended any authors, but still think great care is important to never overstate things.
          And my point is, if I in any way do have a slight point, the peers ARE missing something lately in reviews..

        • Dave says:

          Ian & Ram comet

          Well nobody led astray, well maybe just the mainstream press, that seem to pick up only on a few lines to get the headline they want. This then becomes fact and referred to at future dates.

          Admittedly its difficult to get it right, but caution is best I think.


    • Dave says:

      Ianw16 re the presence of Clathrates.

      Thanks for the links Ian,

      I am a bit surprised that the O2 is not mentioned along with these results especially if there are two sources of water ice in the comet.

      Looking back to the paper – Implications of the detection of molecular Oxygen in Comet 67p by Bieler et al.
      The report says local molecular Oxygen abundances in the range of 1 to 10% (ave 3.8%)
      There is a strong correlation between water and the molecular oxygen in the coma of the comet
      The ratios of Oxygen to water do not vary as a function of heliocentric distance during 7 months of observations.
      also molecular Oxygen detected as close as 10km away from the comet nucleus.

      Although the molecular oxygen discovery was announced quite late, I am surprised it is not included in the sublimation papers, as it seems its in some way proportional to the water produced.

      It seems we are still missing something, if the Oxygen is just trapped in the ice, can it be trapped in the same proportions in both the amorphous and the crystalline ice and would this make sense if the two ices were formed at different distances from the sun?


      • logan says:

        “…It seems we are still missing something, if the Oxygen is just trapped in the ice, can it be trapped in the same proportions in both the amorphous and the crystalline ice and would this make sense if the two ices were formed at different distances from the sun?”.

        Could bet most of the Oxygen is trapped at the interface with ISM, so former amorphous grains. [Besides, a very recent document sighted at suggest the trapping of Oxygen at the outer side of a ‘weaving’, complex proto-planetary disk].

        But here at 67P most of the ices’ grains SEEMS to be actually transmuted into crystalline [or caged] forms. This mutation could be responsible of the intriguing “constant proportion-alization”. Just a hunch 🙂

        This constant is one the biggest discoveries of ROSETTA, in my perspective.

        • logan says:

          “…It seems we are still missing something,”

          On ending this wonderful Journal, we’ll be missing a lot more than We missed on starting.

  • ianw16 says:

    Related somewhat to the paper cited in this article, there was an earlier paper which has some relevance:
    ‘Variegation of comet 67P/Churyumov-Gerasimenko in regions showing activity.’
    N. Oklay, et al.

    Unfortunately it is now paywalled.
    The authors investigated the spectral slope of the comet nucleus in Feb 2015, and could distinguish between active regions, inactive regions and mixed regions (those with isolated bright spots, or containg material ejected from active regions).
    This also allowed them to predict which areas were likely to become active. As they say in the ‘conclusions and outlook’, “In most of the regions detected as potentially
    active in the activity threshold analysis, the activity was detected in spring 2015 either visually or via inversion methods.”

    • ianw16 says:

      Another one I just stumbled across, although not quite as related (but free!):
      “Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko.”
      Lethuillier, et al.

      From the abstract:
      “Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be > 2.45 ± 0.20, which is consistent with a porosity < 50% if the dust phase is analogous to carbonaceous chondrites and < 75% in the
      case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko nucleus is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer."

      Of course, the sintering would be in agreement with lab experiments to which I've linked previously.

      There is also an oral presentation to the EGU 2016 meeting (Rosetta presentations on 21/04):
      "Porosity and water ice content of the *sub-surface* material in the Imhotep region of 67P/Churyumov-Gerasimenko constrained with the Microwave Instrument on the Rosetta Orbiter (MIRO) observations."
      My emphasis.

      And just for an aesthetic sense of how icy a comet can be, I remembered some images released from the Hartley 2 flyby of a "snowstorm" that it flew through. The "snowballs" evidently ranged from golf ball sized to basketball sized:

  • originalJohn says:

    Further to my original comment about the interpretation of VIRTIS images in the published work by Filacchione et al, referred to in this post, it strikes me that the ice sublimation hypothesis, to explain the comet activity and surface properties, is applied, in this case and in others, without any critical appraisal, whereas proposals for alternative explanations are vehemently attacked and dismissed by those who support the ice hypothesis.

    So to redress the balance somewhat let us have a critical look at the ice sublimation hypothesis here. It was originally constructed
    around the idea that heat energy in the Sun’s photon radiation caused the “dirty snowball” agglomeration of ice and dust, that the comet nucleus was held to be composed of, to sublimate and release jets of dust carrying water vapour from the surface of the nucleus.

    However, when insufficient ice by far was detected at the nucleus surface to account for the copious volumes of water being emitted
    the hypothesis received an ad hoc modification. The ice was said to be largely hidden beneath the nucleus surface and the surface heat was transferred to the hidden location. This caused sublimation of the ice, either within existing cavities or pores, or it
    created cavities or pores, which filled with water vapour. The pressure of water vapour in these cavities caused the water vapour
    to break through the nucleus material and form a channel to the surface where it was relased as jets with a velocity close to the speed of sound. This is as I understand it and is how the comet jet emission mechanism is represented in artists impressions and models.

    There is a problem here with the concept of heat transfer in the comet nucleus material. Whether it is ice or rock or an
    agglomeration of ice and rock the heat conductivity would be expected to be very poor. We all know how nice and cool it is in
    a mountain cave on a sweltering hot day. Yet this heat transfer has also been invoked to explain, along with hysteresis, emission of jets in the shadow region of the nucleus. Heat transfer is not the crucial aspect of my argument though so I will not labour it here.

    Most significant is the saturation vapour pressure of water vapour in contact with ice in an enclosed cavity. If we view this useful page from Kaye and Laby:

    we see a helpful little table of vapour pressures and temperatures. At -50 deg C the vapour pressure is 3.94 Pa. This is the
    equilibrium or saturation pressure. At that temperature the pressure cannot go any higher in a cavity. It is self limiting. If the
    temperature increased to -10 deg C the vapour pressure would rise to 259.9 Pa.

    So at such pressures can the water vapour break out through the comet material and reach the nucleus surface? Well, if we assume the nucleus is rock, say a typical basalt we find the compressive strength is 262000777 Pa. So the answer there is a resounding no. Those vapour pressures would make no impression on rock. But what if the nucleus was ice? An equally resounding no I am afraid. The strength of ice is typically 19994796 Pa. And if the nucleus was a frozen agglomeration of ice and dust? What is the strength of that stuff? Well , who knows. But if it were anywhere in the region of 259.9 Pa the nucleus would have trouble retaining its solid integrity. It is likely that the strength of a frozen agglomerate of ice and dust is similar to that of ice, or perhaps higher. So at least in the tens of millions of Pascals. So once again a few hundred Pa makes no impression.

    So what can we deduce from this? We can deduce that any water vapour formed beneath the nucleus surface by sublimation from hidden ice in hidden cavities would remain trapped in those cavities and never reach the surface.

    Some might say that the water vapour escapes through existing fisures. Well, if the fissures were very small it is still likely that the saturation pressure would build up in the cavity and stop the sublimation for long periods, and would the saturation pressure be
    sufficient to propel the vapour to the surface through microscopic fissures?

    And if the fissures were large (not good for the mechanical integrity of the nucleus either) yes the sublimation would proceed but at such a pressure that the vapour would barely seep to the surface and dissipate. It would not form high pressure jets that project tens of kilometres into space. Or would it? Perhaps someone can explain how.

    • ianw16 says:

      I’m afraid you’ve got it all a bit skewed. Ice has been detected below the surface. They know that it is there. It was detected beneath the SH some time ago by MIRO:
      It was excavated at Tempel 1, and there was a veritable “snowstorm” of it at Hartley 2. I think it was also detected after a big outburst at 17P Holmes.
      So I don’t understand why you think it is “hidden”. If it wasn’t “hidden” to our eyes, it wouldn’t be subsurface, would it? By definition, it would be surface ice. Which has also been seen. The ice detected on the surface in this paper is now surface ice. It wasn’t before the dust layer disappeared.
      And nobody is suggesting it needs huge pressures to escape. The overlying material is itself porous, as backed up by lab experiments with ice-tholins mixtures. I’ve linked to the papers a few times, but they are paywalled:
      Suffice to say that the vapour has no problem escaping through the pores, and even dislodging areas of the overlying “crust”. Some of the mixtures even form the hard surface inferred from the Philae results, due to sintering.

      And I’m not sure what you mean by the use of the word ‘rock’. There isn’t any, unless you are referring to the dust, which is to rock as a brick is to Buckingham Palace.
      MIRO can take temperatures at different depths below the surface. It is not that difficult to calculate the thermal inertia from that. It is well constrained. It isn’t that of rock, or anywhere close. Just as it could measure the thermal inertia at asteroid Steins, and see that that was consistent with rock.

      As for major outbursts, they are unpredictable, and so all they can do is measure the increase in gas (if any) within the coma afterwards. I haven’t seen much, but what I have seen suggests (to me) that super-volatiles may be responsible. As for the mechanism(s), there are a number of possibilities, but widening of existing fissures is certainly a possibilty. As per this poster presentation:

      More data looks like it may soon be forthcoming:
      “Investigating the surface brightness profiles, ejected mass and speed from the outburst events of comet 67P/Churyumov-Gerasmenko.”

      Without an astronaut on the surface actually witnessing one of these outbursts, modelling, based on observations of the areas of their initiation, and the gases produced, is the best they can do.

      This spacecraft has been alongside the comet for ~20 months now, and I’ve still yet to see one “alternative” explanation for cometary activity that makes the least bit of scientific sense, either here, or elsewhere.
      That it is due to subsurface ice (observed) is beyond question. We see it below the surface, on the surface, and the vapour it produces in the coma. All happening at temperatures and expansion velocities consistent with sublimation.
      Any “alternative” explanation will need to take that evidence into account.

    • Harvey says:

      Well, up to the quote below, we agree for once. The saturated vapour pressures would not ‘explode there way out of’ almost anything.

      But then you say:
      ‘Some might say that the water vapour escapes through existing fisures. Well, if the fissures were very small it is still likely that the saturation pressure would build up in the cavity and stop the sublimation for long periods, and would the saturation pressure be
      sufficient to propel the vapour to the surface through microscopic fissures?”

      We know relatively little about the internal structure. But we do know is has such a low density it must contain a high pore volume, and from (limited) CONSERT data we know that, for limited paths, there are few if any big caverns. So the evidence favours a rather open structure of interconnected voids.

      In such a structure sublimation will not ‘stop…for long periods’; its rate will simply be reduced by the resistance of the path to the much lower pressure surface. Depending on the pore size and local pressure, the flow regime will probably be laminar or molecular (diffusive.)
      Any pressure gradient at all will drive flow; the question is whether it can drive *enough* flow.

      This is an area where some sensible modelling could be done, of gas transport in a variety of porous structures constrained by the data and material properties, driven by sublimation. It will be a strong function of the assumed geometry of the porosity.

      So there is certainly some detailed modelling for sublimation proponents to do. Are there credible structures which could generate the observed flows? Needs work; however from a lot of practical experience with vacuum systems, my guess would be ‘probably’. Things like ‘virtual leaks’, small trapped volumes, say at the base of a screw in a blind hole, easily come to dominate the system pressure as the gas creeps out round the thread if you don’t design them out. Any sort of ‘open cell foam’ structure would be a vacuum system nightmare. No proof of anything, needs a proper model, but an ‘informed gut feeling’ would be yes, it’s credible.

      An interesting question however is what is the alternative?
      What should we model instead that credibly could lead to the observed facts? The mass outflows? But which also include no emission from the jets, zero or very low magnetic fields, no hot spots, MIRO gas phase temperatures, etc. We could – and I imagine someone will – build the equations for the sublimation through porous material, and make quantitative comparisons to all the data.

      Alternative ‘hypotheses’ have yet to describe a system which could be set up to model, let alone which are consistent with observation.

      • originalJohn says:

        Yep, no point disputing accepted facts, Harvey. Yet you have to dispute something so even though I simply speculate about the structure of the nucleus you prefer to put forward your assumptions about what the sub surface structure might be. There is little to no evidence to support those assumptions. To my knowledge CONSERT was used in just one pass during the brief period of activity of the lander/receiver and from the presentation of those results it is disputable whether any signal was received that passed through the sub surface structure of the nucleus. And as far as porosity goes there is no evidence that any pores are interconnected and indeed no reason why they should be. That is just another assumption favourable to the ice hypothesis.

        In an enclosed cavity vapourisation would stop when the pressure reached the saturation vapour pressure. With connecting microfissures it is feasable that the pressure would reach saturation and sublimation would stop until the vapour pressure fell to below saturation level. This might take a definite period of time. A pressure cycle that would slow down the rate of gas emission, which at just below ice saturation vapour pressure would be vanishingly small anyway at the temperatures we are dealing with..

        Of course I whole heartedly agree that a great deal of further work needs to be done. That is my point after all. There has been and is no critical appraisal of the hypothesis. There is probably something you could contribute there.

        As far as alternative explanations go Harvey I have mine and you know what they are. But do you have any or can you think of absolutely nothing. Anyway if I were you I would start thinking now because the more the ice sublimation hypothesis is examined the less of an open and shut case it will become, and quickly too.

      • ianw16 says:

        If you have access, I would recommend the papers I linked elsewhere a couple of times. Namely: and,

        As a picture tells a thousand words, here is one where I’ve stitched together a couple, along with a table of results from the first of those papers:

        • Dave says:

          re a picture tells a thousand years (Hope the quote does not relate to the 1970s TV programme)

          Thanks for stitching the data together Ian, its very informative and does illustrate a likely method for sublimation where and if type if that surface exists.

          There is still a bit to go though as there seem to be several different types of surface and a seemingly more cohesive crust in many places.

          Then there are the explosive looking flares ups (could be landslide of weathered cliff faces as suggested some months ago) for which there seem to be several muted solutions.

          And then we have the steep sided circular pits, that have been described in at least 1 paper.

          There is a lot of knitting together of the information to come yet


        • logan says:

          Thanks for taking the time for the stitching, Ianw16. Not far from my idea of particulate concretions..

    • loganq says:

      “And if the fissures were large (not good for the mechanical integrity of the nucleus either) yes the sublimation would proceed but at such a pressure that the vapour would barely seep to the surface and dissipate.”.

      That ‘big fissure’ modeling applies only to ‘latticed’, older materials. As for ‘deposits’ [like Imhotep] going with Robin’s material models [foamy ice concretions, progressively sintering].

    • logan says:

      “At -50 deg C the vapour pressure is 3.94 Pa. This is the
      equilibrium or saturation pressure. At that temperature the pressure cannot go any higher in a cavity. It is self limiting. If the temperature increased to -10 deg C the vapour pressure would rise to 259.9 Pa”

      That’s why Ducky should be inhaling at dawn.

      • logan says:

        And there Ducky have its Carnot cycle 🙂 Could it be relevant to heat sinking?

      • logan says:

        “At that temperature the pressure cannot go any higher in a cavity”.

        Yes, it can go higher, if PRESSURED IN 😉

  • logan says:

    Thanks to Emily, and also to Gianrico Filacchione, a, , Fabrizio Capaccionia, Mauro Ciarnielloa and allies for making the huge effort of making this magnificent document an Out-Reaching one 🙂

  • logan says:

    A little of language analysis…

  • logan says:

    Hey, Marco&Cooper 🙂

    “…Co-author Olivier Hainaut (ESO, Garching, Germany), concludes: “We’ve found the first rocky comet, and we are looking for others.”

    “Observations with ESO’s Very Large Telescope, and the Canada France Hawai`i Telescope, show that this is the first object to be discovered that is on a long-period cometary orbit, but that has the characteristics of a pristine inner Solar System asteroid.”.

    Fully open at:


    • logan says:

      “Furthermore, this object displays a very faint, weak level of comet-like activity, five to six orders of magnitude less than that of typical ice-rich comets on similar Orbits coming from the Oort cloud. For the nearly tailless appearance, we are calling C/2014 S3 a Manx object.”

      “We may be looking at fresh forming… material that was ejected… [from our?] pristine inner Solar System…”.

      Congratulations also to Karen J. Meech, Bin Yang, Jan Kleyna, Svetlana Berdyugina and allies.

    • A. Cooper says:

      Thanks logan

      Noted and passed to Marco.

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