Exposed water ice detected on comet’s surface

This article is mirrored from the main ESA web portal and covers the results published in a new  paper in the journal Astronomy & Astrophysics titled "OSIRIS observations of metre-size exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments," by Antoine Pommerol et al. 

Examples of six different bright patches identified on the surface of Comet 67P/Churyumov-Gerasimenko in OSIRIS narrow-angle camera images acquired in September 2014. The insets point to the broad regions in which they were discovered (not to specific locations). In total, 120 bright regions, including clusters of bright features, isolated features and individual boulders, were identified in images acquired during September 2014 when the spacecraft was between 20-50 km from the comet centre. On the left hand side of the image a boulder with icy patches in Hatmehit (top) a cluster of icy features in Imhotep (middle) and a cluster in Khepry is presented; on the right hand side a cluster in Anuket (top), a bright feature in Imhotep and a cluster close to the Khepry-Imhotep boundary is shown. The false colour images are red-green-blue composites assembled from monochrome images taken at different times and have been stretched and slightly saturated to emphasis the contrasts of colour such that dark terrains appear redder and bright regions appear significantly bluer compared with what the human eye would normally see. This imaging technique allows scientists to determine more about the nature of the material; in this case the bluer colour indicates the presence of ice. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Examples of six different bright patches identified on the surface of Comet 67P/Churyumov-Gerasimenko in OSIRIS narrow-angle camera images acquired in September 2014. The insets point to the broad regions in which they were discovered (not to specific locations). In total, 120 bright regions, including clusters of bright features, isolated features and individual boulders, were identified in images acquired during September 2014 when the spacecraft was between 20-50 km from the comet centre.
On the left hand side of the image a boulder with icy patches in Hatmehit (top) a cluster of icy features in Imhotep (middle) and a cluster in Khepry (bottom) is presented; on the right hand side a cluster in Anuket (top), a bright feature in Imhotep and a cluster close to the Khepry-Imhotep boundary (bottom) is shown. The false colour images are red-green-blue composites assembled from monochrome images taken at different times and have been stretched and slightly saturated to emphasis the contrasts of colour such that dark terrains appear redder and bright regions appear significantly bluer compared with what the human eye would normally see. This imaging technique allows scientists to determine more about the nature of the material; in this case the bluer colour indicates the presence of ice.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Using the high-resolution science camera on board ESA’s Rosetta spacecraft, scientists have identified more than a hundred patches of water ice a few metres in size on the surface of Comet 67P/Churyumov-Gerasimenko.

Rosetta arrived at the comet in August 2014 at a distance of about 100 km and eventually orbited the comet at 10 km or less, allowing high-resolution images of the surface to be acquired.

A new study just published in the journal Astronomy & Astrophysics focuses on an analysis of bright patches of exposed ice on the comet’s surface.

Based on observations of the gas emerging from comets, they are known to be rich in ices. As they move closer to the Sun along their orbits, their surfaces are warmed and the ices sublimate into gas, which streams away from the nucleus, dragging along dust particles embedded in the ice to form the coma and tail.

But some of the comet’s dust also remains on the surface as the ice below sublimates, or falls back on to the nucleus elsewhere, coating it with a thin layer of dusty material and leaving very little ice directly exposed on the surface. These processes help to explain why Comet 67P/Churyumov-Gerasimenko and other comets seen in previous flyby missions are so dark.

Despite this, Rosetta’s suite of instruments has already detected a variety of gases, including water vapour, carbon dioxide and carbon monoxide, thought to originate from frozen reservoirs below the surface.

Example of a cluster of bright spots on Comet 67P/Churyumov-Gerasimenko found in the Khepry region (top) and an individual boulder with bright patches on its surface in the Hatmehit region (bottom). The bright patches are thought to be exposures of water-ice.  Both images shown here are subsets of OSIRIS narrow-angle camera images taken on 30 September, when the spacecraft was about 20 km from the comet centre. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Example of a cluster of bright spots on Comet 67P/Churyumov-Gerasimenko found in the Khepry region (top) and an individual boulder with bright patches on its surface in the Hatmehit region (bottom). The bright patches are thought to be exposures of water-ice.
Both images shown here are subsets of OSIRIS narrow-angle camera images taken on 30 September, when the spacecraft was about 20 km from the comet centre.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Now, using images taken with Rosetta’s OSIRIS narrow-angle camera last September, scientists have identified 120 regions on the surface of Comet 67P/Churyumov-Gerasimenko that are up to ten times brighter than the average surface brightness.

Some of these bright features are found in clusters, while others appear isolated, and when observed at high resolution, many of them appear to be boulders displaying bright patches on their surfaces.

The clusters of bright features, comprising a few tens of metre-sized boulders spread over several tens of metres, are typically found in debris fields at the base of cliffs. They are most likely the result of recent erosion or collapse of the cliff wall revealing fresher material from below the dust-covered surface.

By contrast, some of the isolated bright objects are found in regions without any apparent relation to the surrounding terrain. These are thought to be objects lifted up from elsewhere on the comet during a period of cometary activity, but with insufficient velocity to escape the gravitational pull of the comet completely.

In all cases, however, the bright patches were found in areas that receive relatively little solar energy, such as in the shadow of a cliff, and no significant changes were observed between images taken over a period of about a month. Furthermore, they were found to be bluer in colour at visible wavelengths compared with the redder background, consistent with an icy component.

“Water ice is the most plausible explanation for the occurrence and properties of these features,” says Antoine Pommerol of the University of Bern and lead author of the study.

Examples of icy bright patches seen on Comet 67P/Churyumov-Gerasimenko during September 2014. The two left hand images are subsets of OSIRIS narrow-angle camera images acquired on 5 September; the right hand images were acquired on 16 September. During this time the spacecraft was about 30-40 km from the comet centre. The images are false colour red-green-blue composites assembled from monochrome images acquired at different times with the 882.1nm (red), 649.2nm (green) and 360.0nm (blue) channels. Each channel was stretched and slightly saturated to emphasis the contrasts of colour across the scene such that dark terrains appear redder and bright regions appear significantly bluer compared with what a human eye would normally see. While various ices mixed with dust would be consistent with the blue signature, taken with other observations, the various properties of the bright patches point to water-ice. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Examples of icy bright patches seen on Comet 67P/Churyumov-Gerasimenko during September 2014. The two left hand images are subsets of OSIRIS narrow-angle camera images acquired on 5 September; the right hand images were acquired on 16 September. During this time the spacecraft was about 30-40 km from the comet centre. The images are false colour red-green-blue composites assembled from monochrome images acquired at different times with the 882.1nm (red), 649.2nm (green) and 360.0nm (blue) channels. Each channel was stretched and slightly saturated to emphasis the contrasts of colour across the scene such that dark terrains appear redder and bright regions appear significantly bluer compared with what a human eye would normally see.
While various ices mixed with dust would be consistent with the blue signature, taken with other observations, the various properties of the bright patches point to water-ice.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“At the time of our observations, the comet was far enough from the Sun such that the rate at which water ice would sublimate would have been less than 1 mm per hour of incident solar energy. By contrast, if carbon dioxide or carbon monoxide ice had been exposed, it would have rapidly sublimated when illuminated by the same amount of sunlight. Thus we would not expect to see that type of ice stable on the surface at this time.”

The team also turned to laboratory experiments that tested the behaviour of water ice mixed with different minerals under simulated solar illumination in order to gain more insights into the process. They found that after a few hours of sublimation, a dark dust mantle a few millimetres thick was formed. In some places this acted to completely conceal any visible traces of the ice below, but occasionally larger dust grains or chunks would lift from the surface and move elsewhere, exposing bright patches of water ice.

“A 1 mm thick layer of dark dust is sufficient to hide the layers below from optical instruments,” confirms Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research in Göttingen.

“The relatively homogeneous dark surface of the nucleus of Comet 67P/Churyumov-Gerasimenko, only punctuated by some metre-scale bright dots, can be explained by the presence of a thin dust mantle composed of refractory mineral and organic matter, with the bright spots corresponding to areas from which the dust mantle was removed, revealing a water-ice-rich subsurface below.”

The team also speculates about the timing of the formation of the icy patches. One hypothesis is that they were formed at the time of the last closest approach of the comet to the Sun, 6.5 years ago, with icy blocks ejected into permanently shadowed regions, preserving them for several years below the peak temperature needed for sublimation.

Another idea is that even at relatively large distances from the Sun, carbon dioxide and carbon monoxide driven-activity could eject the icy blocks. In this scenario, it is assumed that the temperature was not yet high enough for water sublimation, such that the water-ice-rich components outlive any exposed carbon dioxide or carbon monoxide ice.

“As the comet continues to approach perihelion, the increase in solar illumination onto the bright patches that were once in shadow should cause changes in their appearance, and we may expect to see new and even larger regions of exposed ice,” says Matt Taylor, ESA’s Rosetta project scientist.

“Combining OSIRIS observations made pre- and post-perihelion with other instruments will provide valuable insight into what drives the formation and evolution of such regions.”

More information can be found in the paper, which is available to read online in Astronomy & Astrophysics, here

The individual images making up the context images presented in this release are also provided below. All images credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

 

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.

ESA_Rosetta_OSIRIS_bright_6 ESA_Rosetta_OSIRIS_bright_5 ESA_Rosetta_OSIRIS_bright_4 ESA_Rosetta_OSIRIS_bright_3 ESA_Rosetta_OSIRIS_bright_2 ESA_Rosetta_OSIRIS_bright_1

Comments

90 Comments

  • Phil Stooke says:

    Fantastic result! Thanks for this.

  • Lucas says:

    Hmm so no water ice but bright obcjects was found. So still no proof of water ice on the surface. Or am I wrong ?

    • Marco says:

      Hi Lucas. I don't buy it either. Shadow or no shadow, it is the temperature and vacuum which dictates whether it should sublimate. By their own admission, there was no changes over the period of about a month.
      Also, other than their albedo and spectrum, there is little evidence that points to it being ice. There is a large range of compounds that has that kind of albedo and spectrum. The main argument that they should be assumed ice is the argument that where there is vapour, there is ice. I say fair enough, but not necessarily on the surface. The surface is just not changing enough from month to month to allow for surface erosion.

      • ianw16 says:

        So what is it then? Given the spectral data strongly infers that it is H2O ice? And given that H2O ice was spectrally identified on Tempel 1?
        Any suggestions?

      • Marco says:

        The spectral signal being bluer and albedo being higher would not be evidence for "ice mixed with dust" on any body in which ice was not already expected.

        All we can say is that the substance contrasts somewhat to the rest of the surface.

        I would consider them to be an unidentified, non volatile solid substance. I would quite confidently predict that they would remain unchanged after perihelion when Rosetta can get close to identify them again. If there was demonstrably no erosion, they could not be considered volatile, and other guesses would have to be made.

        • ianw16 says:

          Nonsense. Mass wastage was seen on Tempel 1. Please explain. With scientific references. We all know that isn't going to happen, so how about an introductory course in Planetary Science? Must be something near where you live.

        • Marco says:

          Hi ianw16,
          Two fly byes of Temple 1 leaves a lot to interpretation based on a very small number of images taken from different directions in the two fly byes. Erosion was expected so that was the tentative conclusion on the Temple 1 Cliffs that appear to have changed. We will have the privilege of triple checking precise measurements of presumed erosion on 67P.

          The spectral signature, ie the spectral colour of reflected light off a surface is not a reliable way to identify the substance on the surface. This contrasts to the reliable detection of H2O in the outgassing, which I am not disputing at all.

          For instance, an identical spectrum found on the moon, would not be considered evidence of water. Precisely because we are not expecting water on the moon bar the permanently shaded regions.

          The before and after perihelion photographic evidence will show whether I am right or not as far as the substance that is assumed icy.

  • dave says:

    After all this time it might have been a fuller explanation of surface ice than that presented. The big question remains unanswered, the generation of the columns of gas and dust that we see erupting off the Comet, how does it happen?.
    Its great that we can see ice on the surface but as much as the small patches of ice pictured here is very interesting, what is happening on surfaces hundreds of square kilometers across?
    Erosion /sublimation from these pictured patches surely is not the prime mechanism for the columns of gas, more a side issue.

    Further if thin layers of dust are covering the surface where bulk erosion is taking place, then it would of been lifted by the rising gas/Ice vapour, this then should expose surface ice, If this is happening there must be similar photographic and spectral analysis available. Surely?

    Further earlier papers have suggested a thick crust on the comet, hence most of the discussion in the past months has been about how does the sublimation of ice get from the subsurface to the surface and how is the heat transfer happening. If this is still the expected main method for bulk erosion, then yes it would make sense that boulders from collapsed cliffs could expose what were once subterranean ice.

    Also we can now begin to see that some of the large columns of dust and gas are originating from the whole floor of large craters, what is the mechanism, why are there no pictures of this type of surface to explain what is happening

    I am still confused, I know this is the first paper detailing surface ice, but it is so incomplete, it merely adds to the frustration

    But many thanks for the information and the paper.

    • dave says:

      Re above post from me

      Sorry, hundreds of square kilometers should read - hundreds of square meters

  • originalJohn says:

    Sorry but it is not true that "exposed water ice has been detected on the comet's surface." Bright patches have been detected and there are other explanations for these that have not been considered. Also you state that "based on observations of the gases emerging from comets they are known to be rich in ices". This is not fact either. It is a hypothesis. There are other explanations for the composition of gases emerging from comets. Perhaps the team could explain what led them in the first place to expect that comet nuclei were made of ice, other than the widely repeated idea originating from Fred Whipple. And do they consider any alternative possibilities, or just the one.

    • bob says:

      Reminds me of the white patches on ceres

    • Gerald says:

      Frozen carbon dioxide sublimates much faster than water ice. Therefore water ice is the better alternative for the bright material.
      But I agree, clathrates would be fine to have been considered. The composition of the gasses from the sublimated ices may help - or may have helped - to narrow down the composition of the ice.

    • ianw16 says:

      Well, given that they not only detected surface ice by its spectral signature at Tempel 1 (http://www.sciencemag.org/content/311/5766/1453), but also detected it from the impact ejecta (http://www.leif.org/EOS/Water-in-Comets.pdf), then I think water ice is a pretty good candidate.
      Perhaps you could outline the evidence you have that "based on observations of the gases emerging from comets they are known to be rich in ices", is merely a hypothesis? Because I could drive the mods here crazy by linking to over a dozen scientific papers that indubitably found H2O in cometary environments going back nearly 30 years, not including the indubitable detection at this comet. Unless you have evidence that the signature at ~557 GHz and ~2.7 microns is caused by something other than water?
      As for what evidence led them to expect comets to be made of ice? How about detecting ice on their surfaces, detecting H2O and CO2 in their sublimation products, and their measured densities for starters. Perhaps you could outline the *scientific* evidence that would lead anybody to believe differently. Preferably from someone who is well known within the group of scientists who have spent a large part of their careers studying comets, and have publications in the field. And preferably not going back a hundred years.

      • originalJohn says:

        You do not appear to understand what evidence is w16. It is not for example what is contained in a published paper interpreting something as evidence of the presence of ice when other interpretations are equally or more plausible. Neither is it an interpretation that a comet nucleus is made of ice, based on the occurrence of water in the coma, when other interpretations of the origin of water exist. Neither is it a vindication of the comet made of ice theory to find a thin layer of frost on one per cent of the surface. These and similar would only be entertained as evidence by those wishing to support one theory in particular at the expense of any other.

        • sjastro says:

          And evidently you do not understand that microwave and infra red spectra involve chemical "fingerprints". as explained by ian W16.

          The spectrum of water is evidence and not an interpretation.

    • Kamal says:

      OriginalJohn: The way you say it, scientists since the 1950s have been repeating a mantra from Fred Whipple without critically assessing it, and revising it. This is not true. I doubt very much whether it is the ESA's aim to put Whipple on a pedestal. So many times on this blog we have had people saying that we have moved from dirty snowballs to icy dirtballs. Perhaps after Rosetta we might move to a newer term still.

      I have read that Whipple wrote to the publisher of Velikovsky's book "Worlds in collision" opposing its categorization as science, which may have something to do with the passion with which the electric theorists oppose the dirty snowball theory.

      • Marco says:

        I am completely aghast at electric Universe theories, but saying that, I do concur with them regarding the Whipple model. The model itself - that is of surface or near surface sublimation of volatile solids has not been critically assessed. It is still the *simplest* explanation even given a lot of unexpected data and unpredicted properties, but this is just from Parsimony rather than based on evidence per se. The evidence is not showing surface or near subsurface sublimation. It is still the "simplest" explanation. It increasingly looks like it is much much more complicated than that. For instance, the surface appears to be a brittle organic substance, not a mixture of dust and ices.

        • ianw16 says:

          So how, 10 years ago, do you explain smashing an impactor into a comet, and seeing H2O come out? Seriously, what is the explanation?
          Anything will do, other than unseen and undetected electric woo. Come on, there is a Nobel prize in this for you.
          Please explain. They've found it on the surface, they've found it sub-surface, they've found it in numerous comas. I'm talking *DIRECT* detection. So, please, tell us what you think they are seeing.
          It is ice. End of. H2O or CO2. It is not silicate rock. Nobody is that stupid.
          When I say nobody.................

          • originalJohn says:

            There are at least two possible explanations w16 for the presence of water in the ejecta from the Tempel 1 impact. 1. The nucleus is made of ice. 2. A reaction occurred at the surface a product of which was water. There is no way that you or anyone else could know which is true from the simple presence of water. The investigators and you have picked one possibility as the truth with no justification, swayed by a belief in the ice hypothesis.

            Similarly there are there are at least two possibilities for the presence of water ice on the surface of Tempel 1. 1. the nucleus is made of water ice. 2. water from the coma condensed and solidified on the surface. No way of knowing which represents reality. In science w16 the idea is to find evidence for one or the other, not just to pick the one you like.

          • sjastro says:

            @originaljohn.

            Your alternative explanation of water/ice forming on the surface of Tempel 1 is a classic case of trying fit the evidence into a pet theory rather than the evidence supporting or repudiating the theory.

            The other point is that evidence is used to answer questions not create new ones.
            The most obvious questions that arise when considering this alternative view is why the density of Tempel 1 is 0.62 g/cc and what the comet is made of.

            Note that in the mainstream view the evidence automatically addresses these questions.

        • Kamal says:

          Marco: At the level of general public awareness, "dirty snowball" is an adequate description of a comet. People working in the area will want more refined descriptions. Since I also cannot understand the electric theory for the comet, I looked for a historical explanation for why they are so vehement.

          Velikovsky appears to have been a Freudian psychiatrist who had his bit of fun creating anarchy among scientists by freely mixing myth and history. The electric theorists on this blog also have a disregard for prevailing science. I found a transcript of a talk on the electric theory of comets at the EU conference in 2013 which freely mixes jargon with fact. I see no reason to take seriously any "science" spoken by someone who believes in that kind of stuff.

        • Marco says:

          Hi ianw16,
          I definitely think there is a lot of H2O *well inside* comets, just not anywhere near the surface.

          H2O is 100% confirmed as a major species in the gases very close to the surface and in the coma in general. Direct detection as a solid water ice on or near the surface is NOT confirmed though not disproven, certainly. Silicate rock is also not detected at all on the surface. The surface is mainly solid and organic. To what depth? Nobody knows yet.

          The deep impact impactor, as its name suggests, made discoveries about what is well below the surface. A lot of water and fine organic and inorganic particles. Mud, essentially. No rock, no electricity. If what was exploded out of the internal part of Temple 1 was on the surface, it would be nowhere near as dark. We can safely say that what we see on the surface is very different from what is well deeper inside. The outgassing must be therefore coming from deeper inside....

          • ianw16 says:

            Precisely. And in some places it (H2O/CO2) obviously isn't far beneath the surface. So I'm not sure what your argument is. We know, and have known for some while, that the surfaces of comets are dominated by dust and organics. We know from their albedo, as well as visual evidence from other missions.
            One can check on the thermal inertia of the surface material in previously published data on this comet. There was nothing there that would rule out heat penetrating to reasonable depths, and causing sublimation.
            If a region happened to be dominated by CO2, or had water ice close to, or indeed, at the surface (like Hapi) then that region would show the most sublimation.
            Struggling to see what point you are trying to make.

          • Marco says:

            My points:
            - What is on the surface almost certainly isn't "a mixture of ice and dust" the conclusion of this paper is very premature. Organics, and some unidentified other stuff, which may be lighter colour organics is what is on the surface.

            - The H2O/ CO2 is about 50 metres below the surface. That isn't far compared to the depth of the "sinkholes" of up to 180 metres, but if it was any shallower, we would have seen actual surface changes by now, from the amount of outgassing - either by visible lifting of dust, or erosion. We have seen the dust environment of the coma increase dramatically, but the surface dust still sitting where it was months ago, and no detectable reduction in feature size to point to erosion.

            - The assumption of surface, or near sub-surface sublimation, is skewing data such as thermal inertia and conduction/insulation calculations.

            - Liquid (water and organics) has not been disproven by direct observations. The internal pressure and temperature has not been directly measured 50m below the surface. Therefore, evaporation of liquids rather than sublimation of solids has not been ruled out.

    • Kamal says:

      OriginalJohn: You said it right, "interpreting something as evidence of the presence of ice when other interpretations are equally or more plausible". Unfortunately no other interpretations of this kind exist. Ice is the most plausible interpretation. The ambiguity
      we have regarding Ceres is different: Dawn's pictures are from thousands of kilometres. Rosetta's pictures are from
      tens of kilometres.

      In another post below you suggest that protons from the Sun are responsible for the white stuff. How? The only process I have heard electric theorists mention is EDM. If this is the process you support you need to show how it produces the spectrum seen and why it takes place where it does on the comet. As I said earlier, Hathor seemed like a good candidate for the "discharge machining" process and we don't find any happening there.

      • Kamal says:

        Correction: there seems to be a sinkhole on Hathor according to the recent Nature paper.

        Kamal

      • Harvey says:

        EDM is conducted *under oil* using very short pulse high current capacitor discharges. The resulting pits are on a micron scale. It bears no similarity WHATEVER to anything going on on 67P.

        • originalJohn says:

          The conditions used for EDM are to allow precise control of a precision machining process. That in no way restricts material removal in an electric discharge to those conditions. I am amazed you think that.

          The point you miss, and the reason the comparison is made with EDM, is that despite it being a microscopic process the resulting surfaces share topographical appearance with the surfaces of many planets and moons within the solar system and as we now know with comet nuclei too.

  • originalJohn says:

    Very strong proton activity going on at the Sun at this time with magnetic storms at Earth. Some of the activity is omnidirectional and will impact the comet. Expect those bright patches to get even brighter in the next day or so.

    • Gerald says:

      The currently observed solar eruptions are directed. Since 67P is amost in solar conjunction to Earth, there is probably no direct connection between CMEs experienced by 67P and those hitting the Earth. But since active regions on the Sun are rotating with the Sun, a different eruption may reach the comet.
      Such a CME should change the magnetosphere and maybe the shaoe of the tail of the comet.

    • Harvey says:

      Protons hitting the comet cannot generate the molecular water explicitly observed by MIRO; there are not enough of them by orders of magnitude even in a storm; etc etc. The same repeated argument, without a shred of evidence.
      We've had no plasma data of late, but 67P may well be pretty well screened from the solar wind by now; by the very plasma processes you like!

      • originalJohn says:

        The idea is that the protons are the energy source for the reactions that take place at the surface Harvey. No data is yet forthcoming for proton densities or energy levels within the coma, particularly close to the nucleus surface in the reaction zone. The recent solar activity has given rise to steady streams of protons in the MeV range, as background levels, And once again Harvey I will provide the evidence of what surface reactions are occurring when I have control of the measuring equipment.

        Let me make it clear too that I do not like or dislike particular processes as you suggest. I do however consider their relative plausibilities.

        The plasma envelope called the coma will form a boundary with the plasma of the heliosphere but not necessarily a screening boundary, quite the reverse in fact. It is likely to be an accelerating boundary. A screening effect could arise from the magnetic fields induced by plasma current filaments but you have already suggested that any magnetic fields surrounding the nucleus are vanishingly weak so what then is your screening effect, that protects the nucleus from protons travelling at a million+ mph and protects your crumbling theory accordingly.

        • Harvey says:

          Well, we can agree on one thing; unusual. Bow shock or not, the MeV component would penetrate, but it's tiny and transient.
          Of course this event is very unlikely to hit 67P anyway, given the relative positions of the comet and Earth. Can't easily check, travelling.
          As to what effect it will have, well not so long ago it was confidently asserted that the protons sputtered off oxygen which formed the water. Now we have a good test of that to hand, called our Moon.
          It certainly was hit by this week's CME; were there any reports of the Moon generating a coma this week - or indeed any other week?
          (Yes, it does indeed produce umpteen order of magnitude weaker emissions; I'm talking 67P scale, Halley scale, water emissions.)

          • originalJohn says:

            It is still proposed that the solar protons release oxygen from the rock Harvey. The crucial difference from the original proposal is that the source of hydrogen in the water is not the protons. It is the hydrocarbon layer coating the comet nucleus. Hence the combustion reaction.

            You have put forward the Moon as an example before but you should understand that it is not a relevant comparison. The Moon is in a stable near circular orbit and within the heliosphere following the Earth's orbit. It encounters therefore a stable potential. Comets on the other hand move on an eccentric orbit through the potential gradient of the solar system, hence the discharge voltage.

            The other key factor lacking on the Moon is the dense hydrocarbon layer that coats all comets and many asteroids. So no formation of combustion products at the Moon surface and no plasma discharge. Without these two factors no coma, regardless of the proton density. The oxygen released could react with protons at the surface but producing minute amounts of water compared with a comet and what amounts that did form would remain on the surface.

            The presence of and characteristics of a comet therefore modify the properties of the solar wind plasma and promote the continuation of the surface reactions and discharge. The Moon does not do this so we do not see it displaying the characteristics of a comet. So, pretty much, forget about the Moon.

        • Gerald says:

          Originaljohn, the total energy of solar protons arriving at the nucleus is a tiny fraction of the energy of solar illumination.
          Solar protons of the intensity you need for your suggested effects would almost immediately destroy the technical equipment orbiting the comet.
          Much lower intensities would e.g. already invalidate the images due to oversaturation by SEP events.
          You may occasionally see these radiation-induced blips in images made in the Van-Allen-Belt, and you will see it in images to be made near Jupiter.
          The cameras can be used as particle detectors. So by just looking at the images you see, that radiation is on a reasonable level, similar to that in outer space elsewhere.

          • originalJohn says:

            What you don't appear to understand Gerald is that a body inserted into a plasma is enveloped its own local environment. If the body is disharging there is further modification. This creates the coma of a comet. From the surface moving away from a comet nucleus the environment is likely to be extremely variable and nothing like the heliosphere background. That is why analysis of the boundary environment of the nucleus is so important.

            The proton energies and densities required for the combustion reaction at the nucleus surface need only be local to the nucleus. Even a few metres away let alone a few kilometres the flux intensity is likely to be completely different. The orbiter is a different body with its own local environment and surface conditions.

            As for the lander who knows. And who knows what is left of it. Once again it depends on the particular local conditions very close to where it is. If it happened to be near, or in the middle of an intense glow discharge it chances of survival would be minimal. If it was an arc discharge they would be zero.

            Amazing that you would rely on a secondary effect in cameras as a measure of proton intensity rather than employ the specialist equipment you have there.

        • ianw16 says:

          Lol. Crumbling theory. Coming from someone who supports a load of neo-Velikovskian nonsense, without a whit of evidence to back it up.
          Other than what Uncle Wal says, of course.
          Brilliant.
          And yes, there are data to say what the solar proton flux is. Not only at the distances Rosetta got to the comet (~8km), but from Philae, as it descended. Not to mention the lack of a magnetic field. Not to mention that Philae is still alive, which it wouldn't be in those particlle fluxes. But, hey, why let science get in the way of a good yarn?
          Sorry old boy, but you are still batting zero, as our American friends would say.
          Some evidence would be good. Not holding my breath though. As soon as any does come up, they'll just invent a story for the Faithful, just as dear old Wal did regarding the Deep Impact mission.
          Like I have said before, it is faith based nonsense. End of.

      • originalJohn says:

        The hydrogen comes from the hydrocarbons Harvey, millions of tons on each nucleus.

        • Harvey says:

          How, exactly, do you form H2O from a high energy proton hitting a hydrocarbon?
          There is a slight problem of a lack of oxygen!
          Aside, as ever, for the low sputter yield and inability to form H2O molecules even if there is oxygen.
          It does not work,mend of story.

          (One should always be sceptical of 'Vicar of Bray' arguments BTW. Not long ago there was total confidence it was protons hitting silicates; which, it seems, are in short supply.)

          • originalJohn says:

            You are confused Harvey. The viable hypothesis is that the oxygen comes form the rock. The energy to release it comes from the protons. The water comes from the combustion reaction that takes place at the interface between the rock and the hydrocarbon layer. That has always been, since its inception, and still is the proposal. It does indeed work. Beginning of story.

  • Sovereign Slave says:

    Wow, seems they've found Waldo after all, which is great. One small step for cosmology, one giant step for sublimation. At least ice is apparently what makes the most sense for these findings. Was especially encouraged when I read that "The team also turned to laboratory experiments that tested the behaviour of water ice mixed with different minerals under simulated solar illumination in order to gain more insights into the process." Would be great if they're taking that a few steps further and are actually testing the pressurization and funneling process of frozen volatiles mixed with dust to help decipher what we're seeing with the jets/columns.

  • nenad says:

    Exciting to see ice on a comet

  • Robin Sherman says:

    Thanks Emily. Its nice to know that the speculations and ideas of many here have been confirmed by the OSIRIS team. Congrats to Kamal for I think first suggesting here the sublimation residue, as being the source of the surface "dust" layer. A lot of us thought it made a lot of sense then and it seems the OSIRIS team agree. The question still remains as to how the bulk cometary material became such a homogeneous mixture of volatile ices, dust and organics.

    Its also nice to see some OSIRIS images which in addition to showing surface ices also clear up the "black holes" we have commented on before. In the bottom right of the Hatmehit image there are many examples and it can be seen they are just shallow pits created by the sublimation of small pockets of volatiles. They are clearly not the source of the large jets we see, but just little sprites like we see in the first Imhotep image. It would seem some of the escaping Water vapour recrystallises before sublimating again further from the surface. This must say something about vapour pressures and temperatures just above the surface. I wonder if this is connected to the ALICE team's finding of a change in the coma at a certain height above the surface.

    The first image focuses on what was dubbed "Malmer's Glint", an early candidate for Phillae, which is visible in many NAVCAM and OSIRIS images made publicly available. This is within a few tens of metres of Phillae's final touchdown. We can see close up the rocky, rugged terrain that Phillae is situated in, its no wonder it has been difficult to spot our favourite little robot.

    Many thanks to the OSIRIS team too for allowing this paper to be freely available. It clears up one of the big questions, "Where is the Ice?". Now we know, though the cliff fall we see in the Imhotep - Khepry image was pointed out by Claudia in an early blog post as being a good possibility for surface ice and the VERTIS(?) presentation at the AGU showed it was likely to be Water ice too, about 3% Water I seem to remember.

    • dave says:

      Robin, nice considered post, and as you say many, many people speculated that if there was ice under the surface, then it should show on the boulders under the cliffs.

      However since people have been speculating this for 6 months or more, I don't understand why these pictures have been held on to for 7 months, there seems no great secret revealed and no damage that would of been done to a scientists career, especially if the pictures were released with out spectral analysis.

      There is no mention of crystallisation in the article, and of course its possible that that's at least part of the what we are seeing where these boulders have been in the shade, maybe there are more pictures available just after shade and just before shade to help clear things up, who knows?

      I wonder what else is just needlessly been kept under wraps.

      its good to see a calmer approach than mine though.

      regards

      • Kamal says:

        Dave: Because of this piecemeal information coming out one is losing track of the big picture. Perhaps people who have kept track of the many things which have been reported might like to speculate on what Rosetta may see from 67p over the next couple of months, keeping in mind that it might soon have to move away to a safer distance. We have already seen Imhotep and then Hatmehit open up as big jet regions.

        Since we don't know much about Philae's status it seems difficult to guess at present what it will see. I will only hope that it sends us its current data, survives perihelion and manages to send us the data it gathers over the perihelion period when Rosetta is close enough later on, perhaps in November or December?

      • ianw16 says:

        Sorry Dave, but why do things need clearing up?
        They smashed a 360 kg impactor into Tempel 1. What came out? Water. Indubitably. H2O. How many times do they need to detect it on the surface, below the surface and in the sublimation products, for people to actually get it through their heads that there is a lot of ice in comets?
        Add all that to the *measured* density, and I'm really not sure what your point is.
        "Needlessly being kept under wraps"?
        Do you have evidence of this? It sounds like you're accusing them of something.
        Perhaps you'd like to spell it out?

    • Kamal says:

      Robin: You posted about a "dust cycle" long before I said anything. But let us not lend much credence to our speculations. It is the Rosetta and Philae teams which are doing the real analysis of the data and building firm theories. Gerald hinted in an earlier post that the identity of some of the hydrocarbons may be in the offing. It would be nice to get that in as well.

      Kamal

  • Dave says:

    Great science , but what's happening with Philae on the surface - the silence over the last few days has been deafening - is all well?

  • Harvey says:

    Well it had to be there, but it has kept a low profile; nice. to actually see some 🙂

  • Ramcomet says:

    Boom! Crack! Ice Ice Baby!

  • OzObserver says:

    Just amazing! Well done OSIRIS team! Keep up the good work!

  • ianw16 says:

    Must be electrical discharges, surely? Oh, come on guys, this surely can't be right. Can it? 🙂
    This is the 10th attempt. Great captcha system, chaps

    • Kamal says:

      IanW16: Am waiting to see when the captcha asks me to recognize the chapatis!

  • Bill Harris says:

    Being your basic old-schooler, I'm accustomed to thinking of surface ice in a near-vacuum environment quickly sublimating There is more to it than it just going *poof*.

    Here is a paper from Icarus describing the behavior of ice under Lunar conditions:

    http://people.nwra.com/resumes/andreas/publications/Icarus_Moon.pdf

    A good primer for this flatlander... 😉

    --Bill

    • Harvey says:

      Bill. Ice can exist in vacuum if it's cold enough; the sublimation rate is then limited by the heat input available to supply the latent heat of evaporation. The surface cools as it sublimes.
      In fact in (terrestrial!) vacuum systems with internal water cooled parts, if you get a water leak it sometimes self-seals with ice, and with a big pumping system can sometimes hold a surprisingly good vacuum as the ice cools down due to sublimation cooling of the surface. It is I agree kind of counter intuitive, but does indeed happen.
      The comet has the Universe as its pumping system 🙂

  • Lucas says:

    “Water ice is the most plausible explanation for the occurrence and properties of these features,”

    I thought that rosseta is searching for the truth and proof not plausible explanations

    Great work anyway!

    • Harvey says:

      Lucas.
      Rosetta has a certain set of instruments. We have a limited set of measurements from them. It's capabilities are great - but still limited.
      Scientists tend to use cautious, perhaps over cautious at times, language. What those instruments can provide may often fall short of 'truth and proof' so we speak of 'most plausible explanations'.
      That's not a failure; it's careful recognition of the limitations of the data we have.

  • Graham Hall says:

    One in the eye for the EU boys. " There is no ice they proclaimed" - when will they wake up to reality?

    • dave says:

      Re EU boys

      EU or not Graham, most people have expected Ice or ices to be detected and most would not quibble with the cautious conclusions. Also most of us are glad to see the indication of some ice on the surface. It has been amazing that after all this time this is the only ice on the surface, almost beggars belief it has been so hard to find.
      However
      The paper uses phrases like
      ' appears to be the most plausible hypothesis'

      So there is a bit more to come yet.

      regards

    • ianw16 says:

      Probably when they realise that scientists - real ones, that is - have been directly detecting H2O and CO2 in comets for nigh on 30 years.
      They just aren't told that. Ergo, if it hasn't come from one of the trusted few, it isn't believed. They just make stuff up to try and explain it away.
      Hey ho, some people believe in astrology.

  • Kamal says:

    Is there a kind of blue shadow of the white stuff in the first Imhotep picture?

    • emily says:

      Hi Kamal,
      I am not 100% certain for this particular feature, but I do know that because the colour images are assembled from individual frames taken at different times and then re-aligned, they are subject to artefacts/mix-matches...

  • Bill Harris says:

    Agreed. The Red and Blue fringing is common on these tri-color constructs.

    --Bill

  • Laurence says:

    I'm in continual ecstatic amazement at the new things cropping up all the time. This is SO thrilling.

  • Dave says:

    Ian 16,
    Re sorry Dave why do things clearing -

    Well you can't be satisfied with what had been released Ian, surely?
    Is the ice. In these areas intrinsic in the boulders, or is it just recrystallisation of water in the coma as the areas come in and out of the shade?
    These areas don't look like they are responcible for the large columns of dust and gas, so where is the ice that forms these and how is it released?
    Re kept under wraps -
    I just think it's rediculous for. These close up photos for 7 months when many people had all ready guessed there may be ice there. You seem paranoid, I don't think there is a conspire or they are hiding any thing, there policy has been clear from the beginning, but it's also clear from this paper that the policy is a bad one.
    I'm fine that there is water on the comet, it's clearly a long way from a dirty snow ball, but how could there not be some ice on it.
    We are still waiting to know how much and where- we will clearly have to wait for more data.
    I am not accusing them of anything, well maybe that holding pictures for more that 6months is not a good policy.
    Lighten up

    Regards

    • ianw16 says:

      @Dave;
      "Where is the ice the ice that foms these......"?
      How about sub-surface? How about I don't have to keep posting the link to the Tempel 1 data? How about actually acknowledging that they've spectrally identified water ice on the surface of a comet before? How about actually accepting that they've directly seen the H2O spewing out after impacting it? And have seen the impossibly large crater that wouldn't have been made in rock? And have directly observed water in the comas of many comets?
      And accepting that EU doesn't have a single line of evidence for its nutty ideas, and continually has to obfuscate and deceive its acolytes in the nonsense it writes on its website?
      Or do I need to start linking and quoting that rubbish as well?
      Thankfully, *real science* is either unaware of it, or totally ignores it, and seems to do just fine.

    • Kamal says:

      Dave: Sorry, but it is you who are not satisfied. In January we saw posts on "icy alcoves"
      http://www.esa.int/Our_Activities/Space_Science/Rosetta/Getting_to_know_Rosetta_s_comet
      and your comment was that this was not enough to explain the jets. Now we have more locations and you say that this is still a long way from a dirty snowball. Agreed, the surface of the comet does not look like a dirty snowball, but no one knows what is in the interior, so whether it is or isn't is yet to be resolved. Given that the jets produce so much water, and given the data we have from comets earlier, common sense suggests that there must be a lot of water in the interior.

      If you think that common sense is wrong and protons from the Sun are producing the water from oxygen that is locked up in rocks on the surface or in the interior, it is possible, but there has to be evidence, at least indirect, and that is lacking. For example, you could plot the variation of the protons being released from the Sun over the last year, and correlate it with the number of white spots seen on the surface a few days later when you expect the protons to reach the comet. Without such correlations it is difficult to hypothesize a causal connection. Unfortunately your theory complicates matters by also considering regions of charge in space which is data not accessible to any one, and makes the theory non-testable.

      • dave says:

        Kamal,

        l - 'common sense suggests there is a lot of water in the interior'

        You seem to have misunderstood

        The finding of ice on the surface however cautiously stated is fine.

        It does not yet get us to the point where we understand how the large columns of dust and gas are produced and maintained.

        The point is that common sense will not do. We need to know what the mechanism is that drives the production of those jets.

        Thats all I have said, our curiosity and the project itself needs to know whats happening - therefore we have to wait for more data.

        regards

      • Kamal says:

        Dave: They have another post on this which should have a better explanation, but let me say this here. There are at least two phenomena, gas jets and dust jets. My understanding is that the pictures (for definiteness, the Navcam ones) only show the dust shining in a "jet" because of sunlight reflecting off it. The gas is inferred from other instruments. The dust particles that we see in the jet might have got there at different points of time. Some of them may be shot off the surface. We also seem to have some evidence that by now the whole atmosphere around 67p is cloaked in a fair amount of dust. So some of the dust may have been there earlier, maybe there wasn't as much as to reflect enough to be caught by Navcam, or maybe it primarily reflected at a different wavelength. Exactly how the integrated phenomenon of a jet is seen may require putting together many things.

        You seem to be presupposing that the only mechanism is that there is a large body of volatile, which is expelled quickly and which takes a large amount of dust out of the surface. My guess would be that there are several mechanisms.

  • Kamal says:

    From the paper: "As shown by laboratory experiments, large chunks of the dust mantle are relatively easily removed because the mantle itself can show a strong internal cohesion but low cohesion with the icy material underneath." I interpret this to mean that if the mantle top can be blown off one should see the ice below. Does one expect a perihelion passage to provide this kind of energy from the Sun?

    From the post: "Some of the isolated bright objects are found in regions without any apparent relation to the surrounding terrain. These are thought to be objects lifted up from elsewhere on the comet during a period of cometary activity." Malmer's glint below Hatmehit is a good example of such an object. Can we predict that the ice will heat up and disappear as we approach perihelion?

    • dave says:

      Hi kamal,

      It did say that did say 'low cohesion with icy material underneath'

      However looking at the pictures I was not so convinced that the loose conglomerate under these cliffs was necessarily all bound by ice.
      We have seen lots of these collapsed cliffs, where indeed the boulders below the mantle seem to have loose cohesion with anything, themselves and mantle included. I am not necessarily convinced the cohesion is provided by the ice, the reason for this is because how well hidden it has been, it seems to suggest the proportion of ice to rubble is small, ie prone to collapse under any conditions.
      Of course not all the cliffs are like this suggesting there is a big variation in the distribution of this low cohesion material (may be, its even evidence of high porosity below the surface in these regions).

      Anyway I look forward to further results so that we can understand all that is happening on the surface

      regards

  • Rod says:

    There is ice on the surface and there is a thin coating over it!
    Excellent!, but can someone please explain to me the huge jets coming from the comet as per all the pics.
    If this is the ice sublimating from the surface why doesnt it just billow out in some sort of way?
    What are the jets made from and why are they so energetic when the top surface which is presumably hottest doesnt appear to be doing much. Is it something in the core with a much lower boiling temperature?

    • ianw16 says:

      Well, that would kind of depend on how thick the insulating layer of dust was, and what was underlying it. In the neck region there is very good evidence that the ice is at the surface. Hence the outgassing in that region. At 103P/ Hartley 2, for example, they spectrally identified that the jets there were composed of CO2, which does have a lower vapourisation temperature.

    • Gerald says:

      In the near vacuum water ice sublimates near -70°C, sufficient to eject dust from the comet. It doesn't need to be highly energetic. A few meters per second are sufficient to eject dust from the comet due to the very low gravity.
      The distance of the comet to the sun is between the distance of Earth and Mars to the Sun.
      Think of the Sahara or Atacama desert on Earth, or of Death Valley. That's regions on Earth, where the atmosphere is dry (most of the time), and the sunlight is just moderately filtered.
      The surface is heated up in these deserts. You burn your feet at noon in those deserts. Think of the surface of the comet in a similar way. Now it's easy to reach the necessary minus 70 degrees Celsius in such an environment, necessary to sublimate water ice.
      It's even more easy to sublimate ices of carbon dioxide or carbon monoxide.
      Small grains of dust are set free as the embedded or embedding volatiles sublimate.

      • dave says:

        Gerald,
        -to eject dust from the comet water sublimation does not need to be energetic.

        I think this is the crux of the matter Gerald, The dust should quickly clear the surface, even the comet as soon as sublimation starts (Gas speeds recorded are very high) but ice is so very difficult to detect, we should be blinded by it, if sublimation is at the surface as you would expect it to uncovered.
        .
        If the gas is coming from underneath the surface based on the model of a cavity with gas ejecting through a smaller hole, then we would not expect the ice to be exposed, however it is very difficult to imagine how this method can make those large diameter columns.

        regards

        • ianw16 says:

          It really isn't that difficult to detect. Try reading that Tempel 1 paper that I'm getting sick of linking. It's there, shed loads of it, beneath a surface of varying thickness and composition.
          Perhaps, at this stage, you might want to spell out the predictions of the EU hypothesis of what we would find at this comet? And just how those predictions are going?
          There is a bloke you may have heard of, called Dave Talbott, who made some predictions on a science forum I frequent
          This was some time before the craft caught up with 67P. They really aren't looking too good, to be honest. Perhaps I need to quote and link the post? Not that the bloke has any scientific qualifications, like.

        • Gerald says:

          The ice doesn't need to be pure. It's more likely mixed with grains of black dust. And even worse, just 1/4 or a little less of the mix is ice on average. So, unless the cometary material is highly heterogenious on a scale of at least several centimeters, it's far from self-evident, that bright patches of ice will be exposed.

          As a terrestrial model take wet soot or wet charcoal and freeze it. Exposure of pure ice is possible, but not necessary, when such a mix is going to sublimate/erode.

  • Jon says:

    On June 17th Jean Pierre explains that the OSIRIS images depicting light or bright spots are merely by-products of the processing to view surface features. He says the light spots are 3 times darker than the moon's albedo - so no surface ice. So why did ESA post 120 pics from September about bright spots that they "suggest" might be ice ???

    They lost me. Anybody make sense of this without stirring up any dust? light humor!

    Jean Pierre about the 1:06:30 mark comments:
    http://www.esa.int/spaceinvideos/Videos/2015/06/Replay_of_Rosetta_conference

    • Gerald says:

      The explanation in the press briefing refers to the particular image. In this images the brightes spots are still dark, and don't show surface ice. This applies to many other images, as well. But there are some spots, most of them in shadow, with a much higher albedo, and with a hue shifted to bluish. Those latter spots have been "suggested" to be rich in ice.
      Without information about the radiometric calibration of the images the grey scales are difficult to interprete as absolute albedo values.

    • Kamal says:

      Jon: Sorry, but I will have to quote from the paper, I take the liberty of editing out unnecessary stuff. Please trust me that I am not erasing anything important. Please don't try to take quotes out of context and try to fit them to your pet theory.

      "Global color images of the nucleus reveal a generally dark and homogeneous surface with the exception of the Hapi region, which displays a slightly higher reflectance and is notably bluer than the rest of the surface. [These] might indicate the possible presence of a small fraction of H2O ice mixed with refractory material at the surface of Hapi, an interpretation tentatively confirmed by VIRTIS. The surface is strongly backscattering and shows a significant phase reddening. At a small scale however, the surface is much more heterogeneous. As the spatial resolution of the images was increasing from June to October 2014, more and more variability at the smallest resolved scale could be observed in the form of isolated or clustered bright spots with a strong contrast of the albedo compared to the surrounding surfaces and a distinctive bluer color. In this article, we analyze the properties of these small bright features"

      • Jon says:

        Thanks Gerald and Kamal.

        The problem I still have is that this report, from this article above is from data gathered in September and yet Jean Pierre reports about 3 weeks ago that there are NO bright spots on the comet and that in fact the lightest areas detected on 67P are 3 times darker than the moon.

        fyi - I personally have no pet theory on comets, as I prefer to follow the exploration to see where it leads.

        Is it possible that teams may not "see" the comet the same way?

  • Dave says:

    Hi kamal,
    I agree with you, there must be different types of erosion, we know the geology is different across the comet so would not expect deep pipes every where, We have now seen close up detail of two terrains.
    It's important to know how the whole comet works and I would say extra important to be sure how those big jets form, they are the the things of myth over the years. Many years ago (more than 50) when I first read of halleys comet I counted down the years until it arrived to experience the horizon to horizon tail, of course I was disappointed, so now we can fly along side a comet I look forward to learning how it works however many different processes there are.
    Regarfs

  • What I cannot quite understand: Water in open space under near vacuum conditions, yet above absolute zero Kelvin always has some vapor pressure. And I assume these icy deposits are, like their comet/meteor substrates, millions of years old. Would not all water have completely vanished = evaporized over such a long time span? Like the washing on the line dries even in the coldest winter?

    • Harvey says:

      Maureen. The point is that the vapour pressure of ice falls very rapidly with temperature, so if it is cold enough it only sublimes very, very slowly. The rate it can evaporate at is limited by the energy input to provide the 'latent heat of sublimation' - this is why say alcohol on your skin feels cold; it takes heat from your body to evaporate it.
      So the calculations show that when it is far out in the solar system the sublimation is almost zero; it's so cold, with so little heat input.

      I work on vacuum systems, and I've seen lumps of ice in a high vacuum chamber! A water leak had cooled by evaporation, frozen, and then cooled further by sublimation - the vacuum was surprisingly good. I just had a (big) vacuum pump; the comet has the Universe as its vacuum pump!

      Only close to perihelion does it warm up enough to become active, sublime. Indeed it may not last all that long, after a quite modest number of orbits a comet can become 'dried out' and inactive. But new comets get 'flicked' into these short period orbits by long period comets and other objects interacting with Jupiter mainly, so there is a continuous supply to replace the dried out ones.

      • Jon says:

        Pluto is not a comet, but some of the topography shown to date kind of resembles 67P and Pluto orbits between 30 and 49 AU and yet planetary scientists hypothesize that the frozen plain could be sublimating. Is that in line with accepted theory?

        "This fascinating icy plains region -- resembling frozen mud cracks on Earth -- has been informally named “Sputnik Planum” (Sputnik Plain) after the Earth’s first artificial satellite. It has a broken surface of irregularly-shaped segments, roughly 12 miles (20 kilometers) across, bordered by what appear to be shallow troughs. Some of these troughs have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain. Elsewhere, the surface appears to be etched by fields of small pits that may have formed by a process called sublimation, in which ice turns directly from solid to gas, just as dry ice does on Earth."

        One other observation. I posted a few weeks ago about Bibring' comments on albedo and surface ice. He says there is NO evidence for any surface ice and he also states that the lightest spots - albedo (not the average albedo) is 3 times darker than the moon. He also states that the icy dirtball theory (80% ice) that he wrote many times about himself and "invented" two to three deacdes ago is wrong. Are we finally moving off this theory?

        I wonder where Dr Sierks is in these media pressers of late?

    • originalJohn says:

      Not everybody, Maureen, believes, that the source of comet water is ice in the nucleus. It is a hypothesis that is rapidly becoming obsolete for lack of evidence. There are alternative more likely hypotheses and one that I have proposed is the production of water from a surface combustion reaction between oxygen released from the nucleus rock and the hydrocarbons which coat the surface. Protons from the Sun provide the energy to separate the Si-O bonds.

      • ianw16 says:

        Which is obviously nonsense, as the solar wind proton flux is nowhere near high enough. As has been measured. Yawn.

Comments are closed.