COSIMA watches comet shed its dusty coat

Early results from Rosetta's COmetary Secondary Ion Mass Analyser (COSIMA) are published today in the journal Nature. The study covers August to October, when the comet moved along its orbit between about 535 million kilometres to 450 million kilometres from the Sun, and when Rosetta was orbiting the comet at distances of 30 km or less.

This news item is also published on the ESA Portal

COSIMA is one of Rosetta’s three dust analysis experiments. It started collecting, imaging and measuring the composition of dust particles shortly after the spacecraft arrived at the comet in August 2014.

The scientists looked at the way that many large dust grains broke apart when they were collected on the instrument’s target plate, typically at low speeds of 1–10 m/s. The grains, which were originally at least 0.05 mm across, fragmented or shattered upon collection.

The fact that they broke apart so easily means that the individual parts were not well bound together. Moreover, if they had contained ice, they would not have shattered. Instead, the icy component would have evaporated off the grain shortly after touching the collecting plate, leaving voids in what remained. By comparison, if a pure water-ice grain had struck the detector, then only a dark patch would have been seen.

Introducing the crumbled dust grain Eloi (a) and the shattered dust grain Arvid (b), two examples of the grains helping scientists to decipher the characteristics of 67P/C-G's dust. For both grains, the image is shown twice under two different grazing illumination conditions: the top image is illuminated from the right, the bottom image from the left. The brightness is adjusted to emphasise the shadows, in order to determine the height of the dust grain. Eloi therefore reaches about 0.1 mm above the target plate; Arvid about 0.06 mm. The two small grains at the far right of image (b) are not part of the shattered cluster.  Credits: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/ BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S

Introducing the crumbled dust grain Eloi (a) and the shattered dust grain Arvid (b), two examples of the grains helping scientists to decipher the characteristics of 67P/C-G's dust. For both grains, the image is shown twice under two different grazing illumination conditions: the top image is illuminated from the right, the bottom image from the left. The brightness is adjusted to emphasise the shadows, in order to determine the height of the dust grain. Eloi therefore reaches about 0.1 mm above the target plate; Arvid about 0.06 mm. The two small grains at the far right of image (b) are not part of the shattered cluster. Credits: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/

The dust particles were found to be rich in sodium, sharing the characteristics of ‘interplanetary dust particles’. These are found in meteor streams originating from comets, including the annual Perseids from Comet 109P/Swift–Tuttle and the Leonids from 55P/Tempel–Tuttle.

“We found that the dust particles released first when the comet started to become active again are ‘fluffy’. They don’t contain ice, but they do contain a lot of sodium. We have found the parent material of interplanetary dust particles,” says lead author Rita Schulz of ESA’s Scientific Support Office.

The scientists believe that the grains detected were stranded on the comet’s surface after its last perihelion passage, when the flow of gas away from the surface had subsided and was no longer sufficient to lift dust grains from the surface.

While the dust was confined to the surface, the gas continued evaporating at a very low level, coming from ever deeper below the surface during the years that the comet travelled furthest from the Sun. Effectively, the comet nucleus was ‘drying out’ on the surface and just below it.

“We believe that these ‘fluffy’ grains collected by Rosetta originated from the dusty layer built up on the comet’s surface since its last close approach to the Sun,” explains Martin Hilchenbach, COSIMA principal investigator at the Max-Planck Institute for Solar System research in Germany.

“This layer is being removed as the activity of the comet is increasing again. We see this layer being removed, and we expect it to evolve into a more ice-rich phase in the coming months.”

The comet is on a 6.5-year circuit around the Sun, and is moving towards its closest approach in August of this year. At that point, Rosetta and the comet will be 186 million kilometres from the Sun, between the orbits of Earth and Mars.

As the comet warms, the outflow of gases is increasing and the grains making up the dry surface layers are being lifted into the inner atmosphere, or coma. Eventually, the incoming solar energy will be high enough to remove all of this old dust, leaving fresher material exposed at the surface.

“In fact, much of the comet’s dust mantle should actually be lost by now, and we will soon be looking at grains with very different properties,” says Rita.

“Rosetta’s dust observations close to the comet nucleus are crucial in helping us to link together what is happening at the very small scale with what we see at much larger scales, as dust is lost into the comet’s coma and tail,” says Matt Taylor, ESA’s Rosetta project scientist.

“For these observations, it really is a case of “watch this space” as we continue to watch in real time how the comet evolves as it approaches the Sun along its orbit over the coming months.”

“Comet 67P/Churyumov–Gerasimenko to shed dust coat accumulated over the past four years,” by R. Schulz et al is published in the 26 January issue of the journal Nature.

COSIMA was built by a consortium led by the Max-Planck-Institut für Extraterrestrische Physik (Garching, DE) in collaboration with Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, CNRS / Université d’Orléans (FR), Institut d’Astrophysique Spatiale, CNRS / Université Paris Sud (Orsay, FR), Finnish Meteorological Institute (Helsinki, FI), Universität Wuppertal (Wuppertal, DE), von Hoerner und Sulger GmbH (Schwetzingen, DE), Universität der Bundeswehr (Neubiberg, DE), Institut für Physik, Forschungszentrum Seibersdorf (Seibersdorf, AT), Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften (Graz, AT) and is lead by the Max-Planck-Institut für Sonnensystemforschung (Göttingen, DE).





  • dave says:

    I don't understand where the ice is going to come from when the dust lifts.
    It seems that the Vitis instruments are in direct opposition to these results. Vitis, says no or very little ice on the surface or just below it.
    Even now the dust on the neck lifts up into the coma and drops back to the ground again, so there is a process of sublimation of any ice left on the grains and presumably the crust beneath it, why is there going to be ice under the dust, When there is none under the crust down to a few hundred cm
    Am I missing something?

  • originalJohn says:

    ESA, why would these dust particles cluster in the first place despite the observed jet velocity, Then why would they separate on contact with the plate. Any charge detected ?

    • Kasuha says:

      Gravity on the comet's surface is very small, only very small force (of released gases) is needed to release and accelerate these particles. It's similar to when in winter you can sometimes see large clumped snowflakes which also break up when they hit the ground.
      Static charge may play some role in their formation. Both the comet's surface and released dust are bombarded by solar wind consisting of charged particles. So individual grains of dust may acquire tiny charge of either sign from it and may be pulled together like a clump of little magnets. It is even possible that the clumping process continues even after the grain was released from comet's surface, both due to random collisions and further neutralization of its residual charge. But "mechanical" forces, similar to these holding Earth dust clumps together, likely play their role too.

  • Robin Sherman says:

    Fascinating images Emily, thanks to Martin and yourself for the info. So now we have a lot better idea of the nature of the top surface dust blanket. As suspected the dust grains are extremely fragile, held together by weak forces or even by just physically fitting together like a jigsaw puzzle with the rough surfaces of the tiny smaller particles interlocking.

    We see what happens to comet material when the ices sublimate from around the silicate dust. It just crumbles into a pile of rubble. The same thing as appears to be happening on a vastly larger scale on the comet. Once bits of debris start flying about and hitting the comet surface one can imagine vast quantities of dust are going to be created. What a shame we don't have video, the activity close to Perihelion could get quite dramatic.

  • Gerald says:

    As trivial as these images might look. It's simply fascinating to see this fluffy cometary dust impacted with a few meters per second instead of tens of kilometers per second.

    Sodium enrichment sounds like interaction with water ("aqueous alteration"), and few space weathering, if I rely on papers about planetary dust:

    Finding phyllosilicates ("clay") on a comet due to silicate - water interaction would be fascinating, as well, since the environment looks so different from Earth.

    Clay minerals from meteorites is something I need to get used to, in the geological context of planets. Kind of primordial clay, possibly older than basalt.
    At the end we might get habitable environments, where noone really would have expected.

    • Gerald says:

      interplanetary dust, of course

    • THOMAS says:

      "...since the environment looks so different from Earth".

      Are you kidding? IMHO, we have never acquired images of any other heavenly body which consistently shows such absolutely Earth-like (or Mars-like) features: stratified, fractured rock-faces everywhere, piles of rubble and boulders heaped up at the base of sheer cliffs, spectacular rugged outcrops on every sky-line, a truly impressive canyon.... There are many images which, without knowing their origin (and without the black “sky”), could be taken for landscapes snapped by tourists in some of the more inhospitable regions of the Earth.

      When I made this point once or twice a few months ago, after seeing the first pictures, I was sent packing by defenders of the standard theory: not because my comparison with Earth-like features was wrong, but because it was completely “irrational” to simply believe the evidence of one's eyes, since we already knew that comets are dirty snowballs visiting us from the outer confines of the Solar System…!

      Coming back to your original point, I am confident that “clay” is indeed one of the constituents of 67P, as it is down here on Earth, on Mars and on the only other comet nucleus we have actually made physical contact with, Tempel 1 (of 2005 “Deep Impact” fame):

      • Gerald says:

        Thanks for the pointer to the clay on Temple 1, THOMAS!

        Since our solar system is made of interstellar dust and gas like other solar systems, and clay minerals are found on that broad range of objects in our solar system, this hints towards rocky exoplanets may be more likely to be Earth-like, in the mineralogic sense, than random assumptions would expect.
        I'm thinking at the factor n_e of the Drake equation:

        Adding expeced results of the organic chemistry on 67P/C-G could even add constraits to the factor f_l.
        Great to see some substantial progress on this challenging puzzle!

      • Judy Hawkins says:

        I've got several different ways I can see these pictures of the comet, depending on which perceptual patterns I'm in, mentally. The finest resolution we're getting is, what, 0.67 meters? so, like, if a box of chocolates was sitting there we wouldn't see it, not as a box of chocolates. It's all a blur, from that perspective.

        Another mental perspective I have leads me to compare what I see with things like bread dough, which is a finely grained composite of organics (flour and organisims I can't resolve visually) and water. The crust that forms in baking has some features that are similar, especially if it gets bubbles, that get dried and crack open.

        Or making short bread -- cutting up butter into flour and sugar, and getting a mixture of lumps and fine stuff. I want the big lumps to come to the top so I can cut them up some more, so I rap the bowl, shake it back and forth a bunch of times, until all the small stuff has trended down ward and the big stuff has stayed up top.

        Some of those craters on the comet, with the collections of lumps off to one side of the crater, in a rather circular pattern -- that's what it looks like to me, lumps that have been getting jiggled gently and persistently until the larger ones have collected on the surface, all to one side of the depression they are in. I'm not sure what would be doing the jiggling -- it doesn't seem like the rotation of the comet would do it, so -- it's hard to tell, whether I'm seeing a pattern that matches what I see in my baking bowl, or not? But I am coming from the experiences I have, so that's what I see.

        Or there's the perspective I get from reading papers. I can't visually resolve the organisms in my sourdough bread, but I know they are there because of what I know about biology. I sometimes wonder what people thought of bread before they knew of the existence of tiny, essentially invisible organisims -- magic is a reasonable conclusion to make, I think.

        But when I look at the comet pictures thinking about the papers I have read -- Greenberg's writings on interstellar grains is one set of papers I've found very interesting -- I see stuff that isn't just what's visible in the images. All the work people have done with spectrometers, trying to characterize the materials that the comet is putting up into the coma, and now the materials at the surface of the comet -- that's similar to using microscopes to observe things we can't see with our eyes directly.

        I've been working my way through this paper,

        I like reading what we're sure of, and what we're unsure of, and what we don't know; I like reading things that refer to stuff I've already read, and I like reading stuff that refer to things that I'm clueless about, because then I can take the words that are meaningless to me and stick them in an internet search with pdf tacked on and if I'm persistent I get to find out what the answers are, at least to the best of current research knowledge.

        It's this huge puzzle that I'm watching a whole bunch of people, each working on their own corner of it, sometimes disagreeing about how it actually should be put together, but there's this slow patient pattern of looking as closely as we can at the "box top" -- the evidence we get from the best tools we can build, which can take a lot of very painstaking and careful interpretation.

        All the interpretation is no good unless you can test it, so there's a lot of different things people do -- lab experiments as well as observations -- to learn as much as we can about what materials do, and what our instruments tell us about them. I vaguely knew about all that, in the past, but getting interested in the comet has led me into learning a whole lot more about how this process of science works, and it is a lot of fun to watch.

        • Prof Harvey Rutt says:

          Judy, I love your cooking analogies, as a keen amateur cook myself!
          But I would reiterate a (strong) word of caution. You will often see statements here along the lines of 'believe your eyes, it's obviously rock'; I'm afraid that is very unreliable advice.
          Our eyes, experience, 'common sense' are entirely conditioned by terrestrial conditions. Roughly one bar pressure, one g gravity, 300K give or take a bit. No exposure to hard UV or high energy particles, and (despite the weather!) relatively constant conditions, even over geologically significant timescales.
          Conditions on 67P are simply wildly, utterly different. Gravity is tiny, and doesn't always 'point down' to the surface under your feet' other forces (yes, including electrostatic) become important. Even when active, the pressures are a minute fraction of a bar. Out-Flows very quickly expand and become very low density, molecular flow, not the viscous flow we are used to. The temperature varies wildly over an orbit, it's exposed to hard UV, etc etc.
          Having spent a lifetime around vacuum, cryogenic and high temperature systems, including a lot of plasmas, you would be amazed at the bizarre visual effects you come across at times.
          So our common sense, comparisons to things which are 'obviously similar', are very dangerously missleading.
          So I love the cooking comparisons for fun, but be very wary of reading too much into them - or any other comparison to earth conditions phenomena.

          • THOMAS says:

            @ Prof Harvey Rutt

            “But I would reiterate a (strong) word of caution. You will often see statements here along the lines of 'believe your eyes, it's obviously rock'; I'm afraid that is very unreliable advice.
            Our eyes, experience, 'common sense' are entirely conditioned by terrestrial conditions.”

            Harvey, the cautionary advice which is the logical corollary of all this is presumably: “Above all, go on believing in the Creationist theory of the Big Bang, along with all the required attendant entities called ‘black holes’, ‘dark matter’, ‘dark energy’, ‘neutron stars’, etc. which many here (and elsewhere) will claim to have never been observed (because unobservable, thus unfalsifiable) and to violate the first principles of physics. But be assured that the current decades-old paradigm and the vast body of supporting, concordant equations have conclusively proved these entities to exist (even if it is true that, to achieve the desired result, the equations have sometimes had to incorporated unknown variables based on prior expectations or personal assumptions, the most flagrant example being Albert Einstein’s famous on-off ‘cosmological constant’).”

            I am a plain man and I side wholeheartedly with Samuel Johnson’s position in his famous disagreement with Bishop Berkeley’s ‘immaterialism’: I will always believe that a stone is a stone and a rock is a rock, wherever they are in the Universe and whatever the force of gravity they are being subjected to.

            My neighbour believes that the invisible gnomes at the bottom of his garden are neither ‘black’ nor even ‘dark’: He’s almost certain that, like all gnomes, they are necessarily green (with red hats). The only problem is that he’s never seen them and neither have I because, as I say, they’re invisible.

      • Harvey says:

        The presence of true 'clay', normally formed with liquid water present, would be fascinating.

        So I followed the reference; its just to Wiki, and in turn to the NYT reporting an interview with one C L Lisse, a credible & prolifically publishing 'comet'scientist'.
        I dont really count the NYT as an authoritative scientific reference I'm afraid.
        There the trail goes *almost* cold. Extensive searches of WoS with all the different permutations & synonyms I can think of, produced just one, uncited. conference paper. It does not seem to have been followed up with a (generally regarded as more authoritative) Journal paper that I can find. The authors were not as far as I can see members of the Deep Impact team.

        Lisse himself does not appear to have published any confirmation 'clay' was observed.

        The paper below gives some very poorly resolved spectra in the 9-12um region, which are said to be a 'good fit' to a clay/organic mix.
        But frankly they are pretty unconvincing; 'vaguely consistent with' lots of things, in particular silicates that do not necessarily require liquid water. He sites three references in support; none of which mention 'clay' in their abstracts. One *does* support phyllosilicates, of which clays can be formed.
        Unambiguous confirmation of 'clay' from this remote sensing data is going to be difficult; it's only with the Mars Rovers on the surface it became certain there.

        I note the authors, who do 'follow a particular line' shall we say, as well known proponents of 'panspermia'.

        Overall, I would say they are entitled to say that a clay/organic mixture is reasonably consistent with the data. But they are very far from showing that it is a unique explanation of that data.

        Its possible I missed a paper, didnt get my search terms quite right, or there is some other term I dodnt know; if so please point me to the reference.

        Liquid water in comets: implications for astrobiology
        (Clay mentioned in the abstract.)
        By:Wickramasinghe, JT (Wickramasinghe, J. T.)[ 1 ] ; Wickramasinghe, NC (Wickramasinghe, N. C.)[ 1 ] ; Napier, WM (Napier, W. M.)[ 1 ]
        Edited by:Hoover, RB; Levin, GV; Rozanov, AY; Davies, PCW
        Book Series: Proceedings of SPIE
        Volume: 6694
        Article Number: 66940A
        DOI: 10.1117/12.768445
        Published: 2007

        • Harvey says:

          This seems to be the strongest support for it; it includes the statement quoted below. But if you look at his Fig 3, & recall that you could have tried lots of other things in the fit, I rather think the ~8% error bar extends as far as zero.
          But *yes*, is is 'consistent with'; but far from proof.

          This Lisse paper is heavily cited & clearly well regarded. A few of its citations do mention phyllosilicates in other extraterestrial contexts; it's still a live subject; but it's all 'consistent with', 'resemble'; tantalising, but not proof.

          "Phyllosilicates. We found that ∼8% of all silicates (by surface area) occurred in the form of phyllosilicates; the best match in our spectrum is the Fe-rich Na-bearing nontronite, a smectite-group mineral formed as an aqueous alteration product of basic rock. We did not find good fits with Mg-rich saponite or serpentine. The existence of hydrated silicates in comets is provocative, because it would suggest the presence of abundant amounts of reactive water in the formation region of the comet or in the cometary parent body. On the other hand, Brownlee (18) noted that there is a minority component of phyllosilicates in IDPs of likely cometary origin. "

          Spitzer spectral observations of the Deep Impact ejecta
          By:Lisse, CM (Lisse, C. M.); VanCleve, J (VanCleve, J.); Adams, AC (Adams, A. C.); A'Hearn, MF (A'Hearn, M. F.); Fernandez, YR (Fernandez, Y. R.); Farnham, TL (Farnham, T. L.); Armus, L (Armus, L.); Grillmair, CJ (Grillmair, C. J.); Ingalls, J (Ingalls, J.); Belton, MJS (Belton, M. J. S.)...More
          Volume: 313
          Issue: 5787
          Pages: 635-640
          DOI: 10.1126/science.1124694
          Published: AUG 4 2006

        • Gerald says:

          Since comets are made of interstellar dust, I've googled for
          "interstellar dust clay", and besides my post here I found this paper:

          It contains references to other papers with findings of clay minerals in meteorites.

          Googling "interstellar dust phyllosilicate" returned e.g. this paper:

          Finally a more specific search "carbonaceous phyllosilicate pdf":
          "Spectral reflectance properties of carbonaceous chondrites 4: Aqueously altered
          and thermally metamorphosed meteorites".
          Since cometary origin

          If there is an overlap of the origin of comets and carbonaceous chondrites, this would at least provide some plausibility, that comets may contain phyllosilicates, too, despite possible lack of direct evidence.

  • logan says:

    ...We have found the parent material of interplanetary dust particles,”

    Why not just one big brother?

  • Robin Sherman says:

    Related to the current increase in activity we are seeing, 67P passed a significant milestone over the Christmas period. Before its close encounter with Jupiter in 1959, 67p's Perihelion was at about 2.7 AU, roughly 400 million Km from the sun. This distance was matched around Christmas day. We therefore now have a good idea of the activity levels of the comet, prior to it's change in orbit. It would seem that, even at Perihelion in this previous orbit, the vast majority of any activity would have been limited to the Hapi or neck region.

    However it is not known how long 67P was in this orbit, nor is it known if at some time in the past it orbited closer to the sun, maybe even closer than it does now. We also don't know how much the 8 visits closer to the sun since the orbit change, have changed the activity profile of the comet. Just another puzzle to throw into the mix.

    The comet, and Rosetta of course, are currently at about 2.4 AU (360 million Km) from the Sun, roughly twice the distance at Perihelion for the current orbit. This means the energy 67P receives from the Sun today is roughly one quarter of that it will receive in 195 days time at Perihelion.

    • Kamal Lodaya says:

      Robin: Prior to the change in orbit did 67P have a similar dust insulation? Not very clear, so its behaviour could be different. Unfortunately Earth is on the other side of the Sun so no hope of any remote pictures of 67P for some time. I tried figuring out whether Dawn or one of the Mars spacecraft could see the comet but my guess is it would be difficult for them to take a long exposure image.

  • Rod says:


  • Neil Creamer says:

    Obviously, the presence of phyllosilicates on 67P is an indication that the material of the comet was in a wet environment at some point. However, the absence of evidence for water ice in the solid material of the comet suggests that the water seen in the coma is not due to sublimating ice. Furthemore, there is an indication that the dusty material is carrying a charge and I think it is time that researchers started to take seriously the possibility that it is electrical interaction with its environment that is causing the comet to evolve not water but hydroxyl ions, probably from silicates, which are meeting the solar proton flux and forming water in the coma.

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