Extremely dark, dry and rich in organics: VIRTIS view of 67P/C-G

Based on the press release of the National Institute for Astrophysics in Italy covering the results of the VIRTIS instrument – the Visible and Infrared Thermal Imaging Spectrometer on Rosetta – that were published last night in the journal Science. The results are based on data collected by VIRTIS between August and December 2014.

The first surprising result emerging from VIRTIS’s study of Comet 67P/Churyumov-Gerasimenko is the measurement of its albedo, or how much sunlight is reflected by the surface of the nucleus. With an albedo of only 6%, about half as much as the Moon's, 67P/C-G is one of the darkest objects in the Solar System.

Such a low reflecting power indicates that the surface of the comet contains minerals such as, for example, iron sulfides, but also carbon-based compounds. The low albedo also indicates that there is little or no water ice on the outermost layers of the surface of the nucleus.

“This clearly doesn't mean that the comet is not rich in water, but only that there is no water ice in the outermost shell, just over one millimetre thick,” explains Fabrizio Capaccioni, VIRTIS Principal Investigator from INAF-IAPS in Rome, Italy. “The reason for this is rooted in the recent history of the comet's evolution, since repeated passes in the vicinity of the Sun cause surface ice to sublimate.”


On the left, an image of the nucleus of Comet 67P/C-G obtained with the Navigation Camera (NAVCAM) on Rosetta. On the right, in a similar orientation, a map of the spectral slope of the surface of the nucleus. The spectral slope is used to extract information about the composition of the material present on the surface. Small values of the spectral slope (in blue) are clearly seen in the 'neck' region, which is the one that, to the day, has shown the largest degree of cometary activity in terms of gas and dust emission. Credit: ESA/Rosetta/NAVCAM(left); ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR (right). Projections of VIRTIS data on the shape model were performed using the MATISSE software developed by A. Zinzi and collaborators at the ASI Science Data Center (ASDC, Rome).

Another interesting result based on these infrared observations concerns the discovery of macromolecular organic compounds over the entire surface of the comet's nucleus. Some of these compounds are similar to the carboxylic acids – or actually to polymers of carboxylic acids – that are present in amino acids.

While amino acids were already observed in cometary materials and in primordial meteorites, this is the first time that such compounds are directly observed on the surface of a comet nucleus. In addition, the global distribution of such compounds on the surface suggests that they were abundant in the material that assembled to build up the nucleus of the comet.

“The formation of such compounds requires the presence of ices of volatile molecules such as methanol, methane or carbon monoxide, which only freeze at very low temperatures,” explains Capaccioni. “Therefore, these compounds must have formed at large distances from the Sun, during the early stages of the build up of the Solar System. This suggests that we are facing a comet that locks up, in its interior, traces of primordial chemical compounds that date back to the formation of our Solar System, or possibly to an even earlier epoch.”

“The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta” by F. Capaccioni et al. is published in the 23 January issue of the journal Science.



  • Bill Harris says:

    Interesting. The spectral slope can be thought of as a measure _at this point in time_ of the activity of the comet. Blue is fresher areas which are active and currently producing gas and dust, Red are inactive areas which are receiving dust currently or during an earlier perihelion. It matches up so well with what we see on the comet now. And I suggest that this will change as we approach this perihelion.


    • Graham Hall says:


      The illumination is from about 1 o'clock and the neck area doesn't get as much illumination as the areas on the top of each lobe. yet these top areas areas have a different composition to the more active areas. Does this suggest that originally there was an outer shell composed of the material in the red areas, which over time has eroded to leave exposed the inner areas which are perhaps of a marginally more volatile composition?

  • Cometstalker says:

    Congratulation to the awakening from picture only mode to science fact behavior. Keep on going like this for the future please.

  • Lucas says:

    So what does this mean? Is there any water(H20) found on a comet? For me as a layman all looks like a rock, and what about craters? Some are deep but all with flat bottoms, lots of hexagonal craters are there impact craters? If so how can so many hexagonal craters form on one small remote body?

    • Graham Hall says:

      Yes there's water ice in a comet, plus lots of other frozen materials too. But no liquid water! The water ice and other substances can sublimate in the virtual nil pressure environment directly to gas (vapour) at temperatures we would consider cold but on the comet are not. The material that isn't frozen gas is termed dust, but its very loosely bound together and quite fluffy - think more of candy floss rather than earth rock. Its thought that the interior of the comet is composed of this dust bound (stuck) together with small amounts of the frozen gases, covered perhaps by a shell comprised of the same material but packed more tightly.. Then cover the shell with a layer up to 20cm deep of dust without the frozen gases.

  • Robin Sherman says:

    Myself and others suggested that Thollins would be a large part of the organic soup that covers 67P. The VIRTIS spectra suggest otherwise. Thollins contain many Nitrogen containing compounds, among them the much sought amino acids. Looking at the Infra Red spectra the main absorption is from C- H, C-C and C-O bonds. Nearly every IR spectra I see has a huge absorption from water as its dominant feature, but these show virtually none. Also noticeable is the very low visible emissions, which means very little colour other than orange and mainly red.

    Attempts were made to match the spectra with some likely analogue materials. The surface is described as "dusty" so some likely dark minerals containing Iron Sulphides, Carbon Black, Insoluble Organic Matter from carbonaceous chondrite meteorites, an archetypal mixture of Thollins manufactured in the lab simulating outer solar system conditions, and a mixture made in the same conditions, but without Ammonia (NH3), in the starting material and with Acetic Acid and Alcohol added to the usual gas ices of Water, Carbon Dioxide, Carbon Monoxide and Methane. This last mixture of dark goo has an IR spectra that matches quite well, throw in small amounts of the others and it should be possible to get a good match.

    Their does seem from the IR spectra good evidence for significant amounts of aromatic carbon compounds and the team suggests polymers too. From a biogenic point of view this is good news, though the low amount of Nitrogen compounds is not. There are significant amounts of Sulphur present on 67P that is also encouraging.

    ROSINA has definitely found Ammonia in the coma so it must be present currently on the comet, but where the original material that made up the comet formed it seems to have been in limited supply along with Nitrogen (N2). This would seem to place some constraint on where in the solar nebula the majority of the material that makes up 67P formed.

    Did Saturn, Uranus and Neptune sweep up all the Ammonia and Nitrogen? Neptune's moon Triton is strongly suspected to be a captured Kuiper Belt Object and it has huge amounts of Nitrogen locked up in its Nitogen Ice crust. So did 67p material come from even further out? That high H/D ratio suggests this also.

  • Ross says:

    Why does the presence of macromolecular organic compounds require the presence of ice? Please someone correct me if I'm wrong, but if the surface is known to have carbon compounds, and the comet is interacting with the plasma environment, couldn't hydrocarbons be formed? It seems that prior theory of comets as building blocks is shaping the observations currently being made by dismissing the opportunity for alternative ideas. The dirty snowball paradox is full of researcher's bias.

    • Robin Sherman says:

      As many members of the team have said before the "dirty snowball" theory is WRONG. For visual confirmation of this, in this talk that Claudia Alexander gave to the SETI Institute in early December, she could not be more emphatic in her dismissal of its relevance. Unsurprisingly, the talk from Jens from DLR, mentioned in the video, has not appeared on You Tube. A lot of the information in this talk is out of date.


      Your query about complex carbon compounds Ross. For a start you need some Carbon, Hydrogen, Nitrogen and Oxygen atoms to make these compounds. If the method of producing them relies on the energy from UV, Cosmic and Solar Radiation, those atoms need to be in very simple molecules, the flux of energy is just not sufficient to break multiple bonds in complex molecules. The simplest molecules and the most abundant, are Water (H2O), Carbon Dioxide (CO2), Carbon Monoxide (CO), Ammonia (NH3) and Methane (CH4). At the temperatures found on 67P those molecules will be found as Volatile Ices, loosely called Ices by most. It has been established by repeated laboratory experiments, spectroscopic studies of interstellar gas clouds and other solar system bodies, that in the presence of interstellar dust, UV and Cosmic/Solar radiation, these ices will react to form more complex Hydrocarbon molecules. So your method for forming hydrocarbons does happen, but Carbon atoms in the form of say Carbon Black, finely powdered graphite, was shown by VIRTIS not to match the spectra from the surface and its presence is only in insignificant amounts. This is why you need Ice to provide the raw materials to make complex carbon molecules. As stated above also very little Water Ice was seen too at the very surface, but exposed patches of sub-surface were shown to have significant amounts of Water Ice.

      The evidence appears actually to be showing that the complex carbon molecules probably mainly formed before the comet did and the whole body of the comet is made up of a mixture of volatile ices, dust and organic molecules, with a currently suggested ratio of about 3 to 4 parts dust to one part ices, with the organic carbon molecules as a small, single figure, percentage. That ratio would certainly explain why the comet material does "look like" rock, around 60-70% of it is rock, at least the minerals that make up rock.

      It would seem that bodies that form in the outer, colder parts of the solar system, trap ices between the dust grains to stop them consolidating into rock, whereas in the warmer inner solar system the volatiles remain as gases and the dust squashes together to form rocky bodies, Asteroids. No doubt some objects formed in intermediate regions and so contain variable amounts of ices and rock, hence the recently discovered asteroids with comet like tails.

  • masanori says:

    Sorry for such a stupid question!

    Are there no possibility that those traces of primordial chemical compounds are from (much) later stage in the formation/evolution of the Solar System?? If the answer is no, why?? (Sorry I have not and cannot read the result in Italian.)

    • A.Cooper says:


      I read the Italian. It doesn't say anything extra over and above the translation that would answer your question. I'm afraid I can't answer it either!

  • THOMAS says:

    Regarding the presence of carbon-rich compounds on and in the comet nucleus:

    The EU model postulates that 67P and all other short period comets (together with the asteroids composing the asteroid belt) were blasted off the surface of one of the inner rocky planets by catastrophic electric discharge events in the recent past. The vast majority of those rocks would necessarily contain varying amounts and types of hydrocarbons as is the case down here on Earth. The precise amount of such hydrocarbon content naturally varies according to the rock-formation concerned. At the top end of the scale, we are talking in terms of quantities which, on Earth, are sufficiently high to both enable and economically justify their industrial extraction (shale gas/oil).

    When these rocks are subjected to high temperatures and electrochemical reactions as in the case of prolonged, intense exposure to electric discharges to the comet surface, their physical properties are modified to form a black, carbon-rich surface coating (which may be even less than 1-millimetre-thick as hypothesized for 67P in the article) and produce the “outgassing” of innumerable extremely noxious-smelling compounds.

    Now these are precisely some of the most “surprising” observations made by different Rosetta teams concerning 67P (6 % albedo, making it “one of the darkest objects in the Solar System » - see above; and the “perfume » of the comet reported by Katrin Altwegg way back in September: http://blogs.esa.int/rosetta/2014/10/23/the-perfume-of-67pc-g/); “surprising”, that is, for researchers seeking, as Ross rightly says, to consolidate the standard model of a dirty snowball which has supposedly brought its 4.5 billion-year-old pristine contents in from the outer confines of the Solar System for us to study….

    I would therefore argue that for the EU model, the visual and olfactive signatures of organic-carbon-rich compounds on and around the comet are an actual *requirement* of the model. It is their *absence* which would be problematic for the EU model.

    I've no idea how their presence can be accounted for in the standard model.

    • Kamal Lodaya says:

      Thomas: What is meant by "recent past" - would a few million years do? Oil and gas deposits are formed from remains of dead creatures. Are you therefore suggesting that 67P was blasted off from Earth, or does the EU model also postulate life on Mercury, Venus and Mars, from which 67P could have been blasted off?

      • THOMAS says:

        @ Kamal Lodaya


        Actually, a mere few thousand years will do, within the history and memory of human existence. The oral traditions of unconnected primitive civilizations in every part of the world, along with the strikingly similar motifs they left in their rock-carvings, tell the same story everywhere of inter-planetary electric discharges, which they could necessarily only interpret in their own terms.

        So I think we can safely say that, judging by your comment, you (and all other followers of mainstream astrophysics) and I (and other proponents of the Electric Universe theory) do not live in the same world.

        The astrophysical mainstream lives in a reassuringly ordered Universe organized by the relatively weak force of gravity, inspired by the theory of uniformitarianism (or ‘gradualism’) formulated by Charles Lyell in the mid-Nineteenth Century: a Universe in which, astronomically, biologically and geologically speaking, nothing sudden or surprising has ever happened, since Lyell taught that all significant change has only ever taken place over between millions and billions of years, including the formation of the so-called ‘fossil fuels’...

        For my part, I live in a more dynamic but much less reassuring Universe organized by the vastly more powerful electromagnetic force acting at every scale, from the very small (e.g. the existence and organization of the cells of every living organism down here on Earth) to the very large (e.g. the curious alignment of strings of active galaxies observed in the Local Super Cluster). The electromagnetic force can generate catastrophic and unforeseeable events anywhere in the Universe at any instant, just as lightning does down here on Earth.

        As for the hydrocarbons which we have always been taught to believe to be the “remains of dead creatures” and plants, I take them to have an abiotic (inorganic) origin in the bowels of the Earth: Russian scientists back in the late 70s intuited then confirmed this theory when the Soviets, from economic necessity, started exploring then industrially extracting oil from fields located several miles below miles-thick granite formations…. Our basement, in many places, is awash with the stuff, which is also why the USA is currently flooding the world market with shale gas/oil (the consequence, of course, being the 50 % reduction in oil prices over the past 6 months…). The Big Oil “fossil fuel” argument currently seems to be both scientifically and economically challenged.

        The presence of hydrocarbons in rock thus being totally independent from anything to do with “life” as we know it (as is presumably also the case on Saturn’s moon Titan, unless we assume that Titan has the same sort of biological history as Earth does…), the EU model has no problem accounting for the presence of hydrocarbons in the rock of 67P either.

        • Kamal Lodaya says:

          Thomas: That's a bit strong, surely we can live in the same world and disagree. I am not an astronomer, I have no problem with alternate theories, but when they require changing so many perceptions one already has. it becomes difficult to discuss things, because every now and then I will find a statement which leads me to rethink my position. I don't have the energy to start redoing my science from school upwards just so that some idea you have is to be satisfied. 67P offers interesting data to revise one's existing conceptions of comets, but your goal is to revise it so that it fits your preconceived theories regarding the entire universe (and not just comets).

          • THOMAS says:

            @ Kamal Lodaya

            By « the same world », I simply meant the same way of seeing the world/Universe we live in; I’m delighted to be able to exchange views and ideas with other people on this blog, even if we do not share the same views.

            The idea I was seeking to get across via this rather “strong” formulation was simply that the standard model and the EU model are totally and utterly irreconcilable: if one is more or less “right”, the other is automatically completely wrong. What is at stake here is a complete paradigm shift, as momentous in its implications as Galileo’s observation of Jupiter’s moons which made it logically impossible from that moment on to believe that everything in the Universe turned around Mother Earth: the consequence was a shift from geocentrism to heliocentrism, nothing less.

            For in the same, if you have an electric comet, this is necessarily proof that we all live in a Universe driven essentially by the electromagnetic force at all levels. You just can’t have an electric comet in a gravity-only, Big Bang Universe. This is precisely why the very idea of the EU model is anathema to mainstream astrophysicists and why all evidence of electrical discharge activity of P67 is denied or simply ignored.

            I’m not forcing anyone to do anything or believe anything, Like several others on this blog, I’m just trying to make various aspects of the EU model better known and understood so that, above all, fewer people consider it to be “utter nonsense” and the citizen scientists who defend the idea to be “cranks” from “Crazy Town”…

          • Kamal Lodaya says:

            Thomas: It appears you are trying a paradigm reversal from heliocentrism to geocentrism. Earth is where all the action (like comets flying off) happens, the Sun is just notionally the centre of the solar system.

            After Alfven's ideas, Feynman did calculate whether the universe could run entirely on electromagnetic principles, and came to the conclusion that gravitation was a necessary force. I am not a physicist, so I don't know the calculation, but it seems reasonable to me that a serious physicist would take another well-known physicist's ideas seriously, and convince themselves of their correctness or otherwise.

            On the other hand, if I propose a theory that I can run faster than an electric current (indeed this happens in an old Bollywood movie from around the time of Alfven and Feynman) you would certainly be entitled to a bit of skepticism.

          • THOMAS says:

            @ Kamal Lodaya


            Are you kidding? I suggest you take a look at this picture of a solar Coronal Mass Injection: http://www.cmso.info/cme. THAT is where all the action is!

            This one was actually bigger than the Sun itself. CME’s are gigantic, catastrophic electromagnetic discharges from the surface of the Sun which have absolutely nothing to do with “gravity”. The tens of billions of tons of plasma ejected do not simply fall back onto the surface of the Sun under the influence of gravity, they don’t even slow down: the protons and ions ejected accelerate away through the Solar System in the form of particularly energetic electric currents which the standard theory euphemistically calls the solar “wind”. Some wind!

            When a Solar System body is in the direct line of sight of a CME, the results are necessarily significant. The firs t such event recorded in modern times was the well-documented “Carrington Event” of 1859, of which Wikipedia says (http://en.wikipedia.org/wiki/Solar_storm_of_1859):
            “The solar storm of 1859, also known as the Carrington event, was a powerful geomagnetic solar storm in 1859 during solar cycle 10. A solar coronal mass ejection hit Earth's magnetosphere and induced one of the largest geomagnetic storms on record. The associated "white light flare" in the solar photosphere was observed and recorded by English astronomers Richard C. Carrington and Richard Hodgson.
            Studies have shown that a solar storm of this magnitude occurring today would likely cause widespread problems for modern civilization. The solar storm of 2012 was of similar magnitude, but it passed Earth's orbit without striking the Earth.”

            In other words, a very near miss. We are at the mercy of the totally unpredictable and uncontrollable electromagnetic tantrums of the Sun, one of which, if suitably directed at us, could wipe out a large part of the world’s power grid and worldwide telecommunications systems. (See http://en.wikipedia.org/wiki/Solar_storm_of_2012 ).

            No-one denies the reality of gravity, which is obviously (and fortunately!) a force to be reckoned with, even if still we haven’t the faintest clue about what it actually is. But it pales into insignificance compared with the power of the electromagnetic force acting at all scales. We may even ultimately discover that gravity is actually just one manifestation of the electromagnetic force.

        • Jacob nielsen says:

          @THOMAS, "a few thousand years will do." Etc.... What can I say? I guess this time, In short: "no."

  • THOMAS says:

    I’ve just got round to reading the full ‘Science’ article, “The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta”.

    In their conclusion, the authors state:
    “The compositional homogeneity of the surface observed by VIRTIS, despite the rejuvenating processes acting on the nucleus (at least on the active portion of it) at every passage close to the Sun, indicates that space weathering plays only a minor role in determining the observed composition. The refractory compounds so widespread on the surface of 67P are then representative of the bulk pristine material of the nucleus.”

    Perhaps I’m reading this wrong (please correct me if I am…), but what I understand in plain man’s language is:
    1/ the solid material we see *everywhere* on the surface is basically what the interior of the comet nucleus is *also* made of,
    2/ the nucleus of the comet is made homogeneously of a *solid material capable of withstanding extremely high temperatures*.

    Sorry to again bring up an obvious, much-debated paradox, but how does all this square with the sacrosanct ‘0.4g/cm2’ density finding?

    It is certainly what several of us on this blog have been consistently arguing ever since Rosetta arrived at the comet. It is also completely consistent with the initial CONSERT findings of a homogeneously solid interior. There seems to be an overall picture emerging….

    • dave says:

      Hi Thomas

      the density seems to have moved up a bit over the last 2 months.

      the research article- On the structure and activity of comet 67p vol 347 No 6220

      seems to be using 470 Kg/cubic meter plus or minus 45

      Stills seems very low but it is increasing

      • THOMAS says:

        Hi Dave,

        I noticed that too. It's clearly a step in the right direction.

        But in my book, there is still a vast quantity of “missing mass” to be accounted for. I'm sure the behaviour of the comet itself over the coming months will prove its existence by demonstrating sufficient electrical discharge activity to convince even the most fervent believers in the standard “dirty snowball” model.

        Above all, I’m confident that the proof of the P67’s “missing mass” will be much sooner coming than that of the putative “dark mater” (not to speak of “dark energy”, et al.) which mainstream astrophysicists have been looking for in vain for the past half-century…

        • Marco says:

          Hi THOMAS,
          Note that the error bars would have gone down also. I repeat again that this low density taken at face value does not falsify EU theory. A very porous rock (say pumice, or "aerogel") has similar brittle hard properties and perhaps electrical EU properties. One particular feature being the strata is *very* indicative of 67P being a fragment of a large parent body. Certainly much more likely a Centaur type body rather than a rocky Earthlike planet parent body. There is no reason to suppose that cliffs formed with such low density porous solids would be any different to Earth/Mars cliff features and rubble below cliffs. I really can't see why you are defending the indefensible parts of EU theory.

          • THOMAS says:

            Nice to be able to discuss actual observations again with you, Marco, without having to worry too much about numbers and equations, which should always come only “after the event”, as useful tools to help us to better understand the paradigm implied by the observations.…

            You say:

            It is something I’ve been arguing for several months now on this blog and which Wal Thornhill et al. have been actually *predicting* for decades, within the framework of a secular tradition of catastrophism.

            “Certainly much more likely a Centaur type body rather than a rocky Earthlike planet parent body”.

            Why? Because Centaurs are half-asteroids, half-comets (presumably with wildly varying density values between each phase?), for reasons the standard theory is floundering to explain? How does that help?

            I believe that the “vertical” strata seen in the cliffs on either side of 67P’s neck canyon (and everywhere else on the comet…) are more comparable to the “horizontal” strata observed everywhere on the cliff-faces imaged by NASA in the Valles Marineris canyon on *Mars*: http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17570 (Just flip the image through 90° for easier comparison). On this Mars image, you can even see numerous examples of that distinctive pattern of 3-5 closely juxtaposed ‘drill-holes’ in the strata which are also visible everywhere on 67P, from the smallest to the largest scale. Is all this just a coincidence?

            I honestly believe in the virtues of observation and pattern recognition as the very basis of the scientific method.

          • Marco says:

            The Centaur Chiron is certainly big enough to generate "strata" that are concentric, but at 67P's scale would appear planar, or close to. Also, Chiron is displaying fast rotation (less than 6hour period) and has rings. All indicative of plausible shredding torque capable of shedding kilometre sized fragments. All indicative of similar processes to comets, but bigger.

  • Chemically speaking, I don't understand the sentence : Some of these compounds are similar to the carboxylic acids – or actually to polymers of carboxylic acids – that are present in amino acids,
    What doesn't it means ?
    A carboxilic acid (caracterized by COOH goups) can't
    polymerize as itself ! In a carbon chain, to produce polymers, some other goups are needed, able to react with COOH, such as NH2 (amino group) in aminoacid, OH /SH (hydroxy) in idroxy acids: in dry conditions these compounds can polycondensate to produce poly-aminoacids , polyesters, and so on.
    A polymeric material can be formed by photochemical polyaddition, if some dubble/ triple Carbon or Carbon-Nitrogen (CN) bonds should present.

    • Toni says:

      Looking at life in earth, it could be life there also. Cometas have all that life need. Life can be hybernating during years to be awake for a brief time when light and temperature are optimals. That would be a beautiful discovery.

  • Glen says:

    Here's an interesting history lesson written in 2000 (see below) and now other theories of comet formation are being questioned. The EPH fits with many new observations - it may be time to revisit and update it!

    "With the discovery of the second asteroid in 1802, Olbers proposed that many more asteroids would be found because the planet that belonged at that distance must have exploded. This marked the birth of the exploded planet hypothesis (EPH). It seemed the most reasonable explanation until 1814, when Lagrange found that the highly elongated orbits of comets could also be readily explained by such a planetary explosion. That, unfortunately, challenged the prevailing theory of cometary origins of the times, the Laplacian primeval solar nebula hypothesis. Comets were supposed to be primitive bodies left over from the solar nebula in the outer solar system. This challenge incited Laplace supporters to attack the exploded planet hypothesis. Lagrange died in the same year, and support for his viewpoint died with him when no one else was willing to step into the line of fire." (http://metaresearch.org/solar%20system/eph/eph2000.asp)

  • Dave says:

    Reply to Glen,
    What a gem of a paper you included in your blog. It nicely shatters the current std model for the origin of the comets and much more.
    The last time I examined the Titus - bode law of planetary spacing was probably 40 years ago, this year 2000 update was truly astonishing and is well worth reading for both the std theory proponents and those from the EU crowd.
    Salt water in the tails of some comets was a surprise for me and a nice explanation of the long troughs with a rock at the end of them. Plus of course the detailed analysis about what force splits up comets and of course where why and how comets may have came into existance, great

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