CometWatch closeup: Looming over Aten

With today's CometWatch we take another trip back in time, to 27 October 2014 when Rosetta was in a bound orbit around Comet 67P/Churyumov-Gerasimenko. At the time this single frame NAVCAM image was taken, the spacecraft was only 9.8 km from the comet centre. Due to the viewing geometry, most of the scene is in fact closer to the camera in this view. The image scale is about 75 cm/pixel, and the image spans 770 m across.

ESA_Rosetta_NavCam_20141027_LR

Single frame NAVCAM image of Comet 67P/C-G taken on 27 October 2014 from a distance of 9.8 km from the comet centre. The image has been lightly processed to reveal details on the comet's surface. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

This view highlights the dramatic boundary between two neighbouring but morphologically different regions, Babi and Aten, which are on the comet's large lobe. In the upper part of the image, the two seemingly smooth and bright areas covered in brittle material belong to the Babi region, which overshadows part of the narrow and elongated depression of Aten.

ESA_Rosetta_NavCam_20141027_context

Today's CometWatch entry in context. Credit: ESA

The rougher terrains of Aten are visible just below the steep cliff that separates it from its neighbour. In contrast to Babi and the neighbouring Ash region – small portions of which are visible in the lower part of the image as well as towards the top right – Aten is not coated in smooth dust but appears to host many boulders.

As suggested in a paper by Nicolas Thomas and colleagues, the depression of Aten might be the result of one or more episodes of major mass loss in the comet's history. The volume of the depression amounts to about 0.12 km3, which is equivalent to almost 50 times the volume of the Great Pyramid of Giza.

Getting_to_know_Rosetta_s_comet_region_maps2

Regional map of Comet 67P/C-G. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The regional map of 67P/C-G shows the shape of the Aten depression, wedged between the neighbouring regions of Babi, Ash and Khepry. The complete depression can be clearly seen in the images from some of our previous blog posts: CometWatch 28 February and 12 May.

Lurking in the upper left corner of today's CometWatch image are parts of the comet’s small lobe, including the Ma'at region and hints of Hathor's cliffs.

Today’s image is one of many NAVCAM views that will be released at the end of this month in the Archive Image Browser. That release will include the entire collection of NAVCAM images taken from the 10 km orbit last year, along with images taken in the period leading up to and just after comet landing.

The original image is provided below:

ESA_Rosetta_NavCam_20141027

Comments

62 Comments

  • Jeff says:

    Amazing!!!

  • Robin Sherman says:

    Thank You Claudia for your little teaser. I am counting the days until the next archive release.

    One of the problems with speculating on processes and morphology on 67P, is we know nothing of time scales. Are these formations the result of rapid, dynamic events that only occur around Perihelion, or is it a millennia long process that takes place over thousands of orbits. If Rosetta can get back to these close bound orbits next year it will be very interesting to see how much has changed. Up to this point few obvious changes in the surface topology have been seen. However signs and formations clearly show fluids of some description play a large part. What we can't see in the images are the flows of very cold dense gases flowing over the surface.

    Most of us have seen how dry ice melts, the sublimating gas hugs the cold surface of the solid Carbon Dioxide and "sinks". On the surface of 67P at a certain energy flux the density of the sublimating gases is sufficient to create this flowing fluid. There is not enough energy to transport dust away from the surface, but fine dust , the dust of streamers and coma fallout, lightly settled on the surface could certainly move. At which point that dust becomes an abrasive and a source of erosion, similar to a river.

    Many of the semicircular or horseshoe shaped features which come in all sizes, resemble retreating waterfalls. The Aten region above shows a series of circular regions along its length, 6 or 7 separate stages of Aten's evolution. So here's my crazy theory for today. Aten is the result of the retreat of a high density, very cold gas, fluid layer running over a cliff edge resulting in a continually retreating "gasfall". This process would require millennia to show readily observable progress, but as I said above nobody knows the timescales of the surface forming processes. Given the lack of energy reaching the comet for the vast majority of it's lifetime, one has to think such slow low energy processes are the cause of the majority of formations on 67P.

    Just as on Earth, these slow, geological time scale processes, are interspersed with more dynamic geological processes, "Cometquakes" and "Cryovolcanoes", just like on Earth, possibly the result of fluids below the surface.

    • Robin Sherman says:

      These rivers of very cold gas, could also be responsible for the transport and erosion the many isolated boulders. The examples in an earlier Cometwatch, they could well be like rocking stones on Earth that have been left by fluid erosion. They appear to be stranded in a river of dust, maybe thats exactly how they formed, differential erosion in a river of fine dust carrying, gas.

    • Gerald says:

      This is a really nice story!

      But near-vaccum and low gravity won't allow for these nice rivers of dense carbon dioxide gas.
      After sublimation and exposure to the surface the gas will expand rapidly to space.

    • Gerald says:

      ... nevertheless a mix of dust and sublimating ice grains might form slides similar to your scenario.

    • Harvey says:

      I'm not convinced this mechanism will work like that on 67P.
      On Earth CO2 is subliming into a one bar, 1G environment, at 300 K or so.
      But on 67P the background pressure is very low, and the gravity minute; and we are only talking some 100K colder.
      The movement of the gas will be dominated by the pressure gradients and diffusion; the thermal velocity is way above escape velocity.
      It won't behave anything like subliming CO2 on earth and should have no tendency at all to hug the surface.

      • Robin Sherman says:

        Thanks guys. Intuition told me it was an improbable/impossible scenario, hence the "crazy idea". The OSIRIS team do seem quite keen on fluids of some sort having a bearing on the morphology, hopefully by eliminating each possibility, the number of viable processes can be narrowed down.

        So far the only surface process I can believe from the visual evidence is the creation of cracks and fissures due to thermal expansion and contraction, leading to the large areas of scree and rubble at the bases of cliffs.

        Sublimation of volatiles is also clearly occurring resulting in gas and "dust" being removed from the surface, together with the logical inference that 67P's gravity will mean a percentage of the "dust" will fall back to the surface. Such a layer of refractory, insulating material is clearly going to have an influence on energy fluxes and hence, together with compositional variations, have an affect on sublimation of the volatiles below. This highly porous layer must also have some influence on the flow of escaping gas as it flows through it.

        High speed escaping gas carries a proportion of dust with it and has been seen in images to collide with the surface leading in all likelihood to erosion by abrasion and further sublimation of volatiles from the energy within the gas column.

        These are the basic "knowns" from the evidence we the public have seen. The consensus seems to be that these processes alone can't explain the surface topology and the shape of 67P as we see them today. This leads to the inevitable conclusion that subsurface processes that we can't see with the available imaging, also have a part to play. The recent OSIRIS paper also suggests that 67P's orbit may have been chaotic in the past, with past orbits possibly having Perihelions closer to the Sun than the current orbit. I think therefore the chances of understanding how the comet got to look like it does, are pretty slim without a lot more information about the internal structure and composition. WAKE UP PHILLAE!

    • Marco says:

      Sorry Robin, but gas cannot flow in a vacuum. A low triple point substance like ethane, in a liquid state might just flow, but your crazy theory would require much higher pressures than a vacuum could permit. A transport theory based on spin up and partial fragmentation (then spin down) is much more plausible crazy theory, because there is data that proves both spin up and spin down in turn, and it doesn't break any physics principles...

      • James B says:

        The subsurface of 67P has been shown to be highly diverse. While it is highly doubtful that there is any "flow" on the surface, its possible that "pockets" of volatile dense areas are packed inside. Comets/asteroids are conglomerates and it would only make sense for it to be highly non-homogeneous. Also, due to the porous nature, the subsurface sublimated gasses would take the path of least resistance to the surface. If the density of the outcroppings are greater than the surrounding areas it would cause cliffs to occur. The larger boulders and sloped horseshoe areas would then be results of landslides.

        The boulders are most likely eroded by a "sandblasting" effect of nano-meter sized particles
        caught in the sublimating jets over decades.

        On another note, 67P is theorized to have been transported into the inner solar system in the last century. Any significant spin-up/down would require a much longer period then that.

        • Marco says:

          Hi James,
          The boulder erosion theory that is espoused does not match with the observations of both these boulders in particular, that have evidence of being part of the same large boulder split into three, and a great many others that have evidence of being split off from nearby cliffs. They cannot have then eroded. The issue is not finding mechanisms that can explain the shapes of boulders, but finding explanations of their obvious relationship with other boulders and surface features that contradict the most obvious mechanisms. For instance, one of the cliffs surrounding the amphitheatre could not have been caused by collapse or erosion if it matches a monolith nearby that must have broken away from that surface.

          On another note, spin up and spin down has been shown possible to happen over periods shorter than a century, and/or with perihelions past Saturn. Gerald explained mathematically how very tiny the asymmetry of outgassing needs to be to explain the currently observed spin down. A slightly higher asymmetry would spin up the nucleus to required fragmentation speed within a few orbits.
          The Centaur (comet) Chiron is spinning so fast it has generated rings. It never gets closer to the sun than Saturn, but outgasses significantly..

          • Gerald says:

            As skeptical as I am about the stretch hypthesis, a contribution of changing slopes (with respect to equipotential planes) and gravity minus centrifugal force due to spin changes, to the cause and progress of mass wasting events is certainly an option which isn't easily ruled out.

          • Marco says:

            The evidence for stretch theory is what has pointed us, especially A. Cooper to discover mass movements *precisely* where the mechanisms of stretch theory leads us to look for them. Given conventional explanations for surface features, there would be absolutely no reason to look for matching features between particular rocks and nearby cliffs and scarps. The North Polar Plain is a case in point. Being near the rotation axis, movements of dislodged boulders is limited by the smaller radius of rotation. OSiRIS images could be verifying or even falsifying these matches that appear certain even given NAVCAM resolution, but instead have concentrated on supporting contact binary hypotheses.
            http://arxiv.org/abs/1505.07021
            The main argument appears to rest on the contrasting plane alignments between the lobes, which is more about disproving erosion theories than it does proving contact binary. Also a reliance on models such as this one:
            http://nccr-planets.ch/how-comets-were-assembled/ http://twitter.com/BBCAmos/status/604222178796683264/photo/1

            Which is all well and good, but it is the starting size and relative velocity which is improbable, not the mechanism once running.

        • Gerald says:

          Spin-up / spin-down rates depend very much on the overall symmetry of the outgassing, particularly near perihelion.
          For a 1 kg/s outgassing rate I got a lower bound for the spin-down time of about 100 years as an order of magnitude estimate:
          http://blogs.esa.int/rosetta/2015/04/20/osiris-catches-activity-in-the-act/#comment-456976
          Take a conservative 100 kg/s outgassing during a 1 month period near perihelion, then 12 orbits would sum up to 1 year needed at least to spin down the comet to zero. This corresponds to 12 x 6 years = 72 years. Since the Jupiter encounter in 1959 its 2015-1959 = 56 years. That's the same order of magnitude.
          Therefore I'm hesitant to make any prediction about significant spin changes during these latest 8 or so perihelion passes.
          The only available numbers are a spin-up from 12.76 to 12.4 hours rotation period during the latest perihelion:
          "After Churyumov–Gerasimenko's 2009 perihelion, observers found that its rotational period had decreased from 12.76 hours to 12.4 hours. It is believed this change resulted due to sublimation-induced torque.[5]"
          http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko#Orbital_history
          At this rate a spin-down to zero would need 17 orbits. or 17 x 6.44 = 109 years.
          Detailed calculation:
          Rotational period is inversely proportional to (angular) velocity, energy is proportional to the square of velocity, hence the energy quotient between a 12.76 and a 12.4 hours rotational period is (12.76 / 12.4 )² = 1.029² = 1.059.
          The relative energy difference is hence 1.059 - 1 = 0.059.
          Assume the same absolute energy difference for each orbit. With 1 / 0.059 = 17 the energy differences sum up to 1 ( -times the rotational energy of the comet with 12.4 hours rotational period) within 17 orbital periods.

          • Harvey says:

            The last paragraph appears to be wrong; you can't linearly sum a square law function, but I'm not clear what you are trying to deduce.
            But more importantly, there is no real reason to assume the same spin up, or down, will occur on each orbit. That assumes exactly the same torque acts each time, the same jets doing the same things, which seems unlikely.
            One could perhaps use a 'sailor's random walk' argument, and argue the change of angular momentum will go as the square root of the number of orbits.
            I'm not clear that's valid as there are three axes.

          • Gerald says:

            Regarding linear summing of a square-law function: I've used the square-law function to derive the change of rotational energy within one orbital period, and extrapolated the change of rotational energy.
            The rotational momentum would sum up over about 35 orbits by 12.4 / (12.76 - 12.4) = 34.4, or 35.4 with the respective reciprokes. This would still be roughly the same order of magnitude.

            Regarding the random walk: This would apply to statistically independent behaviour between orbits. But since the overall geometry and composition of the nucleus should correlate, if not strongly correlate, between orbits, I've considered a systematic behaviour over longer periods of time as likely.

            Of course it's questionable to extrapolate from a 1-sample statistics. But that's the only clue I see to start with.

          • Gerald says:

            To prevent complaints about the sloppy math with reciprokes:
            (1/x)/((1/(x+h))-(1/x)) - x/h = 1, provided all denominators 0.
            Therefore the difference of 1 between the sloppy 34.4 and the correct 35.4 orbits.

          • Gerald says:

            ... of course provided all denominators not equal to zero! The character squence "less than" "equals" seems to have been swallowed by the text processing pipe.

          • Gerald says:

            ... I should mention, that I doubt, that a constant angular momentum transfer per orbit, independent of the spin rate, is plausible, since a slowly rotating nucleus would heat up to deeper layers on the insolated side. It should hence be able to convert solar energy more efficiently to sublimation, and hence to a change of angular momentum in case of asymmetries, than a fast-rotating comet.

          • Harvey says:

            Gerald, is it really credible the jets will act coherently so to speak over many orbits? As the comet ablates and its internal and external geometry changes? I could imagine some consistency over a few orbits maybe, but over many? We've seen jets stop and start.........
            Pure speculation of course.

            We know nothing of the Young's modulus, modulus of rupture etc of the material, but even with the coherent spin up are the forces actually big enough to credibly do anything? (Actually I think I did see a paper estimating those, but can't access it here.)

            And I still don't see how it spins down again.

            I'm travelling, badly jet lagged and deprived of my usual sources of information (limited hotel wifi) so maybe just being dim!

          • Gerald says:

            Harvey, If the nucleus is actually a contact binary, or made of larger fragments of different origin, I can't rule out systematic spin changes by sublimation-induced torque over several or many orbits.
            Behaviour would change, after a larger ressource of volatiles is consumed.
            Of course actual evidence of spin-up/spin-down for 67P is missing.
            At the end I can't readily dismiss this part of the "stretch hypothesis".

            Regarding rotation, all I can deal with, are asteroids. They appear to disintegrate on speed-up:
            "Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids."
            http://en.wikipedia.org/wiki/Asteroid#Rotation

            So my reasoning is an extrapolation, hence not quite water-proof. But since there seem to be no exceptions among asteroids, I think the principle has good chances to apply to comets, too.
            Meaning, that a spin-up, sufficient to deform the nucleus, would lead to a loss of material, instead.
            If the comet could change its shape by spin-up, it would also get a roundish shape by gravity with the present spin. Hence the presumed internal rubble pile structure should adhere at least to a degree, that it doesn't change its shape under its own gravity, e.g. by friction. But it would be unusual, if it could withstand tension.

          • Harvey says:

            Gerald, I've also not completely dismissed 'stretch', but it still seems to me unlikely.
            One of its merits is that it is a stable process .
            I take your point about loss of material etc.
            But surely that's very different to saying an already formed object spun up, stretched, and then span down again.

            Once the object has accreted, by whatever mechanism, it acquires some certainly very low tensile strength. It then has to spin up to stretch; and my own guess-timates (not to hand, travelling) said it had to spin up a lot even with very low strength. (I found some estimates in the literature.) Then it has to stretch - but not snap - and slow right back down again.
            It also seems to need a plastic core and brittle shell, at comet temperatures.
            So whilst I can't see a way to say 'impossible', it seems very unlikely to me.
            ( I'm also far from sure the claimed matches would survive a full 3D and proper statistical analysis, but that's another matter.)

      • Robin Sherman says:

        A simple fundamental I forgot, but hey I was just trying to create some food for thought. Your point and Gerald's made me think that a fluidised bed may be a better analogy. The low energy gas being just enough to mobilise the dust, the dust being just impermeable enough to create a vapour pressure to enable the fluid mixture to flow. With no numbers and little direct evidence, it can only be a suggestion, but I seem to remember the OSIRIS team illustrated the surface in places showing evidence of fluidised flow in their first paper in Science. It sort of got lost behind the dinosaur eggs and large chunks of displaced comet surface.

  • A lot of the boulders in the image appear to be quite rounded and are probably eroding from the cliffs. Are these similar to the 'gooseberries' seen in one of the pits and if so what is the current thinking on their origin. Do they predate the striated or layered structures you can see in many images?

  • Sol88 says:

    Absolutely gorgeous pictures! "Rocky" looking stuff everywhere and a few of those brighter areas MUST be "ice".

  • A.Cooper says:

    In the second picture showing the shape model, the left hand edge of the head lobe and the left hand edge of the body lobe exhibit a remarkable similarity in straightness and alignment. In fact, they are exactly parallel. If this is a contact binary the two rocks, coming from disparate ends of the Kuiper belt, display the uncanny ability of being able to choreograph their final approach so as to line up perfectly- as well as having some sort of action-at-a-distance shape telepathy long before they ever met.

    • Marco says:

      The OSIRIS team begs to differ http://arxiv.org/abs/1505.07021

      And this following piece models the formation of a contact binary, visually.
      http://nccr-planets.ch/how-comets-were-assembled/ http://twitter.com/BBCAmos/status/604222178796683264/photo/1

      Contact binary seems to be the researchers' conclusion. They seem to have ignored the evidence that refutes it on 67P.

      • Harvey says:

        Whilst 'stretch' doesn't break fundamental laws of physics, I remain very sceptical. The evidence of the 'matches' is very much 'in the eye of the beholder'. I see, when guided to them, some 'matches' (though I'm not convinced they would all survive a full 3D analysis); but I see lots of things that don't match too. As I've pointed out before, if you have structures with similar basic patterns and allow yourself to look for matches and ignore non matches, a very natural and easy thing to do, it's very deceptive.

        It seems to me very difficult to come up with a credible 'stretch mechanism', on multiple grounds.
        Firstly the need to spin up *dramatically* - and then spin down again. I don't think any comet has ever been seen with the sort of spin needed. So it must be argued comets 'used to spin much faster' and have now slowed; why?
        Then there is a big material properties issue. To get matching fracture surfaces needs brittle fracture. But then it would break in two. So it seems we need a brittle surface and a plastic core. At comet temperatures, most things are going to be brittle. Maybe there is some credible comet core substance that's plastic, but what?
        Also it needs 'special pleading'; it spun up enough to stretch, but stopped just short of breaking in two; why?
        So personally I find the 'visual evidence' far from compelling, but worth proper analysis (and I don't personally know how to do that). But I find envisaging a credible mechanism even harder than accepting 'contact binary'.
        Jury still very much out; personally I'd not rule out some selective ablation mechanism yet.

        • Marco says:

          The evidence for matches are not in the eye of the beholder. They are measurable, and new matches need to be in a precise distance from the previous one. For instance the matches from Serqet to Anubis needed to start from the last matching point previously visible as the areas slowly come into sight. They did and continue across that side unlocking detail about the exact nature of the fracture. Things that look enigmatic and indecipherable looked at conventially like the features and cracks in Anuket tell a definitive story with stretch in mind.

          I'm not sure when you say you see things that don't match - are they things you would expect to match?
          With both erosion mechanisms and contact binary you wouldn't expect anything to match. The chances of even the parallel lines visible between on one lobe and the other at the accuracy evident is infinitesimal given either option of CB or erosion.

          That being said the time difference between Alfred Wegener amassing a mountain of evidence for continental drift, and a proven mechanism for it was several decades. He did not live to see his theory be confirmed.

          If the matching evidence is solid enough, sooner or later the mechanism will reveal itself. The mechanism for continental drift was implausible, and as you noted the stretch mechanism challenges what we think we know about the material properties of comets among other things.

          I don't think stretch theory needs special pleading. I think that modellers really hate modelling objects which have contrasting material properties internally as they do externally, so have never done it. This gives us the false impression that models could never show it to occur, when we don't really know.

          • Harvey says:

            They are 'in the eye of the beholder' because you are selecting the things to measure which look similar, retaining those that fit,discarding those that don't.
            Surely on a pair of brittle fracture surfaces *everything* should match. If they don't one will have to plead 'ah that got eroded'.....and one could fix anything like that.

            It needs some more analytical technique to convince me that this is not essentially chance; I'm not saying that it is chance; just that I've not seen proof it isn't. That needs to include consideration of the likely intrinsic similarities of surfaces with similar intrinsic structures. As an *extreme* example, if I simply erode a groove in a sphere of concentric layers, the opposite sides of the groove will match nicely; but nothing stretched.

            Yes, the comparison the tectonic plates is reasonable.
            But the mechanism problems here are at least as formidable, and multiple, the very separate issues of the material properties and spin up/down.
            As I've said before, I can't see that it's utterly ruled out by anything, but I'd not put money on it.

          • Marco says:

            Hi Harvey,
            We are not "discarding the things that don't fit" It appears that none of them eroded, and all of the ones that are expected to match, match - without any ambiguity that needs to be explained with hand waving or erosion as you suggest.
            I am satisfied with the statistical analysis I have explained previously, that utterly rules out chance just for the sheer weight of numbers of consecutive matching points. The probability of it being chance is inversely proportional to the power of the number of consecutive points. Number of points is often referred to in partial fingerprint matching. Realistically, the probability that 12 consecutive points matching is chance is in the order of 1 in 10^12. As new parts of the shear line become visible, the number of matching points just keeps increasing to about 40 now I think, which is just academic at that point. Stretch hypothesis explains a great deal more than just the match. Once you understand what follows on from that, rotation, mass loss (huge slabs disappearing and some rocks transported) cracks in the neck, lack of erosion overall, 50 metres devolatised crust, and even evidence of internal liquid becomes evident. Stretch hypothesis explains the sorts of features that have the experts scratching their heads, and then some. Of course if you get stuck on having to prove a mechanism before you believe the evidence, cometology will be stuck just like geology was pre continental drift confirmation. Not knowing where to dig for gold.

          • Gerald says:

            Marco, I don't see the matches you're presuming.
            It would be a lot simpler, if you could provide composite images,e.g. of this type:
            http://i.imgur.com/ivuEbUt.png
            and additional references to the source images.
            With the now available bonanza of images this should be an easy exercise for someone who sees the matchings.
            By an RMS analysis it would be close to a routine job to verify, whether these matchings are significant, on an accepted or at least acceptable and reproducible scientific standard.
            If the matching turns out to be significant - far from sure in my opinion - the next step would be looking for appropriate interpretations and possible causes.

          • Marco says:

            Comments not working for me:-(

        • Gerald says:

          The spin-up and spin-down could possibly be constructed by appropriately coreographed heterogeneities or collisions, if necessary over long timescales. I don't think, that that's the weakest point of the "stretch hypothesis".
          I'd think, that the brittleness of the cometary material would lead to a disruption or at least to mass loss in the "negative-g " zone on sufficient spin-up, not to a significant stretching.
          In the intermediate zone just before disruption, and for a sufficiently soft comet, lots of large fractures, and at some point large fragments, would form; I'd think this would show up rather unambiguously.

          • Marco says:

            Hi Gerald,

            I think, If I understand what you mean, is that there should be evidence of mass loss where the comet (or comet part) is brittle (there is evidence - Hatmehit, and Imhotep are obviously from mass loss events, and they are at the outer, where centrifugal force was the greatest)
            And for a soft comet (or comet part) there should be evidence of fractures (there is evidence - Anuket and Hapi on the neck are the soft areas which would have stretched, and they have cracks)

            The mass loss from missing slabs is quite extensive, but limited to the 50m of brittle, volatile depleted crust of the pre-stretched comet.

            Hathor is evidence of the violence of the fracture pre-stretch, and it extends a kilometre into the comet. The neck was very obviously soft and ductile as it has an extruded look, but the surface of the neck has become depleted of volatiles and has also hardened and become brittle.

            If I understand what you are saying, the evidence is rather unambiguous as soon as you actually consider it as a possibility and look for the circumstantial evidence you mention in your comment..

    • Gerald says:

      Taking the huge number of Kuiper / Oort objects, and at least millions of years of random motion, slow off-center collisions aren't that unlikely.

      Starting with a protoplanetary disk of smaller objects it's just straightforward, that the objects will collide frequently, until the number of objects is reduced by accretion and ejection.

      Those frequent collisions induce kind of friction into the protoplanetary disk and adjust velocities, such that slow-velocity collisions become increasingly likely.
      A bottom-up composition of larger objects from smaller objects, including possible contact binaries, sounds consistent.

      This doesn't mean, that a contact binary is the only possibility for 67P/C-G, just that it's a reasonable hypothesis from the dynamical point of view.

      I'd preferred a mostly erosion-driven explanation, but relevant are evidence-based results.
      First collect data, then reduce and analyse the data, then fit the models into this analysis.
      Don't start with a singular model, and ignore all data, which don't fit.

      I'm sure, the contact binary approach needs to be adjusted by sublimation/erosion, since we observe emitted gas and dust.
      We'll see to which degree.

      • Gerald says:

        The authors of the paper are well-aware of these uncertainties:
        "We tentatively conclude that a thick surface layer has likely been eroded from the 67P nucleus by previous gas-producing activity, although the above-mentioned estimate of several hundred meters should perhaps be regarded as an upper limit. In any case, this cautions that activity-driven erosion may result in complex landscapes (Malin and Zimbelman 1986; Thomas et al. 2005), which could affect our ability to interpret geomorphological features in terms of primordial processes. This is particularly true for equatorial regions and the southern hemisphere due to their computed higher erosion rates (Keller et al. 2015)."
        http://arxiv.org/pdf/1505.07021v1.pdf, p.3

      • Marco says:

        Hi Gerald,
        When you say "slow, off centre collisions are not that unlikely", the paper in question (correct me if I'm wrong) considers collisions less than 10 m/s to be slow enough for anything between a head-on collision and a glancing touch.
        Considering the escape velocity of the objects of 1m/s, and that simulation video necessarily being also less than 1 m/s, the 10 m/s is unrealistic for off-centre collisions to result in anything but a kiss, pirouette and escape from each others tiny gravity field. The number of plausible 1m/s collisions is orders of magnitude less frequent than the faster ones in the models I've heard about (again, check them yourself - I could be wrong)

        The models used in the OSIRIS paper use very optimistic assumptions to come to the conclusion that suitable collisions may be likely in the proposed environment they were in when they supposedly came together.

        We'll see soon enough that there is little to no erosion in the equatorial regions to worry about. The implausibility of the collision is one of the many major issues of CB.

        • Gerald says:

          The collision would be non-elastic, so most of the kinetic energy is transformed to heat, some to change of angular velocity.
          Besides this, the early solar system should have been populated much denser by protoplanetary bodies than today, including comets and their predecessors.

          So if I would be ony 10% as tolerant regarding unresolved issues with contact binaries than with stretch hypothesis, I would still clearly prefer contact binaries.

          The next-coming option to "stretch" would be a larger comet, initially formed from cometesimals, later disrupted by spin-up, either sublimation-indued or by collisions, and leaving the starting point of this assymetric body we see by now, followed by millions of years of phases of sublimation and phases of dormancy, depending on the respective perihelion distance in a long-term chaotic orbit,

          • Marco says:

            Hi Gerald,

            I think you are missing the point that I am trying to make regarding assumptions of the collision model. It doesn't matter how non-elastic the collision is, nor how densely populated the Solar system was with these kinds of bodies. The point is that faster collisions (10 m/s) would be way more common than slow enough collisions (1 m/s). The idea is that the ending shape needs to keep some of the initial spherical shape of the two impacting bodies. This is completely unrealistic with most impacts being well above the mutual escape velocity. How does a glancing blow absorb more than 90% of the impact velocity?

            Stretch theory is kind of just the same thing in reverse, except the two bodies don't have to choreograph an impact as they are already in the same orbit as each other.

            Also, we don't have to assume the evidence of the original impact has been eroded away. Any erosion falsifies stretch theory, at least Is there a falsifiability criterion for CB?.

          • Gerald says:

            Hi Marco,
            where should a spherical shape come from? The gravity of minor bodies is too weak to round them.

            Whether a collision is elastic or non-elastic matters, since an elastic collision above escape velocity wouldn't form a contact binary, in contrast to a non-elastic collision.
            http://en.wikipedia.org/wiki/Inelastic_collision#Perfectly_inelastic_collision

            The density of the population matters, since in dense populations high-velocity collisions become less likely after initially assumed chaotic motion, due to gas drag and conversion of relative velocity to heat by collisions.
            Accretion of planetisimals starts after adjustment of orbital trajectories of dust grains (if necessary at all, depending on initial conditions of the collapsing interstellar dust and gas cloud forming the solar system).

            Initial high velocity collisions with fragmentation aren't ruled out by a later slow inelastic collision of two bodies leading to the presumed CB.
            Later higher velocity collisions are possible, too, as long as the impactor hasn't been too large. Craters have been eroded away later, driven by sublimation.

            You can't simply reverse order of an inelastic collision. That contradicts the second law of thermodynamics:
            http://en.wikipedia.org/wiki/Second_law_of_thermodynamics

            Rosetta's instruments are measuring emitted dust. That's definitive evidence for ongoing erosion, not an assumption.
            Another definitive evidence for erosion is an almost complete lack of impact cratering. Non-eroding minor bodies are heavily impact-cratered.

            CB isn't easy to falsify, since comets must have formed somehow from smaller pieces, or at least their predecessor objects, if the comets would turn out to be a result of fragmentation.
            But there are severeal criteria which would provide evidence for a contact binary. Lack of this evidence wouldn't falsify CB, but it wouldn't allow to claim CB as evident.
            A strong hint to a CB would be a systematically different composition of the components, pointing to a different origin. This evidence seems to have not been found thus far.
            The collision of the components could leave systematic changes of their components, like fractures or shatter cones:
            http://en.wikipedia.org/wiki/Shatter_cone
            I wouldn't rule out, that the parallel streaky features at Hathor could be interpreted as shatter cone features.
            If it could be shown, that no initial conditions can be modelled which start with two bodies and end with the observed state, this would - at least almost - rule out the CB version. But such a model has been found, as you know.

          • Marco says:

            Hi Gerald,
            I am continuing conversation down the bottom once they get moderated......

        • Ken says:

          I agree Marco - a very low speed collision is not likely at all - it would require matching orbits and a host of unlikely circumstances - versus a stretch (or I think we are seeing spinup, separation, reorientation and reconnection) seems much more likely statistically speaking.

  • Marco says:

    Hi Gerald,
    Follows is an analysis and review of the calculations they make on the actual paper. It probably would only make sense if you read those specific parts of the paper that talk about collision probabilities.

    The 'R+R' equation for hits in the early Kuiper belt was for any sort of collision right up to 0.6km/sec radiant (approaching) velocities. This generated the average 16 or so collisions that may have occurred to bodies above the size of 67P. The vast majority of these would have to be assumed to be pretty violent smashes but some may have resulted in a slowdown of some of the fragments to a point where they might recoalesce.

    The problems with this approach are:

    a) there is still the assumed (or at least implied), 10m/sec speed threshold, implied because it was the upper limit for crushing/deforming stated earlier in the paper. There's no mention of the 1m/sec that's an absolute constraint in all recoalescing scenarios for 67P lobes except almost head-on collisions.

    b) the R+R element was set at 67P dimensions and above. But a collision at 0.6km/sec for 67P size or similar would surely obliterate both bodies. You couldn't assume anything the size of the two 67P lobes to remain as recoalescing candidates after that sort of collision. In fact such candidates would be ruled out for (I would guess) all collisions that were both above 0.1km/sec and below about 5 or 10 km R value. That would most likely rule out most of the 16 collisions.

    They based their hypothesis for viable residual fragments on a paper they cite that says something along the lines of 'some bodies deform enough in collisions to allow undamaged fragments to escape' and they said they'd 'repurposed' that paper for slower, Kuiper Belt speeds. There are no figures given for what repurposed speeds and fragment sizes might apply in the KB. I doubt if they are exactly the same as the sizes and speeds they cite as being their reasonable inputs (up to 0.6km/sec;67P size and above) for an entirely different equation/scenario, that is, the collision liklihood.

    In other words, just because 16 collisions are found to have happened to such bodies with those radiant velocities and sizes, doesn't mean that all 16 also produced viable recoalescing candidates. You have to find the number of collisions and then apply the "repurposed" findings of the other paper to each and every collision, using its particular radiant v and pair size. That way can you see how many of those 16 collisions involved radiant velocities that were too high or body sizes that were too low. This would probably more likely be a generic distribution of radiant speeds and sizes rather than a particular model run output.

    Then, even when you've found a few collisions that do produce two undamaged candidates the size of the 67P lobes, they also have to be travelling within 1m/sec of each other as they leave the collision. Not only do they have to do that but they have to remain close together as they travel away or their mutual g acceleration drops away with distance squared and the 1m/sec becomes 0.5 or 0.1 m/sec.

    An alternative is that they disperse into somewhat faster orbital speeds around a large parent residual. That way they can leave in different directions and eventually cross each other's path- but only if these two orbits allow sub 1m/sec approach speeds. That's much more likely in say, 5m/sec orbits but those speeds require a parent fragment that is tens of km across, of which there a relatively few even before the known collision that likely obliterated it.

    As you can see, the problems are manifold.

    This is why they are suggesting 67P formed way out further than they thought "based on the solar system formation scenarios assumed in this paper". That's because to have formed in situ without recoalescing scenarios and then survived the 10^-4 chance of not having a collision, 67P had to be sailing around in pristine space where it couldn't have a collision in the first place. That in turn means that, because they haven't noticed the comet has stretched, they are prepared to rewrite the text book on solar system formation. So they've spent a billion euros on a probe that's now misreading the 'Rosetta stone' it was sent to decipher.

  • Marco says:

    Hi Gerald,
    When you say
    "Whether a collision is elastic or non-elastic matters, since an elastic collision above escape velocity wouldn't form a contact binary, in contrast to a non-elastic collision."
    And
    "You can't simply reverse order of an inelastic collision. That contradicts the second law of thermodynamics:"

    You are ignoring the conservation of AM. When a collision occurs well above the mutual escape velocity and is off centre, there is a great deal af angular momentum that has to be conserved as well as the momentum. Look at models or do the maths - they won't stick unless they are spinning just the right amount before they impact. This is because there is greater (angular) kinetic energy in the spinning joined object that has AM conserved, than had the two individual objects pre impact. *THAT* would contradict the second law of thermodynamics.

    67P could be stretching right now without breaking the second law of thermodynamics. Spin is the key for the thermodynamic situations to be reversed..

    • Gerald says:

      Marco, the momentum is not an issue for inelastic collisions, by definition.
      Angular momentum can, but doesn't need to be an issue.
      The escape velocity from the smaller of the two bodies isn't relevant, hence in general, there is no mutual escape velocity. Relevant is the escape velocity of the merged body. As the mass ratio is sufficiently different from 1:1, higher impact velocities may, but don't necessarily increase the angular momentum of the larger body to the disruption limit.
      I've done a lot of math thus far; it's now time for you to do some exercises in physics and math. Try to show, that the provided simulation is wrong, but please with sufficiently accurate calculations, not just armwaving.

      • Marco says:

        Hi Gerald,

        This is one that the challenge is in finding just the right combination of material and relative radius size that *Could* stick. It's not so much about understanding the maths, but looking at a lot of simulations and getting a feel for what sort of requirements there are for two bodies to coalesce in a collision.

        With bodies going at 10 times the escape velocity, and the offset at say 1 Radius, the initial reaction forces are at around 45 degrees, and the frictional forces are acting to match each other's rotational speed, rather than just damping the overall speed. The resultant combined body would be spinning faster than any known comet or asteroid of comparable size, assuming it was made of putty, or superglue... And thus able to stick in the first place. The centrifugal forces would be greater than gravity in any imaginable combination of typical off centre and high velocity sticky, damping impact.

        It is difficult to explain mathematically, but off centre collisions severely limit the probability of collision speeds higher than escape velocity resulting in enough damping for the bodies to combine. Obviously many runs of simulations will show the acceptable level of off centre for any particular collision speed. The assumption that it doesn't matter doesn't stand up to that kind of scrutiny.

        • Gerald says:

          Hi Marco, if you would have done the math yourself, you would have seen, that your conclusions are not true in general.
          Consider an inelastic collision between two spheres, say one of radius 2 km, the other of radius 1 km, both the same density, the collision off-center, such that the velocity vector of the center of the smaller sphere points would miss the center of the larger sphere by 1 km, if the spheres could penetrate each other.
          The first thing to note is, that the mass of the smaller sphere is just 1/8 of the larger sphere.
          The second thing is the surface normal at the first contact. It's just about 20 degrees off central collision.

          Look at the moments of inertia:
          http://en.wikipedia.org/wiki/List_of_moments_of_inertia

          For a sphere of mass m radius r it's 2mr²/5.
          For a point mass m at distance r it's mr².

          Now use the masses of the example, say the small sphere has mass M and radius R, the large one mass 8M and radius 2R.
          With the above formulas the moment of the large sphere is
          2x8Mx(2R)²/5 = 64MR²/5,
          that for the off-center collision is
          MR²
          The moment of inertia of the large sphere is hence 64/5-times equals 12.8-times the moment of inertia of the off-center collision.
          If the two spheres would fuse to one sphere of mass 9M and radius 2Rx(9/8)^(1/3) = 2.08R, the resulting moment of inertia would be
          2x9Mx(2.08R)²/5=15.58MR², i.e. the 15.58-fold moment of inertia of the off-center collision.

          More accurately using the reduced mass (Mx8M)/(M+8M) = 8M/9 of the two-body system
          http://en.wikipedia.org/wiki/Reduced_mass
          for the moment of the collision, the more accurate factor is 15.58 / (8/9) = 17.5, meaning the moment of inertia of the fused collision product is 17.5-times that of the colliding system.
          Hence the angular momentum
          http://en.wikipedia.org/wiki/Moment_of_inertia#Angular_momentum of the fused product is 17.5-times that of the colliding system.
          The angular velocity is hence reduced by a factor of 17.5 by the collision.
          Applied to the radius 2.08R of the fused sphere the maximum surface velocity is 2.08 / 17.5 = 0.1189-times that of the velocity of the collision.
          The maximum collision velocity of two non-rotating spheres of the example settings not to lose mass by escape is hence the 1/0.1189 = 8.413-fold of the escape velocity of the fused sphere in rest.
          At higher collision velocities the fused body may lose some of its material, distant-most of the axis of rotation. A bit below this velocity it may form a binary, temporarily at least.

          Now you may play e.g. with radii of the initial spheres or the distances of the centers of mass before collision, etc., to get the scenarios likely leading to contact binaries, and those leading to fragmentation.
          You'll see, that the outcome of the collision is very sensitive to the ratio of the radii of the two colliding bodies.
          A 400 m diameter impactor on 67P would need about 1km/s velocity to disrupt significant parts of 67P by centrifugal forces, due to a radius ratio of 1:10, hence a mass ratio of 1:1000.

          • Marco says:

            Hi Gerald,

            Looking at the comets we know that are bi lobed - Halley, Borrely, Hartley and C-P and perhaps a couple of others. Is a mass ratio of lobes 8:1 typical or unusually high?

            Have you tried the maths for the two lobes of C-P in a 10m/s off centre collision?

            Is a gravitationally bound pebble pile, or bodies with the outwardly brittle nature of 67P's surface likely to tend to collide with plastic (ie. Inelastic) collision?

            The high speed collision of a very tiny impactor of Deep Impact resulted in a great deal more loss of material from the inside than it gained from the impactor. This is probably not a typical collision either, but would have to be more indicative of a typical impact of a small meteorite or fragment.

            The maths can be made to work if you can cherrypick examples, but the overall collisional problem is not addressed. The ending point is a large proportion of bi lobed comets which could not have had a low speed impact (or destructive impact)of similar sized bodies for billions of years, yet right at the end of their stay in a chaotic accretion disc which somehow had their probability of a soft collision with a similar size body enhanced, they have had no such collisions, nor destructive ones, for billions of years.

            Back to the problem at hand. Even at 20 degrees initial offset, the initial reactive force of impact would be in a direction pushing the lobes away from each other with any portion of the force that transmits through a body firm enough to stay together, such that the smaller body would pull away, unless it was completely liquid, in which case a bi lobe shape would not ensue.

            With CB we need to cherrypick material properties as well as collisional probabilities to suit the final product - More so than stretch hypothesis, which is, yes, requiring the same kind of damping stretchy interior when the exterior needs to be brittle.

            Thanks for the maths, but CB requires similarly sized bodies colliding at low speed and relatively centred to explain real bi-lobed comets. Your mathematical argument does demonstrate that non contact binaries may well be more likely as there is generally a greater disparity in sizes between parent asteroids and their moonlets.

          • Gerald says:

            Hi Marco, I've just given you an idea of how to narrow down the conditions needed to form CBs. I still prefer a mostly erosional explanation, since over billions of years thousands of perihelion passes closer than 2 AU look likely due to the probably chaotic orbit of 67P over long timescales. This would lead to significant surface erosion. E.g. a 16 cm loss per perihelion would lead to a 160 m loss of surface material for 1000 perihelion passes. After a few 1000 passes the original shape of the comet would be going to vanish.
            For the 16 cm I'm assuming sublimation of 0.1m³/s over 116 days (corresponding to 1e6 m³ of solid ice) per orbit and a surface of 5e7 m² (leading to 2 cm loss of solid ice), together with an additional 3-fold volume of dust relative to ice, and a density of 450 kg/m³ (together a factor of 8 relative to the loss of solid ice).

          • Marco says:

            Hi Gerald,
            That's fair enough. There are real issues with CB, but at the same time, there are real issues with erosion that are indeed pointed out in the Osiris paper. They only consider evidence in terms of these two competIng theories, and the paper finds the disproof of erosion to be more compelling than the issues with CB.
            I contend that this is because CB is less falsifiable. Not because of anything the OSIRIS team has discovered.

            But I am in a really good mood because PHILAE WOKE UP!!!!

            Can't complain about more ground truth data.

          • Gerald says:

            Hi Marco,
            really good news, yes!
            With Philae they can hopefully complete the CONSERT measurements, and we will get more evidence about the internal structure of the nucleus.
            ... And all the other questions about the surface composition and properties...

  • Marco says:

    Hi Gerald,

    You say

    "Rosetta's instruments are measuring emitted dust. That's definitive evidence for ongoing erosion, not an assumption.
    Another definitive evidence for erosion is an almost complete lack of impact cratering. Non-eroding minor bodies are heavily impact-cratered."

    All I can say is that I mean physical surface modifications involving removal of features at the scale of the resolution of OSIRIS or more ( say anything more than a metre or so) would falsify stretch theory.

    Why such plentiful emissions would not change the surface can only be explained by the emissions originating deeper and coming up through geysers or similar. Either way, it is the visible erosion that I am talking about. Given that virtually all scientists are predicting visible erosion by post perihelion, this particular criterion, which I am confident in, given the faithfulness of the matches, will have a profound impact on the science of comets.

    As far as the lack of impact cratering goes - that goes to how recent, relatively speaking, the stretch event was. Also almost all features and regions of the comet are explainable by stretch theory, and many are from mass wasting (crust slab removal and fracture) events coincident with fracture and stretch.

    • Gerald says:

      Hi Marco,
      so you are suggesting a recent reworking of the whole cometary surface by stretch. If it would be possible, this would however have destroyed any older structures including matchings.

      • Marco says:

        Hi Gerald,

        The matchings are not older than the stretch event. They are part of the incontrovertible evidence that stretch occured. Large parts of the cometary surface (Hatmehit, Imhotep, and others) are morphologically different because they represent areas where a thick crust has lifted off. Those formed at the same time as the stretch event, or close to. Those areas not subject to such mass wasting coincident with stretch, have evidence of previous events possibly, but haven't suffered erosion as such, but other previous events that caused the surface morphology, rather than impacts.

  • Marco says:

    Hi Gerald,

    You say,

    "Marco, I don't see the matches you're presuming.
    It would be a lot simpler, if you could provide composite images,e.g. of this type:"

    The problem is that 2D images cannot capture 3D matches in this way. You have a fracture in 3D, the ridges are over a kilometre apart and are rotated about 30' in one direction and 15' in the other, so angles and lighting is going to contrast even if the matches are on the same photo. when Rosetta is at the exact right angle, the continuing ridges from head to body can be evident.. I suggest, if you haven't already, to take some time and navigate through the blog link that follows and understand the methodology in verifying a tally of matching points.

    https://scute1133site.wordpress.com/2014/12/14/67pchuryumov-gerasimenko-a-single-body-that-has-been-stretched/

    As for appropriate interpretations and probable causes. We have moved on from proving stretch to interpreting features in light of stretch, and keep finding more corroborating evidence that explains all sorts of things, such as moving rocks, the flatness of Imhotep and Hatmehit, the lines and shapes on Hathor, the cracks on Anuket and Hapi, and so on.

  • Robin Sherman says:

    The recent paper suggest two possible scenarios and various bits of visual evidence suggests other possible processes, such as spin effects and collisions from impactors may also have contributed to 67P's shape and appearance. So if I may, a "story" that allows a contribution from each may be the answer.

    The primordial solar system has a disc of gas, ices and dust which through proposed theoretical models clump together into larger pebble sized objects, which through some process not clearly understood, gradually accrete to form larger objects. The evidence from the Oort Cloud and the Kuiper belt is that there must have been millions and millions of these objects. Collisions between them are inevitable, particularly if the migration of the Gas giant planets did indeed take place as theorised.

    The scenario suggested of a very low speed collision, is therefore not beyond the bounds of possibility. The chances may be 1 in a million or 1 in a billion, but at this time there were surely more than enough objects of suitable size to mean these odds were met on numerous occasions. The question then becomes, how many would survive? That is where I think this scenario becomes more improbable.

    The second possibility is that the contact binary was formed from fragments of a larger object disrupted by a destructive collision. This seems more plausible to me. The composition of the two fragments would be very similar and they could even have been adjacent to each other on the original larger object meaning there could be topographical matches. Although not convinced by Marco and A. Coopers theory to explain the matches, I can see that their visual evidence has some merit.

    So two large fragments are broken off the larger body, or the larger body is broken in two, their solar orbits could well be very similar and the difference in velocities minimal meaning the scenario in the video mentioned, is a reasonable possibility. However there is likely to be a significant amount of other debris from the collision also in very similar orbits and I would think it more than likely that other smaller fragments impacted the infant 67p. The delta V between these objects and 67P would be small and insufficient to cause its destruction, but large chunks could have been knocked off the "corners" resulting in the shear planes we see. Such glancing impacts could also result in big changes in the spin of 67P leading to mass loss due to spin effects and large fissures, particularly in the smaller head lobe. One such collision may even have resulted in some stretching and twisting of the neck. The stratification seen in both lobes is evidence of possible shock compression, either from the initial contact binary collision/collisions or later impacts.

    The original larger object would have already cleared a significantly wider orbit of debris, reducing the chances that the infant 67P would be impacted again. If the original destructive impact occurred late in the Late Heavy Bombardment process, objects crossing 67p's orbit could also have been very much reduced and the chances of another destructive impact very much lessened, or 67p's orbit so radically changed by the original impact, it was removed from the firing line into the inner solar system.

    A lot of "possibles" and "maybes", but remembering the vast number of objects flying around in the outer reaches of the solar system around 4.5 Billion years ago, these are by no means improbable events. Only a minute fraction of the objects have survived and these must be considered therefore, as being the highly unlikely result of a long sequence of "lucky breaks". It would seem therefore that 67P could only have survived until now if it had been the beneficiary of such a long list of "possibles" and "maybes".

    Such a numerically deficient hypothesis is undoubtably a bit of a fudge, but its a sure thing that a simple "one size fits all" theory is the least likely final explanation.

    • Gerald says:

      I agree, 4.5 billion years of history make a complex scenario likely, which no one can reliably guess, at least not in detail.

      If Philae awakes, and CONSERT will be able to make a detailed tomography, we may get some more clues to constrain down the history.

    • Marco says:

      I disagree completely. A fudge is just idle speculation at this level. Careful analysis of data, and perhaps extrapolation backwards as to the changes that happen over a perihelion cycle will be way more fruitful.

      The danger of going backwards billions of years is that anything is explainable as a maybe. No one can say for sure what was possible or not way back when nor the likelihood that features have been retained faithfully since that time.

      • Gerald says:

        By impact crater count you can easily say, that none of the billions of years old surface features survived.

        • Marco says:

          Except by invoking CB as a shape explanation for something that happened billions of years ago, that is an impact (just not a crater) event. That (bi lobe) feature is the feature that I'm talking about.

          • Gerald says:

            Large-scale structures might have survived, but modified by geologically "recent" processes. No "old" surface features are intact. Otherwise they would be impact-cratered.
            I would accept remnants of densified shatter cones from impacts for several outstanding surface features as a possible interpretation, with the surroundings eroded away over time.

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