Comet sinkholes generate jets

This story is mirrored from the main ESA web portal and is based on a paper just published in the journal Nature.


A number of the dust jets emerging from Rosetta’s comet can be traced back to active pits that were likely formed by a sudden collapse of the surface. These ‘sinkholes’ are providing a glimpse at the chaotic and diverse interior of the comet.

Rosetta has been monitoring Comet 67P/Churyumov–Gerasimenko’s activity for over a year, watching how its halo of dust and gas grows as the comet moves closer to the Sun along its orbit.

High-resolution view of active regions in Seth as seen with Rosetta’s OSIRIS narrow-angle camera on 20 September 2014 from a distance of about 26 km from the surface. The image scale is about 45 cm/pixel. The Seth_01 pit is seen close to centre and measures approximately 220 m across and 185 m deep. The image also includes Seth_02 and Seth_03 to the ‘top’ of Seth_01, and Seth_04 and Seth_05 ‘below’ (see context image for guidance). The image has not been enhanced to reveal the jets of activity, but instead focuses on the rich diversity of the comet’s geology both inside the active regions and in the surrounds.  Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

High-resolution view of active regions in Seth as seen with Rosetta’s OSIRIS narrow-angle camera on 20 September 2014 from a distance of about 26 km from the surface. The image scale is about 45 cm/pixel. The Seth_01 pit is seen close to centre and measures approximately 220 m across and 185 m deep. The image has not been enhanced to reveal the jets of activity, but instead focuses on the rich diversity of the comet’s geology both inside the active regions and in the surrounds. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

From a distance of a few hundred kilometres, Rosetta observes an intricate pattern of the dust jets emitted from the nucleus as they stream out into space. But now, thanks to high-resolution images from the OSIRIS camera from distances of just 10–30 km from the comet centre last year, at least some of these dust jets can be traced back to specific locations on the surface, the first time this has ever been seen.

In a study reported today in the science journal Nature, 18 quasi-circular pits have been identified in the northern hemisphere of the comet, some of which are the source of continuing activity.

The pits are a few tens to a few hundreds of metres in diameter and extend up to 210 m below the surface to a smooth dust-covered floor. Material is seen to be streaming from the most active pits.

“We see jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles trapped under the surface can be warmed more easily and subsequently escape into space,” says Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research, lead author of the study.

Active pits detected in the Seth region of Comet 67P/Churyumov¬Gerasimenko can be seen in the lower right portion of this OSIRIS wide-angle camera image. The largest, Seth_01, measures 220 m across and 185 m deep. Seth_02 and Seth_03 lie immediately to the left and measure 110 m and 140 m across, respectively.  The contrast of the image has been deliberately stretched to reveal the details of the fine-structured jets against the shadow of the pit, which are interpreted as dusty streams rising from the fractured wall of the pit. The image was acquired on 20 October 2014 from a distance of 7 km from the surface of the comet.  Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Active pits detected in the Seth region of Comet 67P/Churyumov¬Gerasimenko can be seen in the lower right portion of this OSIRIS wide-angle camera image. The contrast of the image has been deliberately stretched to reveal the details of the fine-structured jets against the shadow of the pit, which are interpreted as dusty streams rising from the fractured wall of the pit. The image was acquired on 20 October 2014 from a distance of 7 km from the surface of the comet. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Scientists analysing the images think that the pits are formed when the ceiling of a subsurface cavity becomes too thin to support its own weight and collapses as a sinkhole. This exposes the fractured interior of the comet, allowing otherwise hidden material to sublimate, thus continuing to erode the pit over time.

“Although we think the collapse that produces a pit is sudden, the cavity in the porous subsurface could have growing over much longer timescales,” says co-author Sebastien Besse, of ESA’s ESTEC technical centre in the Netherlands.

The authors suggest three possible ways the voids are formed.

One idea is that they have existed since the comet itself formed, as a result of very low-speed collisions between primordial building blocks tens to hundreds of metres in size. The collapse of the roof above such a void could then be triggered through weakening of the surface, perhaps by sublimation or via seismic shaking or impact from boulders ejected from elsewhere on the comet.

Another possibility is the direct sublimation of pockets of volatile ices like carbon dioxide and carbon monoxide below the surface, heated by the warmth of sunlight penetrating an insulating top layer of dust.

Alternatively, sublimation could be driven by the energy liberated by water ice changing its physical state from amorphous to crystalline then sublimating the more volatile surrounding carbon dioxide and carbon monoxide ices.

If either of the latter two processes is the driving force, then the fact that the pits are not seen everywhere may indicate an uneven distribution of ices inside the comet.

Graphic explaining how Comet 67P/Churyumov–Gerasimenko’s pits may form through sinkhole collapse. The graphic shows a dusty surface layer covering a mixture of dust and ices. 1. Heat causes subsurface ices to sublimate (blue arrows), forming a cavity (2). When the ceiling becomes too weak to support its own weight, it collapses, creating a deep, circular pit (3, red arrow). Newly exposed material in the pit walls sublimates, accounting for the observed activity (3, blue arrows). Credits: ESA/Rosetta/J-B Vincent et al (2015).

Graphic explaining how Comet 67P/Churyumov–Gerasimenko’s pits may form through sinkhole collapse. The graphic shows a dusty surface layer covering a mixture of dust and ices. 1. Heat causes subsurface ices to sublimate (blue arrows), forming a cavity (2). When the ceiling becomes too weak to support its own weight, it collapses, creating a deep, circular pit (3, red arrow). Newly exposed material in the pit walls sublimates, accounting for the observed activity (3, blue arrows).
Credits: ESA/Rosetta/J-B Vincent et al (2015).

“Regardless of the processes creating the cavities, these features show us that there are large structural and/or compositional differences within the first few hundred metres of the comet’s surface and the cavities are revealing relatively unprocessed materials that might not otherwise be visible,” adds Sebastien.

The authors note that the internal features revealed on the pit walls vary quite significantly from pit to pit, and include fractured material and terraces, horizontal layers and vertical striations, and/or globular structures nicknamed ‘goosebumps’.

“We think that we might be able to use the pits to characterise the relative ages of the comet’s surface: the more pits there are in a region, the younger and less processed the surface there is,” explains Jean-Baptiste.

“This is confirmed by recent observations of the southern hemisphere: this is more highly processed because it receives significantly more energy than the northern hemisphere, and does not seem to display similar pit structures.”

Pits Ma’at 1, 2 and 3 on Comet 67P/Churyumov–Gerasimenko show differences in appearance that may reflect their history of activity. While pits 1 and 2 are active, no activity has been observed from pit 3. The young, active pits are particularly steep-sided, whereas pits without any observed activity are shallower and seem to be filled with dust. Middle-aged pits tend to exhibit boulders on their floors from mass-wasting of the sides. The image was taken with the OSIRIS narrow-angle camera from a distance of 28 km from the comet surface.  Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Pits Ma’at 1, 2 and 3 on Comet 67P/Churyumov–Gerasimenko show differences in appearance that may reflect their history of activity. While pits 1 and 2 are active, no activity has been observed from pit 3. The young, active pits are particularly steep-sided, whereas pits without any observed activity are shallower and seem to be filled with dust. Middle-aged pits tend to exhibit boulders on their floors from mass-wasting of the sides. The image was taken with the OSIRIS narrow-angle camera from a distance of 28 km from the comet surface.

The active pits are particularly steep-sided, whereas pits without any observed activity are shallower and may instead indicate regions that were active in the past. The team suggests that the active pits are the youngest, while middle-aged pits exhibit boulders on their floors that have fallen from the sides. Meanwhile, the oldest pits have degraded rims and are filled with dust.

“We are continuing to analyse our observations to see if this theory holds true, and if this ‘time series’ is related to the internal thermal evolution of the comet, for example,” adds Sebastien.

“But we think that most of the active pits must have been present for several orbits around the Sun already, or else we would have expected to see a number of outbursts as their collapses were triggered this time around.”

Rosetta did witness one outburst during its approach to the comet in April 2014, which is thought to have generated between 1000 kg and 100 000 kg of material. The authors state that a pit collapse could have been the driver for this outburst, but only a small fraction of the total volume of a typical pit could have been liberated at the time.

For example, given the measured average comet density of 470 kg per cubic metre, the rapid evacuation of a typical large pit 140 m wide and 140 m deep would result in the release of around a billion kilograms of material, several orders of magnitude greater than was observed in April 2014.

“We are very interested to see how these active pits evolve and maybe we’ll even witness the formation of a new pit,” says Matt Taylor, ESA’s Rosetta project scientist.

“Being able to observe changes in the comet, in particular linking activity to features on the surface, is a key capability of Rosetta and will help us to understand how the comet’s interior and surface have evolved since its formation.

“And with the extension of the mission until September 2016, we can do the best job possible at unravelling how comets work.”

Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse,” by Jean-Baptiste Vincent et al is published in Nature.



  • Bill Harris says:

    Bravo. This is good science.

    We have been monitoring the pits at Ma’at since August 2014 and the pits at Seth since October 2014 and now the observations are paying off.

    Still, it’s the age-old chicken-egg conundrum: are the jets the result of the active pits, or are the pits the result of erosion from the jets?

    Cluck-Cluck. 🙂


  • Felix says:

    For what it’s worth, here is my “therory” on jets 🙂

    First, without interacting particles, then interaction is enabled and i see some “beautiful” jets, especially in the neck region.

  • Dave says:

    It’s nice to see a theory beginning to get traction, looking forward to more.


  • Guili says:

    I see Ma’at 01, 02, 03, aligned. I know it might be coincidence. But it makes me think about the volcanos appearing at regular intervals above the ocean surface: there is a hot spot on the Earth mantle below, which is regularly puncturing the mantle and creating volcanos.

    • Ramcomet says:

      Nice observation, Guili!
      And the obvious ages are also consecutively demonstrated as youngest to oldest.
      Although in this case, I suspect heat source is from above, with the sun’s maximum heat moving along the surface, (much like the terminator shadow moves across 67p).
      Then, the distinct separation of the three equal sized pits might be explained by cooling effects of venting gasses from the inner perimeters, so, instead of a continuous oblong pit, the next sublimation sink hole pit starts farther away.
      And… Repeat.

  • ianw16 says:

    Nice work, and very good paper. Real science being done by real scientists.

  • A. Cooper says:

    The three pits on Ma’at need to be looked at in the context of the section directly beneath all three that was stretched upwards just before shearing from the body. There is ample evidence of this happening. Firstly, three thin sets of ‘gull wings’, two on the head and one on the body that were once sandwiched together loosely like puff pastry. This would have been when the head lobe was seated on the the body. Secondly, there are two 40-metre high slurry deposits sitting next to (and half under) the set on the body, implying massive outgassing in this exact spot in the past.

    The section of puff pastry layers is rectangular with the gull wings set at one end. The slurry pushed its way between the layers emerging mostly at the gull wing end, pushing them up into their current gull wing shape. The top two layers, including their gull wing shapes sheared away from the bottom layer and rose with the head lobe. This left the top surface of the bottom layer exposed on the body directly below. It’s a visible rectangle with a fluted end which is the gull wings seen from above. It has two clear slurry piles at one end.

    Prior to shearing, the head lobe stretched under the tensile stress of spin-up (this generated the heat for the outgassing). This is why the topmost gull wing shape has delaminated from its partner below and is now at a 45° angle, nestling under the most active of the three Ma’at holes. The second set of gull wings is nestled under the rim directly below it. Their outermost tips join. The lower head set matches perfectly to the set below on the body.

    The three holes in Ma’at are situated directly above the rectangular formation. The most active hole is directly above the end of the top set of angled gull wings. When seated on the body and before the gull wings stretched to 45°, this hole was directly above the place where the two slurry piles are situated. It was very likely a short-cut escape route for the gases emerging under pressure with the slurry.

    The other two holes would have served the same purpose but to a lesser extent, hence their more stunted appearance. Their lesser activity was probably due to less slurry ejection along the outer, long side of the rectangular formation (this left its own elongated slurry pile and it pushed up the head lobe rim into the frill we see today).

    All three holes would likely have continued being active but their differing activity levels are probably due to the circumstances of their initial formation and not their age which is likely to be exactly the same for all three.

    Although, there is evidence of outgassing via dykes and slurry deposits all around the shear line, this particular section shows that far more activity occurred here than anywhere else. It also exhibits the only three well-defined sublimating holes on the entire head lobe (except one at Bastet, the second highest generator of slurry going by the slurry and dyke evidence). This is most unlikely to be a coincidence. All this information has been documented and was linked to on this blog in January 2015.

  • Kamal says:

    Robin Sherman suggested (I can only find his flickr page ) from an Osiris image published in March that the whole Hapi region may have undergone a gigantic roof collapse, which may have led to the big jets from there. Is that too much of a stretch of this theory? Alternatively, does stretch, as promulgated by A. Cooper and others, extend to an explanation of the Hapi jets using the mechanism explained above?

    • A.Cooper says:


      I can answer your question above and also your question regarding the “tombstone” on Anuket in one go because they are related.

      Stretch theory can’t support the idea that the sunken nature of Hapi is attributable to it being giant sink hole. However, it does explain why Hapi is sunken. It’s to do with all the apparently lost material from Hapi being swept up to form the neck during the stretch (see latest post, pt 25). There is much evidence for this, mainly in the four long, quasi vertical lines in the neck at Anuket, which contain material that’s been lifted 1 km from the body and is now embedded in the neck matrix just below the head rim. One of these many pieces is the large protrusion near the top that exactly matches both head rim above and body ‘shear line’ below. That is the “tombstone” you mention (Cometwatch 23rd June) that you couldn’t identify.

      The reason for the Hapi jets being so disproportionately active would be due to the fact that Hapi and the neck constitute the core of the comet, exposed by virtue of the nucleus shearing into the two lobes. Both Hapi and the neck matrix are exposed core but the neck matrix is, in effect, reworked core material. This is especially the case at Anuket where the head tipped up the most and where more material was swept up into the neck and at a faster rate. This explains the ‘dry-mix concrete’ appearance of Anuket.


  • masanori says:

    All of a sudden, any depressios of 67P started looking like (old) pits!!

  • Sovereign Slave says:

    Very interesting and exciting findings presented in the paper, and some basic facts found by Osiris – dust emerging from pits, pit sizes determined, a description of the varying visual characteristics of the different pit walls and bottoms, jets issuing from the walls of the pits, active pits being particularly steep-sided whereas pits without any observed activity are shallower. However, those are some pretty sparse facts upon which to base the many speculations, interpretations and conclusions found in this paper. In fact, in many ways these findings seem quite puzzling, and again raise many more questions than the speculations attempt to answer.

    • dave says:

      Sovereign Slave ‘findings quite puzzling.

      You are right, but this adds to the excitement of something new I guess.
      For me, how the sun can reach 200 meters down a steep sided pit for long enough to maintain continuous heating and sublimation, also the constant diameter pit at that depth is even more surprising/spectacular than the columated jets.
      Given the loose cohesion of the material under the crust from recent Osiris pics, I cannot yet imagine what is stopping the walls collapsing while gas, dust and debris accelerates up the tube.
      Even if the pits are hang overs from the formation of the comets, how on 67P do they stay vaguely circular and straight to those depths.
      Its going to be a lively & very interesting topic.


    • Sovereign Slave says:

      Actually, my first thought when I read this paper and looked at the pictures was that the high resolution Osiris camera must be experiencing the same defect as the Navcam. Somehow it too is making the comet look exactly like rock and dust. Hopefully they’ll correct the issue soon so that what the comet looks like matches what we “know” it is.

      Also, this paper is yet another excellent example of how speculation becomes “scientific fact.” This paper is being regurgitated around the globe and now the holes on P67, and perhaps by extension every other comet, are being called “sinkholes” based on the highly speculative musings of this one initial paper. It’s already happened. But if you distill the findings from the Osiris pictures to what you could identify as actual facts, there aren’t that many of them, and they certainly don’t inevitably point to the sinkhole conclusions found in this paper. But to the world comet sinkholes are now indeed a scientific fact to be added to black holes, big bang, etc etc, as shown below:

      “On 2 July 2015, scientists reported that active pits, related to sinkhole collapses and possibly associated with outbursts, have been found on the comet by Rosetta.” – Wikipedia

      Rosetta’s Comet Is Developing Giant Sinkholes Before Our Eyes – Gizmodo

      Rosetta Found Sinkholes on the Surface of Its Comet – Wired

      Rosetta Spots Huge Sinkholes On Comet 67P – Forbes

      Rosetta spacecraft spots enormous sinkholes on comet 67P – The Guardian

      On and on.

  • Harvey says:

    I would agree that it’s unfortunate that the popular press seizes on such things implying more certainty than actually exists. But there is a need to publish material short of ‘fully proven’ to stimulate debate, usually (sometimes excessively) qualified regarding its certainty. The press wants to sell newspapers etc; scientific caution doesn’t do that. I have no idea what can be done about that; probably nothing.

    As has been pointed out scores of times what it ‘looks like’ is utterly irrelevant. It was formed and processed in conditions radically different to earth, and appearances mislead. The density is 470kg/m^3 and it degasses large amounts of water; yes, H2O; that’s not rock. Not to mention the fact that we are looking mainly at organic material anyway.

    Incidentally, Wickramasinghe has started another unwarranted media firestorm claiming 67P is biologically alive, on roughly as much evidence as there is it is rock.

    • Marco says:

      Hi Harvey,

      The panspermia crowd has been virtually nonexistent as far as comments on this blog go, while the EU community has been out in full force.

      As far as the sinkholes go, I am quite surprised at how the vertical walls and flat bottoms of the sinkholes hasn’t made a mention as being strange for a body that supposedly has seen no liquids.

      What is the origin of the flatness of surfaces that doesn’t involve liquids in the past? Most asteroids have no flat surfaces to speak of, while this comet has plenty.

      • Harvey says:

        The only reason I bother to counter the EU twaddle here is to avoid the ‘interested public’ being mislead. No real scientist gives it more than a second’s thought. I’d never heard of it till I started getting interested in Rosetta/67P; a couple of minutes at the Thunderbolts web site had me rolling in the aisles. But posts here often sound authoritative and are in pseudo-scientific language, which could easily mislead.

        Thanks for the link, yes I have access. In many ways the plasma data to date is very much as expected; I’d love to know the status of any bow shock. Most surprises – unsurprisingly as we had virtually no previous data – relate to the morphology and how it links to comet activity.

        The morphology of the sink holes, if that’s what they are, with such vertical sides and being so circular, certainly is a puzzle. But nothing whatever undermines the fundamental premise, that it’s all sublimation driven, and there is no viable alternative to that.

        However the EU community will never be convinced. It will always be SOLID ROCK, DISCHARGES, whatever the data says. We took the wrong measurements, hid the data that proves them right, etc etc. Who needs science when you are a fully paid up fantasist.

        • originalJohn says:

          “The EU community” What is that Harvey. Not something I know anything about. Certainly rational objective scientists would never be convinced by the weak ice sublimation argument and would feel compelled to look for alternative explanations.

          It will be solid rock when there is evidence of solid rock, and discharge when there is evidence of discharge. Until then they are proposals.

          The electrical explanation fits the observations best and along with the combustion hypothesis deserves serious investigation. Any reluctance to gather data in those areas will of course be commented on.

          I think the public does not require your protection from objective alternative hypotheses. It would be better if you were to explain to them clearly what is a hypothesis and what is a fact. Then perhaps you could go on to explain how ice sublimation is the only plausible water source without the confirmed presence of ice.

    • ianw16 says:

      Well said, Harvey. I’d be interested at this point for the EU fantasists to actually spell out the evidence that they have for this neo-Velikovskian nonsense. Zero is the answer.
      Here’s a paper you may have access to that throws further cold water on it. Not that it needed any. Unless there is some evidence somebody would like to link to?
      Paywalled, but I imagine you can get access to it.

  • Harvey says:

    On interesting issue the paper does rather dodge.
    Lets assume its a sublimation driven cavity whose roof falls in; fine.
    But I can see no reason why that intial hole should be quite accurately circular, as many pits seem to be.
    So we need a mechanism to circularise the pit once its formed, and for any diameter growth to preserve that circularity.
    Whilst I can begin to see mechanisms that might do that, its not that obvious to me they are strong enough to be credible.
    Thoughts anyone?

    • dave says:

      I have already stated Harvey as much Harvey,

      It is hard to see a creditable method, the first roof collapse, yes we can accept that, but then how does it get bigger and deeper at the stated ratio of diameter & depth? How is it that there is a ratio for young or old craters but nothing for middle age? How is it the sublimation of the walls are not badly effected by the relevant incident angle of the sun? and lots more. I was rather hoping there would be more from the mission scientists, to further explain.

      Sorry they are all rhetorical, I have run out of ideas.

      Looking across at the Nasa projects at Ceres & Pluto, they are refusing to comment until they get more data on the unexplained geologies they are witnessing with their own projects. may be this is the most sensible route when their is doubt. The Nasa guys even appear to be backing out of the water theory for the white spots. seems there are many surprises on all projects.

      Maybe this will stimulate a new exploration spike, if our instruments still cant define these structures or what is happening, if we cant describe the geology, how can we put an age on them?

    • ianw16 says:

      Natural shape, Harvey. Should it be square or oblong? I’m thinking that this is just the natural shape of things in vacuum. Spit out a bit of water in zero G, it forms a sphere. Nothing too mysterious here.

      • dave says:

        well circular is popular in space, but if you look at 67p you can also see many very flat surfaces, some look like nicely placed cantilever bridges, and where these flat plates have sheared off there are plenty of oblong/square plates just lying on the surface, maybe nature likes several shapes.

        What is odd from our earthy viewpoint is not that the pits are round, its how deep and parallel they seem to be with what looks like odd depth to diameter ratios. Ok Harvey showed some similar looking sink holes on earth, but they seem to be formed by liquid and there is often a cave system underneath the sink hole also formed by liquid.
        So yeah nothing too mysterious about circle or a sphere, but there appears to be nothing too mysterious about oblong either.


    • Marco says:

      Hi Harvey,

      Even more of an issue for me are the flat bottoms, not only in these supposed sinkholes but flat areas on 67P in general – like Imhotep, Hatmehit and the amphitheatre. Flatness on the earth (strata, salt lakes etc.) as well as other bodies such as the moon (mare etc.) involve liquids in their formation – the flatness coming from the equilibrium of a liquid in a containing formation.

      Combining the need for liquids in their formation to explain the flatness, and a hotspot in the centre radiating out to explain circularity, and the vertical cliffs around as well and the need to modify the sublimation model becomes great. I think the corollaries to stretch theory have a good chance of explaining all features on the comet, including these, but if one hasn’t accepted the strong scientific evidence for matching, I hardly see that circumstantial evidence for liquids is going to sway one.

      • Harvey says:

        I agree entirely the flat bottoms are also a considerable puzzle.
        But Ian, I don’t see any inherent ‘vacuum’ reason for them to be round.

        The hypothesis is that some buried concentration of volatiles sublimes, leaving a cavity. I can think of no particular reason why that should lead to a (surprisingly accurately) round hole with a flat bottom.
        Seems more likely some mechanism operates to circularise the hole and flatten the bottom after the collapse, and I’ve very little idea what that can be.
        Well I do have some ideas, but I don’t present inadequately thought out hypotheses,min like some here!

        I don’t really see that allowing fluids would help a lot, though it adds new possibilities and complexities, it doesn’t, to me at least, immediately proffer an explanation. And there are big problems with having liquid; possibly in pores etc inside the comet, but in a big open pit?

        As I said before, I don’t *exclude* stretch, but I don’t find the evidence as convincing as you do, and I have big problems with a mechanism/material properties. But still, maybe.

        • Sovereign Slave says:

          Harvey, perhaps you could shed some light on a few other things that don’t make sense to my layman’s head. If these are sink holes that were created by sublimation, for depth to be created, the sublimation must be regularly taking place on the floor of the pit once it’s formed, and the floor is getting further and further away from the overlaying roof as it deepens, creating a very large dark vacuum space between the floor and the roof. How then is heat penetrating through the cap down to the floor through this vacuum, which should not be an efficient heat conductor at all? Also, how is the dust escaping in sufficient quantity to dig a pit – once it sublimates from the bottom of the pit into the vacuum of the pit, what force would then cause it to push through the cap? Also, if sublimation is now happening from the side of the open pits, why is the direction of the jets columnized straight up, as we see it doing? Seems the dust would normally be directed at some angle across the pit, and not make a 90 degree turn straight up. And what would these very large caps be made of that have sufficient strength to resist even the slight gravity of P67? Certainly not volatiles and dust. And why would whatever the caps be made of be so fundamentally different from what the materials below them are, which obviously would have to be sublimating? Countless more questions, but will stop here.

          • Harvey says:

            Indeed, countless questions any theory must ultimately answer. Lots of work for the ‘sublimationists’ to do.

            But at least in sublimation we have a rational basis to try, which fits the overall behaviour, matches the degassing species, is energetically possible, and is in the right temperature range. Grossly, it works, in detail, much to understand.

            What exactly is the alternative?
            Basically a completely irrational ‘theory’, which cannot explain at the crudest, most basic level how it is supposed to work. Which is in obvious violation of the ‘laws of physics’ and of many observations. Which has no ‘numbers’ at all.

            If someone proposes a rational, science based alternative to sublimation/evaporation which fits the *gross* observations, I and everyone else will be very pleased to consider it. And of course ask exactly your list of questions of it.

      • Marco says:

        Hi Harvey,
        I think you are just about there with realising why this puzzle challenges the fundamental “solid and gas state only” limitations of standard cometary synthesis.

        Leaving “possibilities and complexities” aside, the fundamental thing we are looking at is that flat things in nature require, however fleetingly, a liquid state which solidifies to maintain the flatness that it derived from the liquid state at gravitational equilibrium. On the moon it is assumed to be lava that solidified to create the mare. Similarly the flat bottoms of impact craters. Strata of rock became planar by being deposited from water and then solidifying later on.

        The difficult question is, what black organic substance could be liquid temporarily on the vacuum surface of a comet, and then become solid after a time of exposure to a vacuum and sporadic solar energy?
        Crude oil perhaps? It would be an easy experiment, but I suspect the light, sweet parts of the crude oil would evaporate, leaving behind longer chain organic tar and other high molecular weight hydrocarbons. Could that (assuming the possibility of crude oil on the surface) explain the flatness and blackness? Could anything else reasonably expect to explain it?

        • Harvey says:

          I’m sure you could find some organic compounds which would be liquid a comet temperatures with low enough vapour pressures to exist for some time. Even things like octane are in the right ballpark maybe. Whether they are credibly present I’ve no idea. But to be involved in the production of the pits, you’d need a heck of a lot of it, enough go to put a decent layer into these sometimes huge pits. So far no hint of liquids in any images.

          At the moment, I’ve a completely open mind on how these formed. As we are now seeing on Pluto, there are a heck of a lot of different mechanisms operating on these bodies. As I’ve said many times before, the most dangerous thing is to make any kind of deduction that relies on 300K, one bar, 1G informed intuition and ‘common sense’, what things ‘look like’.

        • Marco says:

          Hi Harvey,

          The deduction of surface liquids going to solid is independent of earth-like conditions or common sense. Flat things in nature require liquids. For natural things to stay flat requires them to go solid and avoid erosion until the time it can be observed. Thus the flat areas could not have eroded in the timescale since they were formed flat, however long that is. Surely you can see this as a solidly based deduction?

          The corollary to this is that the surfaces are not eroding from the surface down as expected, if they are eroding at all. This is the same conclusion from the head to body shear line matches. When head was in contact with the body, mirror imprints occurred on the shear line. For them to be matching still after many further perihelions, the surfaces must have hardened, and could not have eroded.

          Everything else about the morphology of the comet follows on neatly after realising that the emissions are not remodelling the surface at all. The surface is remodelled only with shear events, stretch events and general removal of the crust and exposure of short lived liquids, which form most of the flat and smooth areas…

    • logan says:

      _In this little world where planes are the rulers. Curves, swirls, concavities, convexities are the phenomena.

      You are right, Harvey.

  • Harvey says:

    Well I have no idea if these Arctic craters really have any relationship to 67P; very probably not. But there is a passing visual similarity, and decomposition of clathrate a may be involved. Some of the proposals for their origin do indeed involve cavities with collapsing roofs!

  • Dr. Prasanta K. Mukhopadhyay (Muki) says:

    The development of craters within the smooth surface reminds me of hydrocarbon (possibly mixture of oil, gas, and clay minerals) outburst? Do you have any chemical composition of the gases that comes out with possibly the water vapor?

    Muki (Mukhopadhyay)

  • logan says:

    Rain. Rain. Does it rain below our feet?

    • Marco says:

      Hi Logan,

      Welcome back. Have you heard of the Great Artesian Basin? A lot of water under our feet here in Australia.

      • logan says:

        Hi Marco. Far from dry the Continent.

        Wanting to point at ‘sphericity’ of wall indentations at Seth_01. Same is seen at neck in first GIF of 67P’s arrival.

        Think those ‘sphericities’ are very important to their genesis.

        They talk of static, very focused failure points.

        Don’t know for sure where exactly our local comets were, really. Scientists shouldn’t project too far in time. Comets are specially metamorphic. Their travels should be too. So, neither know for sure the speed of processes and the amount of internal stress. Fluids? Why not?

  • Harvey says:

    Marco, re liquids.
    Even if they exist – a possibility maybe – don’t forget that their behaviour is quite unlike what we are used to.
    Our thinking on how liquids behave is strongly dominated by gravity.
    But under the very low g conditions of 67P, surface tension effects, in particular Marangoni flow, become much, much more important and can dominate liquid behaviour; this can be quite problematic in spacecraft system designs involving fluids.
    So it’s not so clear that fluids would immediately help with the ‘flat’ problems; they might, but they might not.

  • Harvey says:

    Btw the only commonplace example I know of for Marangoni flow is ‘tears of wine’ , where you see an alcoholic beverages climbing the side of your glass and trickling back down in streaks. It’s caused by evaporation induced variations in ethanol concentration changing the surface tension. In spacecraft, small temperature differences cause the surface tension differences and can cause big effects.
    It needs some fairly serious sums doing to see if under the gravitational field of 67P a fluid would give a flat surface with modest temperature gradients present; it’s quite possible it would not – but it needs a sum doing.
    Trouble is we’d need to know what the liquid was, and it’s surface tension and viscosity etc as a functioning temperature.
    To be clear, I’m *not* asserting that it definitely will not form flat surfaces; but pointing out that there are very definitely credible mechanisms that may prevent it doing so which need checking out.

    • Marco says:

      Hi Harvey,
      Thanks for the reply. As it happens, I have been chatting with a chemist who has a very clear idea of what kind of liquids we are talking about. A mixture of hydrocarbons is the substance that would be exposed on the surface, and the viscosity would be dynamic, as the lighter hydrocarbons evaporate, viscosity would increase progressively until it hardens into a tarry coal-like substance. Thus the hardening aspect is a dynamic function of the liquid mixture being exposed to the surface, which supports the hard, flat evidence.

      Since the surface quickly solidifies, liquids come from below and then set like concrete.

      As far as whether mechanisms would prevent flat surfaces being formed, I can only say that I consider liquids, or liquid like substances, are necessary but not necessarily sufficient for flat surfaces.

      • Harvey says:

        Necessary but not sufficient a rather strong claim!
        Crystal cleavage planes can be atomically flat over significant areas, with not a fluid in sight. I guess one would take ‘shaken sand’ to be covered by ‘fluid-like’, but that’s rather catch all. It won’t show Marangoni flow, helpful, but would orientate normal to the local force vector. In some ways it might be a better candidate than a true liquid.

        One counter example of course removes ‘necessary’. I’d certainly agree they are *commonly* involved in the generation of flat surfaces *under high g conditions*. But that is not the case on 67P; beware earth-bound intuition/experience and common sense.

        Cleavage is not relevant to 67P of course, but whilst fluids are an interesting possibility, and I agree there would be organic candidates, I’m far from convinced they are the only game in town.

        Incidentally re the orientation one would also have to correct for the ‘centrifugal force’. The orientation of the flat areas relative to the local force vector would be a very strong argument for/against a fluid (be it true liquid or shaken sand) origin.

      • Marco says:

        Hi Harvey,

        I stand by the claim of necessity, , and I don’t think it should be controversial. For instance, you mention crystal cleavage, but how would a crystal that large form without liquid in the first place? Sure it is theoretically possible for a large pure crystal to form without liquid, but that is neither consistent with anything about a comet and requires an extremely contrived unnatural situation.

        Basically, flatness that is not engineered by humans, on earth or in space, low gravity or high has an origin with liquids in them. Either comets have intelligent engineers or has had liquids somewhere in the past that has been a factor in forming the flatness. Whether it was forming the strata or crystal that has cleaved or hardened on the surface. I think the former presence of liquids is the less crazy idea.

        • Harvey says:

          It’s perfectly possible to form crystals from the vapour phase. As I said, crystal cleavage is not relevant to 67P; I was just being unnecessarily rigorous about ‘sufficient’.

          I think you are underestimating the low g problem. I rather strongly suspect a true liquid will not readily form a flat surface on 67P. That liquid=flat thing is a result of our one g environment. in low g effects like Marangoni flow can be totally dominant. (I came across this year’s ago when involved in spacecraft design.)

          But a layer of dust, possibly ‘fluidised’ by gas moving through it, might explain things and would not show Marangoni flow. But would also be fairly ‘routine’ as an explanation.

          A proper, detailed analysis of the orientation of the flats would tell.

          • Marco says:

            Hi Harvey,
            I still don’t understand why you are twisting it into a *low g* problem when it clearly is a flatness problem.

            If erosion is the main force at play, and solar radiation is the main forcing cause of erosion, erosion is proportional to the amount of solar radiation and the volatility of the ices. The tendency is that these factors destroy flatness, and any flatness which erosion exposes, predates the erosion and needs to be explained without reference to the erosion exposing it.

            For instance, the Colorado river exposed strata visible in the Grand Canyon through erosion, but the root cause of the flatness of those strata has absolutely nothing to do with the erosion from the river.

            When the morphology of Imhotep is explained, the question of why erosion is not making it un-flat is what should be asked, rather than assumed to be sufficient to explain the flatness.

            A plausible explanation could be that the strata are due to 67P being originally a chunk of a bigger object that had developed strata conventionally due to higher gravity and rounder shape.

            With the fluidised dust explanation, why would the flatness persist unless the dust solidified in an analogous way to sedimentary rock?

            Jets are originating from the bottom of these sinkholes. Wouldn’t the erosion from that destroy the flatness?

            Flatness as evidence, is evidence that gradual erosion from the surface is not happening. Stretch theory explains all this, and infact, requires a lack of surface erosion. It is a rational, science based alternative to theories which require surface erosion.

          • Harvey says:

            Macro, I’m not sure if we are talking at cross purposes in some way.

            The connection between ‘flatness’ and ‘liquid’ is gravity; without it, or with very little, there is very little connection of ‘liquid’ to ‘flat’.

            Gven the very low density of the comet and. *probably* very low strength, it seems implausible it is a fragment of something formed under high g conditions.

            Low g could keep a fluidised dust bed flat covering an irregular under-the-dust eroding surface. I don’t see that it needs to ‘solidify’. We may be looking at just that surely.

            True liquid free surfaces may well have very little tendency to be flat on 67P. Sedimentation followed by solidification (‘sedimentary rock’ style) in a liquid might give flat layers as it dodges surface tension effects; but it’s an awfully long way from anything we’ve seen. Even then you’d need to think very carefully as to whether there are other weak processes that might prevent flat layer formation. Lots of counter intuitive things may be happening in a mucky liquid on 67P, if it exists or existed.

            Also it’s not necessarily true that all erosive processes lead to severely non flat surfaces. Things like river erosion certainly lead to grand canyons – but require a high g environment again. Actually seems to be a subject of some argument in terrestrial geomorphology; but as ever, the processes operating are so different that’s little help.

            The flats are a puzzle, as is the circularity of the pits. A true fluid, as opposed to fluidised dust, looks within the bounds of possibility, so not excluded as yet, but I’d not put any money on it.

          • Marco says:

            Hi Harvey

            Will continue conversation at bottom of comments….

  • logan says:

    Very important to distinguish between liquids and fluids. Fluids only require small amounts of non-froze-able lubricant substances.

    • Harvey says:

      An interesting point Logan. In fact they don’t necessarily require any at all; shaken particulates can behave very like a true liquid, and don’t show some of the awkward effects like Marangoni flow.
      A fluid like dust layer covering a rougher bottom might be more credible than true liquid.
      However the orientation argument would still apply and needs checking out.
      BTW welcome back, I hope wherever you’ve been out of contact was pleasant/successful/whatever is appropriate!

      • logan says:

        Have seen particulate to pile up at neck’s grinding. So lubricant needed [beyond the natural continuously active tremor], even if it is only out-flowing gas.

  • Harvey says:

    A further thought re liquids.
    Actually it should be quite easy to give a strong boost to a ‘liquids’ theory if someone can be bothered to do the analysis – or indeed to deny it.
    On earth, large diameter high-g environment, we are used to liquids forming ‘flat’ surfaces with a surface normal to the gravitational field; a tangent plane to the sphere.
    But on 67P gravity often does *not* ‘point straight down’ into the surface but is all sorts of odd directions,

    So if the flat areas are liquid generated they should be orientated normal to the local gravitational field & *not* locally tangential to the comet surface, where those two differ as they certainly do in some areas.

    Locate such areas & if they contain a flat feature check, & fluids will be quite strongly supported – or pretty much rules out.

    • Marco says:

      Hi Harvey,

      That is a fabulous idea right there. Of course, the mission scientists, have jealously guarded global (duckyal?) gravity maps if they have them. They would be interesting in their own right.

      A rough calculation of local gravity field looking around the comet would indeed lean towards there being a strong case for liquids. Especially the flat circular features on Imhotep. The planes are close to parallel to nearby planes, and normal to what you expect the gravitational vector to be in those locations. Most flat areas barring Hathor would arguably be close enough within error measurement.

      • Harvey says:

        It needs correcting for the rotation too; but ultimately this would pretty much prove/disprove ‘fluids’.
        Fluid like behaviour from a dust layer might explain things perfectly well, and would really involve no surprises or need for a rather constrained (but not impossible) true liquid.

    • Marco says:

      I thing the outgassing jets are parallel to the gravity field at the point of origin also. I can’t wait until the scientists map the jet directions, origin against gravity field.

    • logan says:

      At least for Imhotep central plains gravity slope is 5º or less.

      • logan says:

        Obviously the gravitational slope will change slightly at every perihelion. So the fails drawing its surface 😉

  • logan says:

    On consolidating graphic memories, think icy/ceramic/salty behavior dominates over organic. Organic behavior is decanted at ‘certain’ layers and was more evident at cold arrival.

  • logan says:

    Just remaining every soul that as near as Nov-Dec there were particulate pile-ups at Ducky’s neck. So not quite sure ‘shaking’ is enough for fluid-like behavior.

  • Marco says:

    Hi Harvey,

    With liquids and flatness the cross purpose is that I am trying to bring it back to the fundamental physics reason things are flat. First of all flatness is an approximation. In the case of earth, the layers follow the curvature of the Earth, so are spherical sub segments. For the purpose of 67P the sub segment is assumed to be a curved surface, approximating to flat, but the slightly curved plains of Imhotep are leading.
    In the case of microgravity, a single drop of liquid will go to a sphere, and have a modicum of cohesion, while dust would not.
    Imagine a space station experiment where you are trying to make a rough surface of an object look smooth (like Imhotep) Being microgravity, sand would not be helpful. In fact there is a better chance of dust/sand acting like a fluid able to naturally go flat in a much higher G situation, because it is not naturally sticky, and static will be an enemy.

    Whether in an experiment in the small scale microgravity of a space station or an imagined bigger experiment. The connection between liquids and engineering a flat (or rather, smoothly curved) surface from a random surface, the requirement for high G or any G is nonexistent.

    Your arguments seem to imply that liquids are so impossibly unlikely that the above thought experiment must be wrong.
    Yes. Most of our Earth examples and Moon examples and for that matter, Pluto examples have big spheres and reasonable gravity. Big spheres mean a segments are closer to Euclidean planes than the curves associated with 67P.
    I think the more important property of liquids is that they spread and fill in holes as they do. Thus the natural cohesive nature, surface tension, and the tendency to find an equilibrium makes flatness a natural consequence of the properties of being a liquid. Thus flatness would be an expected property even with negative gravity (eg. On the surface of a comet spinning such that the centrifugal force is bigger than the gravity) because the cohesive forces would stick the liquid to the surface, but as a liquid it would spread flat. Extremely contrived situations would be required for “fluidised dust” or any other substance and mechanism to naturally form a smooth gently curved surface without at least some requirement for liquid.

    • Harvey says:

      Sorry, just noticed this.
      But I’m afraid in many ways we have to agree to differ.

      In the high G environment we are used to, setting aside the flat/sphere thing which doesn’t matter much, it’s the high G that causes the free surface to be ‘flat’.
      In zero G, *with no perturbations’*, it’s a sphere.
      But a very, very delicate sphere, watch the videos.

      In very low G, in contact with a complex geometry, and probably with temperature and possibly compositional gradients present, all bets are off on what shape the surface is. Surface tension effects will become dominant, capillary action, huge meniscus, Maragoni flow, etc etc.
      This is the big problem in liquid system design for spacecraft, which as I said I had some involvement with years ago. You avoid free surfaces at all costs, because they are highly unpredictable.

      A gently fluidised layer of dust in in the bottom of a pit might go fairly flat, as it doesn’t have surface tension to mess things up.

      Fluids don’t seem impossible, but they do seem pretty difficult , and I’m still far from convinced they actually help.

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