Two Knobs joined by a Neck
The day after tomorrow (August 6, 2014) the Rosetta spacecraft arrives at Comet 67P/Churyumov-Gerasimenko, and to celebrate, we’ve got another image to look at. This one was taken on the 3rd with the NAVCAM system, and it makes the comet look more intriguing than ever.
What intrigues ME most about this image, aside from the craters and ridges (which are pretty darned interesting in their own right), is the “neck” of material joining the two knobs. I imagine the mission astronomers are talking a lot about it, too. In this view it appears smoother in some places than in others. That makes me wonder at the processes that smoothed it out. Were they part of an event that “stuck” two pieces together in a collision? Is this nucleus part of a larger one that broke apart along some interior seams?
Comet nuclei date back to the formation of the solar system, and in 4.5 billion years, they can be affected in many ways as they orbit in the Oort Cloud/Kuiper Belt regions. Collisions can and do occur, which shape these icy chunks as they age. If they get close to the Sun, their surfaces are heated, which causes them to lose ice and dust. This comet is certainly doing that. Comet nuclei can also be hit by smaller pieces, which explains the craters you see here and there along this one’s surface.
As the spacecraft approaches the comet’s nucleus, Rosetta is measuring and mapping it. The VIRTIS instrument onboard has taken the comet’s temperature and measured it at -70 C. That’s actually fairly warm for an icy body, and this suggests that the surface is covered with dusty, dark-colored crust. VIRTIS measures the thermal infrared (heat) profile of the comet, and in this case, the dark crust is emitting more heat that the instrument can collect. Dust emits more heat than ice does, and this is why scientists are suggesting the dusty crust idea for the surface.
Of course, in a couple of days, we’ll all know MUCH more about this comet as Rosetta arrives and starts taking in situ measurements and images. The European Space Agency will be running a live feed during the arrival, so if you’re interested in watching, check out their web page for more details. The webcast will occur from 10:45 – 11:45 CEST (5:45 – 6:45 EDT). Also, the ESA Rosetta blog posts the latest images, so be sure and read it, too.
Yes, the contact region will always be interesting. As I mentioned in an earlier post, the smooth-surfaced ‘neck’ morphology is what is expected to collect at the junction of contact binary bodies over the eons. It would consist of loose material – debris and dust acquired not only from impacts (the small proportion of low-energy debris that does not escape) but from the grinding action that must invariably occur between the lobes, induced by an impact or more rarely (if not more regularly) by passing through a sufficiently high gravitational gradient (via close flybys of one or another planet during its past history) which could induce a tidal strain on the loosely bound lobes. The resulting ‘bump-and-grind’ action over long periods would build up a mantle or ‘neck’ of dust and debris at the junction, exactly as seen.
The most likely scenario for the occurrence of contact binaries is from relatively slow mutual approach speeds which were common between rocky and icy bodies which were still capable of growing by accretion during the early formative epoch of the Solar System before large planetary masses developed. Afterward, the growth of massive planets gravitationally scattered such planetesimals into mutually higher-energy elliptical and higher inclination orbits, which would have increased the incidence of violent impacts. But while things were still energetically ‘cold’, objects of considerable size – many kilometers across, can have bumped into each other at relatively low velocities and have nullified most of their initial mutual velocity. Many can have bumped themselves into mutual orbit in this way.
Some fair fraction of contact binaries initially started by losing enough momentum from these low-energy collisions, initially resulting in close-orbiting binaries. Through a variety of processes these close binaries can lose their orbital momentum and gradually link up to form ‘contact binaries’ that continue to spin at a rate that does not exceed the centrifugal tendency to separate them.
This is well understood.
The other popularly expressed idea, that such double-lobed morphologies may arise exclusively or at least substantially from erosion from solar heating or the solar wind, isn’t at all consistent with any of the observations we have thus far made of small bodies.
First, even rocky bodies (not considered to currently be, or have recently been, comet-like in behavior) are commonly found to exhibit such a morphology. It seems to happen with a high frequency without significant volatile involvement.
Second, if such erosion were a big player, we would expect to have found many shapes considerably more complex than the binary lobe we so frequently see as an alternative to a single body: so why would such erosion preferential select for binary morphologies instead of, say, some complex crenulated object that may be described as a ‘kiki’ as opposed to a ‘bouba’? Or if not a spiky ‘kiki’, maybe something toward the opposite character that resembles the structure of a vesicle-ridden pumice rock, with near-surface pockets of volatiles boiled off leaving deep hollows (NOT impact craters)? Or with 3 or 7 strangely-shaped lobes sticking out at odd angles? Why would the binarity be a common motif selected by erosion?
Third, there is no obvious correspondence between observed shapes reputed to be produced by erosion and the disposition of the objects rotation axes with respect to its orbit’s ellipticity and inclination with regard to that axis. Indeed, it is significant that 4 out of the 6 comet nuclei now visited by spacecraft exhibit unambiguous contact binary morphologies: 1P/Halley, 19P/Borrelly, 103P/Hartley 2, and 67P/Churyumov-Gerasimenko. The other two – 81P/Wild 2 and 9P/Tempel 1 – are single bodies that exhibit surface features do not suggest they have been any more or less ‘eroded’ than their contact-binary counterparts…which begs the question: why haven’t they been shaped to bi-lobed structures?
We must also note that icy bodies formed in the outer Solar System, as opposed to the rocky planetesimals that formed in the inner Solar System. The significance of this is that relative velocities between icy planetesimals is much lower in the outskirts than they are for rocky planetesimals farther inward. It, thereby enhancing the probability of low-energy contact between objects farther out. Until the big guys formed (and to be sure, there were probably many growing ‘planetary-scale’ cores that were brought into the ultimate billiard slamming that eventually resulted in Neptune, Uranus, Saturn and Jupiter) the protoplanetary disk was a relatively peaceful environment that condoned nudges over the big slams later on.
The erosion notion? Its a fake theory that has been repeated in the internet echo chamber. I think the source comes from a few initially off-hand remarks by people who may or may not know better, but it gets reinforced by others who take the suggestion up and recall a vacation spot or photos they’ve seen in which erosion caused by wind, water and gravity vaguely resembles the shapes we are only now beginning to understand the actual nature of.
BTW, I should point out another interesting aspect – that the general shape of the smaller lobe follows along with another observation back when Rosetta flew within 800 km of the asteroid 2867 Steins, which was popularly described as “A Diamond in the Sky”, because of its distinctive shape. At that time I suggested that the morphology was very suggestive of the shapes that accrue from high-energy impact shock in terrestrial and lunar rocks, specimens that can range between micro/crystaline scale up to many meters, called ‘shock cones’. At 4×6 km in size, 2867 Steins seemed a shockingly, implausibley, giant example of such an outcome. Yet the shape of the smaller lobe on 67P/Churyumov-Gerasimenko [roughly about a kilometer across] exhibits a striking resemblance to that same distinctive shape [no puns intended]. This second potential example may be decent grounds for suspecting that (at least) pseudo-shock cones of such a size can be produced by on-center collision, and figures in the shaping of a significant proportion of the population of planetsimals.
This may also say something important about the previous history of at least that small member: it may have been involved with a much higher energy collision well before it bumped alongside the larger slab-shaped companion…OR it is possible that they are BOTH the fragmentary consequence of a violent impact that knocked a larger object apart, and these two main fragments came back togeher in much the configfuration we see now..
At any rate, finding two fairly explicit examples of a distinctive cone-like shape with very similar surface-profile angles in a sample of barely over a dozen is reason to keep it in mind. It may be that collisional shockwaves may sculpt similar shapes over a greater range of scale than we may be accustomed to expect from what may turn out to be small-scale examples of a heretofore unanticipated range. Its seems plausible that With solid objects of sufficient tensile strength, it is possible that catastrophic near-center collisions can not only have preserved the largest fragments, but have given them the rough but distinctive ‘diamond’ shapes. There may be a fair population of such “Diamond-shaped” asteroids and/or cometary bodies waiting to be discovered.
With regard to the apron of dust and debris the ‘neck’: as I alluded to in my earlier post, that would not be a good landing site for Philae for the reasons stated…but I would definitely SCREAM ADVOCACY for landing on a hard surface close to its boundary: the science from seeing the transition region together with the additional imagery provided by the visual return from that position being (in principle) able to see a dramatically ‘overhanging precipice’ of the opposite member holds the prospect and bonus of becoming the greatest on-site science as well as imagery of the century so far.
Alas, I am unable to determine the extent to which Philae’s camera system may provide wide-angle panorama shots like those we have become so accustomed to seeing from the Mars rovers…If anybody out there happens to know, please do chime in here.
Sorry about the long post…and any inevitable typos.
Adoph, you raise a number of very good points and I am looking forward to seeing with the mission team comes up with to explain the odd shape of this comet nucleus!