This week marked the end of the Rosetta spacecraft, sent by the European Space Agency (ESA) to uncover what it could of a comet, specifically 67P/Churyumov-Gerasimenko. Rosetta’s purpose was to orbit the comet, send back information from the Philae lander, take images, and analyze the gases in the tail of the comet. Now it was time to die, and ESA crashed it into the comet. The reason: Rosetta’s activities were powered by electricity from solar panels, but now the comet was getting sufficiently far from the sun that solar power would not suffice much longer, so if there was anything left to do, now was the time to do it. As a consequence of this visit to the comet, it sent back a huge amount of information, allegedly enough that it would take decades to analyze it all.
So, what have we learned so far? First, this may not be an “average” comet, because apparently it originated from the region around Jupiter, as opposed to much further out. Second, we got some idea of how a comet gets its tail. One characteristic of the comet was that it is covered with pits. What appears to happen is that gases below the surface get heated by the sun, the pressure breaks the surface and the gases are ejected. Pits in the same region tend to be the same size, mainly because the size depends on the strength of the surface covering and about a million tonnes of matter come from each pit for this comet. In some ways, this suggests the volatile material is not uniformly distributed, but during comet formation, some sort of separation of volatiles from solids such as silicates went on.
One fact that I found interesting was that the emitted gases were mainly water, carbon monoxide and carbon dioxide. The comet apparently had very little nitrogen or other volatiles in it. To me, that is important, because in my ebook “Planetary Formation and Biogenesis” I pointed out that if my mechanism of the accretion of bodies was correct, in the Jupiter region there should be very little nitrogen, or ammonia, because it is too close to the star, and hence too warm, for them to accrete as such. That is one of the reasons why I assert there can be no life on Europa. In that context, in the very wispy atmosphere of Europa more sodium has been detected than nitrogen. There are very small amounts of nitrogenous material, such as isocyanates in the comet, though. On the other hand, I would not have expected carbon monoxide either, unless carbon dioxide could have been reduced subsequent to cometary formation.
There were also significant amounts of silicates, mainly as finely divided material. This is consistent with the concept that the original dust in the accretion disk contained such finely divided silicates, and in all probability, the dust acted as nuclei for ice condensation. Generally speaking, when something crystallizes out from another phase, it needs something in the other phase to get started. It is sometimes quite easy to make supersaturated solutions of something, and these solutions refuse to crystallize, and then when a suitable piece of dust or seed is added, it all simply crashes out of solution.
One of the other things I found to be of great interest was the shape of the nucleus of the comet, because it shows (as far as I am concerned) how accretion might have progressed. In my ebook, I proposed that what happened was that small particles would impact on the surface of a growing body, and one of two things would happen. The first was that nothing would happen, and gas would eventually abrade the surface and it would fly off (at least until the object became big enough that gravity would be strong enough to hold it.)
The second option was that at a certain temperature, an ice within the object would absorb the energy of impact, melt, then cool and re-freeze, thus melt welding the two bodies together. That seemingly may well have happened on a somewhat larger scale with this comet, as it has the appearance of two large bodies seemingly having collided and stuck together. (There are further smaller examples of seeming attached roundish objects, not visible in the given image.)
As an aside, that would be a mechanism by which volatiles might separate and concentrate in the small areas that would later generate the pits. For a collision to result in no subsequent separation, the collision had to be inelastic. To be inelastic, all the kinetic energy of the collision has to be absorbed by the objects as heat, and to keep the bodies together, rather than have them fly off again through centrifugal force as the body rotates, something has to hold them together. Ice melting and re-freezing looks a good option to me, but then I am biased. It is also interesting that there are no pits on the face facing the junction. Localised heat may have blown such gas away at the time of collision.
In my opinion, this was a great technical achievement, and ESA should be complimented. This was a truly complicated procedure to get the vehicle to orbit the comet, because the gravitational field of the comet is not exactly strong. Everything had to be done exactly right, and it was. We must now await further results.