The big news on the science front this week was the landing of Philae on a comet, and I found it interesting to note how the news media reported it. One of the most fascinating things for me was the announcement of the comet’s velocity. Yes, it is travelling very fast, but what is important when it came to landing on the comet is not how fast the comet is going, because that is meaningless. The important thing to consider is the relative velocity of Philae and the comet, and that was rather small.
There is no absolute velocity; velocity only has meaning when you consider it in the frame of reference of something else, in which case it is called the relative velocity. It is from this sort of thinking that we get relativity, the first version of which came from Galileo when he noted that if you are inside a ship you cannot tell how fast you are going – and even outside the issue is questionable. Outside, you can see how fast you are travelling relative to the water, but the water may be travelling relative to the land. In the case of trying to land a vehicle on a comet, the trick was to match the mother ship’s velocity as near as possible to that of the comet, and then let Philae approach and land at a relatively small change of velocity. In fact the hardest part of this was not to get Philae safely down, but rather to keep it down. The danger was always that it would bounce off, as the comet has only an extremely small gravitational force. In fact it appears that Philae did do some bouncing, and unfortunately it landed in a shady spot from where it cannot easily recharge its batteries through its solar cells. That means the information we get back will be dependent on what it could do for the day or so the original charge in the batteries lasted.
So, what will Philae tell us, assuming all goes well with its experimental equipment. We probably will not get an analysis of gas coming off the comet, because gas emissions probably do not get underway sufficiently well until the comet gets closer to the sun. However, we should get a good account of the composition of the very top surface of the comet. Unfortunately, that may not be very informative because the interesting more volatile material has probably been given off on previous encounters with the star, and any organic material there will have been subjected to considerable UV radiation. Comets spend their lives in very cold places, and the colder the site, the slower chemical reactions are (although this does not apply to photochemistry, because the energy to get these started comes from the light). The problem is, the comets would have formed when the solar system formed, which was 4.5 billion years ago, and while such reactions might be slow, 4.5 billion years remains a long time.
I have seen some reports saying that it will show that comets brought water to Earth. It will show nothing of the sort. At best, it might show comets are a possible source, but the evidence that we have so far is that cometary water has too much deuterium in it, so that is not where our planet’s water came from. It might have organic molecules in it, such as amino acids. It might, but that does not mean that is where the chemicals that permitted life to form came from. These issues are very complicated, and for those interested, I outlined the issues in my ebook Planetary Formation and Biogenesis.
Finally, what I would like to see is evidence of neon secreted into the ices. My mechanism for how planets accreted involves the melt fusion of bodies with a particular ice encased in water ice acting as the fusing agent. The mechanism is a little like that of forming snowballs, and the outer giants in this solar system are predicted to total four, based on four different temperature ranges of known ices. However, there is a possible fifth: neon. Was neon encased in ice? For neon to act as such an agent, the ice probably had to be below fifteen degrees above absolute zero as it started to approach the star. Would it? If it did, the comet might contain neon, but whether we can detect it is another matter because any such neon would have been lost from the surface of the comet in previous stellar encounters. Degassing from the centre might show it, but Philae probably won’t have the power to tell. For me it is important because if neon was encased in the ices that were in our accretion disk, then there is a possibility of another planet out there, probably in the order of 3-4 times further from the sun than Neptune. It would be a lot smaller than Neptune (because growth there is slower because everything is more dilute) so it would not be easy to find, but it would be nice to know if there were a reason to look for it.