Sometimes, scientists announce things that are breathtakingly stupid. The Guardian, which generally has pretty good science coverage, has an article up reporting that some top scientists believe that the comet 67P may harbor lots and lots of life. The purported evidence for life is the presence of complex hydrocarbons on the comet's crust. Of course, this article is just based on a press release, and the data won't be available until it's presented later today at a meeting of the Royal Astronomical Society.

But The Guardian could at least have done some background reading on the person behind the claim, Chandra Wickramasinghe. It would have found that he has a long history of making claims about extraterrestrial life (and that he testified in favor of teaching creationism in US classrooms). Or, the reporter could have talked to someone who knows a little bit about surface chemistry—like me.

I am here to make a prediction: this claim will vanish, never to be heard from again. If skepticism were radioactive, a crowd of lead-suited firefighters would be sacrificing their lives to bury me in concrete as I typed this. At this point, you should be thinking, who the hell is this guy to say that an astronomer is wrong about something astronomical? Surely, Chris-the-physicist is out of his depth here?

Well, to be honest, I know very little about astronomy. The amount I know about comets would almost cover the bottom of a tiny thimble. (I know enough physics to say that the coverage would only be complete if we used a liquid with low surface tension, though.)

I do, however, know something about how carbon grows on surfaces.

My day job is surface chemistry, though I'm not really a chemist, and we don't do the sort of chemistry that your average chemist would think of as real chemistry. Typically, we work in vacuum conditions and study how ionizing radiation initiates chemical processes on surfaces. One thing you very quickly learn when you do these experiments is that, unless you have relatively strongly oxidizing conditions—and that means having either a lot of water or oxygen around—then you always get a layer of carbon. Sometimes we study this, but mostly it is just a damn nuisance.

This nuisance is pretty widely known. For instance, ask anyone who works with an electron microscope or synchrotron, and they will tell you that carbon growth is a problem. Indeed, the problem is so well known that there are now modifications to electron microscopes to use background carbon to grow structures deliberately. What is less well known—and, to be fair, not really well understood—is that these carbon layers have a huge amount of variation in them.

In an electron microscope, what you normally get is a carbon that is rather graphitic in nature: it looks like the soot you get in an engine's exhaust. But it doesn't have to be that way. If you change the exposure conditions, then you can get carbon that looks much more like a flexible, transparent polymer. And, with a different set of conditions, you can get a carbon layer that looks very much like diamond. In other words, given some time, I can produce any carbon in the carbon phase diagram using some light organic molecules, a bit of water, and some energy.

The other thing about these layers is that they are highly dynamic: the very processes that build them up will also tear them down. So, if you only think about them from a static point of view, it can be quite confusing to find a rather fragile carbon layer on a surface exposed to such a corrosive environment. However, I would be surprised to not find such a layer, and even less surprised to find that the layer properties changed vastly over time.

As it came in from the cold, 67P was in almost perfect condition for carbon growth: it has spent a lot of time sweeping through the Solar System, gathering organic molecules as it went (which it seems to have done)—its surface is so cold that most molecules will hit and stick. In other words, the comet has simply been gathering all the ingredients that it needs to start performing chemistry.

The comet 67P is approaching the Sun, so the surface temperature is starting to increase. As it does so, the surface mobility and local partial pressure of hydrocarbons in the almost non-existent atmosphere will increase. The intensity of ion bombardment and ionizing radiation will also grow as the comet hits its perihelion (its closest point to the Sun). That means all the stuff that I observe in the lab will now be happening at top speed on the comet surface.

With such a reactive mix, almost any carbon layer with any structure you care to think of can and will form. What's more, that structure won't be there for very long as different processes come to dominate over those happening now. Indeed, depending on the distance of closest approach and the maximum surface temperature, I would imagine that by the time the comet starts to disappear back out into the cold depths of the Solar System, most of the carbon will have been burnt off.

So let's not go shouting about life, when what we actually know is that we have some strange lumps of carbon.