rfmcdonald | [LINK] "Now you don't" (Reply)

The Economist's article examining the question of life on Mars--is the methane detected in the Martian atmosphere product of life, or of non-living processes--made me happy. I like coming up with reasons for going to our sister planet.

One of the things that everyone agrees about methane on Mars is that it has to be short-lived. Though Mars has relatively little oxygen in its atmosphere (just 0.13% of the total) that atmosphere nonetheless provides what chemists call oxidising power, which gives it the ability to pull methane molecules to pieces. The predicted lifetime of Martian methane, based on observations of the Earth’s atmosphere and experiments in laboratories, is just 300 years. This is one of the reasons why methane was seen as an exciting discovery—its constant oxidation in the atmosphere means it would have to be replenished by some occult process. Even if there was no life involved that would, at least, require that some novel chemistry was going on.

The problem is that observations of methane on Mars imply that its lifetime must be far shorter than a few hundred years. They indicate that the gas comes and goes on a seasonal basis, with much more of it around at some times and places than others. If methane waxes and wanes with the seasons, then its lifetime must be on a par with the length of a season—several months, not several centuries.

Various suggestions as to how this might be possible have been made. There could be special catalysts in the soil, for example, perhaps created by the static electricity whipped up by dust devils. Dr Zahnle and his colleagues argue that such ideas merely pass the buck. Whatever oxidises the methane would itself get used up in the process, and so would also need to be replenished. To oxidise the methane at a high rate the planet would need to make new oxidising chemicals at that same high rate. There is no evidence of its doing so.

The known way in which oxidising power gets added to the Martian atmosphere is through the destruction of water molecules by ultraviolet light. This creates hydrogen and oxygen. Left to themselves they would recombine, producing no net change, but some of the hydrogen leaks out of the atmosphere into space, leaving oxygen—the paradigmatical oxidiser—behind. This process works too slowly, though, to account for the sudden drops seen in the methane level. Explaining these requires new ways of producing oxidising power which do nothing to alter the balance of other chemicals in the atmosphere. That seems a tall order.

There are other things which might be happening to the methane. It could be stored on the surfaces of minerals, or locked into exotic ices, or even eaten by yet more bugs. But all these, too, are inconsistent with the big picture. Mars has xenon in its atmosphere, and xenon atoms are similar enough to methane molecules that a physical process which locked up methane would lock up xenon too. Yet the xenon persists, airily unfettered. As to methane-eating bugs, a look at the planet as a whole again seems to rule that possibility out. Carbon monoxide offers a lot more energy per molecule to hungry microbes than methane does—but Mars’s carbon monoxide level is stable and much higher than the claimed methane level. Martian life might be different from terrestrial life in many ways, but it is hard to conceive of any form of life that would spurn a rich and abundant energy supply in favour of a scarce and less fulfilling one. Darwin would certainly not have approved of such pickiness.