One of the early results I think we're expecting from Curiosity is an analysis of trace gases in the Martian atmosphere -- including the level of methane. There was a time when I, and this blog, were obssessed with the news of methane on Mars and what it meant for the likelihood of a cryptic microbial biosphere in the Martian subsurface (full archive). Indeed it is possible, I can't say for sure, that I may have been the origin of the martian-methane-expressed-as-a-number-of-cows meme found recently in an excellent piece by Dick Kerr (summary|paywalled full text). In the excitement, and newly enamoured with the fun of blogging, I was quite the believer.
More recently I have become more sceptical, very largely as a result of this article (pdf) by my friends Kevin Zahnle and David Catling and their colleague Richard Freedman. My take on the article and its arguments, by which i was and remain broadly convinced, appeared a couple of years ago in The Economist. I won't recap it here, but if you're interested it's worth a look. And I liked the conclusion:
The debate carries a worthwhile scientific lesson in itself. Observations, which to an outsider might sound like simple things, are often remarkably difficult, and depend on complex models to make any sense at all. Thomas Huxley, Darwin's ally in the fight to get evolution accepted, spoke warmly of the facility with which ugly facts can kill beautiful theories. But that fatal ability should not hide the fact that well-applied theories, beautiful and otherwise, can play a crucial role in deciding which observations get treated as facts in the first place.
Pretty soon after landing, insha'Allah, we should know whether the level is in parts per billion or parts per trillion. It may, indeed, be one of the mission's crucial results (unless something satisfies the "Knoll criterion"...)