In the latest Curiosity rover status update, Pan Conrad, discusses the work done using the SAM instrument suite to characterize what the Martian atmosphere is made of.
One mystery for scientists studying Mars is the methane detected from Earth and by using the Mars Express orbiter. We think of the molecule methane (CH4) as a signature of life as life generates it. Cows burp tons of it on Earth. Methane also is quite unstable in any atmosphere and breaks down over time. So, any methane detected in the Martian atmosphere would have been added to it recently by some process.
Likely, the methane on Mars isn't from little green men (or green cows). It's probably from inorganic processes (around 10% of Earth's methane is generated this way) but that doesn't rule out that there could be something interesting going on. We can be sure that any changes over time in methane and other gasses will be closely monitored by the Curiosity team. So far, no significant quantities of methane have been found.
Curiosity will give scientists an opportunity to study the changes in methane and other aspects of the Martian atmosphere over the many seasons it will operate. The work done with the SAM (Sample Analysis at Mars) instrument will ultimately inform us about the atmospheric cycles going on at Mars.