Methane
Methane has been detected in three areas on Mars; one of which is in the Elysium quadrangle. This is exciting because one possible source of methane is from the metabolism of living bacteria. However, a recent study indicates that to match the observations of methane, there must be something that quickly destroys the gas, otherwise it would be spread all through the atmosphere instead of being concentrated in just a few locations. There may be something in the soil that oxidizes the gas before it has a chance to spread. If this is so, that same chemical would destroy organic compounds, thus life would be very difficult on Mars.
Read more about this topic: Elysium Quadrangle