The Martian atmosphere, thin and inferior though it is, does contain a few gems. One of these is a very special trace gas, one often associated with life.
It has been known for nigh on a decade that the Martian atmosphere contains methane. Now, methane is an important gas to look for. Usually, it is produced by volcanic activity, or biological processes. The volcanoes on Mars are thought to have gone cold long ago. Therefore, if the methane had been a byproduct of vulcanism, it would mean Mars still had a molten core, which would be very exciting to geologists.
Obviously, if the methane were produced by life, that would be even more astounding.
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Unfortunately, it seems the methane was produced by another, less common process. Under the harsh Martian sun (sunlight on Mars is very weak, but with no ozone, ultraviolet radiation easily reaches the surface. This actually makes sunlight on Mars more damaging than on Earth), micrometeorites are broken down by UV radiation. Some of these meteorites contain carbon molecules, which are synthesized into methane by sunlight.
This does not mean there are no microbes hiding in the crevices of some canyon, but it does make them less likely. We won't know for certain until we find some. Now, if there was free oxygen on Mars as well as methane, the case for life would be overwhelmingly strong. Unfortunately, the only oxygen on Mars is tightly bound to the rocks and thin soil.