methane and organics
It isn't quite a smoking gun, but... there's too much methane on Mars. Several recent papers have addressed the Mars Express results, looking at necessary conditions for both biogenic and abiogenic generation mechanisms. Problem is, we have searched and haven't found any of the abiogenic sources (hydrothermal vents, current volcanics, etc. No one quite wants to claim that this favors the biogenic explanation... too controversial.
And since radiation destroys even chondrites on the surface in a few million years... one has to go down at least a meter under the Martian surface to find any possible surviving organics in the soil. This gives a boost to the need for drilling in future missions... subsurface access in multiple locations was what the surface science working group here listed as its highest priority today.
And since radiation destroys even chondrites on the surface in a few million years... one has to go down at least a meter under the Martian surface to find any possible surviving organics in the soil. This gives a boost to the need for drilling in future missions... subsurface access in multiple locations was what the surface science working group here listed as its highest priority today.