I was reminded recently of an idea that seems to have simmered quietly in the background the past few years. If you take a walk outside you will see evidence everywhere that life is actively rearranging the surface of this planet - quite literally. Microbes, plants, and animals all act to change the topography of the world. This can range from the small scale - a clump of grass over time traps soil particles and builds a small lump in the ground - to the large scale, when the erosion processes of entire swathes of land are governed by surface plant life and subsurface sticky microbial shenanigans. A thought provoking and unusual paper appeared a few years ago by Dietrich and Perron in the journal Nature. Entitled 'The search for a topographic signature of life' it explored the ways in which life quite literally molds the outward appearance of the Earth.
What really wowed me when I first read this paper was the fact that mathematics can be applied to quantifying the effects of 'geomorphic transport' - basically descriptions of how material is eroded and moved around by various mechanisms, from water to wind. This lets one evaluate things like the frequency with which certain geographical features arise, and their physical scales, depending on whether there is life or not. One of the conclusions of this work was that in order to apply these tools of analysis to Mars we really need global topographical maps to better than a few meters resolution. Obviously with Mars Reconnaissance Orbiter - in orbit since 2006 - the data is getting pretty close to this requirement.
So, can we search for signs of ancient surface life on Mars in the undulations of the terrain, or the way water-cut channels meander across the landscape ? It's going to be tough, and needs something else to encourage us. The extra possibility would be that if surface life was once reasonably abundant on the martian surface then it could have left a distinct chemical footprint. Imagine if a rich lagoon of martian water existed for thousands, or even tens of thousands of years, full of microbial life. Layer upon layer of biologically dirtied sediment would be deposited at the bottom. The chemistry of this muck would surely differ from its surroundings. Fast forward a few million years, the dried up remains of the lagoon might be visible, and there could be subtle topographic variations in its shape due to that earlier life. Although beaten and stirred by radiation, and a later harsh surface environment, the biological detritus could conceivably also present an unusual chemical or spectroscopic signature. In combination these two factors might be interesting enough to warrant further inspection....I just wonder what the magic statistic is ?