Planetary fumigation

Where do you go on Mars to look for life ? Some pretty big decisions are looming for NASA's next Mars rover - the plutonium powered, SUV-scale beast known rather delicately as Curiosity. The choice of landing sites is currently down to four, and much depends on clues to the past history of these regions. Some show signs of a watery era, with the possibility of rich sedimentary rock - the kind of places where the detritus from an ancient martian biosphere would have ended up. Curiosity will be armed with an array of instruments, some capable of analyzing the detailed organic chemistry being trampled underfoot.

There has been considerable debate in the community about where to land, how to avoid chemical and even biological contamination of where you land, and exactly what instruments to carry - given the very real limitations to hauling every gram of gear to Mars. There's no doubt that Curiosity represents the next great stage in exploring another planet, and looking for the signs of extinct, or even extant, life. We'd like to get it as right as possible.

Thinking about all this raises something else, that gets less attention. There is a supposition that if Mars once had a watery surface environment - even if only fleeting - and some kind of biosphere, then there must surely be traces left. Obviously the geochemical alteration of the surface is likely to be noticeable, but why do we assume it is hard to erase the signs of life from a planet ? It may be an Earth-centric bias.
What would it take to erase life from the Earth, and perhaps also any sign that it was ever there ?

For everything on the surface then we can imagine burning, exploding, acidifying, and irradiating until we had just an amorphous, crispy, residue that would be entirely sterile and uninteresting. However, then there's the subsurface - kilometers deep rock, oceanic sediments and porous material, isolated subterranean water. All of this is chock full of slow-living microbial life. A solution would be to 'go Venus', melt the surface and keep up strong volcanism for billions of years - heat and pressure over time will transform just about anything. Time alone might do it. If you could wait another five billion years or so (at which point the Sun will have engulfed the Earth anyway) then perhaps enough of the molecular handprint of life will have fizzled away to be unrecognizable. In short, without a cataclysm then it's hard to imagine a way to remove all traces of past or present life on Earth.

But what about Mars ? If a planet never gets a fully entrenched, deeply embedded biosphere does it retain the same stain ? Suppose for a while, in the distant past, Mars did have a wetter and more obviously nurturing surface environment. Perhaps microbial life got going across a number of optimal regions, but was then cut off - as if life on Earth had stopped 3 billion years ago. It seems it would be much easier for time and chemistry, not to mention volcanism and asteroid impact, to lay down the white-out. It sounds pessimistic, but it's really more about tackling the fundamental problems of molecular paleontology - you need to find an understanding of what might be blocking your view in order to reconstruct what was once there...