Nick,

Excellent points.

I've worried about focussing too closely on "life" as the organizing principle for our exploration, especially to the extent that policy makers get the idea that maybe sometime in the next few missions we'll find fossils, or at least fossil hot springs full of chemically interesting ooze. That leads to a notion of quickly trying to cherry-pick some magic spots where you think you can find those revolutionary rocks.

But, if you don't find what you're looking for quickly, that leads to a pretty frustrating situation. Gee, that didn't pan out. Maybe we should throw another dozen rovers at the next most likely spots, and hope they hit paydirt? Or once the idea sinks in that you're not likely to hit gold quickly, maybe the whole enterprise is halted from above.

In the long run, I'd rather we focus on understanding the global evolution of Mars. It's a really fascinating body that's intermediate between the mostly dead Moon, and larger, still-active, volatile-rich bodies like Earth and Venus. It uniquely preserves a surface that's transitioning between the early impact dominated era as seen on smaller bodies, and a surface dominated by volcanic/tectonic modification and fluvial/aeolian weathering.

Once we understand the bigger picture, maybe we'll have some better ideas on where to look, and where not to look, for anything that might have once been alive.

But for astrobiology generally it is important to understand how different sorts of planets evolve, including lifeless ones.

As for Mars, one thing I'd add to Nick's list are all of those light colored layered deposits. Whether they are sandstone (from wind-deposited dunes) or rhyolite tuff or mud from CO2 lakes or dinosaur bones I don't care, but they are hard to explain just from impacts and basaltic volcanism. Something interesting clearly happened, what was it?