Since Mars apparently still has a small amount of volcanic activity (a few small lava flows on it seem to be less than 10 million years old), it's even possible that subsurface regions geothermally warmed above freezing might still serve as an environment for live Martian microbes today. (Indeed, it's thought that all of Mars' rock below its 1 or 2-km thick permafrost layer is still warm enough to have liquid water in its pores.)

A poster by Jeff Plescia proposed the Cerberus Plains -- an area of lava flows only 140 million years old -- as one region that could serve as such a redoubt for subsurface life.

A new idea that turned up in a whole series of Conference posters is that volcanoes might not be the only Martian areas geothermally warmed enough for liquid water to exist -- big meteor craters might have been warm enough after their formation to maintain near-surface springs, or even lakes, for tens of thousands of years.

Another poster by Plescia suggested the Hellas Basin -- Mars' most gigantic impact crater, in the southern hemisphere -- as the biggest such region; it is flanked by ancient volcanoes and valley networks suggesting just how warm it made its surroundings.

C.S. Cockell pointed out that ancient impact craters on Earth have made the local igneous rocks porous enough to serve as a habitat for bacteria.

And Teresa Segura suggested that a single Mars impact that gouged a 50-km crater could raise the entire planet above freezing for decades.

Virtually all these impacts occurred during the "heavy bombardment" period in the early days of the Solar System, though; there have been no big ones for hundreds of millions of years.

But Hellas is interesting for another reason.

Robert Haberle pointed out in his talk that the common belief that Mars' air pressure is too low for liquid water to exist anywhere on its surface today is not quite true -- there are still a few lowlands where the air pressure is just high enough for it to exist briefly.

He identified seven such regions where the temperature is high enough, for at least a few weeks each year, for liquid water to exist -- but the air pressure is still so low that it evaporates after only a few minutes, which means that it would have to be replenished from some groundwater source.

Five of these lowlands are in the equatorial regions and have long since dried out, but two others -- the huge Hellas and Elysium craters -- are in the far south and may still have a substantial amount of ground ice mixed into their soil (originally poured into them by runoff from those valley networks created by the heat of the giant impacts that formed the basins). If their soil is actually moist, even for a few weeks a year, it might allow live microbes to thaw out and carry out their reproductive cycle before refreezing.

And Nathalie Cabrol's poster stated that there is geological evidence that at least a few big Martian craters were still filled by lakes of liquid water (probably covered by a layer of ice) as recently as a few hundred million years ago -- and that these did exist in the lowlands identified by Haberle.

(Haberle has said elsewhere that these lakes might even have been formed, not by water heated by some geological freak event, but by local warming caused by the slow increase and decrease in Mars' axial tilt over regular cycles of millions of years.)

There is another problem, though: one of the biggest geological puzzles on Mars today.