But if it is water, then -- even though the eruptions seem to take place only in small, limited areas, and may take place only every few tens of thousands of years -- the implications are enormous, both for the possibility that Martian microbes may still be alive under the planet's surface, and for the ease with which we may be able to discover them.

The assumption up to now has been that Mars probably does have a large supply of liquid groundwater left, but that it's in the pores of the rock layers kilometers below the surface, where the remaining geothermal heat coming out of the planet's interior can still keep it liquid.

Above that level, the belief was that it is entirely in the form of a very thick layer of permafrost (a "cryosphere") -- and that Earth investigators would have to drill down that deep to reach it.

However, the new discoveries suggest that a substantial supply of liquid water may (at least sometimes and in some places) lurk only a few hundred, or even a few dozen, meters below Mars' surface -- perhaps because it's mixed with the plentiful salts in Mars' soil which could possibly reduce its melting point to as low as minus 60 deg C.

Such salty brines certainly wouldn't be a complete obstacle to the existence of live Martian germs -- there are many species of "halophilic" bacteria on Earth that get along just fine in such saline environments -- and even if Mars' near-surface liquid layers are frozen for hundreds of thousands of years at a time, there are many species of Earth bacteria that can survive such long periods frozen in simple suspended animation or in spore form and then come back to life and reproduce during the brief periods when their environment thaws out and turns friendly again.

And, of course, it would be tremendously easier for Earth expeditions to drill down these relatively shallow distances to look for them. In fact, drilling down a few dozen meters will very likely be practical even for unmanned probes -- whereas drilling down kilometers is such a massive and complex project that it would very likely require a manned team.

So how do we start investigating these spots? One clear conclusion is that immediately dispatching landers to the "gully" locations located by MGS is probably not the best way to approach the problem.

In the first place, we can't currently land spacecraft on Mars with enough accuracy to bring them down anywhere near the runoff sites -- our current landing accuracy is only about 30 km.

One important goal for the next U.S. Mars lander (whenever it flies) will be to reduce this error to at most 10 km, and hopefully just a few kilometers. But even then, a fairly long-range unmanned rover will clearly be necessary to reach the runoff sites.

And such a rover, even when it got there, would have considerable difficulty investigating these runoff sites in any detail -- the slopes on which they occur are usually kilometers long, and as steep as 30 degrees, making it very hard to drive down them even in Mars' light gravity.