They don't seem to be located near areas where Mars might still have a significant amount of underground geothermal activitity -- and they are mostly at high altitudes, rather than lowlands where the force of gravity on the higher-altitude underground liquid water table in nearby highlands might increase the water pressure under the lowlands to cause eruptions of pressurized water there ("Artesian wells").

And how does the liquid water even stay liquid long enough to get so near the surface in these frigid regions, rather than freezing thousands of meters underground?

The latter mystery may be solved by the fact that any groundwater on Mars is likely to be very salty. Mars' soil is thought to be extremely rich in salts -- some estimates say that as much as 30% of it may be magnesium sulfate ("Epsom salt"), and another large part may be calcium sulfate. And such brine solutions can stay liquid at far lower temperatures than plain water.

In an abstract published at this March's Lunar and Planetary Science Conference, Russian scientists R.O. Kuzmin and E.V.Zabalueva point out that a solution of magnesium sulfate melts at only -35 deg C, a calcium sulfate solution melts at only -55 deg, and a mixture of the two stays liquid down to only -63 deg.

This could mean that liquid groundwater on Mars may be much nearer the surface than most scientists have thought up to now.

The puzzle of what's forcing the briny water upward may be explained by another abstract at the same conference.

Jeffrey S. Kargel and Kenneth Tanaka pointed out that virtually all Mars specialists think that the Martian subsurface contains not only large amounts of liquid or frozen water, but also large amounts of carbon dioxide released from volcanic vents during Mars' early history.

Mars' buried subsurface groundwater may be highly carbonated salty seltzer -- and the lower parts of the layer of permafrost above it may often be a mixture of water ice with CO2 incorporated into it (a "clathrate").

But if this material is significantly heated, the CO2 will come violently out of solution and could build up a tremendous underground pressure -- a pressure great enough to force deeply buried subsurface briny water up to near the surface, or even out of it to form natural soda fountains!

What could thus heat it? There are two possibilities. First, it could be that various areas of Mars undergo intermittent periods of greater subsurface volcanic heating even today.

But another possibility has to do with the well-confirmed fact that Mars' axial tilt (its "obliquity") -- unlike Earth's -- slowly but very greatly increases and decreases over a cycle of about 124,000 years (due to the fact that Mars lacks the tides from a large Moon as a stabilizing mechanism).

During this cycle, its axial tilt (which is currently 25 degrees) increases and decreases over the range from 15 deg to 35 deg -- and more recent simulations suggest that over longer cycles of tens of millions of years, the tilt may wobble all the way from 0 deg to 60 deg (that is, there are periods when Mars, like Uranus and Pluto, is keeled over on its side).

Now, when this happens to a planet, very strange things happen to its climate.