The northern residual cap is made of naked water ice. But the southern one is covered with another layer of frozen CO2 that never does vaporize away, even in midsummer -- despite the fact that, thanks to Mars' lopsided orbit and the direction its spin axis is tilted at the moment, the south pole currently has much warmer summers than the north pole. And this extra layer conceals the underlying southern water-ice cap from view by orbiting space probes.

In fact, it's only recently that we got proof beyond doubt that there really is another water-ice cap at the south pole, concealed under that CO2 "dry-ice" layer. (Odyssey's THEMIS played a major part in that proof, photographing deposits of ice exposed around the edge of the southern residual cap in summer which its infrared camera showed are much too warm to be frozen CO2 -- they must be frozen water.)

Well, Jakosky's computer climate simulations show that if something happened to strip away the residual south polar cap's overlying layer of CO2 ice, so that exposed water ice could vaporize into Mars' air during the southern summer as well as during the northern one, near-surface water ice COULD exist even near the equator right now. This is because this exposure of southern-icecap water ice would raise the air's humidity level to the point that water ice elsewhere on the planet would find it harder to evaporate -- meaning that water ice even at the equator, if it was shallowly buried and thus shielded from the peak noontime heat, wouldn't evaporate at all.

By his calculations, if pure ice at the equator was buried under only 10 cm of dry dirt, this would protect it enough to last for a thousand years -- and if it was buried a bit deeper, it would last far longer.

And we do now have strong evidence that the residual south polar cap's veneer of frozen CO2 may indeed not be nearly as durable as had been thought. Mars Global Surveyor's super-detailed photos of the southern residual cap have shown much of it to be covered with "Swiss cheese ice" -- circular holes hundreds of meters across in which the two to eight-meter thick layer of CO2 ice has vaporized away, revealing the flat surface of the water-ice cap underneath.

And they're growing -- fast -- even as MGS watches: one to three more meters of frozen CO2 vaporize away from their edges every Martian year. If you assume that they always grow at that rate, the holes didn't exist at all just a century or two ago -- and the entire permanent southern CO2 ice cap will vanish completely in just a few more centuries!

They may be growing unusually fast right now, because the only thing that's kept the southern CO2 ice layer cold enough to survive the southern hemisphere's warmer summers is the fact that it's much lighter in color than Mars' polar water-ice caps (maybe because any dark dust particle that lands on it gets warm enough from absorbed sunlight that it immediately vaporizes the CO2 ice directly under it and quickly sinks out of sight). But this may not be able to preserve it forever; the cap may be shrinking fast right now because it's been darkened by dust from the great global storm that completely blanketed the planet back in 1971.

And if the so-called "permanent" southern CO2 ice layer actually is very unstable and easily destroyed by such dust storms, and so often covers little or none of the southern water icecap before growing back, this could mean that a lot of the time Mars has a lot more humidity in its thin air than it has right now -- perhaps enough to preserve, for millions of years, areas of equatorial near-surface permafrost that happen to be buried beneath a particularly thick layer of windblown dust or a surface crust of sulfate salts.

Anyway, it's now virtually certain that --while Mars' still-existing liquid water in the pores of its rocks kilometers underground, kept melted by its remaining internal heat, has been sealed off from the planet's surface for billions of years by the kilometers-thick layer of permafrost above -- obliquity-cycle related changes in local surface temperatures keep perpetually shuffling a thin veneer of water ice around from place to place at the planet's very surface, by vaporizing it in one place to refreeze in another.