If Michael Carr and James Head are right, the northern lowlands during the Noachian may have been filled with a great sheet of ice made from water that gradually drained into it from the occasional, sparse outflowings of the southern valley networks -- and that northern ice sheet might well have been kept warm enough at its bottom by Mars' internal warmth to allow a thin but long-lasting layer of liquid water to exist there.

However, it's also possible that the northern rocks are andesite -- dry since it was erupted onto the surface as silica-rich lava -- and that even Mars' northern lowlands were also frozen and devoid of liquid water during the Noachian. But in that case, how do we explain the fact that Mars' very large supply of fine, reddish dusty soil unquestionably HAS undergone a good deal of water weathering?

Mars' soil is known, from the element measurements by the Viking and Pathfinder landers, to be very rich in sulfur, probably in the form of sulfate salts -- which implies that those salts were dissolved out of some of Mars' rocks by exposure to liquid water and then recrystallized. These salts seem to have cemented a lot of Mars' surface dust together into brittle sheets of "duricrust", which -- judging from infrared mapping from orbit -- cover a large part of its surface.

(In another poster at the DPS meeting, G.A. Marzo of the University of Lecce announced tentative evidence from Mars Global Surveyor's IR spectra that there may be a significant amount of gypsum -- calcium sulfate -- on the floor of Gusev crater, the landing site for the first of NASA's two current Mars rovers. Indeed, this may be concentrated enough that it is actually a remnant of Mars' Noachian days, when Gusev -- which has a very large Noachian drainage valley leading into it -- is thought likely to have been one of the ice-covered crater lakes of the time. This is precisely why the rover is headed there.)

We also know -- from both Mars Global Surveyor's long-wavelength IR spectra and the shorter-wavelength near-IR spectra obtained by Earth-based telescopes -- that Mars' dusty soil also contains some mineral which has been chemically hydrated by exposure to liquid water, although we're still not sure what that mineral is. (It may just be those sulfate salts, in a hydrated form. It may be clay minerals, although infrared spectra indicate that Mars has at best a small amount of clays. Or it could be "zeolites" -- claylike minerals made when the volcanic glass in solidified lava and volcanic ash is water-weathered.)

And we now know with reasonable certainty from IR spectra that Mars' powdery soil contains a few percent of carbonate minerals that must have been formed by liquid water. But those carbonates seem to be totally absent in the planet's solid surface rocks, which at least makes it certain that no part of its Noachian surface was exposed to liquid water that wasn't near-freezing, and MAY mean that it was never exposed to liquid water at all.

And post-Noachian Mars has had such a low air pressure that it's almost impossible for liquid water to exist on its surface, even briefly -- instead of melting when the temperature gets warm enough, solid ice instead sublimates directly into vapor. (In fact, at Mars' low air pressures, ice boils directly into vapor at temperatures considerably below the melting point of water.) So, during the rocking-horse movement of water back and forth across Mars' post-Noachian surface during its obliquity cycles, one would think that the water has always been in the form of either ice or vapor.

So does the existence of a lot of water-weathered material in Mars' soil prove that Carr and Head must be right, and that -- at least in Mars' northern lowlands -- a large part of its rocks WERE weathered by liquid water (albeit very cold liquid water) during the Noachian? No. There's an possible alternative explanation.

When water is trapped in the tiny spaces between fine soil particles, it can often stay liquid at temperatures far below its normal freezing point. Films of liquid water a few thousandths of a millimeter thick can stay liquid at soil temperatures down to -20 deg C (- 4 deg F) -- and even thinner films of water, only a few molecules thick, can stay liquid all the way down to -80 deg C (-112 deg F)!

Moreover, instead of saying that post-Noachian Mars' air pressure is too low for liquid water to exist without instantly boiling into vapor, it's more accurate to say that it's hovering at the very brink of that point (the so-called "triple point" of water). There are lowlands on Mars right now where, if the surface was warm enough, liquid water could exist for a few minutes without evaporating, and there may have been phases during Mars' obliquity cycles when its air pressure -- though still extremely low -- was several times greater than it is now. Moreover, water mixed with a large amount of those sulfate salts has both a lower melting point and more resistance to evaporating into low-pressure air.

So it seems likely that, during its obliquity cycles, there have frequently been times when Mars' soil did at least have very small amounts of liquid water moistening its particles -- enough, during the soil's repeated brief exposures to such traces of water over billions of years, for Mars' soil particles to be weathered in a way that its rocks have not been weathered. This is a lot more likely to happen in Mars' polar regions -- they get warmer in summertime during the planet's high-obliquity periods than its low-latitude regions ever get at any time -- but soil that was thus water-weathered near the poles may since have been blown all over Mars by the planet's winds.

(If Aaron Zent is right and the near-surface ice layer at Mars' high latitudes was drawn up from below as an "ice lens" rather than being deposited from the air as an ancient snowfall layer, it's also these small traces of liquid water between the particles in the slightly warmer soil below the cold surface that would make such upward wicking possible.)

Studies of the soil in Antarctica's "dry valleys" -- probably the closest Earthly environment to that on Mars -- show that, even in soil that's been far below freezing for millennia and deprived of any visible trace of liquid water, substantial weathering can occur through this process. Microscopic films of water clinging to soil particles even in that cold weather tend to rust iron minerals into oxides, turn minerals into clays and zeolites, and extract dissolved salts (which then tend to concentrate in a layer at the soil's top). To some extent, simple exposure of minerals on Earth and Mars to plain water vapor can have this effect -- but exposure to such periodic tiny traces of liquid water over the eons further encourages it.

So Mars' rocks, instead of being water-weathered into powdery soil, could have been originally pulverized into soil by purely mechanical processes:

(1) The rain of meteoroids down onto the planet over the eons (both the thundering giant impacts that kept crashing down onto it during the Noachian era, and the steady rain of smaller impacts during the 3.8 billion years after the Noachian ended and most of Mars' meteor-blocking atmosphere disappeared);

(2) Wind erosion during the Noachian, when Mars had a dense atmosphere (and something was unquestionably eroding its surface features at a far higher rate than they have been since);

(3) The periodic violent torrents of rain that must have pelted down onto Noachian Mars' surface for years to centuries after any giant impact -- which could have greatly eroded its surface mechanically while still lisaving the rocks exposed to a total of only a few thousand years of liquid water, not enough to weather them chemically;

(4) A lot of the soil could have been belched out of Mars' volcanoes as ash in the first place;

...and then those endlessly repeated short exposures of the soil grain's surfaces to tiny films of liquid water, throughout all the following 3.8 billion years of Mars' existence, could have weathered a lot of the mechanically ground-up soil chemically as well.