by Bruce Moomaw
Cameron Park - June 27, 2000 - When the eagerly-awaited press conference was held Thursday on the new revelations by Mars Global Surveyor Surveyor, it quickly became clear that -- instead of discovering clear and unambiguous evidence of present-day near-surface Martian groundwater-- MGS had discovered something much more ambiguous and mysterious.
The new photos certainly show eruptions of something from strata a few hundred meters below the Martian surface in some areas -- something that has flowed down slopes in a way that looks very much like flows of released groundwater on Earth -- but they are being seen in precisely the least likely areas: the coldest slopes on Mars rather than the warmest.
All the observed eruptions occur at latitudes higher than 30 degrees (except for those on the walls of the Nirgal Valley, which occur at 27 degrees South). Twenty five percent occur on the walls of pits in Mars' south polar regions. While overall some ninety of them occur in Mars' southern hemisphere -- covered with high-altitude highlands -- rather than the lower-altitude plains of the northern hemisphere. And fifty percent occur on slopes facing poleward, as compared to only twenty percent on slopes facing toward the equator.
As discoverers Michael Malin and Ken Edgett noted in their "Science" article, "These features spend as much as half a Martian year at or near the freezing point of carbon dioxide [at Martian pressures, -125 deg C]."
Yet they do look exactly like the arroyos carved on earth by running water released from springs. The scientists at the press conference can be excused for expressing puzzlement as to just what's going on here.
At the moment there are several competing theories. The one favored by discoverers Michael Malin and Ken Edgett themselves is that what we're seeing might be called "miniature violent liquid-water outbursts".
In this model, liquid groundwater -- which, most scientists already believed, exists in Mars' warmer subsurface layers two or more kilometers down -- is forced upward to near Mars' surface by some process and trickles through the cracks and pores of a layer of bedrock until it reaches a slope where the rock layer is exposed to Mars' surface.
The first water to reach this area instantly boils ("sublimates") into vapor in the low pressure of Mars' near-vacuum atmosphere -- but as it does so, it drops sharply in temperature.
And on colder slopes, this lowers its temperature enough that much of the liquid water immediately behind it freezes solid, forming a plug of ice that seals off the pores leading to the outside. (The backpacks of the Apollo moonwalkers used exactly the same setup in their cooling systems, with a tank of water separated from the lunar vacuum only by a porous plate whose pores were plugged by ice.)
Thus, while on warmer slopes there is a continuous slow stream of released water vapor as more liquid water is transported up from underground, on the colder slopes the pressure behind the plug builds up until the plug finally bursts explosively -- releasing an explosive spurt of liquid water, mud and rock fragments that pours for hundreds of meters down the slope before the liquid water has time to boil completely into vapor.
Then the temperature drop from this explosive depressurization causes a new ice plug to form in the vent area, and the process begins all over again.
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