For how long did the Martian waters run?

For how long did the Martian waters run?

Although the atmosphere of Mars is now too thin and cold to sustain liquid water on the surface, such features as branching channels, teardrop-shaped islands and braided slit deposits leave the widespread impression that the amtospheric pressure and temperature were great enough during the planet's early history for water to have flowed freely. However, Michael H. Carr of the U.S. Geological survey in Menlo Park, Calif., suggests that the same features could have formed even if running water was "only intermittently present" at the time.

If the atmosphere was thick enough for water to be stable at the surface, Carr notes in the June ICARUS, carbon dioxide--the atmosphere's main constituent--might have been locked up in the form of carbonate rocks. Some carbon dioxide could have been released by the impacts of meteorites massive enough to be driven well below the surface, where the heat and could free the carbon dioxide instead of making carbonate, he says. But such cratering could not have generated much carbon dioxide between the time of Mars' formation and the time when many researchers say the solar system's heavy bombardment by meteorites ended, less than a billion years later. Even with an additional supply released by volcanic activity, says Carr, the total carbon dioxide would still be "barely sufficient" to keep water "continuously present" at the surface.

This leaves the problem of explaining the water-worn valley networks and other features. Carr suggests that during the heavy-bombardment period, atmospheric temperatures were "just below those required for liquid water to be stable at the surface." From time to time, carbon dioxide released by volcanoes and impacts would cause the atmosphere to thicken and warm, allowing water to exist periodically as a liquid with only slight fluctuations, carbon dioxide could then be lost and temperatures again fall below freezing. Finally, says Carr, when the number of meteorites declined, "the equilibrium would no longer be maintained." The decline in meteorities and volcanism would recharge the supply less often, with an amount that "could not keep pace with even the low carbon dioxide fixation rates $(into carbonate rocks$) that apply when mean temperatures fall well below freezing."

And so the irregular cycles would have continued, sometimes allowing water to cut drainage channels, sometimes leaving nothing but the hard, dry wasteland. "The atmosphere, therefore, slowly thinned, global temperatures fell, ground-water could only rarely access the surface and the planet assumed its present frigid state," Carr suggests.