Runoff Channels on Mars

In the highland plains of Mars in the equatorial region there are many small channels which look like the runoff channels which carry water from rainstorms. These channels are typically a few meters deep, tens of meters wide and some 10 to 20 km long. These runoff channels are part of the evidence that Mars had much surface water at a previous period of its history.

Some indication of the age of a feature can be obtained from counting craters. The crater density in the runoff areas is greater than the lunar maria but less than the lunar highlands. Lunar maria dates extend back to 3.9 billion years, so the implied age of the runoff channels is somewhat older than that.

The runoff channels are much smaller than the apparently younger outflow channels.

There are no runoff channels in the northern plains, which are dated at more than 3 billion years old, so it has been a long drought - no rain for over 3 billion years. The conditions which gave runoff channels required a much more dense atmosphere than the present one, so the rough chronology that one can get from crater counting tells us something about when the atmosphere was lost.

Outflow Channels on Mars

Distinctly different from the small runoff channels which are observed on the Martian surface are the large features called outflow channels. These features are 10 km or more in width and may be hundreds of kilometers long. They appear to be features which are carved by very large volumes of running water.

Judging that the outflow channels are much too large to have been caused by flooding from rainfall, other sources for the water were sought. One model involves large amounts of water frozen as permafrost in the soil. When a major source of local heating was produced, perhaps in the formation of the volcanic plains of Mars, there was melting and catastrophic flooding.

Evidence for Present Water on Mars

The present atmospheric pressure on Mars, about 5 mmHg, is too low for water to exist in liquid form. There is a large amount of water ice in the north polar cap. We can get a reasonable estimate of the amount of water there because the summer temperature warms up enough to evaporate off the dry ice seasonal cap, leaving only water ice. That 1000 km permanent cap with depth approximately 3 km contains some 1018kg of water, an amount which Fraknoi, et al. liken to the amount of water in the Mediterranean Sea.

The south polar cap always has frozen carbon dioxide on it, so we can't get a clear estimate of the amount of water ice there. There is some water in the atmosphere, as evidenced by the water ice clouds which form around mountains like Olympus Mons.

In 2002 additional data was collected by the Mars Odyssey mission which attempted to find evidence for sub-surface water on Mars. The instruments were the Gamma-Ray Spectrometer, the Neutron Spectrometer, and the High Energy Neutron Detector. Those instruments mapped the flux of gamma rays and secondary neutrons, which can serve as evidence for hydrogen in the top meter of the martian surface. The data support a water ice-rich layer about 30-60 cm deep in the region above 60° latitude, near both poles. The surface material of Mars, called regolith, in these regions may be 20 to 35% by weight of water ice. The amount of evidence for water diminishes near the equator where the regolith layer extends to about a meter in depth.

Photo credit: ESA/DLA/FU Berlin (G. Neukum)

The above photo from the European Space Agency's Mars Express mission was taken on Feb 2, 2005. It shows a sheet of residual water ice in the crater. The crater is in the far northern latitudes of Mars. Reference: http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_1.html

Permafrost on Mars?