Understanding when and where liquid water existed on Mars is crucial to determining whether the Red Planet ever housed living organisms. A new study in Nature suggests that there was liquid water under the surface of Mars three to four billion years ago.
Satellites and Mars rovers have observed valleys and deltas on the surface of Mars. They have found clay, a combination of rock and liquid water, in the older parts of Mars' surface. These features indicate that Mars was at one time warmer and wetter and housed flowing liquid water. The timescale for these conditions are estimated by counting impact craters — the older a geological feature, the more craters it will have.
The current atmospheric conditions on Mars do not allow water to reach its liquid phase on the surface of the planet. Instead it forms ice and sublimates to water vapor. Some models suggest that early in its history, Mars had an atmosphere with higher temperatures and pressures that allowed liquid water to form clays. But these models are problematic because it is hard to explain where the warm conditions came from, and what happened to the proposed atmosphere.
This study set out to examine the clays and determine the conditions under which they were formed and to shed light on whether there was surface or subsurface liquid water in Mars' history.
The characteristics of the clay are determined by environmental conditions like temperature, pressure, water pH and the amount of water present. Another factor is whether the clay is being formed in an ‘open' or ‘closed' system. An ‘open' system interacts with other reservoirs like the atmosphere, whereas a closed system does not. These conditions produce distinctive types, or assemblages, of clay.
The scientists examined the assemblages of clays that they could see. They knew what the clay would look like and contain if it were formed at the surface of the planet or underground, respectively. They used data from the Mars Express OMEGA spectrometer and the Mars Reconnaissance Orbiter CRISM spectrometer. These visible/near-infared cameras are on satellites orbiting Mars and have accumulated thousands of images of clay on Mars' surface.
The data indicates that there was a period of a about a billion years between Mars' Noachian and Hesperian periods, which occured three to four billion years ago, in which clays formed under anoxic (no oxygen), high pH and high temperature conditions. These conditions characterize the subsurface of Mars rather than an open, atmospheric reaction environment.
They also found clays in the deepest surfaces, usually completely covered by lava but exposed by craters that broke through the crust. These clays were buried too deep to have been formed by liquid water on the surface.
All of this evidence suggests that liquid water was not held on the surface by the atmosphere but instead flowed underground during the Noachian and Hesperian periods.
Another Mars rover, called Curiosity, is scheduled for launch in late 2011. One of its missions is to analyze clays in deep craters. The authors expect to use data from this mission to further test their hypothesis about the history of water on Mars.
