Liquid water on the Martian surface was probably never around for very long, meaning life would have struggled to gain a foothold there.
A new NASA interpretation of mineral-mapping data from more than 350 sites suggests that it was generally found underground, appearing for only short periods on the surface.
The new findings have implications for scientists studying whether life existed on Mars. They support the hypothesis that the many features of water erosion were carved during the brief periods when liquid water was stable at the surface.
"If surface habitats were short-term, that doesn't mean we should be glum about prospects for life on Mars, but it says something about what type of environment we might want to look in," says Bethany Ehlmann, assistant professor at the California Institute of Technology.
"The most stable Mars habitats over long durations appear to have been in the subsurface. On Earth, underground geothermal environments have active ecosystems."
Clays are formed by the interaction of water with rock, with different types of clay minerals deriving from different types of wet conditions.
Over the past five years, researchers have used OMEGA and NASA's Compact Reconnaissance Imaging Spectrometer, or CRISM, instrument on the Mars Reconnaissance Orbiter to identify clay minerals at thousands of locations on Mars. And the types of clays found indicate that there were only small amounts of water on the surface.
Another clue is detection of a mineral called prehnite, which forms at temperatures above about 400 degrees Fahrenheit - typical of underground hydrothermal environments, rather than surface waters.
"Our interpretation is a shift from thinking that the warm, wet environment was mostly at the surface to thinking it was mostly in the subsurface, with limited exceptions," says Scott Murchie, principal investigator for CRISM.
One of the exceptions may be Gale Crater, the site targeted by NASA's Mars Science Laboratory mission. Launching this year, the Curiosity rover will land and investigate layers that contain clay and sulfate minerals.
NASA's Mars Atmosphere and Volatile Evolution Mission, or MAVEN, set to launch in 2013, may reveal more information, says NASA.