Today's cold, dry, and likely lifeless martian surface extends back in martian history past the time when life was taking hold on Earth, according to a new study. But researchers have also found that liquid and likely warm water persisted kilometers below the surface at the same critical time for life. Not exactly Darwin's "warm little pond" for the beginnings of biology, but it might well have served.
The key to Mars's water history is clay. The familiar mineral forms only when liquid water has plenty of time to change the chemical structure of rock into that of clay. And the particular sort of clay that forms depends on the type of rock as well as the prevailing temperature and chemical conditions. So planetary scientist Bethany Ehlmann, now at the California Institute of Technology in Pasadena, and her colleagues considered the martian clays identified by their spectral colors as observed by NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express.
In their paper published online today in Nature, Ehlmann and her colleagues report finding two sorts of clay formations going on at different times on early Mars. Clays indicative of warm, even hot, water persisting over millions of years show up where deep martian crust is exposed today, especially where large impacts have blasted into the crust. These clays formed about 3.7 billion years ago and earlier, when the earliest hints of life on Earth first appear in the geologic record. Other clays, as well as salts such as sulfates that suggest formation on or near the surface under intermittently wet conditions, are found in sediments washed into ancient lake basins, especially around 3.7 billion years and later.
The new findings probably deliver the final blow to the possibility that the surface of early Mars was a "warm and wet" incubator for any martian life. They are consistent with a history in which Mars has been almost always rather cold and dry, Ehlmann says. The later weathering of rock into salts seems to have occurred during geologically brief intervals when water flowed on the surface, she says. "The most stable, [habitable] environments may have been underground."
The new interpretation of the spectroscopic observations tends to confirm what planetary geologists have been suggesting, says planetary geochemist Scott McLennan of Stony Brook University in New York. They have been arguing for intermittent water flows around 3.7 billion years ago because the river channels, deltas, and sedimentary fans typical of that period look as if they formed within millennia, not over millions of years.
The inference that martian clays are pointing to persistent liquid water deep beneath the ancient surface is more tentative, McLennan says. "A clay cycle on Earth is really complex," he says. The new study, by contrast, "is a very simple view. You can't get too elaborate when the data is so immature." The clays that the group sees in crustal rock, for example, may have formed elsewhere and later been deposited in the crust.
Planetary scientist Ralph Milliken of the University of Notre Dame in Indiana agrees. "It's certainly not the end of the story," he says. "It puts forth a testable hypothesis." And it could be seriously tested quite soon. NASA's Curiosity rover (formerly Mars Science Laboratory) is scheduled to launch toward Mars as early as the 25th of this month, destination Gale crater.