Martian soil data collected by five robotic missions indicates that rain fell on the Red Planet billions of years ago. The findings provide no new insight into the possibility of martian life, but they do suggest that further clues to Mars's past could be found right here on Earth.
There's little doubt now that Mars once was wet. The twin Mars Exploration rovers--Spirit and Opportunity--have been finding signs of water-associated minerals for 4 years now. And less than 2 weeks ago, the Phoenix Mars Lander struck water ice while digging at the north polar region (ScienceNOW, 20 June). What remains to be determined is where this wetness came from and how long it lasted. Preliminary investigations by Mars mission scientists, as well as high-resolution images taken by orbiters, have suggested that water on Mars surged up from deep below the surface, sometimes carving extensive channels and gullies (see photo).
Now, a team led by planetary geologist Ronald Amundson of the University of California, Berkeley, has found indications of rain by studying our own planet's geochemistry. Analyzing soil samples collected by five previous missions, including the 1976 Viking and 1997 Pathfinder landers, the researchers found a distinctive pattern of chloride and sulfate deposits. In all of the samples, the data show that the chlorides tend to stay nearer to the surface, whereas sulfate concentrations increase with depth. That's the same pattern found in extremely arid places on Earth such as Antarctica's dry-valley regions and Chile's Atacama Desert. In these areas, rain is light and infrequent, but over millions of years it can change the chemical makeup of soil by depositing chlorides near the surface and by transporting the less soluble sulfates farther into the soil.
So the picture emerging is that by about 3 billion years ago, the biggest bodies of water on the martian surface, which were derived from groundwater, had mostly frozen or evaporated, the researchers report online this month in Geochimica et Cosmochimica Acta. Then a prolonged period of intermittent drizzle and dew began. That climate apparently continued long enough to alter the chemistry of surface minerals, creating the pattern detected by the analyses.
It's a convincing argument, says planetary scientist Itay Halevy of Harvard University: "Atmospherically delivered water and downward migration of salts, both common processes on Earth, played a part in the formation of martian soils, too." Moreover, the results "provide further chemical support for what previous studies have found: that during early parts of Mars's history, liquid water existed on the surface for geologically significant periods of time."