Even as planetary scientists were going into raptures over images from the Mars Global Surveyor (MGS) that suggest water may have flowed on Mars in recent years, a lonely rover stood poised on a Martian crater鈥檚 edge, ready to tackle another big question: was the Red Planet once washed by oceans?
Last week鈥檚 pictures from the orbiting MGS spacecraft revealed fresh, light-coloured material in two of the gullies it was monitoring. Over the past three years, NASA鈥檚 rover Opportunity, operating on the Martian surface, has found physical and chemical evidence showing that water was once present. This could have been groundwater that never appeared on the surface for any significant length of time, but the MGS images now raise the exciting possibility that the marks they show were left by flows of liquid water, indicating that groundwater might still be surfacing.
Tim Parker, a planetary geologist at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, is also interested in water on Mars, but on a much larger scale. He says there is an even chance that as Opportunity descends into Victoria crater, it will reveal features on its cliffs that could only have been formed underwater. Parker thinks an ocean once occupied much of Mars鈥檚 northern hemisphere and that the Meridiani Planum area, where Opportunity has been roving, was a shallow coastal platform under that ocean.
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Parker claims that features already seen by Opportunity in the cliffs of the crater can test this idea. 鈥淲e鈥檙e seeing large-scale crossbeds鈥� 鈥� layers of rock at steep angles to other layers. These crossbeds could only have been laid down as deposits driven by wind or water to form a sloping layer on the underlying surface. The general consensus is that these deposits are dunes that solidified into rock early in Mars鈥檚 geological history, says Parker. 鈥淏ut dunes can form underwater as well as on dry land,鈥� he points out.
鈥淭he general consensus is that the rock formations are dunes. But dunes can form underwater as well as on dry land鈥�
It may be possible to determine what exactly happened by close-up examination of the exposed cross-sections of these fossilised dunes 鈥� if that鈥檚 what they are. Small ripples on the surfaces of both sides of the dune-like structures would be a clear sign that they formed underwater: dunes that form from wind-blown material collapse on the downwind side by slumping, so if that is how the Martian structures formed they should have ripples on one side only, Parker reasons.
Other features of Victoria and its vicinity, such as the unusual flatness of the entire crater rim, are also hard to explain unless they were formed by water. Impact craters always form with prominent rims, Parker says. While wind can flatten surfaces, this usually leaves some raised areas, and any wind-driven process strong enough to erase the entire rim should have filled up the crater with the resulting debris. This has clearly not taken place at Victoria.
Now Parker, who is presenting his idea at the American Geophysical Union鈥檚 autumn meeting in San Francisco this week, is hoping that Opportunity will firm up the evidence. 鈥淲hat I鈥檓 hoping to see is ripple festoons on those crossbeds,鈥� he says.
