A U.S. rover on Mars has discovered more evidence of ancient water, revealing that the liquid was once widely distributed on the red planet. The other U.S. robot is about to begin a risky trek into a crater from which it might not return.
The rover Spirit has found strong concentrations of epsom salts on the bottoms and sides of trenches it has dug a couple of kilometers from Gusev Crater, where it landed in January. Cornell University mission geologist Steven Squyres says the concentration of the mineral salts is about three times higher than usually appears in soil. He calls it clear evidence that water carried the salts up from basalt rock below ground and left them behind on the surface when it evaporated.
"Now, we're not talking about big standing bodies of liquid water. You don't need a lake to do this," says Mr. Squyres. "You can perfectly well do this with small amounts of water percolating through the soil, maybe going along grains in rocks, maybe coating the outer surfaces of rocks and going through the soil as well."
Mr. Squyres says Spirit has measured salt in varying concentrations in several trenches it has dug near Gusev Crater, but the latest evidence is the most compelling. The findings are similar to what the twin Opportunity rover discovered on the other side of Mars at the Meridiani Plains in March. The difference is that the Meridiani site appears to have been covered by a large standing body of salt water, perhaps large enough to be called a sea.
"What is emerging here is the story of a planet that is made mostly of basalt, where the water flows through that basalt and then dissolves stuff out of it," says Mr. Squyres. "As it comes up to the surface, it leaves some of those salts behind. [It has] two very different appearances at these two places, but the underlying chemistry might have some intriguing similarities."
The Spirit rover discovered the mineral salts on its way to Columbia Hills, a low martian mountain range it will inspect so scientists can determine what they are made of.
The Opportunity rover, meanwhile, is ready to enter a deep crater named Endurance - an appropriate name because the robot may have to endure the rest of its existence there if it cannot roll back out. The crater's rocky sides slope about 20-degrees, relatively steep for the six-wheeled rover. An identical test vehicle at the Jet Propulsion Laboratory in California has demonstrated its ability to maintain traction and retreat from such terrain, but mission controllers say they will drive Opportunity cautiously.
The appealing target at Endurance Crater is an exposure of layered bedrock several meters down the slope. The head of the U.S. Mars Rover project, Iranian-born Firouz Naderi, says the rock layers represent different ages of Mars and will let scientists see further back in geological time than they have been able to do so far.
"If you think of the layered rocks as history books that tell you about the martian past, we have read the top layer and now we want to look even further back in the history of Mars, and so we're ready to turn the page," says Mr. Naderi.
Mission scientists are not worried that the rover might become stranded in the crater. Because the depression is geologically rich, they say it would be like a history lover being confined to a library.