Even though NASA's Mars rover Spirit has been trapped in the sand for months, the robot has still managed to report new facts about the red planet — all by just spinning its wheels.
These findings shed light on the history of water on Mars, which might once have supported life.
For nearly six years, Spirit roamed Mars, experiencing a number of close calls. In fact, the solar-powered robot has driven backwards since its right front wheel jammed in 2006.
Spirit's most challenging ordeal yet began in April, when it got bogged down in a patch of loose soil on the edge of a small crater. As scientists plotted Spirit's escape for months, they dubbed the area Troy, after the city the ancient Greeks struggled against in myth for a decade.
As frustrating as Spirit's dilemma has proven, it has yielded an unexpected insight.
"Spirit had to get stuck to make its next discovery," said geologist Ray Arvidson of the Washington University in St. Louis. "The rover's spinning wheels have broken through a crust, and we've found something supremely interesting in the disturbed soil."
Spirit broke through a dark reddish-brown crusty surface that was an inch or so thick, exposing loose, sandy material. As the rover tried to break free, its wheels began to churn the soil, uncovering even more sandy material, bearing "a higher concentration of sulfate that seen anywhere else on Mars," Arvidson said.
"Sulfates are minerals just beneath the surface that shout to us that they were formed in steam vents or hydrothermal pools, since hot water associated with these systems has sulfur in it," he explained. "These deposits are evidence of water-charged explosive volcanism. Such areas could have once supported life."
"Also, the robot found that the top of the sulfate material is crusty," Arvidson added. "Ancient sulfates probably formed this crust as they were processed by variations in climate associated with changes in Mars' orbit over millions of years."
The angle at which Mars' axis tilts can vary extremely, and during periods when its axis is highly tilted, the pole facing the sun gets warmer during the summer, shifting water to the equator as snow. The scientists think the warm soil causes the bottom layer of the snow to melt, and the water trickles into the sulfates, dissolving the water-soluble iron sulfates and forming a crust with the white calcium sulfates remaining.
"By being stuck at Troy, Spirit has been able to teach us about the modern water cycle on Mars," Arvidson said. The robot's saga at Troy has given scientists material evidence of past water on Mars on two time scales — ancient volcanic times, and cycles ongoing to the present day.
"We were never in one place long enough to really look at the layered nature of these deposits, which all fits with water migrating downward, giving us real insights into the history of water on Mars," Arvidson said.
As useful as all this time spent at Troy has proven, "we've sat here for more than six months," Arvidson said. "That's a long time to take measurements. We've learned a lot. Troy is a good place to be under siege, but we're ready to leave."
Spirit may be able to break free to continue its journey after all.
"The right front wheel almost magically turned on a few days ago after we ran some voltage through it," Arvidson told SPACE.com. "Hopefully it can give us significant traction — the probability of extricating Spirit has gone up."