The presence of a particular type of hematite, a mineral mostly associated with water, along with large, sandy areas, indicates that the red planet once had a supply of the liquid, said Robert Denise, a scientist at NASA's Jet Propulsion Laboratory and a member of the flight development software team for the Mars Exploration Rovers program. Denise is attending the Hot Chips conference taking place this week at Stanford University.
The data comes from the Mars rover expeditions launched last year and continuing today, said Denise, who added that the missions have been remarkably successful. The two rovers, Spirit and Opportunity, were each supposed to travel about 60 meters over 90 sols--the name for the Martian day, which lasts 24 hours and 40 minutes. Instead, the rovers have been on the surface for more than 180 sols and have covered 3.2 kilometers.
The rovers, which started to roam last April, are now going through the worst of the Martian winter. Funding for the program, barring another extension, ends Sept. 13, which coincides with a period in which Mars will be on the other side of the sun, making contact difficult. Technically, though, there is a good chance that the mission can continue.
"The true life of these rovers is not determined. If we get through September, things will start to get warm," he said. "We have surpassed our success criteria multiple times with both rovers."
The evidence of water in the soil is based largely on deductive reasoning. Hematite on earth generally forms near water. The mineral can be formed through volcanic eruption, but that type of hematite exhibits concentric circles.
Images of blueberry-shaped nodules of the mineral dug up by Opportunity do not have concentric circles. "This adds credence to the idea that the hematite nodules were formed in the presence of water," Denise said.
Similarly, the idea that soil patterns show that water existed on Mars is also dependent on something that's missing. In this case, it's rocks. Mars, like Earth's moon and like Mercury, is pockmarked with craters formed by meteors and comets run aground during the early days of the solar system. When this space garbage hits, it leaves circular rock patterns from the impact.
Most of the Martian surface is strewn with these rock fields. Both the Viking lander in 1976 and the Mars rovers that landed in 1997 came down in such rock fields.
Opportunity came down in a sandy field very thin on rocks. There are two theories to explain this, Denise said. One is that Opportunity landed in a rare spot, where meteors didn't hit. That seems unlikely. "Meteors don't aim," he said.
The second is that water covered them up. The area, he noted, is quite soft and sandy. When the rover would press into the surface, centimeter-size pebbles would sink and become invisible. Evidence of meteor hits on Earth--meteors that may have hit our planet after rebounding off Mars--has mostly been washed away by the oceans.
"The operating hypothesis is that this part of Mars was once covered with a shallow, salty sea," he said.
Things happen slowly on Mars, Denise added. One of the rovers came across a serpentine sand dune. On Earth, the surface of these types of dunes is currently circulated and reformed by wind and erosion. The serpentine dune on Mars has a crust that's millions of years old.
"This is a prehistoric dune that has not moved," Denise said. Dust, he added, covers a huge portion of the planet. Uncovered rocks are grey in color, but dust gives objects a red appearance.
The data obtained by the rovers comes from cameras and a robotic tool on the front that can bore holes into rock, sweep surfaces and take microscopic pictures of the bore holes. The engineers also learned that the wheels of the rover can be turned in such a way to dig 8-centimeter trenches in the soil.
The closest signs of intelligent life the rovers have found are things they brought themselves. Strange flower patterns on the surface of the planet turned out to be the indents left by seams in the airbags used for landing. Shiny spots in the distance picked up by cameras turned out to be discarded heat shields.
"We spun donuts, dug trenches and left a landing device," he said. "If you did this in a National Park, you'd get a hefty fine."
Blue screens on a red planet
Despite the technical sophistication of the project, the Spirit rover almost came to a halt because of a programming problem. The rover contains 128MB of DRAM memory and 256MB of flash memory, which is used to store images and the system's directory. Technicians on Earth would continually download images from flash. The directory, a map of the computer's files, however, did not update itself to acknowledge that files had been downloaded and deleted from the rover.
As a result, by sol 18, the computer on the Spirit rover was convinced that it had maxed out on memory. When new data was harvested, the rover would reboot itself. Suddenly, researchers on Earth could not obtain data from the rover, and the rover was dangerously running low on power because of all the energy sucked up by the reboots.
Researchers on Earth finally shut it down and created a utility to scrub out old files from the directory, which was based on DOS.
"The Spirit was willing, but the flash was weak," he said. "This was a feature of the DOS files system that we failed to accommodate."