Soil stuck to the wheels of a U.S. rover on Mars is causing mission scientists to wonder if it is mud. Two rovers are on the red planet seeking evidence of liquid water that might once have supported life.
The Spirit rover has arrived at a small depression in the ground nicknamed Laguna Hollow and has wiggled its wheels to disturb the very fine soil. This kind of maneuver allows its instruments to sense minerals below the surface. When the vehicle backed up, scientists noticed that some of the fine soil stuck to its wheel.
NASA researcher David Des Marais is curious about why it is so sticky. He says fine dust can hold together and adhere when compacted, but he points out there may be another explanation involving water.
"It could also have salt in it or for that matter a brine or a little bit of water to give it moisture," he said. "In any case, we're interested in why that surface material is sticky, it's sticking to the wheels."
The search for water is the motivation for the $820-million mission. Two identical six-wheeled rovers are sensing the landscape on opposite sides of Mars to determine the environment in which the soil and rocks formed.
If the sticky soil has water, Mr. Des Marais says it probably is not much. "To caution, it could be just enough to cause a moistening and a dense concentrated brine," he said. "I wouldn't expect to see a pool of water when we dig. You don't need to have that much to explain these properties that we see."
If it is water holding the dust together, might it be a remnant of the oceans, lakes and rivers scientists believe once existed on Mars?
One clue to Mars' watery past might come from the next rock inspection by the twin Opportunity rover, which is in the shallow crater where it landed in late January. It is about to scrape into an outcrop of exposed bedrock that has a lot of sulfur on its surface. The lead mission scientist, Steven Squyres of Cornell University, says the presence of the sulfur could mean the rock has embedded sulfate compounds, which can form in volcanic conditions or in water, depending on the specific sulfate. His team will be able to tell which when Opportunity's instruments look inside the bedrock.
"So, for example, if we look inside and we see evidence for a sulfate that is the kind that forms only in the presence of liquid water, that would be an extraordinarily exciting finding," he said. "It would probably be the most interesting thing we have found yet here."
Mr. Squyres was not able to explain the presence of what appear to be puzzling thread-like features seen in Opportunity's microscopic pictures of soil. He suggests that they might be threads from the airbags which cushioned the rover's landing. The airbags, he says, took an awful lot of beating and may have lost fibers in the soil. "I don't know that these things are Martian," he said.