After a long hibernation period during its second Mars winter in the northern regions of the Home Plate, NASA's Mars Exploration Rover Spirit is again on the move and is now investigating the Gusev Crater where it found what appears to be a deposit of pure silica similar to the one created around hydrothermal vents on Earth in the Yellowstone National Park, US. The discovery was made last year, although NASA couldn't afford to make a detailed investigation at the time, knowing that the sunlight levels would decrease with the onset of the winter season on the Red Planet.

The discovery once again shines light on the possibility that the respective area may contain preserved traces of past life.

"On Earth, hydrothermal deposits teem with life and the associated silica deposits typically contain fossil remains of microbes. But we don't know if that's the case here, because the rovers don't carry instruments that can detect microscopic life. What we can say is that this was once a habitable environment where liquid water and the energy needed for life were present", says Jack Farmer, professor of astrobiology in ASU's School of Earth and Space Exploration, and part of the College of Liberal Arts and Sciences.

Spirit, along with its twin, the Opportunity rover, found on the opposite side of the planet, arrived on Mars in early 2004. They originally had a mission of only 90 days, but they proved so efficient that they are still operating on the Red Planet. Opportunity was more fortunate, since it is investigating an area of the planet with gusts of wind that clean its solar panels, while Spirit's solar panels are somehow dirtier, which is why it was put in a hibernation mode for the last five months, in order to conserve power.

Gusev Crater, where the Home Plate plateau is located, measures about 170 kilometers across and is filled with a relatively flat lava plain that has baffled the minds of researchers for years, since its origin is unknown.

"We were going back to an area of exposed soil called the Tyrone site, which we didn't have time to investigate before winter began", notes Steven Ruff, associate researcher at ASU's Mars Space Flight Facility. The soil in the Home Plate regions contains high concentrations of sulfate mineral, suggesting that liquid water might have existed in the area, because sulfate mineral mostly forms in the presence of water.

"While parked next to Tyrone, we used the Mini-TES to look at some nearby light-toned and knobby outcrops. It wasn't clear what we were seeing in the knobby outcrops because they were contaminated with dust and wind-blown soil. But I thought they might be silica-rich", says Ruff.

As Spirit returned to the site where the Mini-TES instrument had identified the soil as possibly rich in silica, its crippled right front wheel dug up a trench several centimeters deep in the soil, exposing the deposits below.

"The trench looked bright white, but we thought initially it was just more sulfate minerals. We aimed Mini-TES at the trench and it showed a clear silica spectrum. This prompted us to drive back to it, where the rover's Alpha Particle X-Ray Spectrometer told us the white soil was more than 90 percent silica. That's a record high for silica on Mars. On Earth, the only way to have this kind of silica enrichment is by hot water reacting with rocks", Ruff added.

Since the Home Plate region is known to have volcanic features, Ruff believes that it might have been created in the outcome of a volcanic event involving either liquid or ice water, throwing rocks and ash into the air around the vent. "It's not just the soil in a trench in one place. It's a broader story of outcrops that extend 50 meters away from Home Plate. It's not a small scale, modest phenomenon", Ruff said.

The occurrence resembles the geothermal heat and water combination that created the hydrothermal systems still active today in the Yellowstone National Park.

The reference samples for the Mini-TES instrument was provided by Farmer from his own laboratory collection of rocks, including samples from Yellowstone and various regions of New Zealand. "The best fit we got was with siliceous sinter", says Farmer. Such hydrothermal systems giving off dissolved gases, silica and other minerals would have been perfect for organisms to live in. Whether they did or not remains unknown for now, but future instruments sent on the surface of the Red Planet should be able to answer the question.