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Deposits of nearly pure silica discovered by the Mars Exploration Rover Spirit could have formed when volcanic steam and hot water percolated through the ground, mirroring the processes that occur at Earth’s Yellowstone National Park today. Despite struggling with a broken wheel, the Spirit rover emerged from its second hibernation through a harsh Martian winter unscathed, and headed straight for an exposed patch of soil - the Tyrone site - that the science team hadn’t had time to examine before winter began. The area, located on Home Plate within the Colombia Hills, had already proved rich in sulfate minerals at other locations in the Columbia Hills, a tell-tale sign that water had once been present on the now dry and dusty planet.
As Spirit drove away from Tyrone, its broken right front wheel dug this trench, which was found to have a high silica content. Image: NASA/JPL/Cornell University. While Spirit was parked next to Tyrone examining some nearby outcrops, the rover’s jammed right front wheel inadvertently produced the eureka moment, having gouged a trench a few inches deep into the soil as the rover drove ahead in reverse, dragging its crippled wheel behind. "The trench looked bright white," says Steven Ruff of Arizona State University’s (ASU) Mars Space Flight Facility, "But we thought initially it was just more sulfate minerals." But the science team got curious and aimed Spirit’s ‘Mini-TES’ (Miniature Thermal Emission Spectrometer) instrument at the trench, revealing 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." Making that amount of pure silica requires a lot of water; on Earth, the only way to have this kind of silica enrichment is by hot water reacting with rocks, such as that observed at hot springs like Yellowstone. The Tyrone silica deposit is linked to Home Plate, which is already known to be a volcanic feature. "Home Plate came from an explosive volcanic event with water or ice being involved," says Ruff. "We saw where rocks were thrown into the air and landed to make small indentations in the soft, wet, ash sediment around the vent."
The silica rich deposits occur in Eastern Valley, flanking Home Plate in the inner basin of the Columbia Hills. Spirit spent the winter on Low Ridge. The first silica-rich outcrop was spotted at site A, with subsequent sites at B and C. D marks the location of the silica rich trench dug by Spirit. Image: NASA/JPL/University of Arizona. Once the science team knew what to look for, they found more silica in many other nearby locations, adding to the global picture that Mars was once much more hospitable than it is now. But how long ago these hydrothermal deposits were formed is another question, as there is no way of accurately dating them. “It's fairly clear that they were buried by other rock and are now being exhumed,” Ruff tells Astronomy Now. “We do know that the Columbia Hills, which contain the Home Plate feature and adjacent silica deposits, are older than the plains of Gusev crater, perhaps even a couple of billion years old. But this also may have been during the hypothesised "warmer, wetter" period of Martian history, which may bode well for the possible presence of ancient microbial life in what could have been a habitable environment.”
Terrestrial experience shows that microbes thrive in hydrothermal environments and can easily be trapped and preserved in hydrothermal deposits. Unfortunately, the microscopic imagers on the current Mars rovers cannot resolve the microbial remains seen in terrestrial hot spring deposits, but imagers in development for future missions should allow scientists to detect such features. "We just need to deliver such instruments to the right place,” says Professor Jack Farmer of ASU’s School of Earth and Space Exploration. “The discoveries at Home Plate have helped us know where to go next."
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