Even though the Mars Rover Spirit is asleep, a new look at old data from one of her instruments confirms the presence of large amounts of carbonate-rich rocks, which means that regions of the planet may have once harbored water. The Miniature Thermal Emission Spectrometer, or Mini-TES, instrument on the rover looked at an outcrop of rocks called “Comanche” back in 2005, but the instrument was partially “blinded” by dust. Only when scientists developed a special calibration to remove the spectral effects of the dust on the instrument was the spectral data revealed to show evidence for carbonate-rich outcrops in a range of low hills inside Gusev crater on Mars.
Spirit has gone into hibernation because of low power levels during the extremely cold winter months on Mars. She is stuck in some loose sand in the Home Plate region, and the rover teams were unable to get her solar panels in a good position to soak up the sun’s energy.
See more images of Comanche, below, courtesy of Stu Atkinson.
Carbonates are clues to neutral pH conditions, and the mineral forms readily in the presence of water and a carbon dioxide atmosphere. If conditions were right for carbonate-bearing rocks to form, water would have been present, and could have created an environment favorable to life. Yet until now, geologic clues for the presence of carbonates on the surface of Mars have been scarce.
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“Mini-TES got dusted months before Spirit reached Comanche, and we didn’t have a good way to correct for the dust effects at the time,” said Steve Ruff, research scientist at Arizona State University’s Mars Space Flight Facility. Ruff is one of a team of scientists on the paper, whose lead author is Richard V. Morris of NASA’s Johnson Space Center in Houston. “We knew there was something weird about the outcrop’s spectrum as seen by Mini-TES, but couldn’t say what caused it.”
Ruff said that even though Spirit’s Mössbauer spectrometer indicated that carbonate was possible, the team needed more evidence to be convinced. When the calibration method to remove the spectral effects of the dust made that data available, and combined with chemical data from a third spectrometer, “the Mini-TES spectra put the discovery over the edge,” Ruff said.
Scientists have been searching for Martian carbonate rocks for decades because such minerals are crucial to understanding the early climate history of Mars and the related question of whether the planet might once have held life.
Small amounts of carbonate minerals have been detected on Mars before, but Ruff said this new data is different. “We’re seeing a couple of large outcrops of rock poking through the soil of the Columbia Hills,” he said. “The rocks are about 25 percent carbonate by weight, by far the highest abundance we’ve seen on Mars.”
The mineral is rich in magnesium and iron and possibly formed a long time ago by precipitation from the hot, residual waters from leftover magma that flowed through buried carbonate deposits.
NASA’s other Mars rover, Opportunity, has discovered ample evidence for alteration of rocks by water in Meridiani Planum, on the other side of Mars from Spirit’s Gusev Crater. But the water at Meridiani was strongly acidic. While life can evolve to survive in acidic conditions — such as in some of Yellowstone National Park’s geysers and hot springs — few scientists think it can start under those conditions.
Moreover, acidic water quickly destroys carbonate minerals, as for example vinegar dissolves hard water deposits. Thus finding outcrops of carbonate rock shows that the hydrothermal water at Comanche was liquid, chemically neutral, and abundant.
While there’s no evidence for life, Ruff says, the conditions would have been more favorable for it.
Ruff added that more old data from Spirit could hold new clues to Mars’ past. “The Comanche data have been available to scientists and the public for about four years now. The new finding shows that this data set still harbors potentially major discoveries.”
More images of Comanche, rendered by Stu Atkinson: