Rare rocks identified on Mars

Analysis of data collected by the Mars Exploration Rover Spirit in 2005 has confirmed the discovery of carbonates in an outcrop at the base of the Columbia Hills with Gusev Crater.

At the time, the instrument that took measurements of the outcrop at the location known as Comanche – the Miniature Thermal Emission Spectrometer (Mini-TES) – was partly blinded by dust, but calibration tests back on Earth has enabled scientists to get rid of the effects of the dust.

More than four years after Mars rover Spirit visited the Comanche outcrop in Gusev Crater's Columbia Hills, scientists armed with a new instrument calibration have discovered the rocks are rich in long-sought carbonate minerals. Image: NASA/JPL/Cornell University.

“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,” says Steve Ruff, research scientist at ASU’s Mars Space Flight Facility. “We knew there was something weird about the outcrop’s spectrum as seen by Mini-TES, but couldn’t say what caused it. “Spirit’s Mossbauer spectrometer indicated that carbonate was possible, but I didn’t believe it.”

Applying the calibration trick to the Mini-Tes data, and combining the data with two other instruments the detection of carbonates was clear. Carbonate minerals are a crucial for understanding the red planet's early climate and searching for life on Mars, and while small amounts have been detected before, this is the first time such large rocks have been identified.

Seen close up, the Comanche outcrop shows both a granular texture and multiple layers. Scientists think it is volcanic debris draped over preexisting terrain. After it was deposited, the rock was soaked in hydrothermal water rich in carbonate minerals. Image: NASA/JPL/Cornell University.

“The rocks are about 25 percent carbonate by weight, by far the highest abundance we’ve seen on Mars,” says Ruff. “They’re definitely a puzzle to understand. “The outcrops are very rich in olivine, a volcanic mineral, but they appear to have been soaked in water.” Ruff adds that it appears that the entire stack of rocks was flooded with carbonate-rich water, most likely from a hydrothermal source.

Alteration of rocks by water has been discovered at many of the sites examined by Spirit's twin rover Opportunity, but at these locations the water was much more acidic, which quickly destroys carbonate minerals. The sedimentary rocks at Comanche in the Columbia Hills however, must have interacted with a more chemically neutral water, conditions that would be much more favourable to life.

Ruff comments that the new finding complicates the story of the Columbia Hills even more, by adding another dimension to the environmental history of the region. “The Comanche data have been available to scientists and the public for about four years now so the new finding shows that this data set still harbors potentially major discoveries,” he says. “Do other surprises await us? Who knows? But I’ll make a strong prediction: More discoveries will be made with old data.”