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Curiosity Digs the Bed of Gale Lake

Curiosity has completed the first detailed analysis of the bedrock in Gale Crater, and the results were announced in a press conference on March 12.

Curiosity has spent much of the last two months at an outcrop named Yellowknife Bay, examining the oldest sedimentary rocks exposed in Gale Crater. Some of the findings from its work were announced in a NASA press conference on March 12. Since mid-February, most of Curiosity’s work has been focused on “John Klein”, a particularly well-exposed portion of Yellowknife Bay.

Even before Curiosity got busy at John Klein, it was clear that the rocks here had interacted with water. The rocks were made of fine-grained sediments consisting mainly of clays and sulfate minerals. Such rocks, known as mudstones, are typically deposited in calm waters. But that wasn’t the only evidence – the rocks also contained concretions and veins of hydrous minerals. Concretions generally form as water (and the elements dissolved in it) trapped in the sediment causes small clumps to be cemented more tightly together. Veins form as the rock fractures and those fractures fill with water. That water then deposits minerals inside the fractures.

Evidence of water was all around, but what was that water like? To answer that question, Curiosity had to get dirty. John Klein was drilled in three places starting on February 9, and the resulting rock powder was analyzed by two of the rover’s instruments: the Chemistry and Mineralogy Instrument (CheMin) and its Sample Analysis at Mars instrument (SAM). CheMin is an X-ray diffractometer, which measures how X-rays bounce off of  mineral powders. Each mineral has a distinct diffraction pattern based on its crystalline structure. By analyzing the patterns produced by CheMin, mission scientists can determine the mineral makeup of the rock. SAM is a mass spectrometer, which measures relative elemental ratios within the sample. Combined, they are a powerful tool to determine the chemistry present at the time the rocks formed.

Helping the mission scientists along is the fact that John Klein is a mudstone. Mudstones contain a large amount of clay, the composition of which is extremely dependent on the minerals and environmental conditions they form in. What Curiosity found was a particular type of clay called smectite. Smectite usually forms when freshwater interacts with basaltic lavas. The presence of the mineral also indicates that the conditions at the time it formed weren’t extremely oxidizing, acidic, or salty.

The general conclusion from the John Klein samples seems to be that the rocks in Yellowknife Bay were deposited in a freshwater environment, probably in a large lake that filled the crater. “We have found a habitable environment which is so benign and supportive of life that probably if this water was around, and you had been on the planet, you would have been able to drink it,” said project scientist John Grotzinger (Caltech). Further supporting evidence for that finding comes from the mineral veins themselves. The veins are mostly calcium sulfate, which forms in neutral to slightly alkaline conditions. Overall, the findings contrast with rocks sampled by Spirit and Opportunity.  Those rovers found evidence that rocks in their respective areas had at least interacted with acidic waters.

In addition to evidence for freshwater, SAM also found that samples from John Klein contained significant quantities of elements critical for life. Those elements, carbon, oxygen, nitrogen, sulfur, and phosphorus, are key ingredients for forming DNA and amino acids. While the findings aren’t sure signs of life, they do indicate that conditions were ripe for Earth-like life to arise on ancient Mars. Even better for ancient Martian life, Curiosity found that the sediments in John Klein contained a mix of materials in different oxidation states. Differences in oxidation state are frequently used by microbes on Earth as a source of energy, and ancient Martian microbes might have been no different. “The range of chemical ingredients we have identified in the sample is impressive, and it suggests pairings such as sulfates and sulfides that indicate a possible chemical energy source for microorganisms,” said SAM principal investigator Paul Mahaffy.

NASA plans to double-check their findings in the next couple of months with follow-up samples. Since the instrument has only analyzed a few samples, small puffs of gas or contaminants from Earth may still be present in the mass spectrometer line. If that is the case, then a reduction of elements present in the contaminants should be seen in future samples as the contaminants are scrubbed from the system.

Currently, plans are for Curiosity to collect a few more samples at Yellowknife Bay over the next month or two. Science operations have been slow this month due to a computer glitch that forced operators to temporarily suspend science operations for a week while the rover switched to a backup computer. The switch has left the rover operating on only one functional computer for the time being.  Earlier this week engineers shut down the rover for a day as a solar flare passed Mars to keep the remaining computer safe. Similar delays are a possibility until the glitchy computer is restored to service.

Also slowing operations is the fact that Mars is nearing solar conjunction. As Mars passes behind the sun as seen from Earth, communications become difficult. The inability to communicate new commands or receive any data back from the rover means that Curiosity will have the next couple of weeks off. Once operations resume near the end of April, Curiosity will spend the next couple months with follow-up investigations at Yellowknife Bay. Once scientists get a good idea of the geology of the rock unit exposed there, they’ll turn their sights towards Mt. Sharp.

Image credit: NASA/JPL-Caltech/MSSS

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About The Author
Justin Cowart
Justin Cowart is a geologist interested in Earth and Solar System history. As a geologist, he spends hist time looking at the ground, but in his free time he looks to the skies as an amateur astronomer.

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