The Curiosity rover has discovered manganese oxide on Mars. It may indicate possible microbial life billions of years ago

by alex

Martian microbes could use manganese as an energy source

The Curiosity rover has discovered evidence that increases the likelihood that microbial habitable environments existed on Mars billions of years ago.

Curiosity is exploring the 154-kilometer-diameter Gale crater impact crater, where it landed in 2012. Research has already helped establish that the crater was at least partially flooded in ancient times, although the evidence for this is disputed. However, the latest results not only strengthen the case for the existence of an ancient lake, but also suggest that the conditions inside it were favorable for life.

The evidence comes from manganese oxide. Curiosity first detected small amounts of manganese oxide in Gale Crater in 2016, but it has now found much larger amounts in the sedimentary Murray Formation. The Murray Formation is located on the flank of Mount Sharp, in the center of the crater.

Manganese oxide was identified using a device called ChemCam, which shines a laser on the rocks that scientists want to study. The laser heats a small area of ​​the surface, thereby vaporizing it, resulting in a small cloud of plasma that the on-board camera and ChemCam spectrometer can study in situ to determine the composition of the material. ChemCam discovered mudstone enriched in manganese oxide up to 45%.

On Earth, manganese oxide is commonly found in lake beds or river deltas where high oxidizing conditions occur. And microbes existing in this environment are capable of catalyzing the oxidation process.

Typically, this process requires a constant supply of oxygen, which is in short supply on Mars. The small amounts of manganese oxide discovered in 2016 can be explained without significant amounts of oxygen, but the large amounts found in the Murray Formation require very different circumstances. For such a content, the oxidation process will require a significant amount of oxygen.

Manganese oxide is difficult to form on the surface of Mars, so we didn't expect to find it in such high concentrations in coastal sediments. We have no evidence of life on Mars, and the mechanism for oxygen production in Mars' ancient atmosphere is unclear, so how manganese oxide formed and became concentrated here is puzzling», — said lead researcher Patrick Gasda of Los Alamos National Laboratory.

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One clue is the nature of the mudstone deposits in which the manganese oxide was found. Rocks enriched in manganese oxide were found between two geological regions of the Murray Formation. One part of Sutton Island appears to be deposits on the shore of the lake. The other, Blunts Point, is deeper in the lake.

The mudstone, enriched in manganese oxide, is coarser and has larger grains than bedrock in other parts of the crater, where only small amounts of the compound were found. This supports the theory that the Sutton Island and Blunts Point region is either the site of an ancient river delta that once flowed into the lake, or the shoreline of the lake, both of which are places where coarser sediments were preferentially deposited. Larger grains would contribute to the formation of a more porous bedrock than the fine-grained mudstone that is present elsewhere in Gale Crater — mudstone, which is presumably located much deeper in the lake. This porosity would allow groundwater to flow more freely.

According to scientists, manganese could seep from groundwater, passing through coarse mudstone, concentrating in the rocks. However, where the oxygen came from for its oxidation remains a mystery.

The presence of manganese oxide also supports the possibility that microbial life could exist in the lake. Not only can microbes catalyze the oxidation of manganese, but they also have the potential to use manganese's many oxidation states as a source of chemical energy for metabolism, just as microbes do on Earth. In other words, in some sense, the abundance of manganese oxide may be an indirect biological marker.

The discovery of elevated concentrations of manganese oxide in the coastal sediments of an ancient Martian lake indicates that conditions in the reservoir were sufficiently oxygenated and suitable for the possible emergence and existence of microbial life. This is some of the most compelling indirect evidence that life, at least at the microbial level, could once have flourished on Mars. Further research will help shed light on the mystery of the origin of oxygen for the oxidation of manganese and other details of the ancient Martian environment.

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