Manganese has been observed on Mars by the NASA Curiosity rover in multiple places, but a higher-than-usual amount has been detected in the Gale crater.
The ChemCam instrument onboard the rover detected the deposits, which indicate that the sediments had formed in a river, delta, or near the shoreline of an ancient lake.
These sedimentary rocks have larger grain sizes than what is typical for the lakebed rocks in the Gale Crater. This may indicate that the original sediments were formed in a river, delta, or near the shoreline of the ancient lake.
On Earth, manganese becomes enriched because of oxygen in the atmosphere, and this process is often sped up by the presence of microbes, as per the paper.
Microbes on Earth can use manganese’s many oxidation states as energy for metabolism; if life was present on ancient Mars, the increased amounts of manganese in these rocks along the lake shore would be a helpful energy source for life.
The discovery
Within the Murray formation, an ancient primarily fine-grained lacustrine sedimentary deposit in Gale Crater, an up to 45× enrichment in manganese and up to 1.5× enrichment in iron within coarser-grained bedrock targets compared to the mean Murray sediment composition has been observed.
This enrichment in manganese coincides with the transition between two stratigraphic units within the Murray: Sutton Island, interpreted as a lake margin environment, and Blunts Point, interpreted as a lake environment.
This work has important implications for the habitability of Mars to microbes that could have used Manganese redox reactions, owing to its multiple redox states, as an energy source for metabolism.
The implications of the research
The sedimentary rocks explored by the rover are a mix of sands, silts, and muds. The sandy rocks are more porous, and groundwater can more easily pass through sands than the muds that make up most of the lakebed rocks in the Gale Crater.
The research team looked at how manganese could have been enriched in these sands—for example, by percolation of groundwater through the sands on the shore of a lake or mouth of a delta—and what oxidant could be responsible for the precipitation of manganese in the rocks.
The mechanism of manganese precipitation on Mars is likely abiotic, but it raises intriguing questions about its habitability and potential biosignatures on Mars.
Experts on the Manganese deposits on Mars
Patrick Gasda, from Los Alamos National Laboratory’s Space Science and Applications group and lead author on the study, said that it is difficult for manganese oxide to form on the surface of Mars,
Gasda said the researchers “didn’t expect to find it in such high concentrations in a shoreline deposit.”
He said that the findings point towards a large oxidation process happening in the Martian atmosphere, which needs to be studied at length.
“On Mars, we don’t have evidence for life, and the mechanism to produce oxygen in Mars’s ancient atmosphere is unclear, so how the manganese oxide was formed and concentrated here is really puzzling,” he added.
The manganese-rich sandstones indicate that there were once habitable conditions in the Gale Crater.
“The Gale lake environment, as revealed by these ancient rocks, gives us a window into a habitable environment that looks surprisingly similar to places on Earth today,” said Nina Lanza, principal investigator for the ChemCam instrument.
“Manganese minerals are common in the shallow, oxic waters found on lake shores on Earth, and it’s remarkable to find such recognizable features on ancient Mars.”
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