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Wednesday, 21 July 2021

Curiosity discovers traces of ancient Martian life have potentially been destroyed


Even though Mars has met a tragic fate by losing its magnetic field and most of its atmosphere, the planet remains an important site of interest for planetologists and exobiologists. The red planet could indeed have sheltered life in its past, and, if it is the case, could still shelter part of it in its undergrounds. In this context, NASA's Curiosity rover recently discovered that traces of ancient Martian life may have been destroyed by biological processes.

The space agency's Curiosity rover made the surprising discovery while investigating clay-rich sedimentary rocks around its landing site in Gale Crater, an ancient lake created when an asteroid hit the Red Planet about 3.6 billion years ago. Clay is a good indicator of signs of life, as it is typically created when rock minerals erode and break down after contact with water, a key ingredient in life. It is also an excellent material for the storage of microbial fossils.

But when Curiosity took two samples of mudstone, a sedimentary rock containing clay, from patches of the dry lake bed, dated to the same time and place (3.5 billion years ago and only 400m apart), the researchers found that one plot contained only half the expected amount of clay minerals. Instead, this area contained a greater amount of iron oxides, the compounds that give Mars its red hue.

Signs of a past life potentially destroyed by the brine

The team believe the culprit behind this geological removal process is brine: super-saline water that has seeped into the mineral-rich clay layers and destabilized them, flushing them out and cleaning up areas with the both geological and perhaps even biological. “We used to think that once these layers of clay minerals formed at the bottom of the lake in Gale Crater, they stayed that way, preserving the moment in time they formed for billions of years,” said Tom Bristow, CheMin principal investigator and lead author of the paper at NASA’s Ames Research Center in California’s Silicon Valley. “But later brines broke down these clay minerals in some places – essentially resetting the rock record.”

The rover completed its analysis by drilling into layers of Martian rock before using its chemistry and mineralogy instrument, known as the CheMin, to study the samples. The process of chemical transformation in the sediment is called diagenesis, and it could have created new life under Mars even as it erased some of the evidence of old life on its surface. So even though old traces of life may have been erased in the brine areas, the chemical conditions caused by the influx of salt water may have allowed more life to arise in its place.

“These are excellent places to look for evidence of ancient life and gauge habitability,” said John Grotzinger, CheMin co-investigator and co-author at the California Institute of Technology, or Caltech, in Pasadena, California. “Even though diagenesis may erase the signs of life in the original lake, it creates the chemical gradients necessary to support subsurface life, so we are really excited to have discovered this.”

The search for Martian life carried out jointly by Curiosity and Perseverance

By comparing the details of minerals from both samples, the team concluded that briny water filtering down through overlying sediment layers was responsible for the changes. Unlike the relatively freshwater lake present when the mudstones formed, the salty water is suspected to have come from later lakes that existed within an overall drier environment. Scientists believe these results offer further evidence of the impacts of Mars’ climate change billions of years ago. They also provide more detailed information that is then used to guide the Curiosity rover’s investigations into the history of the Red Planet. This information also will be utilized by NASA’s Mars 2020 Perseverance rover team as they evaluate and select rock samples for eventual return to Earth.

“We’ve learned something very important: There are some parts of the Martian rock record that aren’t so good at preserving evidence of the planet’s past and possible life,” said Ashwin Vasavada, Curiosity project scientist and co-author at NASA’s Jet Propulsion Laboratory in Southern California. “The fortunate thing is we find both close together in Gale Crater, and can use mineralogy to tell which is which.”

Curiosity is in the initial phase of investigating the transition to a “sulfate-bearing unit,” or rocks thought to have formed while Mars’ climate dried out.

Reference: 

“Brine-driven destruction of clay minerals in Gale crater, Mars” by T. F. Bristow, J. P. Grotzinger, E. B. Rampe, J. Cuadros, S. J. Chipera, G. W. Downs, C. M. Fedo, J. Frydenvang, A. C. McAdam, R. V. Morris, C. N. Achilles, D. F. Blake, N. Castle, P. Craig, D. J. Des Marais, R. T. Downs, R. M. Hazen, D. W. Ming, S. M. Morrison, M. T. Thorpe, A. H. Treiman, V. Tu, D. T. Vaniman, A. S. Yen, R. Gellert, P. R. Mahaffy, R. C. Wiens, A. B. Bryk, K. A. Bennett, V. K. Fox, R. E. Millken, A. A. Fraeman and A. R. Vasavada, 9 July 2021, Science.

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