Curiosity investigates how rocks on Mars could preserve signs of life

A self-portrait of NASA's Curiosity rover taken on Sol 2082. A Martian dust storm has reduced sunlight and visibility at the rover's location in Gale Crater.
A self-portrait of NASA’s Curiosity rover taken on Sol 2082 (June 15, 2018). A Martian dust storm has reduced sunlight and visibility at the rover’s location in Gale Crater. NASA/JPL-Caltech

Trying to find evidence of life on Mars isn’t a simple matter. If there ever was life on Mars, it was likely microbial and lived millions of years ago. That means that to find evidence of its existence, rovers like Perseverance and Curiosity have to look for clues hidden in rock samples.

But not all rocks retain indications of life, as certain minerals preserve these clues better than others. Now, a new study using data from the Curiosity rover has shed light on what indicators of life have been preserved or destroyed through the history of Mars.

Curiosity is exploring a dried-up lakebed called the Gale Crater, which has clay layers at the bottom. Clay forms in the presence of water and is excellent at preserving signs of life, so it’s a good place to look. But it turns out that these clay minerals aren’t static over time, as was previously thought.

“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, principal investigator of Curiosity’s CheMin instrument and lead author of the paper. “But later brines broke down these clay minerals in some places — essentially resetting the rock record.”

This process, called diagenesis, erases part of the record of organisms that might once have lived there. However, the good news is that the researchers have a model of how to look for life signs in clays by looking at comparable locations on Earth. There are habitats on our planet known a “deep biospheres” that host thriving colonies of microbes in underground environments.

“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.”

All of this means that searching for evidence of life on Mars is even more complicated than we thought — but not impossible.

“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. “The fortunate thing is we find both close together in Gale Crater and can use mineralogy to tell which is which.”

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