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Water in super-salt could have erased some evidence of life on Mars

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The Martian landscape near where Curiosity took the rock samples in 2019.
Picture: NASA / JPL-Caltech / MSSS

Rocks on Mars preserve an overview of the planet’s ancient past, but a surprising discovery made by NASA’s curiosity rover shows that some patches of martian rock have had their stories completely erased.

The primary purpose of NASA’s Curiosity mission is to evaluate the former potential for habitability on Mars, while the newcomer Endurance mission aims at to find actual remains or signs of past lives. To that end, Curiosity has studied sedimentary rocks in the Gale crater, which are filled with clay minerals. Clay is an important marker of habitability as it suggests the past presence of liquid water – an important ingredient for life.

Using its instrument of chemistry and mineralogy, also known as Path, the six-wheeled rover has analyzed drilling samples of sedimentary layers along the lower part of Mount Sharp. In 2019, a random road from the Vera Rubin ridge to Glen Torridon made it possible for Curiosity to study a layer of mudstone that formed in a Mars lake about 3.5 billion years ago.

Sedimentary rock at the site of a former lake in the Gale Crater.

Sedimentary rock at the site of a former lake in the Gale Crater.
Picture: NASA / JPL-Caltech / MSSS

The rover took samples from two areas that were less than 400 meters apart. Research published today in Science describes unexpected differences in these two areas, as a patch contained only half the expected amount of clay minerals. Instead, these ancient mudstones were filled with iron oxides, which, interestingly enough, are the things that give Mars its iconic red hue.

The mudstone from both stains dates back to the same time and place, so they must contain similar amounts of clay minerals. This surprising observation required that scientists, led by Tom Bristow of NASA’s Ames Research Center, be able to elicit an explanation for the lack of clay. In fact, ancient cliffs are known to be archives of history, but as new research shows, natural geological processes can undo this record.

To explain what happened, the team posed a scenario where water leaked into the clay from a sulfate deposit located directly above. The super-salty salt layers seeped through grains of sand at the bottom of the former lake, thus forever changing the mineral-rich layers below.

Cracks in this marsh stone, called

Cracks in this marsh stone, called “Old Soaker”, were probably formed by drying a layer of mud.
Picture: NASA / JPL-Caltech / MSSS

“We used to think that when these layers of clay minerals formed at the bottom of the lake in Gale Crater, they remained that way and retained the moment in time that they formed over billions of years,” Bristow explained in a NASA Declaration. “But later brines broke down these clay minerals in some places – essentially resetting the rock record.”

In an email, Bristow said the new research adds to the ever-growing picture of the old Martian habitability.

“It backs up previous evidence showing that liquids continued to move through Gale crater rocks long after they were deposited,” he explained. “It also shows that there were geochemical gradients – some parts of the rocks were affected more than others, and liquid chemistry changed,” said Bristow, adding that biological organisms “can use geochemical gradients to capture energy.”

This process was not uniform across the bottom of the former lake as it happened after the lake lost its liquid water, according to the research. Groundwater in Crazy craters continued to flow – and transport and dissolve chemicals – below the surface. As a consequence, some pockets of underground mudstone were exposed to various conditions. These pockets exposed to saline underwent a process called “diagenesis” in which the changing mineralogy wiped out geological – and possibly biological – record.

Interestingly, if not ironically, diagenesis could create environments that are friendly to microbes even when deleted potential evidence of lifeaccording to John Grotzinger, a co-author of the study and professor of geology at Caltech.

“These are excellent places to look for evidence of ancient life and measure habitability,” Grotzinger said in the statement. “While diagenesis can erase signs of life in the original lake, it creates the chemical gradients needed to support underground life, so we’re really happy to have discovered this.”

I like this paper for several reasons. First, it improves our understanding of the geological processes on the red planet and its unexpected complexity. Second, it’s a reminder that Curiosity is still doing important work on Mars, even nine years after it first began to roll, and as endurance begins to steal the limelight.

This study can now inform the endurance team as they assess targets for exploration and select rock samples that can eventually be brought to The Earth for further analysis. Excitingly, the two rovers now work as a team (even though they are 2,300 miles apart), and thus they can influence each other’s work.

More: NASA’s ‘other’ Mars rover sends a selfie back to remind us that it still exists.

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