A new paper enriches researchers’ understanding of where the stone record preserved or destroyed evidence of Mars’ past and possible signs of ancient life.
Today, Mars is an extreme planet – it is bitterly cold, has high radiation and is bone dry. But billions of years ago, Mars was home to marine systems that could have sustained a microbial life. As the planet’s climate changed, such a lake – in Mars’ Gale Crater – slowly dried up. Scientists have new evidence that super-salted water or brine seeped deep through the cracks between soil grains in the dried-out lake bottom and altered the mineral-rich layers under clay.
The results, published in the July 9 issue of the journal Science and led by the team in charge of chemistry and mineralogy, or CheMin, instrument ̵
“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,” said Tom Bristow, CheMin’s lead researcher and lead author of the paper at NASA’s Ames Research Center in California’s Silicon Valley. “But later brines broke these clay minerals down in some places – essentially resetting the rock record.”
March: It continues with your fixed record
Mars has a treasure chest of incredibly ancient rocks and minerals compared to Earth. And with Gale Crater’s undisturbed layer of rocks, scientists knew it would be an excellent place to search for evidence of the planet’s history and possibly life.
Using CheMin, researchers compared samples taken from two areas about a quarter of a mile apart from a layer of mudstone that was deposited billions of years ago at the bottom of the lake at Gale Crater. Surprisingly, in an area about half of the clay minerals they expected to find were missing. Instead, they found mudstone rich in iron oxides – minerals that give Mars its characteristic rusty red color.
Researchers knew that the mud stones sampled were about the same age and started the same – filled with clay – in both areas studied. So why did patches of clay minerals – and the evidence they preserve – disappear when Curiosity explored sedimentary clay deposits along the Gale Crater?
Clay keeps track
Minerals are like a time capsule; they give an overview of what the environment was like at the time they formed. Clay minerals have water in their structure and are signs that soil and rocks containing them came in contact with water at some point.
“Since the minerals we find on Mars are also formed in some places on Earth, we can use what we know about how they are formed on Earth to tell us how salty or acidic the water on ancient Mars was.” said Liz Rampe, CheMin’s deputy. lead researcher and co-author at NASA’s Johnson Space Center in Houston.
Previous work revealed that while Gale Crater’s lakes were present, and even after they dried out, groundwater moved below the surface, dissolving and transporting chemicals. After they were deposited and buried, some mudstone pockets experienced different conditions and processes due to interactions with these waters that changed the mineralogy. This process, known as “diagenesis”, often complicates or erases the Earth’s past history and writes a new one.
Diagenesis creates an underground environment that can support microbial life. In fact, some very unique habitats on Earth – where microbes thrive – are known as “deep biobeads.”
“These are great places to look for evidence of ancient life and measure habitability,” said John Grotzinger, CheMin co-researcher and co-author at the California Institute of Technology, or Caltech, in Pasadena, California. “Although 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.”
By comparing the details of minerals from both samples, the team concluded that saline filtering down through overlying sediment layers was responsible for the changes. In contrast to the relatively freshwater lake that was present when the mudstones formed, it is believed that saltwater came from later lakes that existed in a generally arid environment. Scientists believe these findings provide further evidence of the effects of Mars’ climate change billions of years ago. They also provide more detailed information, which is then used to guide Curiosity Rover’s investigations into the history of the Red Planet. This information will also be used by NASA’s Mars 2020 Perseverance Rover team when assessing and selecting rock samples for possible return to Earth.
“We’ve learned something very important: There are some parts of the Mars record that are not so good at preserving evidence of the planet’s past and possible life,” said Ashwin Vasavada, Curiosity project researcher and co-author of NASA’s Jet Propulsion Laboratory in the South. California. “The lucky thing is that we both find each other close in Gale Crater and can use mineralogy to tell which one is which.”
Curiosity is in the initial phase of investigating the transition to a “sulfate-bearing unit” or rock believed to have formed as Mars’ climate dried out.
The mission is led by JPL, a division of Caltech, for NASA’s Science Mission Directorate, Washington. Colleagues in NASA’s Astromaterials Research and Exploration Science Division at Johnson and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, are also authors on paper as well as other institutions working on Curiosity.
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Jet Propulsion Laboratory, Pasadena, California.
Ames Research Center, Silcon Valley, California.