Mars: Curiosity discovers “pickle” rocks

Nuove evidenze da Curiosity che mostrano come l’acqua salata, percolata tra le rocce di Marte, abbia alterato gli strati sottostanti.
Rover Curiosity su Marte. Credit: NASA

Chemistry and Mineralogy (CheMin) instrument aboard the Mars rover, Curiosity, analyzed some rocks. The team of NASA‘s Mars Science Laboratory, published (ref.) results on July 9th in the journal Science. The evidence helps to distinguish where the rock has preserved, possible signs of Martian life. “We thought the clay-rich layers formed on the bottom were unaltered, preserving the same characteristics as the moment they formed billions of years ago” said Tom Bristow, chief investigator at NASA’s Ames Research Center and author of the article.

Mars today

Today Mars is a planet of extreme conditions. The lack of an atmosphere makes it cold, dry, and constantly exposed to radiation. Yet, billions of years ago, Mars had lake systems, fertile ground for forms of microbial life. When the climate changed, one of these lakes, hosted in the Gale crater, slowly dried up. Scientists have found new evidence that shows how super salty water, slowly percolating deep into Martian rocks, has altered the underlying clay-rich layers.

Scientists compared samples taken in two points 400m apart on the bottom of the Gale crater. To their surprise in one area half of the clay minerals they expected to find were missing. Mudstones rich in iron oxides, which give Mars its typical rust color, replaced the clay minerals.

Minerals are like time capsules, they provide an instant snapshot of what the environment was like at the time they formed. Those of clay type, are rich in water and are evidence that these lands came into contact with water. “Since the minerals we find on Mars also form in some locations on Earth, we can use what we know to understand how salty or acidic the waters were on Mars” said Liz Rampe, lead researcher of the Johnson Space Center and co-author of the research.

The clay of Mars discovered by CheMin

Some previous research had already confirmed that water had penetrated the subsoil carrying with it various chemical substances. This mud deposited under the surface layer changed the surrounding mineralogy. This phenomenon is known as diagenesis. Diagenesis creates the ideal conditions for the proliferation of microbial life, as demonstrated in some unique habitats on Earth, called deep biospheres. “These are excellent places to look for evidence of ancient life” said John Grotzinger, CheMin co-investigator and co-author at Caltech. “Even though diagenesis can erase signs of life in the original lake, it creates the chemical gradients needed to support life in the subsurface, so we’re really excited to have discovered it”.

Comparing the minerals of both samples analyzed by CheMin, the team concluded that the brackish water that filtered through the sediments was responsible for the mineral changes in Mars’ subsurface. Unlike freshwater that generates a muddy sediment, it is suspected that the salty water comes from lakes present in a drier environment. Scientists believe that these results are further evidence of the impacts of climate change on the Martian planet billions of years ago.

This information will also be used by the team of NASA’s Perseverance Mars 2020 rover in the selection of rock samples to bring back to Earth. “We’ve learned something very important thanks to Curiosity: not all rocks on Mars are ideal for investigating the presence of any past life on the planet” said Ashwin Vasavada, Curiosity project scientist and co-author. “Our luck is that both types are present in the same crater, and now we can use mineralogy to distinguish them.”

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