Bengaluru: NASA’s Curiosity rover has discovered sediments rich in salt in Mars’ Gale Crater, indicating that it once contained salt lakes which dried up as the planet went through prehistoric climate change. Curiosity has occupied this crater and studied it for seven years.
The finding supports the theory that Mars became arid and dry after the crater was formed, approximately 3.5 billion years ago.
The findings have implications for understanding how Mars evolved and became the dry and cold planet it is today, as well as its past habitability, as salt water bodies on Earth support abundant forms of life. The study has been published in the journal Nature Geoscience this week.
The research, led by a team from the California Institute of Technology or Caltech, discovered that exposed sedimentary rock in the 150 km-wide Gale Crater, called the Sutton Island, was enriched heavily with sulphate salts. These salts have been detected before on the crater floor, but not in such high concentrations.
The salts found in the 150m-tall rock formation were calcium sulphate and magnesium sulphate.
The researchers concluded that the salt was probably deposited along the edges of the crater’s basin, where the water was shallow, and via streams that ran along the crater walls. They also speculate that the deposits might have come from multiple ponds in the crater.
The immediate implication of the findings is that the lake could have been habitable, as the environment would have been very similar to that of salt water lakes on Earth. Sulphur also makes up an important component of life.
“We went to Gale Crater because it preserves this unique record of a changing Mars,” said lead author William Rapin of Caltech. “Understanding when and how the planet’s climate started evolving is a piece of another puzzle: When and how long was Mars capable of supporting microbial life at the surface?”
Curiosity has been steadily climbing the Gale Crater, and as it examines the rock it is on, the concentration of salts has changed.
At the bottom of the crater, where it landed, Curiosity discovered evidence of sediments that were from a freshwater lake, with no salt. But as it moved higher, the sediments started displaying varying concentrations of salt.
This indicates that the crater periodically dried out for salts to precipitate out, much like sea water evaporating and leaving behind salt. Gale Crater in the past would have experienced variations in its water composition, where the lake alternated between freshwater and brine.
These “geochemical fingerprints”, as the researchers describe the findings, show glimpses into the kind of changes in climate that life would have had to deal with as Mars lost its atmosphere and became dry, most likely owing to changes in its axial tilt.
As Curiosity climbs higher, the researchers expect to learn more about how these fluctuations occurred and how the process of loss of water took place.