Curiosity Rover Provides New Insights Into How Mars Became Uninhabitable

Data gathered by NASA’s Curiosity rover on Mars is revealing new information about how the Red Planet became uninhabitable in the distant past, attributed to a changing climate that led to the evaporation of surface water or its entrapment as ice.

Not too long ago, the concept of life on Mars was widely accepted. At worst, Mars was seen as a dying planet, but not entirely lifeless. In his novels, Edgar Rice Burroughs imagined exotic Martians riding eight-legged thoats across barren sea floors, while H.G. Wells envisioned Mars as home to highly intelligent beings constructing war machines to invade Earth. Meanwhile, serious scientists engaged in discussions about the existence of canals on Mars, speculating whether they were extensive irrigation systems meant to transport water from the poles to cities on the equator.

Well into the 1950s and ’60s, the belief persisted that some form of life existed on Mars. Even scientists who argued that Mars was a barren planet still thought it could potentially harbor mosses, lichens, and other limited vegetation.

Today, Mars is regarded as so unwelcoming to life that discovering even a single patch of lichen would likely earn someone a Nobel Prize.

The Search for Life on Mars

More than fifty years of direct exploration of Mars by a fleet of robotic orbiters and landers has confirmed that it is a lifeless planet where, if any life ever existed, it was probably no more complex than bacteria that went extinct around two billion years ago. Nevertheless, some continue to hold out hope that extremophile microbes may still survive in some deep-buried oasis.

The Harsh Martian Surface

Regarding the surface, with the majority of the Martian atmosphere having been stripped away by solar winds, Mars’s mountains and plains are so arid that they make the driest deserts on Earth appear like rainforests. The extreme dryness and constant cosmic radiation have also resulted in a peculiar and destructive chemical composition of the soil.

Around four billion years ago, Mars had a much thicker atmosphere, abundant water, lakes, and flowing rivers, with about a third of its surface covered by a shallow ocean. Although the duration and stability of these conditions are uncertain, it was a much more favorable environment for life compared to today.

Understanding the dramatic climate changes is essential for deciphering Mars’s history. Recent findings from the Curiosity rover, including carbonates collected from the 96-mile-wide Gale Crater formed by a meteor impact 3.5 to 3.8 billion years ago, shed light on this transformation.

Water History in Gale Crater

Geological evidence shows that Gale Crater once contained water, leading to the presence of minerals like clays, sulfates, and carbonates. Curiosity collected these samples using its robotic arm and analyzed them with its Sample Analysis at Mars (SAM) and Tunable Laser Spectrometer (TLS) instruments.

Carbonates are significant for climate studies because heavier isotopes tend to remain while lighter ones escape during formation. The ratios of these isotopes provide a record of the Martian climate, including temperature, water acidity, and the composition of water and the atmosphere.

David Burtt of NASA’s Goddard Space Flight Center stated, “The isotope values of these carbonates indicate extreme evaporation, suggesting they likely formed in a climate that could only support transient liquid water. Our samples do not support the idea of an ancient environment with surface life, although they do not rule out the possibility of an underground biosphere or one that existed before these carbonates formed.”

Insights into Carbonate Formation

NASA notes that the isotope ratios suggest two processes for carbonate formation: one during wet/dry cycles and another in highly saline water under cold, ice-forming conditions. This suggests that Mars may have gone through cycles of habitability and uninhabitability or that remaining water became increasingly locked in ice, with surviving liquid becoming as salty and inhospitable as the Dead Sea. A combination of these processes is the most likely scenario.

“The higher carbon and oxygen isotope values found here compared to any previously measured on Earth or Mars suggest that a process (or processes) occurred at an extreme level,” said Burtt. “While evaporation can significantly alter oxygen isotopes on Earth, the changes observed in this study were two to three times greater. This indicates two things: 1) An extreme level of evaporation was responsible for driving these isotope values to such heavy levels, and 2) The processes that would produce lighter isotope values must have been considerably less significant.”

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