How Did Mars Become Uninhabitable?

Mars has captured our imagination for centuries. Ever since the invention of the telescope our imagination has often drifted toward the possibility of life on Mars. Exploration of the red planet has often revealed that Mars once had plenty of water on its surface but it’s no longer there. Now NASA’s Curiosity rover has found deposits of carbon-rich minerals that could give us a much needed clue.  

Mars Curiosity Rover was launched by NASA to the red planet in 2011. It’s part of the Mars Science Laboratory mission and, like many of the explorers, is a robotic mission. It arrived at Gale Crater on 6 August 2012 and was designed to explore the geological and climatic environments of Mars, search for signs that it was once a habitable world and to that end, was essentially a sampling and analysis mission. Originally it was a two year mission but Curiosity has continued long passed the intended duration, operating now for over a decade.

Mars, the fourth planet from the Sun, has been known to be a fairly hostile world devoid of any signs of life. Gale Crater was chosen as the target for Curiosity because it’s an impact crater formed just under 4 billion years ago. The layered rock formations found on the crater walls make it an ideal location to study the red planets geological history. The analysis is completed using the on board instruments SAM (Sample Analysis at Mars) and TLS (Tunable Laser Spectrometer) that heat up the samples to almost 900 degrees Celsius with the resultant gasses being analysed.

As Curiosity explores Mars it has been measuring the isotopic composition (the ratios of different isotopes) of the minerals found in the crater. Isotopes are elements that have different masses than usual for example as water evaporates, lighter version of carbon  and oxygen were likely to evaporate leaving heavier ones behind. Eventually, over time, the heavier versions (isotopes) became an integral part of the rocks that Curiosity is analysing. The minerals are largely carbon rich but they point toward high levels of evaporation suggesting they could only have formed when Mars could support liquid water. 

David Burtt from NASA’s Goddard Space Flight Centre and lead author of the paper that describes the findings do not rule out ancient life either. He said “Our samples are not consistent with an ancient environment with life on the surface of Mars, although this does not rule out the possibility of an underground biosphere or a surface biosphere that began and ended before the carbonates formed.”

Their paper that was published in the National Academy of Sciences suggests there may have been two possible formation processes for the carbonates. A series of wet-dry cycles within the Gale Crater are one such possibility with the formation of carbonates in highly salty water under cold temperatures the other. 

The two possible formation scenarios point to two different climate models and different habitability.  The wet-dry cycling suggests an alternating climate between more and less habitable conditions. Alternatively the salty water process with cold temperatures would indicate a less habitable environment with most water locked up in ice and the rocks. Neither scenario is ideal for the establishment of complex life forms but as Burtt highlighted, it doesn’t rule out the possibility further back in the history of Mars. 

Source : NASA: New Insights Into How Mars Became Uninhabitable