NASA has just announced two major breakthroughs in the search for life on Mars: the first is that the Mars Curiosity rover has detected the presence of organic molecules preserved within the bedrock of the Martian soil; the other is that Curiosity has also detected a wide variation in the amount of methane present in the Martian atmosphere that fluctuates with the planet’s seasons. Although NASA cautions that these phenomena could ultimately have non-biological origins, they add to the growing evidence that Mars, once home to a habitable environment, might very well have had lifeforms of its own–lifeforms that may still be living there today.

Drawing on samples bored from 3.5 million-year-old bedrock in Gale Crater, the Curiosity rover discovered the presence of organic molecules, amongst them molecules such as the benzene-like compound thiophene; and dimethyl sulfide, a compound typically produced by marine algae and phytoplankton on Earth. Billions of years ago, Gale Crater itself was once the site of a large, open-water lake for at least 700 million years, in an otherwise warm, oxygen-rich environment. The presence of organic compounds in the ancient lakebed raises the possibility of life having existed there dramatically.

Curiosity has also discovered that the amount of methane (CH4) in Mars’ atmosphere fluctuates with the planet’s seasons. This pattern has not only repeated itself for the past five years, but the variation is also quite pronounced, with CH4 concentrations in the Martian summer more than tripling over their winter numbers. Although methane can be produced by non-biological processes, the main source of CH4 on Earth is as a byproduct of biological metabolism. This means that Mars’ methane, whether biological or not, could be from ancient subterranean deposits that release more of the gas when they warm up during summer. Or, there is the possibility that the Martian soil is currently home to lifeforms that release more CH4 as their metabolisms increase with the warming weather.

"The big takeaway is that we can find evidence. We can find organic matter preserved in mudstones that are more than 3 billion years old," explains Rice University geologist Kirsten Siebach, of whom wasn’t involved in NASA’s research. "And we see releases of gas today that could be related to life in the subsurface or at the very least are probably related to warm water or environments where Earth life would be happy living." Siebach adds that Mars’ methane fluctuations provide "one of the most compelling" cases for the existence of present-day life on the Fourth Planet.

This isn’t the first time that Mars has given us the hint that it might be more than just a dead, barren world: in 1976, the landers from the Viking 1 and 2 probes conducted three separate experiments to detect possible signs of life in the Martian soil. The result of one of the experiments, called the labeled release (LR) experiment, came back overwhelmingly positive from both probes’ respective samples, indicating that there was something in the soil that was metabolizing a carbon-14-rich nutrient injected into the samples. However, subsequent nutrient injections failed to produce the same results; additionally, the other two experiments, designed to detect the presence of organic molecules, came back negative. At the time, this prompted NASA to officially conclude that there was no life present on Mars.

However, in 2008, the Phoenix lander detected the presence of perchlorate salts in the Martian soil, re-opening the debate: when heated above 200ºC (392ºF), a temperature well below the 650ºC (1200ºF) that the pyrolytic release (PR) experiment heated its samples to, the substance becomes a powerful oxidizer that would have destroyed any organic molecules present in the soil samples, invalidating the results of the failed Viking experiments. In fact, in 1976 the Viking probes did indeed detect the presence of chloromethane and dichloromethane, two byproducts of such a reaction between perchlorates and organic compounds.

Unlike the Viking experiments, of which used samples taken directly from the surface of the Martian soil, Curiosity drilled down five centimeters (2 inches) into the bedrock to obtain its samples, in a layer that was less likely to be sterilized by the harsh bombardment of ultraviolet light: Mars has no ozone layer, allowing virtually all of the Sun’s UV radiation to bake the surface, rendering it sterile

One has to bear in mind that these findings aren’t direct evidence of life on the Red Planet, as the compounds that have been discovered could also have non-biological origins. However, it appears that "Mars is telling us to stay the course and keep searching for evidence of life," according Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA. "I’m confident that our ongoing and planned missions will unlock even more breathtaking discoveries on the Red Planet." 

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