NASA’s Curiosity rover has found new evidence preserved in rocks on Mars as well as its atmosphere suggesting that the planet could have supported ancient life.
These new findings – ‘tough’ organic molecules in three-billion-year-old sedimentary rocks near the surface, as well as seasonal variations in the levels of methane in the atmosphere – have been explained in two separate papers published on June 8 in the international journal Science.
Organic molecules contain carbon and hydrogen, and also may include oxygen, nitrogen and other elements. While commonly associated with life, organic molecules also can be created by non-biological processes and are not necessarily indicators of life. However, the NASA statement maintains that the findings are a good sign for future missions to the planet.
“Curiosity has not determined the source of the organic molecules,” said Jen Eigenbrode of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is the lead author of one of the Science papers. “Whether it holds a record of ancient life or has existed in the absence of life, organic matter in Martian materials holds chemical clues to planetary conditions and processes.”
Although the surface of Mars is inhospitable today, there is clear evidence that in the distant past, the Martian climate allowed liquid water – an essential ingredient for life as we know it – to pool at the surface. Data from Curiosity reveal that billions of years ago, a water lake inside Gale Crater held all the ingredients necessary for life, including chemical building blocks and energy sources.
“The Martian surface is exposed to radiation from space. Both radiation and harsh chemicals break down organic matter,” said Eigenbrode. “Finding ancient organic molecules in the top five centimeters of rock that was deposited when Mars may have been habitable bodes well for us to learn the story of organic molecules on Mars with future missions that will drill deeper.”
Discovering organic molecules on Mars
To identify organic material in the Martian soil, Curiosity drilled into sedimentary rocks known as mudstone from four areas in Gale Crater. This mudstone gradually formed billions of years ago from silt that accumulated at the bottom of the ancient lake. The rock samples were analyzed by the Sample Analysis at Mars (SAM) instrument suite, which uses an onboard oven to heat samples at temperatures higher than 500°C so they release organic molecules from the powdered rock.
SAM measured small organic molecules that came off the mudstone sample – fragments of larger organic molecules that don’t vaporize easily. Some of these fragments contain sulfur, which could have helped preserve them.
The results also indicate organic carbon concentrations on the order of 10 parts per million or more. This is close to the amount observed in Martian meteorites and about 100 times greater than prior detections of organic carbon on Mars’ surface. Some of the molecules identified include thiophenes, benzene, toluene, and small carbon chains, such as propane or butene.
In 2013, SAM detected some organic molecules containing chlorine in rocks at the deepest point in the crater. This new discovery builds on the inventory of molecules detected in the ancient lake sediments on Mars and helps explain why they were preserved.
‘Mysterious’ methane releases in the Martian atmosphere
In the second paper, scientists describe the discovery of seasonal variations in methane in the atmosphere of the planet over the course of nearly three Mars years, which is almost six Earth years.
Water-rock chemistry might have generated the methane, but scientists cannot rule out the possibility of biological origins. Methane previously had been detected in Mars’ atmosphere in large, unpredictable plumes. This new result shows that low levels of methane within Gale Crater repeatedly peak in warm, summer months and drop in the winter every year.
“This is the first time we’ve seen something repeatable in the methane story, so it offers us a handle in understanding it,” said Chris Webster of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, and lead author of the second paper. “This is all possible because of Curiosity’s longevity. The long duration has allowed us to see the patterns in this seasonal ‘breathing.'”
Finding methane in the atmosphere and ancient carbon preserved on the surface gives scientists confidence that NASA’s Mars 2020 rover and European Space Agency (ESA) ExoMars rover will find even more organics on both the surface and in the shallow sub-surface.
“Are there signs of life on Mars?” asked Michael Meyer, lead scientist for NASA’s Mars Exploration Program, at NASA Headquarters, proceeding with the answer on his own, “We don’t know, but these results tell us we are on the right track.”