The latest evidence for methane and other organic chemicals on Mars isn’t the smoking gun for life on Mars that some folks may have been hoping for, but it gives astrobiologists much more to go on.
Readings gathered by NASA’s Curiosity rover show a seasonal cycle in the rise and fall of methane in Mars’ atmosphere, as well as conclusive evidence of organic molecules in drilled-out Martian rocks. The findings are laid out in this week’s issue of the journal Science.
The reports are consistent with past observations, including previous detections of organic molecules as well as measurements of atmospheric methane going back more than a decade. The readings could be explained by past or present biological activity, but non-biological explanations could serve as well.
The big difference this time around is that the signals of organic molecules are orders of magnitude stronger than those previous hints, thanks to more precise, targeted measurements.
“Both these findings are breakthroughs in astrobiology,” Inge Loes ten Kate, a planetary scientist at Utrecht University in the Netherlands, wrote in a commentary for Science.
The readings came from Curiosity’s locale in Mars’ Gale Crater, which is thought to have been filled with water billions of years ago.
One science team, led by Christopher Webster of NASA’s Jet Propulsion Laboratory, found that Mars’ atmospheric methane levels rise as the planet’s seasons in the northern hemisphere progress from winter to late summer, then fall back down as the seasons cool.
The measurements were made over the course of three Martian years using the Tunable Laser Spectrometer, which is part of a suite of instruments known as Sample Analysis at Mars, or SAM.
Webster and his colleagues say the best non-biological explanation for the phenomenon points to the possibility of subsurface reservoirs of frozen methane. The methane molecules might be trapped within water-based crystals known as clathrates, and might seep through to the surface as the weather warms.
“It’s only when it gets to the surface, where the surface temperature has that seasonal cycle, that that modulates the release of that methane into the atmosphere,” Webster said.
Another science team, led by Jennifer Eigenbrode of NASA’s Goddard Space Flight Center, conducted chemical analysis of Martian rock powder using SAM’s sample-cooking oven.
SAM had detected earlier evidence of organic chemicals, but ten Kate noted that questions remained about those measurements because they could have been due to the contamination or the breakdown of perchlorate salts in SAM’s oven. To avoid raising those questions, Eigenbrode and her colleagues focused only on the gases that were released at higher temperatures.
After careful study, the researchers found clear evidence for three types of organic chemicals, classified as thiophenes, aromatics and aliphatics.
Eigenbrode said the molecules that she and her colleagues identified could have broken off from larger molecules of an organic compound known as kerogen. On Earth, kerogen is found in fossil fuels such as coal and oil shale, but they’re also found in meteorites from space.
“There were a lot of people who thought we weren’t going to find all the organic molecules that we did,” Eigenbrode said. “Now we have a diverse set of molecules [so] that we can now start to understand a little bit more about how this material is preserved and where else we might look to get more.”
In this context, “organic” doesn’t necessarily mean the compounds were created or used by organisms. Rather, the term merely denotes that the molecules contain carbon and hydrogen atoms. But such molecules are considered the foundation for chemicals of biological origin.
Researchers still have to work out the detailed implications of their latest findings. Webster and his colleagues, for example, say the strong pattern in the rise and fall of atmospheric methane “indicates that there remain unknown atmospheric or surface processes occurring on present-day Mars.”
Ten Kate said the newly announced detections have “far-ranging implications in light of potential past life on Mars.” The research builds on previous reports from the Curiosity team about Mars’ habitability in ancient times, she said.
More answers could come as the Curiosity mission continues. And in 2020, another NASA rover and the European Space Agency’s ExoMars rover will focus more sharply on the questions about life on Mars.