Nearly six years into its survey of a site called Gale Crater on Mars, NASA’s Curiosity rover has delivered what may be the biggest discovery yet in its quest for signs of habitability and life: Organic molecules are abundant in Red Planet rocks, and the simplest organic molecule, methane, seasonally blows through the thin Martian air. On Earth, such carbon-rich compounds are one of life’s cornerstones.
Both discoveries emerged from Curiosity’s Sample Analysis at Mars (SAM) instrument, a miniaturized chemistry lab and oven that roasts dollops of air, rock and soil to sniff out each sample’s constituent molecules. Samples of ancient mudstone yielded a diversity of organic molecules in SAM’s oven—and in a separate study, five years’ worth of atmospheric samples gathered by SAM tracked fluctuating levels of methane that peaked in the Martian summer. The results are reported in a pair of papers published recently in Science.
Although tantalizing, the two findings remain far from definitive when it comes to past or present life on Mars. Methane is ubiquitous in places like the atmospheres of gas-giant planets. It can also arise from lifeless interactions between flowing water and hot rocks whereas other simple organic molecules are known to exist in some meteorites and interstellar gas clouds. “Short of taking a picture of a fossil in a rock on Mars, [finding life there] is extremely difficult to do scientifically,” says Chris Webster, a chemist at NASA's Jet Propulsion Laboratory and lead author of the methane study.
Mars’s Missing Carbon
That Mars possesses organic molecules is not surprising. Like every planet in our solar system, it receives a steady rain of carbon-rich micrometeorites and dust from space. Yet when NASA’s twin Viking probes landed on Mars in 1976, their studies suggested something startling: Martian soil, it seemed, contained less carbon than lifeless lunar rocks. “It was a big surprise,” says Caroline Freissinet, an astrobiologist and co-author on Curiosity’s mudstone study at the Atmosphere, Media, Spatial Observations Laboratory (LATMOS) in France. “It slowed down the whole Mars program, unfortunately.”
Ever since, scientists have ardently hunted for Mars’s missing carbon—or at least an explanation for its absence. A crucial clue came in 2008, when NASA’s Phoenix lander found perchlorate salts—highly reactive molecules containing chlorine—in soil samples near the Martian north pole. Combined with high-energy ultraviolet light and cosmic rays streaming in from space, perchlorates would destroy any organic material on the surface, leaving little to be seen by carbon-seeking landers and rovers. Perhaps, some researchers speculated, Mars’s remaining organics—and thus any signs of past or present life—were locked away in its subsurface depths.
In 2015, however, Curiosity made the first tentative detection of organic molecules on Mars, finding evidence of chlorine-contaminated carbon compounds in soil samples heated to more than 800 degrees Celsius in SAM. But early into the rover’s mission, researchers discovered that carbon-rich chemical reagents were leaking out of some of SAM’s components, potentially contaminating nearby samples. To combat the contamination, the Curiosity team focused on finding more chlorine-containing organics, and limited subsequent SAM runs to temperatures between 200 and 400 degrees C.
In their new work the team checked to see what this restrictive process might have missed. After carefully accounting for background contamination from SAM, Freissinet and her colleagues baked 3-billion-year-old mudstone samples at over 500 degrees C, a temperature at which perchlorates should have fully burned away. In the ashes that remained they found thiophenes—relatively small and simple ringlike molecules containing both carbon and sulfur. The latter element, it is thought, came from a sulfur-rich mineral called jarosite that previous Curiosity investigations had revealed in 3.5-billion-year-old deposits in Gale Crater—laid down at a time when the crater was warm, wet and apparently habitable. The researchers suspect the thiophenes’ carbon came from as-yet-unidentified larger organic molecules, which had been trapped and preserved inside the jarosite for perhaps billions of years.
Despite this latest discovery’s patchwork nature, George Cody, a geochemist at the Carnegie Institution for Science who was not involved in the work, considers it an impressive step forward. The presence of these larger molecules, he says, hints at well-preserved reservoirs of carbon hidden at and just below the Martian surface—a prospect that bolsters the case for future missions to collect samples and return them to Earth. “If you can do this on Mars, imagine what you can do with analytical facilities available to us on Earth,” he says.
Methane Spikes and Changing Seasons
In the meantime Curiosity has undertaken what Webster calls “the most important measurements of Mars methane made to date.” The carbon-containing gas is significant because most methane on Earth is produced by methanogen microbes, which are common in oxygen-poor environments. Methane is also quickly broken down by ultraviolet radiation, so any of the gas discovered on Mars was probably released recently. Using SAM, Webster and his colleagues have found a persistent background level of methane in the atmosphere above Gale Crater over the last five years of about 0.4 part per billion—a scarcely detectable trace, to be sure, but enough to pique astrobiologists’ interest. Tellingly, the methane levels appear to periodically spike in time with Martian seasons, being about three times higher in the sunny summertime than in the darker, colder winter.
This periodicity is to Webster the most exciting part of his team’s results. Previous research had seen evidence for sporadic methane plumes on Mars, but never seasonally recurring events. “It’s like having a problem with your car,” he says. “If it doesn’t repeat, you can’t find out what it is.” The methane, he and his colleagues speculate, could come from aquifers melting during the Martian summer, releasing water that flows over rocks deep underground to produce fresh gas. Or it could be ancient, belched out billions of years ago by geologic or biological processes and then trapped in matrices of ice and rock that unfreeze when warmed by the sun. And, of course, there is always the chance that Martian methanogens still slumber in the planet’s subsurface even today, periodically awakening during clement periods to produce their gaseous calling-card.
Other scientists who did not take part in the research had mixed reviews on findings’ significance in the search for life. Michael Mumma, an astrobiologist at NASA Goddard Space Flight Center, considers the measurements important and says they provide ground-truth evidence for his independent (and controversial) detections of Martian methane plumes, using Earth-based telescopes. But Marc Fries, a planetary scientist who curates the cosmic dust collection at NASA Johnson Space Center, takes a more skeptical view. He points out carbon-rich meteorites and dust could generate the reported amounts of methane as they fell into the Martian atmosphere, and that the year-to-year periodicity is not wholly consistent with the timing of the Martian seasons. “A rigorous approach based on available evidence starts with the scientifically responsible default explanation that Mars is and always has been lifeless,” Fries says. “Testing a hypothesis to the contrary requires a very strong body of evidence.” Such tests could come soon, via data from the joint European and Russian ExoMars Trace Gas Orbiter. It arrived at Mars in 2016 and is now mapping concentrations of methane and other gases from on high.
For his part Webster says he has no preference among the different explanations, and believes it will take a long time before any final conclusions can be drawn. Such incremental progress is the whole point of NASA’s Mars exploration program, Freissinet notes. “It’s step by step,” she says. “Mission after mission.”