In its quest to find molecules that could point to life on Mars, NASA's Curiosity rover has struck a gusher. Since Curiosity landed in 2012, it has sifted samples of soil and ground-up rock for signs of organic molecules—the complex carbon chains that on Earth form the building blocks of life. Past detections have been so faint that they could be just contamination. Now, samples taken from two different drill sites on an ancient lakebed have yielded complex organic macromolecules that look strikingly similar to the goopy fossilized building blocks of oil and gas on Earth. At a few dozen parts per million, the detected levels are 100 times higher than previous finds.
Although the team cannot yet say whether these molecules stem from life or a more mundane geological process, they demonstrate that organics can be preserved for billions of years in the harsh martian surface environment, says Jennifer Eigenbrode, a biogeochemist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led a study in this week’s Science. "We're in a really good position to move forward looking for signs of life."
Ever since it reached its primary target, Aeolis Mons, a 5000-meter-tall mountain rising from the floor of Gale crater, Curiosity has spent much of its time driving on a mudstone formed by sediments that settled to the bottom of a lake some 3 billion years ago, when Mars was a more clement place. Mudstones are ideal for trapping and preserving organic molecules. Because ultraviolet radiation and oxidizing compounds in the martian soil would destroy any compounds exposed at the surface, Curiosity's scientists used a robotic drill to penetrate several centimeters into the mudstone. They delivered the fresh grit to an oven inside the rover's belly.
To unlock organic molecules from the samples, the oven baked them to temperatures of between 600°C and 860°C—the range where a known contaminant disappeared—and fed the resulting fumes to a mass spectrometer, which can identify molecules by weight. The team picked up a welter of closely related organic signals reflecting dozens or hundreds of types of small carbon molecules, probably short rings and strands called aromatics and aliphatics, respectively. Only a few of the organic molecules, sulfur-bearing carbon rings called thiophenes, were abundant enough to be detected directly, Eigenbrode says.
The mass patterns looked like those generated on Earth by kerogen, a goopy fossil fuel building block that is found in rocks such as oil shale—a result the team tested by baking and breaking organic molecules in identical instruments on Earth, at Goddard. Kerogen is sometimes found with sulfur, which helps preserve it across billions of years; the Curiosity scientists think the sulfur compounds in their samples also explain the longevity of the Mars compounds.
Earth's kerogen was formed when geologic forces compressed the ancient remains of algae and similar critters. It's impossible to say whether ancient life explains the martian organics, however. Carbon-rich meteorites contain kerogenlike compounds, and constantly rain down on Mars. Or reactions driven by Mars's ancient volcanoes could have formed the compounds from primordial carbon dioxide. Monica Grady, a planetary scientist at The Open University in Milton Keynes, U.K., believes the compounds somehow formed on Mars because she thinks it's highly unlikely that the rover dug into a site where an ancient meteorite fell. She also notes that the signal was found at the base of an ancient lake, a potential catchment for life's remains. "I suspect it's geological. I hope it's biological," she says.
Curiosity has one last tool to help the team find out: nine small cups containing a solvent that frees organic compounds bonded in rock, eliminating the need to break them apart—and potentially destroy them—at high temperatures. In December 2016, rover scientists were finally prepared to use one of the cups, but just then the mechanism to extend the rover's drill stopped working reliably. The rover began exploring an iron-rich ridge, leaving the mudstone behind. In April, after engineers found a way to fix the drill problem, the team made the rare call to go backward, driving back down the ridge to the mudstone to drill its first sample in a year and half. If the oven and mass spectrometer reveal signs of organics in the sample, the team is likely to use a cup. "It's getting so close I can taste it," says Ashwin Vasavada, Curiosity's project scientist at the Jet Propulsion Laboratory in Pasadena, California.
The discovery could give a boost to future Mars exploration plans. Europe's ExoMars rover, due for launch in 2020, will drill deeper than Curiosity, to soil depths better protected from radiation. But detection of past life may ultimately take the precision analysis of labs on Earth, Grady says. "We've got to bring a sample back." In such labs, technicians can dissolve away nonorganic molecules and take a full index of the remaining organic ones, including, say, fatty acids with an even number of carbon atoms—a hallmark of life. Another clue has added to the incentive: In a separate study in this week’s Science, Curiosity scientists report that traces of methane in the martian atmosphere rise and fall with the seasons. Nonbiological processes could explain the signal—but so could seasonally varying microbes. Fortunately, NASA's next rover, Mars 2020, is set to collect some 30 rock cores for return to Earth in subsequent missions. Plans to retrieve those rocks are far from finalized—or financed—but NASA's case has gotten much stronger with the organics discovery, says George Cody, a geochemist at the Carnegie Institution for Science in Washington, D.C. "If somebody asks me to go to Congress and defend a sample return mission, this paper makes that job much easier."
That the rover team found anything at all speaks well to their planning and execution, Cody adds. Imagine drilling in Chile's Atacama Desert, which is often used as a Mars analog. "You'd be damn lucky to detect an ancient kerogen." But the Curiosity team managed it—on Mars. "They lucked out. We lucked out."