In a study released today, the Curiosity science team announced that earlier drill samples revealed evidence of complex organic carbon molecules, the possible remains of past life.

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.

It must be emphasized once again that they have not found evidence of past life. What they have found are the types of molecules that are often left behind by life, but can also form without the presence of life.

This result, from past drillholes in the Murray Formation, explains however why Curiosity headed back downhill to do its most recent drill test.

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 newest drillhole sample has now entered the mass spectrometer. Stay tuned!