Curiosity’s latest data reveal that the watery lake that once filled Mars’s Gale Crater contained complex organic molecules about 3.5 billion years ago. Hints of them are still preserved in sulfur-spiked rocks derived from lake sediments. Sulfur may have helped protect the organics even when the rocks were exposed at the surface to radiation and bleach-like substances called perchlorates.

By themselves, the new results aren’t evidence for ancient life on Mars; non-living processes could have yielded identical molecules. At a minimum, the study shows how traces of bygone martians could have survived for eons—if they existed at all—and it hints at where future rovers might look for them.

“This is an important finding,” says Samuel Kounaves, a Tufts University chemist and former lead scientist for NASA’s Phoenix Mars lander. “There are locations, especially subsurface, where organic molecules are well-preserved.”

Seasons of Methane

In addition to ancient carbon, Curiosity has caught whiffs of organics that exist on Mars today. The rover has periodically sniffed Mars’s atmosphere since it landed, and in late 2014, researchers using these data showed that methane—the simplest organic molecule—is present in Mars’s atmosphere.

Methane’s presence on Mars is puzzling, because it survives only a few hundred years at a time, which means that somehow, something on the red planet keeps replenishing it. “It’s a gas in the atmosphere of Mars that really shouldn’t be there,” says NASA Jet Propulsion Lab scientist Chris Webster.

In addition, methane’s observed behavior on Mars is bizarre. In 2009, researchers reported that inexplicable martian plumes randomly belch out thousands of tons of methane at a time.

Webster’s latest study, also published today in Science, shows that Mars seasonally “breathes” the stuff. Each martian summer, the atmosphere’s methane concentration rises to about 0.6 parts per billion. In the winter, this count ebbs by a factor of three to 0.2 parts per billion.

“We don’t have seasonal variations in many molecules in Earth’s atmosphere, so to have a planet have seasonal variations in chemistry is very otherworldly,” says Eigenbrode. “It’s an astounding observation.”

Webster and his colleagues suspect that the methane comes from deep underground, and temperature swings on Mars’s surface throttle its flow upward. In the winter, the gas could get trapped underground in icy crystals called clathrates, which may melt in the summer and free the gas.

But what’s making the methane? Nobody knows.

“We really can’t tell if this methane we see today is a current product of serpentinization [a chemical reaction between iron-bearing rocks and liquid water] or microbial activity at some depth,” says Michael Mumma, the NASA Goddard scientist who discovered Mars’s methane plumes. “Or is it something that is stored from an ancient time that’s being slowly released?”

Still Looking for Life

Experts have hailed the two new studies as milestones for astrobiology.

“It’s incredibly exciting, because it shows that Mars is an active planet today,” says Caltech planetary scientist Bethany Ehlmann, a Mars expert who wasn’t involved with the studies. “It’s not cold and dead—it’s perhaps hovering right on the edge of habitability.”

But Webster and others stress that the studies themselves aren’t evidence for life on Mars: “The observations we see do not rule out the possibility of biological activity, [but] it’s not a smoking gun for it.”

To get firmer answers, researchers will need to get equipment to Mars that’s sensitive enough to detect life’s thumb on the chemical scales. On Earth, life makes more methane and less of the gas ethane than non-living reactions do. If researchers saw this signature on Mars, the case for life would get stronger.

Future missions will help. The European Space Agency’s ExoMars spacecraft, due to land in 2020, will be able to drill more than six feet down into pristine martian soils and examine samples with its on-board suite of instruments. And NASA’s scheduled Mars 2020 rover is slated to package soil samples for future missions to pick up and return to Earth.

Even now, the ExoMars mission is making strides. The mission’s Trace Gas Orbiter arrived at Mars in late 2016, and it’s currently collecting data that will let scientists map Mars’s methane—and maybe even pinpoint its sources.

“We just a few weeks ago started our measurements in the most sensitive mode, and the teams are working hard on extracting the data on methane,” Håkan Svedhem, the project scientist for the Trace Gas Orbiter, says in an email. “We believe we will be able to present results on this in a few weeks’ time.”