NASA’s Curiosity rover has detected background levels of methane in the atmosphere of Mars, and these concentrations seem to go up in the summer and down in the winter, according to new research. Where the methane is coming from is still a mystery, but scientists have some ideas, including that microbes may be the source of the gas.

Researchers at NASA and other US universities analyzed five years’ worth of methane measurements Curiosity took at Gale Crater, where the rover landed in 2012. Curiosity detected background levels of methane of about 0.4 parts per billion, which is a tiny amount. (In comparison, Earth’s atmosphere has about 1,800 parts per billion of methane.) Those levels of methane, however, were found to range from 0.2 to about 0.7 parts per billion, with concentrations peaking near the end of the summer in the northern hemisphere, according to a study published today in Science. This seasonal cycle repeated through time and could come from an underground reservoir of methane, the study says. Whether that reservoir is a sign that there is or was life on Mars, however, is impossible to say for now.

“Are we alone? Are we the only life form?”

Methane had been detected before on the Red Planet, but the measurements were all over the place. In 2003, for instance, telescopes from Earth mapped plumes of methane of about 45 parts per billion on Mars. Other measurements were taken by spacecraft orbiting the planet. And then in 2013 and 2014, Curiosity detected plumes of methane of 7 parts per billion. Today’s study is the first one to show that methane in the Martian air seems to follow a pattern: it has a seasonal cycle, and it’s not just random. That is key for finally understanding where this methane is coming from, and whether it’s a sign that there’s life on our neighboring planet.

“Most humans, as we crawled down from trees, have wondered about, ‘Are we alone? Are we the only life form?’” says Mike Mumma, a planetary scientist at NASA’s Goddard Space Flight Center, who’s studied methane on Mars and was not part of today’s research. “If we can identify whether on Mars this methane has originated from life, that would be one way of answering that question.”

Since its arrival on Mars in 2012, Curiosity has been making lots of discoveries, including that there’s a big variety of organic matter in the soil, which could contain signatures of life, according to another study published today in Science. The rover has also been detecting methane, a gas that’s produced by many life forms here on Earth, like bacteria digesting food in the tummies of cows. On Earth, 95 percent of all the methane in the air is made by life, says lead author Chris Webster, a senior research scientist at NASA’s Jet Propulsion Laboratory (JPL).

“Methane is a signature of biological activity on Earth,” he says. So since methane was first detected on Mars, “There’s been lots of excitement. Could this methane be produced biologically? Could it be produced by subsurface microbes?”

The first detection actually came in 1969, but it was a false alarm. Back then, researchers announced that NASA’s Mariner 7 probe had sniffed the gas while flying by the Red Planet. But it turned out that it was actually carbon dioxide. Since 2004, however, both ground-based telescopes and spacecraft orbiting the planet have found plumes of methane spiking up in the air. All those measurements were random, though, making it really hard to figure out exactly what is happening on the Red Planet, Webster says.

The rover Curiosity has been a game-changer. The bot was equipped with an instrument designed specifically to sniff methane, called the Tunable Laser Spectrometer of the Sample Analysis at Mars (TLS-SAM). The instrument has a chamber that fills with Martian air and then shines a laser through it to determine which compounds, like methane, are present. Now, data collected by TLS-SAM over five years reveal that there’s a background level of about 0.4 parts per billion of methane at Gale Crate, where Curiosity has been exploring the Red Planet. But Webster and his colleagues also realized that the concentrations of the gas went up in the summer and gradually down in the winter. And that’s a key discovery, Webster says.

“All measurements to date have been tenuous: they come and go, they don’t show dependence on anything. It’s been very frustrating for everybody,” he tells The Verge. “This is the first time that any Mars methane measurements have shown any kind of repeatability.” That may help researchers get a sense of where the methane is coming from. “When something has a pattern, we may be able to explain it,” says Renyu Hu, a planetary scientist at NASA’s JPL, who was not involved in the research.

“When something has a pattern, we may be able to explain it.”

Webster already has an idea. He thinks that the methane may be coming from an underground reservoir that’s slowing seeping through the soil, through pores and cracks, and releasing into the air. “It’s a constant, gentle breathing of the surface into the atmosphere,” Webster says. The methane could be buried in the soil in crystalline ice cages called clathrates, which trap the methane when it’s cold but release it when it’s warm, says Inge Loes ten Kate, assistant professor of Earth Sciences at Utrecht University in the Netherlands, who was not involved in the research. That would explain the seasonal variation as well as the spikes of larger concentrations of methane detected by both Curiosity and other sources. If the methane is seeping through cracks, there could be occasional large bursts of methane, Webster says. “Although they’re very different phenomena, they’re probably related to the same source,” he says.

Here on Earth, in Oman, there’s an area where ancient ocean crust is out in the air and methane seeps through cracks in the ground, Mumma tells The Verge. “You can actually light it with a match and burn it,” he says. “The idea is that, perhaps on Mars, there could be similar kinds of methane coming up from deep down. It still begs the question of what produced it.”

It could be microbes, either living currently on Mars or from an ancient past. Or the methane could be made by rock-forming processes that have nothing to do with biological life. Either way, figuring out the source would be amazing. “If it’s life, that’s a key indication that life is ubiquitous,” says Mumma. “If it’s not life, that’s also important because Mars is geochemically active, and we have a window into the interior now, which is new information about another planet.”

At this stage, these are questions that can’t be answered. The measurements reported in today’s study also come from one area of Mars, and concentrations of methane might be different higher up in the atmosphere or somewhere else on the planet. The seasonal pattern also needs to be confirmed with more measurements. “At the end, the more data points, the better statistics you can do,” ten Kate tells The Verge. “So they just have to keep measuring.”

The findings could soon be confirmed by another source, the ExoMars Trace Gas Orbiter, a European Space Agency spacecraft that’s currently circling Mars, sniffing the atmosphere. The probe has been making measurements since March. “The community is waiting with bated breath on what they’re going to find because they’re looking at the whole planet,” Webster says. “Stay tuned, because this European orbiter is about to announce its results, too. So between the two missions, it’s going to be a very exciting time for the Mars methane story.”