Organic molecules are the building blocks of life, though they can also be produced by chemical reactions unrelated to life. They are fairly certain that it comes from melting water-based crystals, called clathrates, buried just below the planet's surface.
The methane could simply be the product of basic geological processes, but it's possible the gas has origins in biological sources.
Since Curiosity landed on Mars in 2012, the rover has been exploring Gale Crater, a massive impact crater roughly the size of CT and Rhode Island, for geological and chemical evidence of the chemical elements and other conditions necessary to sustain life.
Rocks line an ancient channel where water may have once flowed on Mars.
"This is the first time we've seen something repeatable in the methane story, so it offers us a handle in understanding it", said Chris Webster from NASA's Jet Propulsion Laboratory, the lead author of the paper.
Three Mars years' worth of data shows that along with spikes in methane, levels swing between 0.24 and 0.65 parts per billion, peaking in the northern hemisphere summer. Eigenbrode says the analyzed rocks came from the bottom of what was once a lake at a time when Mars was a much warmer, wetter place.
Several spacecraft including Curiosity have detected whiffs of this gas that "defied explanation", Webster said. The Martian surface is bombarded with radiation that can degrade organic compounds, explains Eigenbrode.
"With this new data, we again can not rule out microbial activity as a potential source", Webster said.
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On Mars, organic molecules could have been produced by some form of either present or past lifeforms. But they're of interest to astrobiologists because they are the essential ingredients in all the chemistry that drives life on Earth.
"Once it's on the surface, the temperature on the surface regulates the way in which it holds on to the methane through 'stickiness, ' or surface adsorption as we call it", he says.
Curiosity dug up samples at Mojave and Confidence Hills near Pahrump Hills. In the ashes that remained they found thiophenes-relatively small and simple ringlike molecules containing both carbon and sulfur. The fact that similar molecules were also present at this new site suggests that this kind of organic material is present in abundance.
Researchers say they can't rule out a biological source.
By examining data spanning almost three Martian years (six Earth years), Webster and his colleagues discerned the first repeating pattern in Martian methane.
"Ideally we want to get to samples that have not been irradiated". Like every planet in our solar system, it receives a steady rain of carbon-rich micrometeorites and dust from space.
While the Curiosity Rover only scrapes off the top 5 centimetres, ExoMars's MOMA lab which is planned to be launched in 2020 will go down 2 metres.
Webster says the rover results don't say whether the methane being released has been trapped for eons or is being generated now.
The Curiosity rover has lasted three times as long as it was meant to and is still going, which helped with the study of seasonal methane cycles. Its two-year mission will explore Mars to see if it's "geologically alive", or active below the surface.