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Over a year ago, the team behind the Mars rover Curiosity declared that they hadn’t found methane on Mars. The gas is one of the potential hallmarks of life, and the discovery was a letdown. But now, the Curiosity team not only says that they’ve detected traces of methane, but also that it’s concentration in the atmosphere keeps mysteriously rising and falling.

Mars’s atmosphere contains an amount of stable methane that’s generated, for example, from the breakdown of dust and organic materials brought to Mars by meteorites. The levels are so low, though—less than half of what they expected—that its unlikely to be the signature of widespread

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NASA Senior Research Scientist Director Christopher Webster and his colleagues have analyzed 20 months of data collected by the Mars Curiosity rover in Gale Crater.

Methane levels are lower than anticipated, but NASA scientists have recorded periods of what could be high-volume methane venting.

But that’s just the background level. Every so often, methane levels detected by Curiosity shot up to ten-times higher in as short as 60 Martian days, a rate which surprised scientists since the gas sticks around for about 300 years. So where’s it coming from? According to Science’s description of the study:

Their results suggest that methane is occasionally produced or vented near the Gale Crater—and that the gas disperses quickly once these episodes of venting or production cease.

These periods of high-volume venting may contribute to a more complex atmosphere than NASA originally estimated. This study, published today in the journal Science, comes about a week after John P. Grotzinger, the project scientist for the Curiosity mission, revealed at a news conference last week that Gale Crater likely had large lakes, rivers, and deltas, on and off, for millions to tens of millions of years. One of the key ingredients to a habitable planet is this presence of water, and a telltale sign of water—past or present—can be found in hydrogen, or rather its isotopes.

In a separate study, also released today, NASA scientists examined the hydrogen isotopes in clay sampled from Mars’ Yellowknife Bay formation. They found that the clay’s ratio of deuterium (a heavy hydrogen isotope) to regular hydrogen is three times more than the ratio in Earth’s ocean water. On the other hand, the deuterium-to-hydrogen ratio within this 3-billion-year-old clay is about half that of the current Martian atmosphere. This suggests that greater amounts of the heavier deuterium remain in the atmosphere while more of the lighter, regular hydrogen has been lost to space. Determining how much has been lost will help them determine when the water started to disappear and why the Red Planet has lost so much of it.