The most recent studies, both published in Science, shed light on that history.
"These clathrates lock the methane inside a water-ice crystal structure and are incredibly stable for millions of years until environmental conditions change and suddenly they can release that gas", says Duffy. If there was, then that life would have left behind organic molecules when it decayed.
If Viking was phase one of our search for life on Mars, and the methodical quest for clues of habitability that followed was phase two, says Grinspoon, "this is the successful culmination of phase two".
"[Curiosity's] molecular observations do not clearly reveal the source of the organic matter in [Gale Crater]".
One big reason, ten Kate said, is that it's not actually that surprising. "It slowed down the whole Mars program, unfortunately". In this case, Curiosity found molecules with names like "thiophene" (C4H4S) and "dimethylsulfide" (C2H6S) that aren't all that rare in the solar system.
While it could be produced by microorganisms under the surface of Mars, it could also be produced by non-biological processes such as chemical reactions in rocks, or the breakdown of organic matter in dust delivered by comets or meteors, by UV radiation.
"The question of whether life might have originated or existed on Mars is a lot more opportune now that we know that organic molecules were present on its surface at that time". The organic compounds aren't even the first molecules of their kind found on Mars, though they are the oldest. But early into the rover's mission, researchers discovered that carbon-rich chemical reagents were leaking out of some of SAM's components, potentially contaminating nearby samples. But that doesn't necessarily mean it is biological in origin - it is also similar to an insoluble material in tiny meteorites that rain down on the surface of Mars. The rover spotted the chemical signature in samples take from sedimentary rocks the formed some 3 billion years ago.
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"The results convincingly show the long-awaited detection of organic compounds on Mars".
Despite this latest discovery's patchwork nature, George Cody, a geochemist at the Carnegie Institution for Science who was not involved in the work, considers it an impressive step forward.
"The wonderful consistency of the results makes me think we have a slam-dunk signal for organics on Mars". Curiosity reports that methane levels on Mars go up and down in a predictable cycle. Much as a detective figures out whodunnit by filling in all the details of a crime first, astrobiologists set about piecing together a picture of the Martian environment to figure out if the planet could even support life, now or in the past. Tellingly, the methane levels appear to periodically spike in time with Martian seasons, being about three times higher in the sunny summertime than in the darker, colder winter.
The results also indicate organic carbon concentrations on the order of 10 parts per million or more. The methane, he and his colleagues speculate, could come from aquifers melting during the Martian summer, releasing water that flows over rocks deep underground to produce fresh gas. "To do that you either have to find an outcrop at the surface that has been recently exposed ... or you have to drill deep", she said.
Other scientists who did not take part in the research had mixed reviews on findings' significance in the search for life.
"We have just satisfied a mission objective for Curiosity", says Jennifer Eigenbrode, study lead author and a member of the Mars Science Laboratory mission team. We might find even more evidence in future missions, too.
Eigenbrode said that regardless where the organic material came from, its existence means that any microbial life found on Mars would have had a food source. "Testing a hypothesis to the contrary requires a very strong body of evidence". This is different from previous measurements of methane on Mars, which could not be repeated.