Mars yet-but a new result makes it seem like maybe, at some point in the planet's history, the conditions were ripe for some extraterrestrial beings.
The traces of methane on Mars are tantalizing to biologists eager for evidence of life. "And we see releases of gas today that could be related to life in the subsurface or at the very least are probably related to warm water or environments where Earth life would be happy living". Organic matter can be one of several things: a record detailing ancient life, a food source for life or something that exists in the place of life. Methane previously had been detected in Mars' atmosphere in large, unpredictable plumes.
As with methane, there could well be non-biological explanations for the presence of carbon-containing molecules on Mars, such as geologic processes or impacts by asteroids, comet, meteors and interplanetary dust.
Using Curiosity's Sample Analysis at Mars instrument - which heats soil and rock samples to examine their contents - astrobiologist Jennifer Eigenbrode and her colleagues were able to identify an array of interesting organic molecules: Ring structures known as aromatics, sulfur compounds and long carbon chains.
Scientists agree more powerful spacecraft - and, ideally, rocks returned to Earth from Mars - are needed to prove whether tiny organisms like bacteria ever existed on the red planet. The 96-mile crater, named for Australian astronomer Walter F. Gale, was most likely formed by meteor impact between 3.5 to 3.8 billion years ago.
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. Pictured, a computer-generated view depicting part of Mars at the boundary between darkness and daylight, with an area including Gale Crater beginning to catch morning light. Over the intervening years, fluid flowing thought it would have initiated chemical reactions that could have destroyed the organic matter - the material discovered may in fact be fragments from bigger molecules. "The first one would be life, which we don't know about".
NASA's Curiosity Mars Rover snaps a self-portrait at a site called Vera Rubin Ridge on the Martian surface in February 2018 in this image obtained on June 7, 2018. The methane signal has been observed for almost 3 Martian years (nearly 6 Earth years), peaking each summer.
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".
But the scientists can not say what the larger molecules were or how they formed. But it is the preservation of the material that is important - if there is this much organic matter preserved close to the surface, then there should be even better protected material at greater depths.
Inclusion of carbon dioxide in the mix could potentially lower the pressures required to form these lattices, allowing methane clathrates to form just a few metres below the surface.
Curiosity has been drilling since 2013.
"We have greatly expanded our search for organic compounds, which is fundamental in the search for life", said Paul Mahaffy, study author and director of the Solar System Exploration Division at NASA's Goddard Space Flight Center.