Scientists for the first time have confidently identified on Mars a collection of carbon molecules used and produced by living organisms.
That does not prove that life has ever existed on Mars. The same carbon molecules, broadly classified as organic matter, also exist within meteorites that fall from space. They can also be produced in chemical reactions that do not involve biology.
But the discovery, published on Thursday by the journal Science, is a piece of the Mars puzzle that scientists have long been seeking. In 1976, NASA’s two Viking landers conducted the first experiments searching for organic matter on Mars and appeared to come up empty.
“Now things are starting to make more sense,” said Jennifer L. Eigenbrode, a biogeochemist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and lead author of the Science paper. “We still don’t know the source of them, but they’re there. They’re not missing any more.”
The data comes from NASA’s Curiosity rover, which has been exploring a former lake bed within the 96-mile Gale Crater where it landed in 2012. The discovery shows that organic molecules can be preserved near the Martian surface, surviving the bombardment of radiation from the sun.
“It’s very exciting for Mars geology and for the search for life,” said Sanjeev Gupta, a professor of earth sciences at Imperial College London in England, who was a co-author on the paper.
A second paper in Science adds wrinkles in the Martian puzzle of methane — a simple molecule of one carbon and four hydrogen atoms — that could also play an important part in figuring out whether life ever arose there and might even persist underground today.
The organic matter was found in pieces of solidified mud that Curiosity drilled into in 2015. The rocks formed about 3.5 billion years ago when Mars was drying out, although Gale Crater was still filled with water for stretches of thousands to millions of years.
The rock fragments were heated to more than 900 degrees Fahrenheit, and the rover’s instruments looked at the molecules that wafted away at the high temperatures. Then the scientists sifted through the results to figure out what might be genuine Martian organics.
The analysis was complicated in part because a cup of solvent within the rover’s mobile laboratory had leaked, contributing misleading signals. In addition, some of the readings could have come from contamination that had tagged along from Earth; others could have been produced in combustion as the sample was heated, which may have been the case in an earlier detection of organics by Curiosity.
“If we weren’t sure, we removed it,” Dr. Eigenbrode said.
In the end, a few smidgens of organics remained, including benzene and propane molecules.
“The detective work they did is worthy of Sherlock Holmes,” said Katherine Freeman, a professor of geosciences at Pennsylvania State University who was not involved with the research. “What they show is that organics were present early on in Mars.”
Intriguingly, the organics Dr. Eigenbrode and her colleagues detected looked like they were pieces that came from more complex material. The molecules could have come from something like kerogen, a component of fossil fuel that is found in coal and oil shale.
But the scientists cannot say what the larger molecules were or how they formed.
“We’ve considered three possible sources for the organics: geology, meteorites and biology,” she said. When they did experiments in their laboratory on Earth to bake samples containing those three types of organic carbon, the readings were all consistent with what was detected on Mars.
That means they do not have compelling evidence for a biological origin of the carbon, but the possibility is not ruled out, either. “It's on the table with all the other ones,” Dr. Eigenbrode said.
In the second Science paper, scientists led by Christopher R. Webster of the NASA Jet Propulsion Laboratory in Pasadena, Calif., find that levels of methane in the thin Martian atmosphere are usually very low, less than 0.5 parts per billion by volume. But with data now extending over five years, the scientists reported that methane levels go up and down by a factor of three, and the variations appear to follow Martian seasons.
“It's very, very fascinating and puzzling,” Dr. Webster said.
Planetary scientists originally expected little methane in the Martian atmosphere, because that molecule is readily destroyed by sunlight and chemical reactions. But in 2003, observations from Earth indicated plumes of methane over parts of Mars. Those readings vanished two years later.
Because methane does not last in the atmosphere, any significant amounts there today must have been released recently. Methane can be created by geological interactions between rock, water and heat, or it could be a product of microbes that release methane as a waste product.
Curiosity added to the mystery when it looked for methane and initially didn’t find any at all. A refined technique was able to measure lower levels, below 1 part per billion. Then in 2013 it recorded a burst of methane that lasted at least two months.
The rover has measured a few more methane spikes, but the new wrinkle is the undulations in the low background levels — higher in summer than winter. With the seasonal patterns, scientists can now begin to test ideas on the source of the methane, Dr. Webster said.
Michael J. Mumma, a scientist at the Goddard Space Flight Center who led earlier Earth-based measurements and who is not involved with the Curiosity research, said the work was carefully done and confirms the low background levels, but that he was not yet convinced of the seasonality of the variations.
He said his team has conducted another round of ground-based measurements earlier this year but they have not yet analyzed the data.
Additional information will come from the European Space Agency’s Trace Gas Orbiter, which began its scientific data gathering a couple of months ago. Eventually, it will produce a global map of methane, but no results have been released yet.
Future missions could also provide additional clues to help scientists solve both the methane and organics puzzles. NASA’s InSight spacecraft, currently en route, will measure marsquakes. It is possible that the impacts of meteors, which InSight might be able to record, rupture the surface and allow underground pockets of methane to rise into the atmosphere.
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The InSight spacecraft is heading to Mars to listen for marsquakes and study the planet’s structure.
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Two rovers launching in 2020, one from NASA and one from the European Space Agency, will also gather better rock samples to study organics. The European one will be able to drill a couple of yards into Martian rock, much deeper than the couple of inches that Curiosity was able to delve into.
The next NASA rover plans to collect rocks that will be brought back to Earth on a later mission where scientists will be able to examine them with a much wider array of instruments.
“Imagine what we can do on Earth in Earth laboratories in 10 years’ time,” Dr. Gupta said.