Life on Mars? Discovery puts it ‘on the table’
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 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, Maryland, 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 154-kilometre 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,” Eigenbrode said.
In the end, a few smidgens of organics remained, including benzene and propane molecules.
Intriguingly, the organics 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,” 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.
Because methane does not last in the atmosphere, any significant amounts there today must have been released recently.