Richard A Lovett
NASA’s Curiosity Mars rover has hit new scientific pay dirt, unearthing “conclusive” evidence of organic molecules — the building blocks of life — in ancient lake bed sediments. It’s also found a pattern to the levels of methane, another chemical often associated with life, in the planet’s atmosphere.
Organic chemicals are ones that contain carbon and hydrogen, often along with other elements, such as oxygen and nitrogen. Methane, the formula for which is CH4, is the simplest such chemical.
Neither discovery proves that Mars has or ever had life. But they are important enough that Inge Loes ten Kate, an astrobiologist at Utrecht University, Netherlands, who was not part of either study, calls both of them “breakthroughs in astrobiology”.
“We found organics from rocks in an ancient lake bed,” says Jen Eigenbrode, an astrobiologist at NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, US, who is lead author of the rock-study team. “These organics could have come from life.” {%recommended 1045%}
Though, she notes, they could also have come from organic-rich meteorites that hit Mars long ago, or been formed by purely geological processes.
“The information we have doesn’t tell us which source is responsible,” she says.
Eigenbrode’s team found these chemicals by careful study of samples drilled out of rocks near the base of Mount Sharp, the 5,500-metre peak that Curiosity has been slowly ascending since 2014.
The analysis was so painstaking that it’s only now that Eigenbrode’s team was confident enough in the results to release them.
“It’s taken a few years to get the results we have today,” says project scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory (JPL), Pasadena, California.
The wait, however, was time well spent. “[The data] provide conclusive evidence for the presence of organic compounds,” ten Kate says.
Among the matter discovered were ring compounds such as benzene and chains such as propane and butene. And because these only baked out of the rock samples at high temperatures, Eigenbrode says, it’s likely that they came from bits of larger compounds, called macromolecules, such as kerogen, a sheet-like polymer found on Earth in coal and black shale.
Of particular interest is a molecule known as thiophene (C4H4S), which has a pentagonal ring-like structure with four carbons and a sulfur atom. It’s an interesting discovery, Eigenbrode says, because such sulfur-containing compounds are resistant to degradation — a possible explanation for how organics survived in three-billion-year-old rocks.
Meanwhile, a second team led by Chris Webster, a senior research fellow at JPL, reported that atmospheric levels of methane vary during the Martian seasons, peaking in the summer and dropping in autumn and winter.
On Earth, methane comprises about two parts per million of the atmosphere and is produced by everything from cow farts to decomposing plants and leakages from natural gas production.
On Mars, methane levels are several thousand times lower — low enough that they may be produced entirely by non-biological processes such as the interaction of certain types of volcanic rock with water. Previously, Webster says, orbital observations have reported short-lived plumes of methane at various places around the planet, but these have been frustrating, because each was a one-off event, with no discernible pattern.
But now, he says, an analysis of air samples taken by Curiosity over the course of nearly six Earth years (equivalent to three Mars years) has revealed that Martian methane levels cycle strongly through the seasons. And it’s not a small effect.
“The seasonal cycle changes by a factor of three,” he says.
Why isn’t clear. Most likely, Webster says, methane comes from somewhere in the subsurface. From there, it slowly makes its way upward — steadily for most of the way, since the subsurface temperature isn’t much affected by the seasons. Then, as it gets close to the surface, it encounters zones where temperatures vary seasonally, allowing more to escape when the surface is sun-warmed, and less when it is colder.
But the important thing is that there’s a pattern.
“This is the first time we’ve seen something repeatable in the methane story,” Webster says. “It gives us something to test our models against.”
As with the organics detected by Eigenbrode’s team, the source of this methane remains to be determined. “We don’t know if [it] is ancient or modern,” Webster says. “We don’t know if it’s from rock chemistry or created by microbes.”
Both studies fuel interest in continued Mars exploration.
“Mars is telling us to stay the course and keep searching for evidence of life,” says Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters, in Washington DC.
Eigenbrode’s and Webster’s findings were published in the journal Science, along with a commentary by ten Kate.
