Follow the Methane!
Scientists ponder their next steps in the search for life on Mars.
Last week’s announcement that the Curiosity rover has seen mysterious surges in the amount of methane in the Martian atmosphere has astrobiologists excited that it may be a sign of past or even present life. But regardless of whether the methane comes from geologic or microbial activity, finding it is difficult. The spikes in methane concentration seen by Curiosity only amounted to seven parts per billion, and that was a tenfold increase over the normal background. And, as if detecting methane weren’t hard enough, determining its origin is even tougher.
NASA says its next step in solving the puzzle is for Curiosity to keep looking for the kind of chemistry that produces methane. Finding organic molecules on Mars would strengthen the argument that the methane is biological in origin, and Curiosity scientists also reported last week that they’ve found such organic material in a rock— nicknamed Cumberland— that it drilled into in May 2013.
Nailing down the presence of organics in Cumberland was complicated by contamination problems with the Sample Analysis on Mars, or SAM, the instrument suite where the samples were analyzed. Inside SAM’s “oven” are a few compartments filled with an organic substance called MTBSTFA, which is intended to make the identification of Martian organic compounds easier when they’re cooked and put through an onboard mass spectrometer. Problem is, MTBSTFA leaked to areas where it shouldn’t be, leading to worries that SAM’s identification of organic material may be suspect.
The long delay between drilling samples from Cumberland and last week’s announcement made some scientists uneasy that the contamination might be a significant issue, says Indiana University geomicrobiologist Lisa Pratt, who chairs NASA’s outside scientific advisory group for Mars. But Cornell planetary scientist Jonathan Lunine believes the team was careful in its analysis. “I don’t have a direct involvement in what they have done, but I know people on the team, and it sounds like they’ve done really a good job in trying to eliminate that contaminant,” he said. “When they say they have evidence for organics now in a sample, they wouldn’t do that unless they had been able to eliminate or correct out this contaminant.”
The two rovers and five orbiters currently exploring Mars will be joined in 2016 by the European-Russian ExoMars Trace Gas Orbiter. Its two spectrometers, and particularly the European-made NOMAD instrument, are designed to measure the constituents of the Martian atmosphere with great sensitivity. The spectrometer has three channels, and will be able to detect methane concentrations potentially as low as 10 parts per trillion.
Even if TGO finds the methane, it will take more work to determine if it’s biological or geological. “If you can make a measurement of the isotopic ratios, you may be able to get a better idea,” says Geronimo Villanueva, a co-investigator for NOMAD at NASA’s Goddard Space Flight Center. “It’s not conclusive, but you may get a better idea if it’s biological or not. But it has a lot of caveats.”
Generally, a biological source should show more of the lighter isotopes of methane while geological sources would have a more evenly distributed ratio, he said. Measuring isotopes, however, requires large amounts of the gas. “It all depends on the methane abundances,” Villanueva said.
NASA’s next planned Mars mission is InSight, a stationary lander that will arrive in 2016 to drill under the planet‘s surface. After that comes Mars 2020, a rover based on the Curiosity design. Europe plans a pair of ExoMars missions—the orbiter in 2016 and a rover in 2018.
Meanwhile, concepts for a new generation of Mars CubeSats are beginning to take shape. Why not, some Mars scientists ask, send a fleet of these tiny spacecraft to canvass multiple areas on the planet instead of sending one large spacecraft to a single location as we have in the past? “I don’t think we need to get bigger. I think we’ve reached the acme of this technology capability,” says Pamela Clark, a geochemist at NASA Goddard. “If we fly a lot more of those bigger systems up, they are cost-constrained. We aren’t going to learn a lot more than we already know.”
Clark and her colleagues have come up with a concept for a spectrometer that she says could fly on a CubeSat. Called BIRCHES (for Broadband InfraRed Compact High-Resolution Exploration Spectrometer), it weighs only 5.5 pounds. Such a flyweight instrument might have trouble picking up tiny amounts of methane, however. By way of comparison, the tunable laser spectrometer on Curiosity weighs 8.2 pounds, and the total mass of the SAM instrument suite is about 10 times that much. A typical mass spectrometer in a laboratory is the size of a washing machine, says Lunine. While it’s possible to miniaturize spacecraft technology, he said there may be a limit to how small spectrometers can get. “Trying to put one of those on a CubeSat is an interesting proposal, I will only say that,” he says.
For now, Curiosity is still on the scene at Gale Crater, adding to scientists’ understanding of Martian geology and chemistry. The recent findings about methane and organics were a coup for a mission that encountered criticism in the past year for racking up travel time at the expense of science. Roving over the rocky Martian terrain tore up the wheels more than expected, and prompted mission managers to choose a gentler route that doubled the expected seven-month journey to Mount Sharp, the informal name for the 3.4-mile-high mountain officially called Aeolis Mons.
Last August a NASA review panel criticized the team’s plan for operations past its prime mission, and recommended that the project shorten the planned traverse distance and spend more time stopping to explore areas where water once existed, which the rover had passed on its way to Mount Sharp.
“I’d say the criticism of the senior review panel, it’s not unexpected in the sense that at some level, it’s true. We spent 14 months not sciencing that much,” said Ashwin Vasavada, the deputy project scientist for Curiosity at NASA’s Jet Propulsion Laboratory. While it was a “risky investment” to push the rover for so hard and so long, Vasavada said his team believes it will be worth it. Embedded in the sediments at the base of Mount Sharp are layers showing the area’s geological history, including how wet it once was.
Aside from the battered wheels and a focusing problem with its ChemCam laser instrument that has forced operators to use a backup method to do science, the rover is in good shape. Its nuclear battery should supply power for another two years at least, after which the rover’s work days will have to be limited.