NASA rover finds no methane on Mars, yet
Non-detection casts doubt on previous claims of methane hotspots due to microbes.
The question of methane on Mars isn’t dead yet, but NASA’s Curiosity rover has at least put a first nail in the coffin.
On Friday, scientists on Curiosity announced that they had not detected methane with any confidence — though they left themselves some wiggle room for revision, saying that the 95% upper and lower confidence limits of the non-detection varied between -2 and 5 parts per billion.
“Bottom line, we have no detection of methane so far,” says Chris Webster, a Mars scientist at the Jet Propulsion Laboratory in Pasadena, California, and principal investigator for the Tunable Laser Spectrometer (TLS), the rover instrument central in seeking the gas. “Mars may yet hold surprises for us.”
On Earth, life is responsible for the vast majority of the planet’s atmospheric methane, which exists at levels of about 1,700 parts per billion. If methane were detected on Mars, microbes could thus be invoked as its source, though trace amounts could also be produced via comet impacts or chemical reactions underground involving rocks and hot water.
Various campaigns in the last decade have claimed to detect martian methane at levels as intriguingly high as 30 parts per billion1 and 45 parts per billion2. But more perplexing was the way that the methane signals sometimes appeared as hotspots, or plumes, and then disappeared — implying both a sudden injecton, as well as an unknown process to destroy the methane quickly, which would otherwise mix in the atmosphere and persist. Skepticism for these claims has abounded (See “Curiosity set to weigh in on Mars methane puzzle” ).
Is there methane on Mars? The question has dogged scientists since 1969, when George Pimentel at the University of California, Berkeley, an instrument leader on NASA's Mariner 7 programme, held a press conference to announce that methane had been detected near Mars’ south polar cap. The revelation came less than 48 hours after his team received the data it was based on; he retracted the finding a month later after realizing that the methane signal was actually coming from carbon dioxide ice.
It is easy to understand why scientists are so keen for an answer. Although there are plenty of ways to make trace amounts of methane, levels of more than a few parts per billion would imply the presence of an unexpectedly active source — and raise the possibility that the planet supports methane-producing microbes.
NASA's Curiosity rover is poised to settle the question as early as this week. But the tale of George Pimentel, and a handful of hotly debated methane detections reported over the past decade from orbiting spacecraft and ground-based telescopes, have instilled a sense of caution in the rover science team. “We’re committed to getting this right,” John Grotzinger, the Curiosity project scientist at the California Institute of Technology in Pasadena, told Nature on 17 October, during a meeting of the American Astronomical Society’s Division for Planetary Sciences in Reno, Nevada.
Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland, is trying to wait patiently. In 2009, he reported finding seasonal plumes of methane following an analysis of observations made years earlier with telescopes in Hawaii1. In 2003, methane levels in one of the plumes reached 45 parts per billion, but three years later the methane had all but disappeared. Now, Mumma says he has results from 2009–10, gathered using even larger telescopes, that indicate no methane, although the upper limits of his error bars reach 6 parts per billion.
Mumma says the new results don’t worry him, but others suspect that what he saw in 2003 was really a mirage. “He’s not seeing anything now, and that’s a comfort to me,” says Kevin Zahnle of the NASA Ames Research Center in Moffett Field, California, who worked on a critique of recent positive detections2.
Zahnle’s main problem with the observations made by Mumma and others is not the existence of methane, but its extreme variability. Mars has an atmosphere that would quickly mix methane. A disappearing plume implies not only a sudden injection, but also a massive sink. The main method by which methane is destroyed — photochemical dissolution in the atmosphere — yields an average methane lifetime of about 300 years. A disappearing plume would require a lifetime on the order of months. Alternative mechanisms have been proposed to account for this, but they also face problems3. Curiosity is poised to break the stalemate.
The rover’s chief tool for spotting methane is its Tunable Laser Spectrometer (TLS). Several times now, during the Martian night, a valve on the rover has opened and let air into a 20-centimetre-long chamber with a mirror at each end. A mid-infrared laser passes through the gas, and if methane is present it will absorb the laser light at particular frequencies. If the instrument is performing as designed, a single 15-minute test should be sufficient to detect methane levels down to 0.3 parts per billion. By looking at the carbon isotopes that make up the methane, the TLS may also be able to distinguish between biological and non-biological sources.
Even the sceptics say that they would not be surprised if Curiosity sniffed a trace presence of methane, on the order of one to two parts per billion. That’s because there are non-biological ways of maintaining such modest amounts. Comet impacts could leave some methane. Hot water, reacting with olivine-rich rock, creates methane along with the mineral serpentinite. And clathrates, icy lattices that trap gas molecules, could be gradually leaking methane produced in previous geological epochs.
In the first test of the TLS, about three weeks after the rover’s 6 August landing, there was a strong methane signal. But that, it later emerged, was from a separate ‘foreoptics’ cell that the laser shoots through on its way to the mirrored chamber. The cell has not yet lost all of its Earth air, which is comparatively rich in methane. “It was a surprise,” says Grotzinger, who adds that the unexpected result gave the team an additional opportunity to calibrate the instrument.
Since then, the group has been silent on the question of Mars methane. At the Reno meeting, when Mumma stood up to ask Grotzinger about TLS results, the response was blunt. “Stand by.”
Now the wait may finally be over. NASA has announced that Grotzinger’s team will discuss atmospheric measurements at a briefing on 2 November. If the rover has detected methane at sufficiently high concentration, or exhibiting temporal variations of the kind that suggests microbial activity, then it will surely motivate a desire to identify and map the sources. “If it’s there, we really ought to figure it out,” says Philip Christensen, a Mars scientist at Arizona State University in Tempe.
But, when it come to methane on Mars, 'if' has always been the operative word. Just ask George Pimentel.
While the Curiosity result would also seem to cast doubt on previous claims, one proponent, Michael Mumma, of Goddard Space Flight Center in Greenbelt, isn’t backing down yet. The plume he detected back in 2003 was in a different part of the planet, and could have dispersed by now, nearly a decade later, at the rover’s landing site. “Gale crater is not an auspicious place to search for current releases,” he says.
The TLS team will continue to take small gulps of Mars air in an effort to beat down their uncertainties. They have used the instrument four times, which works by firing a laser into a small mirrored chamber; if methane is present, absorption lines should appear at key frequencies. But on the first two occasions, a large methane signal of 7 or 8 parts per billion was present. The team quickly realized it was due to contamination by residual Earth air. The team has pumped out this residual air as best they can, and have also devised a workaround — but it’s a more complicated protocol that increases the systematic errors.
The TLS team has another trick that it plans to use in the coming weeks or months that would make the methane show up far more noticeably. By stripping the Mars air of carbon dioxide and concentrating it prior to illuminating it with the TLS laser, Webster says the concentration of methane can be increased by a factor of 10 or more, making it stand out sharply. He says the instrument should ultimately be able to detect methane at levels as low as 100 parts per trillion.
But at that sensitivity — not as good as had been promised pre-launch — many natural processes could be responsible. Moreover, another key capability of the TLS will be mooted: distinguishing between methane with different isotopes of carbon. The team had hoped to see the slightly different absorption lines associated with methane comprised of carbon-12 — the type that microbes tend to produce — and methane made with the carbon-13 isotope, which tends to be non-biological in origin. Webster says these differences would be noticeable only if methane existed at levels of several tens of parts per billion, which is now ruled out.