/Mars methane mystery thickens as newest probe fails to find the gas

Mars methane mystery thickens as newest probe fails to find the gas

For an odourless gas, the presence — or not — of methane on Mars has created a big stink.

Key points

Key points

  • Scientists have widely debated the existence of methane on Mars
  • First results from the ExoMars Trace Gas Orbiter reported no methane in the Mars atmosphere, in contrast to methane detections by the Curiosity rover and Mars Express spacecraft
  • Seemingly contradictory results may be explained by an as-yet-unknown process that removes methane near the planet’s surface

It’s a greenhouse gas on Earth, but it matters to planetary scientists because it may be evidence of life on our neighbour in the solar system.

The newest probe to orbit Mars — the ExoMars Trace Gas Orbiter (TGO) — failed to find any methane in the Red Planet’s atmosphere, according to a paper published today in the journal Nature.

But the gas has been detected several times in the Red Planet’s atmosphere by spacecraft and rovers since 2004.

Just last week, scientists published data that confirmed the 2013 detection of methane by NASA’s Curiosity rover as it trundled across the surface of Mars.

Along with detecting two spikes in methane, the Curiosity rover found low background levels of 0.4 parts per billion of the gas, which rose in the summer months.

These results split the planetary science community, with some suggesting the methane may have come from Curiosity itself — a claim NASA rejected.

Methane mystery thickens

The ExoMars TGO, a European Space Agency and Roscosmos joint project, began looking for methane along with dust and traces of water vapour in the Martian atmosphere last April.

Orbiting 400 kilometres above the Red Planet’s surface, instruments on the TGO look at how the thin Martian atmosphere absorbs sunlight to determine its chemical make-up.

This “solar occultation” method should detect methane levels as low as 0.05 parts per billion.

The sensitivity of the TGO varies depending upon the altitude and atmospheric conditions such as dust, said Ann Carine Vandaele of the Royal Belgian Institute for Space Aeronomy and principal investigator of NOMAD, one of two gas-sensing instruments on ExoMars.

The TGO’s optimal methane detection range is 15 to 25km above the surface, but it could pick up as little as 0.012 parts per billion at 3km altitude.

“Although none [of the observations] go down right to the surface, we went very close.”

Curiouser and curiouser

NASA’s Chris Webster, who is in charge of Curiosity’s methane-sensing instrument, said the measurements gathered by the TGO were “very credible and robust”, but he remained confident of the data collected by the Curiosity rover.

“They are really comparing apples and oranges because we’re measuring a metre from the surface in Gale Crater and they’re measuring above 3 or 4km,” Dr Webster said.

Methane, he added, could be trapped inside the lowest part of the atmosphere, called the planetary boundary layer.

“The planetary boundary layer, which is the air that tumbles around and kisses or touches the surface, is about 2km [altitude] on Mars [during the day].”

And at night, when Curiosity takes its methane measurements, the boundary layer drops to only a few hundred metres above the surface.

“Their arguments assume we would see 0.4 parts per billion continuously, but we don’t, we don’t know what we see during the day,” Dr Webster said.

Even though the TGO did not detect any methane in the Mars atmosphere overall, the results did not rule out the possibility of spikes, Dr Vandaele said.

“We are still leaving open the potential observation of plumes very localised in time and space, but even those would then be at odds with our current non-detection.

“If there are plumes, there should be a process to very rapidly destroy methane, a process which is still to be discovered.”

Planetary scientists think methane can exist in the Martian atmosphere for up to 300 years.

Dr Vandaele said any gas emitted should be spread evenly around Mars before being destroyed, so it should be detectable in “background” methane levels.

If the levels seen by Curiosity were correct, the TGO team argued, there must be some as-yet-unknown process that quickly removes methane from the lower atmosphere or stops it mixing with the atmosphere above.

Dr Webster says the 300-year lifetime of methane might not apply to the lowest layer of the atmosphere.

“Down by the surface there’s the possibility of other processes that may shorten the lifetime,” he said.

“This could include surface uptake and release … or could include other gas-surface interactions.”

dust storm

More results from the Trace Gas Orbiter

  • The methane paper is one of three from the ExoMars Trace Gas Orbiter’s first batch of results
  • Also published in Nature, a second paper tracked atmospheric water vapour during the dust storm that led to the demise of NASA’s Opportunity rover
  • It revealed levels of water and “heavy water”, which contains a hydrogen atom with a neutron in it, quickly increased at the storm’s onset
  • A third paper, which will be published in the Proceedings of the Russian Academy of Science, mapped water in the form of ice and hydrated minerals just below Mars’ surface
  • It confirmed water-rich permafrost at the poles and refined localised “wet” regions around the planet’s equatorial regions, which may have been the locations of the poles in the past

A history of contradictions

Mars exploration has a long history of contradictory results, said Gretchen Benedix, a planetary scientist at Curtin University who was not involved in the methane papers.

You only have to look back a few years to the dark streaks that appeared on Martian hillsides during summer.

“People said ‘oh, that’s liquid water seeping from these areas’,” Professor Benedix said.

But a paper published last year suggested those “briny flows” were nothing but avalanches of dry sand.

This is not the first time instruments have been unable to detect methane levels, said Marco Giuranna, of the National Institute of Astrophysics in Rome, who led the Mars Express observations.

“In the past, Earth-based observations, in-situ measurements by the Curiosity rover and Mars Express remote-sensing observations have often failed to detect methane.”

Dr Webster said it might simply be a matter of time before the TGO finds methane in the Martian atmosphere: it took Curiosity a good six months before it detected methane.

“They’re not covering the whole planet all the time … but sooner or later I believe they are going to see methane,” he said.

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