Methane, a gas that offers a tantalizing hint of possible life below the barren landscape of Mars, displays a now-you-see-it, now-you-don’t quality that is confounding scientists.
Last week, scientists reported that a careful analysis of data collected by the European Space Agency’s Mars Express orbiter revealed a plume of methane rising above Gale Crater near the Martian Equator in June 2013. The measurement occurred just one day after NASA’s Mars rover, exploring inside that crater, had also detected elevated levels of methane.
But now, a newer European spacecraft, the ExoMars Trace Gas Orbiter, has found no methane at all.
“It’s really hard to reconcile,” said Oleg Korablev of the Space Research Institute in Russia and principal investigator of the Trace Gas Orbiter instrument that probed the Martian atmosphere for methane.
The newer spacecraft entered orbit around Mars in 2017 and carries more sensitive instruments than Mars Express, which arrived in 2003. While the Trace Gas Orbiter has so far failed to find methane, that does not mean Mars Express and the Curiosity rover were wrong.
Methane, if it is there in the thin Martian air, is significant, because sunlight and chemical reactions would break up the molecules within a few centuries. Thus any methane detected now must have been released recently.
On Earth, almost all methane is produced by living organisms, although it is also generated by geothermal reactions devoid of biology.
Dr. Korablev and his colleagues reported their findings on Wednesday at a meeting of the European Geosciences Union in Vienna, and in an article published in the journal Nature.
In the Nature article, the scientists said that first batch of scientific data, from April through August 2018, showed no signs of methane. Dr. Korablev said that readings since have also come up empty.
The orbiter’s instrument is sensitive enough to conclude the air contained less than 50 parts per trillion of methane. That appears to contradict Curiosity’s readings, which did detect methane, at levels 10 times higher.
The 2013 plume observed by Curiosity and Mars Express was even more striking, about 100 times as much methane as what the Trace Gas Orbiter data says is not there.
“So what’s going on?” said Dorothy Z. Oehler, a senior scientist at the Planetary Science Institute, based in Tucson. “Nobody really knows.”
Dr. Oehler was involved with the Mars Express research but not the Trace Gas Orbiter findings.
Researchers first reported the detection of methane in the Martian atmosphere a decade and a half ago based on measurements by Mars Express and telescopes on Earth. But because those claims were at the edge of what the instruments were capable of measuring, many scientists remained skeptical.
The Trace Gas Orbiter uses a different technique. From behind Mars, it looks at sunlight passing through the planet’s atmosphere. Specific colors of light absorbed by the air provide fingerprints that identify specific molecules.
So far, the fingerprint for methane is missing.
However, the orbiter and Curiosity are looking at different places. The orbiter has yet to make a measurement directly over Gale Crater. Because clouds and dust block sunlight, the Trace Gas Orbiter is peering at least several miles above the surface, while Curiosity is measuring what is at the surface.
In addition, Mars was enveloped by a giant dust storm much of last year — the same one that ended the mission of NASA’s Opportunity rover. In 2018, the Curiosity rover only made a single methane measurement, in June. Some scientists think that the methane in the atmosphere at that time could have stuck to the storm’s dust particles, speeding up its cleansing from the air.
Still, scientists had expected that any releases of methane would mix throughout the Martian atmosphere, much as a drop of food coloring diffuses in a glass of water.
In an email, Christopher R. Webster, a scientist at NASA’s Jet Propulsion Laboratory who leads Curiosity’s methane measurements, said the new results “show that the Curiosity background levels are not representative of the planet as a whole, and this is a big surprise.”
Dr. Oehler said at first she wondered whether there might be something wrong with the instrument aboard the Trace Gas Orbiter. But scientists working on the mission persuaded her that it was correctly identifying other molecules in the atmosphere.
Methane does seem to waft episodically into the atmosphere, she said, pointing out that both Curiosity and Mars Express observed methane in the same place at the same time in 2013.
“That kind of tipped the balance to me,” she said. “That’s just too coincidental.”
The mystery has scientists wondering not only whether there might be methane-producing microbes living in the rocks of Mars, but “maybe there’s methane-utilizing bacteria that is gobbling it up the minute it’s produced,” Dr. Oehler said.
Lisa M. Pratt, NASA’s planetary protection officer who is responsible for limiting human contamination of Mars, is also intrigued.
“The mysterious appearance and disappearance of methane in the Martian atmosphere remains the most compelling hint of possible life in the subsurface of Mars,” Dr. Pratt said. “What is needed now is a carefully planned and coordinated set of observations between the Trace Gas Orbiter and Curiosity.”