Is there life on Mars? The question has intrigued us for as long as we have been studying our red planetary neighbour. And yet, only one space mission—the twin Viking probes in 1976—has been tasked with the direct objective of searching for life, with inconclusive results. Since then, all that has been done is to assess the habitability of the Martian environment and look for possible indirect signs of life. Among these, one in particular has intrigued researchers for the past 15 years: methane, a gas that may appear through geological processes, but which on our planet comes mostly from living organisms. Could this be a strong clue to the discovery of Martian life?
In 2003, NASA astrobiologist Michael Mumma reported the detection of methane on Mars by terrestrial telescopes. The following year, data from the Planetary Fourier Spectrometer (PFS), an instrument aboard the European Space Agency’s (ESA) Mars Express orbital probe, also revealed the presence of methane in the Martian atmosphere. Those responsible for the study wrote: “The source of methane could be either biogenic or nonbiogenic, including past or present subsurface microorganisms, hydrothermal activity, or cometary impacts.” The authors stressed “that the detection of methane does not imply the presence of life on Mars, now or in the past.”
Methane itself is not indicative of life. But on Earth there are methanogenic archeas (microbes distinct from bacteria, producers of methane) in communities independent of sunlight and oxygen, in deep aquifers and oceanic hydrothermal chimneys. Previously it had been speculated that similar microbes could exist on Mars using CO (carbon monoxide) as a source of carbon and H2 (hydrogen) as a source of energy; both gases are present in the Martian atmosphere.
But after some intriguing initial observations, Martian methane began to play hide-and-seek. While new data supported the earlier findings, other measurements, however, did not find the slightest trace of the gas, as was the case with NASA’s Curiosity rover. The contradictory data puzzled the scientists, until Curiosity itself turned the story around.
A turning point in the story
In January 2015, the journal Science reported that the rover had finally recorded, between late 2013 and early 2014, a tenfold increase in methane levels on four different occasions and at its specific location, the Gale crater. While the proportions were hundreds of times lower than on Earth, the researchers, led by Christopher Webster of the Jet Propulsion Laboratory, concluded that: “Mars is episodically producing methane from an additional unknown source.”
This rapid coming and going of methane in a particular place is a rarity. It would be expected that the gas, whether of geological or biological origin, would be distributed throughout the atmosphere, where sunlight would slowly degrade it over 300 years. None of this seemed to be happening on Mars; instead, methane seems to pop up suddenly in certain regions, where it disappears as quickly and mysteriously as it emerges. “Methane is rapidly destroyed, much faster than photochemistry alone can explain,” Mumma points out to OpenMind. “My opinion is that methane on Mars has been firmly established, but equally firm is the view that we have much to learn about its origin and its destruction.”
In June 2018, new Curiosity data added one more clue to the enigma: methane levels seemed to follow a seasonal variation, with a late summer peak in the Martian northern hemisphere. Mumma and his collaborators had already suggested that methane might be trapped in the subsoil and be released by ground warming caused by the Sun, a hypothesis later developed by John Moores of York University (Canada). Webster, the principal investigator of the Curiosity instrument that detected methane, agrees with this explanation. As he told OpenMind, “the mechanism proposed by Moores et al. associating the seasonal variation with changes in surface adsorption and subsequent release seems promising.”
A scientific puzzle
The Curiosity data has also been confirmed by the PFS instrument of Mars Express. In April 2019, it was reported that the European probe detected from its Martian orbit the same methane peak in the Gale crater reported by Curiosity, one day later. For the first time the same episode was confirmed at the same time by two independent pathways.
However, almost simultaneously, disconcerting news arrived again. Scientists in charge of the ExoMars Trace Gas Orbiter (TGO) orbital probe—a joint mission of ESA and the Russian space agency Roscosmos—capable of detecting tiny traces of gases in the Martian atmosphere, reported that there did not seem to be the slightest trace of methane on Mars, an observation they made last year. “The aligning of TGO and Curiosity results is a big scientific puzzle,” Oleg Korablev, of the Space Research Institute of the Russian Academy of Sciences and principal investigator of the TGO’s Atmospheric Chemistry Suite (ACS) instrument, tells OpenMind.
“The only explanation is that methane, once released, does not mix with the atmosphere, as expected from the results of every atmospheric model, but is instantly destroyed or sequestered near the surface by some yet unknown and unexplained process,” says Korablev. For Marco Giuranna, at Italy’s National Institute of Astrophysics and principal investigator of the Mars Express PFS instrument, the findings from Mars Express and the TGO “don’t contradict each other,” but “are parts of the same story.” As Giuranna explains to OpenMind, “the non-detection of methane on Mars is what we expect to be the rule, not the exception.” “Methane spikes may only be detected occasionally, when rovers, landers or obiters happen to be at the right place at the right time.”
Another big question to solve
But despite the fact that in the end all the data seem to point to the same dynamics of Martian methane, there is still one big unanswered question: what mechanism is capable of removing the gas from the atmosphere so quickly? In June 2019, Curiosity went from detecting the highest concentration of methane it has ever recorded to witnessing those levels drop by a factor of 20 in just a few days.
Now, perhaps the mystery is closer to resolution. Researchers at the University of Aarhus (Denmark) have proposed a process of electrical rubbing of methane by wind, called saltation, which could explain the rapid ionization of the gas to form other compounds. Although at the moment they are only laboratory simulations, “we expect that the proposed mechanism can explain the fast disappearance of methane,” Svend Knak Jensen, the researcher in charge of the research, tells OpenMind. “We are in the process of doing new simulation experiments where we measure the disappearance of methane as function of the time-length of the wind action, and the preliminary data are encouraging.” In addition, adds Knak Jensen, “there is some circumstantial evidence that the proposed mechanism is in operation on Mars.”
But even if this brings us closer to solving the methane puzzle, there is a counterpart: according to Knak Jensen, the compounds formed as a result of this process would be highly toxic to bacterial life. Unfortunately, it seems that the main question, whether there is life on Mars, is still far from being answered.