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The question of whether Mars ever hosted life, or still hosts it today, remains one of the most heated debates in science. At every major planetary conference, scientists present new data that seems to shift the understanding of Martian biology. For a few weeks, it feels like a breakthrough is imminent. Yet, after more than twenty years, one significant mystery persists: the origin of methane detected on Mars.
The debate centers on observations made by the Tunable Laser Spectrometer (TLS) on NASA’s Curiosity rover. This instrument has repeatedly reported several detections of methane in the Martian atmosphere. However, these findings are difficult to reconcile with other data. The controversy actually began much earlier, with the European Space Agency’s (ESA) Mars Express spacecraft.
Mars Express began its orbital science operations in 2004. During its first year of observations, a small spectrometer on the spacecraft detected a faint spectral bump. This feature indicated the tenuous infrared absorption of methane in the red planet’s atmosphere. However, the detections were at the very limits of the instrument’s sensitivity. The Planetary Fourier Spectrometer, as the device was called, struggled to provide clear proof.
At the same time, two independent research groups using ground-based telescopes claimed they could also see Martian methane through Earth’s atmosphere. Kevin Olsen, a planetary scientist at the University of Oxford and a co-investigator for the ExoMars Trace Gas Orbiter’s Atmospheric Chemistry suite, noted the conflicting reports. He explained that while the evidence was not entirely convincing, it sparked a lasting scientific curiosity.
The search for methane is not just about gas; it is about habitability. "The most important thing people care about is whether Mars has been or is habitable, and whether there's microbial life, and in the atmosphere the big thing is methane, which we don't see," says Olsen.
The Trace Gas Orbiter (TGO), part of the ExoMars mission, was designed to address these uncertainties. Its primary objective is to definitively detect trace gases, including compounds containing sulfur, chlorine, and methane. The stakes are high because methane can have two very different sources. One possibility is biological. If methane comes from a biosphere, living things might be producing it as they use energy. Alternatively, ancient biologically created methane might remain in the crust, even if life is no longer present.
If the methane is not biological, the most likely source is abiotic. This involves active rock metamorphosis. This is a process where rock changes from one type into another. It occurs deep underground, under high pressure and heat, with water and chemical energy. Distinguishing between these sources is critical for understanding Mars.
For eight years, the TGO has operated with its instruments working at nominal levels. Despite this long duration, the ESA team has not found any evidence of methane. The contrast with previous missions is stark.
"We have a thousand times better sensitivity than the Mars sample analysis suite on NASA’s Curiosity rover, but we do not see any sign of the absorption of methane in the Martian atmosphere," says Olsen. This non-detection is puzzling. The simplest explanation, according to Olsen, is that methane is not present in the atmosphere in detectable amounts.
However, scientists are working hard to reconcile their observations with those made by other teams and instruments. So far, they have not found a satisfactory explanation. If methane is produced by microbes, other organic molecules are likely produced alongside it. Olsen suggests that if other organics were found on the surface in association with methane, it would strongly suggest methanogenesis. This is the process by which anaerobic microbes produce methane as a byproduct of their metabolism.
While methane remains elusive, the TGO has detected other substances. "We know that there's chloride on the surface of Mars in the form of simple salts," says Olsen. He cites two key examples: the perchlorate measurement from NASA’s Phoenix lander and sodium chloride, or table salt, found in Martian meteorites. Yet, hydrogen chloride is the only novel trace gas the TGO has thus far detected.
Hydrogen chloride is a stable reservoir species for chlorine. Chlorine is important because it is often associated with volcanic activity. "If we went to Mars and found sulfur and chlorine, maybe we’d have some sort of active volcanic activity or magmatic outgassing," Olsen explains. However, the TGO is unable to measure sulfur or methane, both of which can also come from volcanoes. This lack of data leaves a gap in our understanding of Mars' geological activity.
Detecting biosignatures on Mars has proven far more difficult than scientists initially expected. When NASA’s Viking 1 landed on the red planet on July 4, 1976, optimism was high. Decades later, the picture is more complex. "Before our mission started, there were already people that didn't believe the past methane observations," Olsen admits. "But without methane, then we're really back to square one."
This lack of detection continues to challenge researchers. Olsen wonders why other teams see a clear methane signature while the TGO sees nothing. He emphasizes that they cannot simply dismiss the other findings. "We can't be flippant and say that they made a mistake; we've seen their spectra, there's something there that needs explaining," he says. "We don't have that explanation, even though TGO has the sensitivity to detect trace amounts of methane."
ESA hopes to resolve this mystery in the near future. The next major step is the Rosalind Franklin Mars Rover, which is scheduled to launch in 2029. This rover will carry a drill capable of penetrating beneath the surface. By analyzing samples from below the radiation-heavy surface, scientists hope to find direct evidence of organic compounds or other signs of past or present life.
Despite the current silence from the TGO, optimism remains. "I want to stress that I believe that Mars is currently habitable, which is why I am searching for signs of life with the ExoMars missions," Olsen says. "And I haven't given up on methane yet."
The debate continues. As instruments improve and new missions launch, the scientific community remains committed to answering one of humanity's oldest questions: Are we alone? The absence of methane is not the end of the story, but it is a significant clue that requires careful interpretation. Until the Rosalind Franklin rover delivers its findings, the origin of Martian methane remains an open question, balancing the hope for life against the rigor of empirical data.