Methane on Mars: a mere illusion?
PRESS RELEASE: New study casts doubt on detections by NASA’s Curiosity rover
NASA’s Curiosity rover, using its Tunable Laser Spectrometer (TLS), has repeatedly detected methane on Mars, giving rise to speculation about possible biological sources. But how reliable are these detections? Our recent study raises serious concerns, suggesting they may be instrumental artefacts.
Press release for the publication
“Questioning the Reliability of Methane Detections on Mars by the Curiosity Rover”
in the Journal of Geophysical Research: Planets
by Sébastien Viscardy1, David C. Catling2, and Kevin Zahnle3
1Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
2Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
3NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
Methane detections by Curiosity puzzle the scientific community
Because methane on Earth is mainly produced by biological activity, its reported presence in the Martian atmosphere over the past two decades has sparked intense scientific interest. However, despite the excitement surrounding these detections and their potential implications for the search for extraterrestrial life, scepticism remains.
The major paradox is that neither of the two independent spectrometers onboard ESA’s ExoMars Trace Gas Orbiter (TGO) has ever detected a trace of methane since the mission began in 2018. The TGO instruments have instead established a stringent upper limit that is more than ten times lower than the lowest previously reported detection. Attempts to reconcile the TGO and previous reports require that the putative methane vary in ways that cannot be convincingly explained by any known chemical or physical processes.
Detections recorded by the Tunable Laser Spectrometer (TLS) aboard NASA’s Curiosity rover have drawn particular attention since its landing in Gale Crater in 2012 (see Figure 1).
By analysing samples of Martian air taken directly on-site, these in situ measurements have fuelled a staggering number of studies seeking to uncover the source and origin of this methane but, until now, no independent group has scrutinized the underlying dataset.
A hidden methane source inside Curiosity?
To shed light on the apparent inconsistencies surrounding the Mars methane story, our study—recently published in the Journal of Geophysical Research: Planets—reanalyses publicly available TLS data and calls into question the Martian origin of the detected methane.
We found that the foreoptics (FO) chamber—where the laser emits, directly adjacent to the cell holding the sample of Martian air (see bottom of Figure 2)—is contaminated with methane at levels at least 1,000 times higher than in the sample cell. This contamination, initially caused by the accidental introduction of terrestrial air before Curiosity’s launch, has persisted despite repeated attempts to evacuate the gas from the system: after each attempt, methane systematically reappears and builds up again.
We also observed abnormal pressure variations during the measurements (see top of Figure 2), even though both the FO chamber and sample cell are supposed to be hermetically sealed—meaning no Martian air should be able to enter or escape from either compartment. These variations indicate gas exchanges with an unidentified reservoir—most likely the Martian atmosphere—raising concerns about the sealing of the instrument’s compartments and the integrity of the air sample.
Although the source of the methane in the FO chamber remains unknown, its proximity to the sample cell raises the possibility of diffusion across the O-ring seal separating the two compartments (see bottom of Figure 2). We demonstrate that even a very small fraction—less than one thousandth of the methane-rich air in the FO chamber—diffusing into the sample cell could fully account for the previously reported methane detections. Moreover, pressure variations resulting from these internal gas exchanges would be masked by the much larger variations caused by exchanges with the unidentified reservoir.
Doubts raised over Curiosity’s methane analysis method
Our study also identifies potential issues in the way TLS data were analysed in five experiments that were previously published. Methane concentration has been determined by analysing TLS spectra at three infrared wavelengths (see Figure 3a), which form a triplet of spectral lines labelled “e”, “f”, and “g”. In standard practice, a single recorded spectrum yields one methane concentration measurement by considering all three lines collectively.
However, instead of following this conventional approach, each spectral line was analysed separately, resulting in three separate methane concentrations, which were then averaged to yield the reported value (see Figure 3b). This approach is problematic: we find that in most cases, only one of the three spectral lines suggests the presence of methane, while the other two do not. Yet the weighted average leads to a positive detection. In other words, the reported detection relies primarily on one of the three spectral lines, while the other two see nothing.
Furthermore, in the five TLS experiments with publicly available data, the three methane concentration values are not mutually consistent within their respective uncertainties (see Figure 3b). We estimate that the probability of this occurring in every case—by pure chance—is just one in a thousand. Our reanalysis strongly suggests the presence of systematic errors, likely masked by averaging three inconsistent methane measurements.
Our analysis suggests that the methane detected in the sample cell may:
- originate from the rover itself, possibly due to gas diffusion from the FO chamber;
- result from underestimated measurement uncertainties; or
- be due to a combination of both factors.
A proposed test to help determine the origin of the detected methane
Our study was made possible by the public release of key TLS datasets by the instrument team—an openness that deserves recognition.
While our analysis raises concerns about the reliability of Curiosity’s methane detections, it also opens the door to clarifying some of the uncertainties that have persisted for years. We propose a simple yet decisive test in the future that could help determine the true origin of the detected methane.
This test would involve a carefully designed sequence of two TLS experiments conducted on consecutive nights, keeping the Martian air sample sealed in the cell between them. By allowing the sample to remain in the cell for an extended period and comparing the methane concentrations measured across both experiments, this test could determine whether the methane detected by Curiosity truly comes from Mars or from internal contamination.
If conducted, this straightforward experiment could provide crucial insights into the longstanding debate over whether Martian methane is real or merely an illusion.
Press contacts
Scientific contact:
- Dr. Sébastien Viscardy (Sebastien.Viscardy_at_aeronomie.be)
Research group “Planetary atmospheres”, Royal Belgian Institute for Space Aeronomy
Communication cell:
- Dr. Karolien Lefever (Karolien.Lefever_at_aeronomie.be)
Communication and documentation, Royal Belgian Institute for Space Aeronomy
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