Dr Kevin Olsen, UKSA Aurora Fellow, is working on data analysis for the ExoMars Trace Gas Orbiter to help understand the photochemistry, dynamics and climatology of Mars. Here he comments on the latest finding from the TGO and what they mean for life on Mars.
The ExoMars Trace Gas Orbiter has been searching for signs of life on Mars – past or present – since 2018. By carefully studying the composition of its atmosphere, we have been hunting for gases that may have been emitted from living organisms.
A major find that inspired the ExoMars mission has been the detection of methane on Mars. Methane should be destroyed by sunlight, so its contemporary presence hints at an active source – possibly subsurface microbiology or, if not, then active geological processes. which would create chemical energy in an environment that is hot and wet and so be perfect for microorganisms. A third possibility may be active volcanism, which would be very exciting to discover since Mars is thought to be no longer active volcanically. TGO is also searching for gases that could hint at volcanism, resulting in the recent discovery of hydrogen chloride.
Searching for signs of life
My colleagues and I have published three papers recently reporting that the mission so far has not been able to find evidence of methane in the atmosphere of Mars – nor of several other possible so-called biomarker gases that are potential signs of life. Two of these are ethane and ethylene, which may be emitted from a biosphere, but should also be part of the chemical cycle of methane. The third is phosphine – a study that I led – which was recently proposed to be used as a biomarker since it is part of the phosphorous exchange cycle in Earth's biosphere. The claimed detection of phosphine in the atmosphere of Venus was one of the biggest science stories of the past year, but also one of the most controversial.
A major objective of the ExoMars mission is to solve the mystery of Mars methane. Methane in the atmosphere of Mars was announced in 2004 by three independent groups, two of which used Earth-based telescopes, and the third used the Planetary Fourier Spectrometer (PFS) on Mars Express. The signal was not unambiguous within the noise, resolution, and calibration of these instruments, and further observations revealed that the methane had disappeared. In 2009, another report using Earth-based telescopes announced an astounding quantity of methane – 30 and 45 parts per billion by volume (ppbv) – that showed up in plumes, with the amount changing over the surface of the planet and over time. The source of these plumes became a major scientific objective of space agencies worldwide. The Curiosity rover arrived in 2012 and carried a very high resolution instrument cable of measuring methane. Initially, they reported no methane in 2013, but then reported plumes of 6 and 10 ppbv in 2015, and a seasonal cycle of methane in 2018. In 2019, Mars Express and Curiosity reported a coincident observation of 15 ppbv. Meanwhile, the revised PFS data set only contains two definitive detections over 15 years of observation, and no methane has been reported using Earth-based telescopes since 2009.
The Mars methane mystery
To solve the mystery of Mars methane, we need a very sensitive instrument with fine spectral resolution that makes continuous, near global measurements. These capabilities are provided by the ExoMars TGO which entered Martian orbit in 2016 and began its science mission in 2018. The new papers report on three years of observations and only find low upper limits of methane and other trace gases, deepening the Mars methane mystery. The above results from Curiosity and Mars Express used observations made before the arrival of TGO, but the Curiosity team reported this year that they measured the largest quantity of methane – 20 ppbv – while TGO was operating. Will TGO need several more years of observation to capture a rare plume of methane? Are there unknown meteorological phenomena trapping methane near the surface? Is there an unknown mechanism rapidly destroying methane? And where would such a plume come from? Two instruments on TGO have not seen the signal of methane, and their sensitivities lead to detection limits of 0.02 ppbv, several times lower than the reported background quantity.
Aside from methane, TGO instruments are capable of searching for several related trace gases. The first report looks at ethane and ethylene alongside methane, and also found no sign of the presence of either. Both molecules may have biological origins, and are destroyed by sunlight, so must have an active source. Their presence, alongside methane, would lend support to theories of its biological origin.
Biomarkers include phosphine
Phosphine is a very reactive gas that is part of the cycle of phosphate on Earth – indeed, it is an essential molecule for all signs of life on Earth. It was only recently suggested to be used as a biomarker on other planets, but its spectral signatures fall within the range of one of the TGO instruments, the Atmospheric Chemistry Suite (ACS). Following its controversial announcement on Venus, along with my colleagues here at Oxford, we sought its presence on Mars; we didn’t find its signal in the ACS observations and our upper limits are similar for those of ethane and ethylene – between 0.1 and 0.6 ppbv.
The TGO will continue its search for undetected trace gases, and its studies of the Martian climate, its atmospheric chemistry, and its water cycle. It will be followed next year by the Kazachok lander and the Rosalind Franklin rover, the second phase of the ExoMars mission, which will be able to study the atmosphere and make coordinated observations with the TGO. They will also have the capability to look for signs of life below the surface, a first for Mars. Underground samples may be more likely to retain biomarkers, as material is shielded from the harsh radiation environment of space.
Upper limits for phosphine (PH3) in the atmosphere of Mars, K Olsen et al, Astronomy & Astrophysics
A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater, F Montmessin et al, Astronomy & Astrophysics
Comprehensive investigation of Mars methane and organics with ExoMars/NOMAD, E Knutsen et al, Icarus