New spectral signatures: ExoMars sheds light on Mars’ methane and atmosphere

17 July 2020

Dr Kevin Olsen from the University of Oxford’s Department of Physics, working with international colleagues, has spotted new gas signatures that shed new light on the mystery of methane in Mars’ atmosphere – a gas strongly associated with biological or geological activity.

The European Space Agency’s ExoMars Trace Gas Orbiter (TGO) has been observing Mars from orbit for more than two years with a mission to understand the composition of the Martian atmos-phere, searching for trace gases that could indicate whether Mars is an active planet. It has now spotted never-before-seen signatures of ozone (O3) and carbon dioxide (CO2).

Surprising findings

‘These features are both puzzling and surprising,’ says Dr Olsen. ‘They lie over the exact wavelength range where we expected to see the strongest signs of methane. Before this discovery, the CO2 feature was completely unknown, and this is the first time ozone on Mars has been identified in this part of the infrared wavelength range.’

The findings, based on a full Martian year of observations by TGO’s incredibly sensitive Atmospheric Chemistry Suite (ACS), are reported in two new papers: one led by Dr Olsen and another led by Dr Alexander Trokhimovskiy of the Russian Academy of Sciences in Moscow, Russia.

CO2 is the main component of the Martian atmosphere, accounting for 96% of it. It is used by remote sensing missions to gauge temperatures, track seasons, explore air circulation, and more. Ozone – which forms a layer in the upper atmosphere on Mars, as on Earth – is part of the chemical cycles that maintain the stability of the Martian atmosphere. The new CO2 absorption band results from transitions theoretically thought to be forbidden and has never been observed or modelled before. The ozone features that have been observed for the first time at Mars result from a higher-than-expected abundance near the surface of Mars, and were unanticipated.

Unravelling the methane mystery

While also generated by geological processes, the majority of the methane on Earth is produced by life: from bacteria to livestock and human activity. Detecting methane on other planets is therefore xciting; this is especially true given that the gas is known to break down in around 400 years, meaning that any methane present must have been produced in the relatively recent past.

Reported detections of methane on Mars – including those from ESA’s Mars Express and NASA’s Curiosity Rover – are variable and somewhat enigmatic.

’Discovering an unforeseen CO2 signature where we hunt for methane is significant,’ says Dr Trokhimovskiy. ’This signature could not be accounted for before, and may therefore have played a role in detections of small amounts of methane at Mars.’

Dr Olsen continues: ‘These findings are a direct result of the TGO’s hunt for methane. So far, even with the unprecedented sensitivity of TGO, we have not been able to detect any methane at Mars, but these features are what we have found in its place.’

The potential of ExoMars and the search for life in the solar system

Methane aside, these findings highlight how much can be learned about Mars as a result of the ExoMars programme. ExoMars is the flagship mission of the Aurora Exploration Programme, the basis of European efforts to explore the Solar System via both manned and robotic missions.

‘These findings enable us to build a fuller understanding of our planetary neighbour,’ adds Dr Trokhimovskiy. ‘Ozone and CO2 are important in Mars’ atmosphere. By not accounting for these gases properly, we run the risk of mischaracterising the phenomena or properties we see.’

Additionally, the surprising discovery of the new CO2 band at Mars, not in the laboratory, provides exciting insight for those studying how molecules interact with both one another and with light – and searching for the unique chemical fingerprints of these interactions in space. ‘Together, these two studies take a significant step towards revealing the true characteristics of Mars: towards a new level of accuracy and understanding,’ says Dr Trokhimovskiy.

ExoMars is a two-part mission that was realised when the TGO and Schiaparelli lander were launched in 2016. The TGO’s primary objective is to search for any trace gases diagnostic of geological or biological activity, such as methane. It will be followed in 2022 by the ExoMars Rosalind Franklin rover and Kazachok lander which will continue the search for past or present life from the surface.

First detection of ozone in the mid-infrared at Mars: implications for methane detection’ by K. S. Olsen et al., Astronomy & Astrophysics 2020
First observation of the magnetic dipole CO2 absorption band at 3.3 μm in the atmosphere of Mars by the ExoMars Trace Gas Orbiter ACS instrument’ by A. Trokhimovskiy et al., Astronomy & Astrophysics 2020