“Puzzling and Surprising” New Gas Signatures Discovered by ExoMars Orbiter in the Martian Atmosphere

ExoMars Trace Gas Orbiter at Mars

Artist’s impression of the ExoMars 2016 Trace Gas Orbiter at Mars. Credit: ESA/ATG medialab

ESA’s ExoMars Trace Gas Orbiter has spotted new gas signatures at Mars. These unlock new secrets about the Martian atmosphere, and will enable a more accurate determination of whether there is methane, a gas associated with biological or geological activity, on the planet.

The Trace Gas Orbiter (TGO) has been studying the Red Planet from orbit for over two years. The mission aims to understand the mixture of gases that make up the Martian atmosphere, with a special focus on the mystery surrounding the presence of methane there.

Meanwhile, the spacecraft has now spotted never-before-seen signatures of ozone (O3) and carbon dioxide (CO2), based on a full Martian year of observations by its sensitive Atmospheric Chemistry Suite (ACS). The findings are reported in two new papers published in Astronomy & Astrophysics, one led by Kevin Olsen of the University of Oxford, UK, and another led by Alexander Trokhimovskiy of the Space Research Institute of the Russian Academy of Sciences in Moscow, Russia.

“These features are both puzzling and surprising,” says Kevin.

“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 Martian atmosphere is dominated by CO2, which scientists observe to gauge temperatures, track seasons, explore air circulation, and more. Ozone – which forms a layer in the upper atmosphere on both Mars and Earth – helps to keep atmospheric chemistry stable. Both CO2 and ozone have been seen at Mars by spacecraft such as ESA’s Mars Express, but the exquisite sensitivity of the ACS instrument on TGO was able to reveal new details about how these gases interact with light.

Spectral Signatures Carbon Dioxide Ozone Mars

This graph shows an example of the measurements made by the Atmospheric Chemistry Suite (ACS) MIR instrument on ESA’s ExoMars Trace Gas Orbiter (TGO), featuring the spectral signatures of carbon dioxide (CO2) and ozone (O3).
The bottom panel shows the data (blue) and a best-fit model (orange). The top panel shows the modeled contributions from a variety of different gases for this spectral range. The deepest lines come from water vapor (light blue). The strongest O3 feature (green) is on the right, and distinct CO2 lines (grey) appear on the left. The locations of strong methane features (orange) are also shown in the modeled contributions, though methane is not observed in the TGO data. Credit: K. Olsen et al. (2020)

Observing ozone in the range where TGO hunts for methane is a wholly unanticipated result.

Scientists have mapped how Martian ozone varies with altitude before. So far, however, this has largely taken place via methods that rely upon the gas’ signatures in the ultraviolet, a technique which only allows measurement at high altitudes (over 20 km above the surface).

The new ACS results show that it is possible to map Martian ozone also in the infrared, so its behavior can be probed at lower altitudes to build a more detailed view of ozone’s role in the planet’s climate.

Key Methane Measurements Mars

This graphic summarizes significant measurement attempts of methane at Mars. Reports of methane have been made by Earth-based telescopes, ESA’s Mars Express from orbit around Mars, and NASA’s Curiosity located on the surface at Gale Crater; they have also reported measurement attempts with no or very little methane detected. More recently, the ESA-Roscosmos ExoMars Trace Gas Orbiter reported an absence of methane, and provided a very low upper limit. Credit: ESA

Unraveling the methane mystery

One of the key objectives of TGO is to explore methane. To date, signs of Martian methane – tentatively spied by missions including ESA’s Mars Express from orbit and NASA’s Curiosity rover on the surface – are variable and somewhat enigmatic.

Create Destroy Methane Mars

This graphic depicts some of the possible ways methane might be added or removed from the atmosphere. How methane is created and destroyed on Mars is an important question in understanding the various detections and non-detections of methane at Mars, with differences in both time and location. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet’s current state of activity. Credit: ESA

While also generated by geological processes, most of the methane on Earth is produced by life, from bacteria to livestock and human activity. Detecting methane on other planets is therefore hugely exciting. 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 or released in the relatively recent past.

“Discovering an unforeseen CO2 signature where we hunt for methane is significant,” says Alexander 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.”

The observations analyzed by Alexander, Kevin, and colleagues were mostly performed at different times to those supporting detections of Martian methane. Besides, the TGO data cannot account for large plumes of methane, only smaller amounts – and so, currently, there is no direct disagreement between missions.

Carbon Dioxide Spectral Feature Discovered Mars

This graph shows a new CO2 spectral feature, never before observed in the laboratory, discovered in the Martian atmosphere by the Atmospheric Chemistry Suite (ACS) MIR instrument on ESA’s ExoMars Trace Gas Orbiter (TGO).
The graph shows the full extent of the magnetic dipole absorption band of the 16O12C16O molecule (one of the various ‘isotopologues’ of CO2).
The top panel shows the ACS MIR spectra (shown in black) along with the modeled contribution of CO2 and H2O (shown in blue); the model is based on the HITRAN 2016 database.
The bottom panel shows the difference between data and model, or residuals, revealing the structure of the absorption band in detail. The calculated positions of spectral lines are marked with arrows, in different colors corresponding to different ‘branches’ of the absorption band (red stands for the P-branch, green for the Q-branch and blue for the R-branch).
Credit: A. Trokhimovskiy et al. (2020)

“In fact, we’re actively working on coordinating measurements with other missions,” clarifies Kevin. “Rather than disputing any previous claims, this finding is a motivator for all teams to look closer – the more we know, the more deeply and accurately we can explore Mars’ atmosphere.”

Realizing the potential of ExoMars

Methane aside, the findings highlight just how much we will learn about Mars as a result of the ExoMars program.

“These findings enable us to build a fuller understanding of our planetary neighbor,” adds Alexander.

Compare Atmospheres Mars Earth

Mars is about half the size of Earth by diameter and has a much thinner atmosphere, with an atmospheric volume less than 1% of Earth’s. The atmospheric composition is also significantly different: primarily carbon dioxide-based, while Earth’s is rich in nitrogen and oxygen. The atmosphere has evolved: evidence on the surface suggests that Mars was once much warmer and wetter. Credit: ESA

“Ozone and CO2 are important in Mars’ atmosphere. By not accounting for these gases properly, we run the risk of mischaracterizing the phenomena or properties we see.”

Additionally, the surprising discovery of the new CO2 band at Mars, never before observed in the laboratory, provides exciting insight for those studying how molecules interact both with 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 Alexander.

Successful collaboration in the hunt for life

As its name suggests, the TGO aims to characterize any trace gases in Mars’ atmosphere that could arise from active geological or biological processes on the planet, and identify their origin.

ExoMars Orbiter and Rover

Artist’s impression of the ExoMars 2020 rover (foreground), surface science platform (background), and the Trace Gas Orbiter (top). Not to scale. Credit: ESA/ATG medialab

The ExoMars program consists of two missions: TGO, which was launched in 2016 and will be joined by the Rosalind Franklin rover, and the Kazachok landing platform, due to lift off in 2022. These will take instruments complementary to ACS to the Martian surface, examining the planet’s atmosphere from a different perspective, and share the core objective of the ExoMars program: to search for signs of past or present life on the Red Planet.

“These findings are the direct result of hugely successful and ongoing collaboration between European and Russian scientists as part of ExoMars,” says ESA TGO Project Scientist Håkan Svedhem.

“They set new standards for future spectral observations, and will help us to paint a more complete picture of Mars’ atmospheric properties – including where and when there may be methane to be found, which remains a key question in Mars exploration.”

“Additionally, these findings will prompt a thorough analysis of all the relevant data we’ve collected to date – and the prospect of new discovery in this way is, as always, very exciting. Each piece of information revealed by the ExoMars Trace Gas Orbiter marks progress towards a more accurate understanding of Mars, and puts us one step closer to unraveling the planet’s lingering mysteries.”


“First detection of ozone in the mid-infrared at Mars: implications for methane detection” by K. S. Olsen, F. Lefèvre, F. Montmessin, A. Trokhimovskiy, L. Baggio, A. Fedorova, J. Alday, A. Lomakin, D. A. Belyaev, A. Patrakeev, A. Shakun and O. Korablev, 27 July 2020, The Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202038125

“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, V. Perevalov, O. Korablev, A. F. Fedorova, K. S. Olsen, J.-L. Bertaux, A. Patrakeev, A. Shakun, F. Montmessin, F. Lefèvre and A. Lukashevskaya, 27 July 2020, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202038134

The studies utilized the Mid-InfraRed (MIR) channel of the Atmospheric Chemistry Suite (ACS) on the ExoMars Trace Gas Orbiter (TGO), reporting the first observation of the 3000–3060 cm-1 ozone (O3) band and the discovery of the 3300 cm-1 16O12C16O magnetic dipole band (which both overlap with the 2900–3300cm-1 methane v3 absorption band) at Mars.

ExoMars is a joint endeavor of the European Space Agency and Roscosmos.

The ACS instrument is led by the Principal Investigator team at the Space Research Institute (IKI) of the Russian Academy of Sciences (RAN) in Moscow, Russia, assisted by the Co-Principal Investigator team from CNRS/LATMOS, France, and co-investigators from other ESA Member states.

2 Comments on "“Puzzling and Surprising” New Gas Signatures Discovered by ExoMars Orbiter in the Martian Atmosphere"

  1. Methane is one of the most prolific compounds in the Solar system. The gas giants are loaded with it, made of it… in addition we now understand that millions of tonnes of sea water per year are absorbed into the mantle where they are reduced to hydrogen, the oxygen torn away by oxidation with metals and semi metals – leaving free hydrogen to combine with carbon into chains and percolate back up through the crust as oil and gas…

  2. The presence of ozone should imply the presence of some free molecular oxygen, presumably derived from the Sun’s UV photolytic decomposition of water vapor in the Marian stratosphere with the loss of light hydrogen to space. This oxygen presumably was responsible for the oxidation of hydrothermal ferrous iron to hematite in sediments at the surface…making the planet appear red. With the Martian surface water now gone so must the oxygen-ozone be gone?

    “By not accounting for these gases properly, we run the risk of mischaracterizing the phenomena or properties we see.”.

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