The ExoMars Trace Gas Orbiter (TGO) has revealed how oddly ‘light’ carbon monoxide forms in Mars’ atmosphere. The finding paints a better picture of how carbon-containing matter can be formed on the Red Planet without life, and helps clarify a puzzling discovery made by NASA’s Curiosity rover last year.
The TGO observations show that a process at play in Mars’ atmosphere – where carbon dioxide is split apart by sunlight – forms carbon monoxide containing less ‘heavy’ carbon than we would expect.
The finding is consistent with the idea that a combination of sunlight and complex chemistry, rather than life, gave rise to the carbon-based compounds (‘organic matter’) we see on the martian surface.
Tracing Mars’ carbon
Mars’ atmosphere contains both light carbon (carbon-12, which accounts for the vast majority of the carbon in the Solar System) and heavy carbon (the isotope carbon-13: a carbon-12 atom with an extra neutron).
Measuring the relative amounts of these isotopes can reveal a great deal about an environment’s past and present. Many processes, both short and long-term, affect this ratio, including how substances break apart in sunlight, how they escape to space from the uppermost layers of an atmosphere, how they condense or turn to gas, and – excitingly – how they are produced and consumed by forms of biological life.
“Measuring the carbon isotopic ratio in carbon monoxide is a powerful way to understand where the planet’s organic matter came from and reveal Mars’ history of habitability,” says Shohei Aoki of the University of Tokyo and Royal Belgian Institute for Space Aeronomy, and lead author of a new paper published in the Planetary Science Journal.
"In 2021, TGO mapped the ratio of ‘normal’ to ‘heavy’ hydrogen in Mars’ atmospheric water vapour to create a ‘chronometer’ with which to trace the history and evolution of the planet’s water. We’ve now applied the same approach to the carbon found in Mars’ atmospheric carbon monoxide, something we were only able to do thanks to TGO’s exquisite sensitivity and ability to profile many different molecules.”
Shohei and colleagues analysed data gathered across eight TGO orbits in March-April 2022 by NOMAD (Nadir and Occultation for MArs Discovery), an instrument of which BIRA-IASB is the Principal Investigator. NOMAD watched as the Sun’s rays traversed Mars’ atmosphere, a perspective that revealed the amounts, identities and carbon content of the gases present.
A chemical cause
The new carbon measurements help clarify a puzzling finding from NASA’s Curiosity rover last year.
Several of the 3.5-billion-year-old deposits sampled by Curiosity at its landing site, Gale Crater, contained surprisingly low amounts of heavy carbon. The researchers suggested a few possible causes, ranging from interstellar dust clouds periodically raining down on Mars to ancient microbes burping methane. On Earth, depletion of heavy carbon often signals life, as several biological processes preferentially use lighter isotopes of carbon.
“Any phenomenon at Mars that could be caused by life is a cause for excitement, but our findings point in a different direction,” says co-author Yuichiro Ueno of Tokyo Tech. University. “Instead, we see that the cause for the heavy carbon depletion seen in both Mars’ atmospheric carbon monoxide and in Gale Crater could be chemical.”
Molecules of carbon dioxide in Mars’ atmosphere interact with sunlight and break apart to form carbon monoxide that is depleted in heavy carbon – something we also see happening in Earth’s atmosphere.
The researchers modelled how this process would affect Mars' carbon monoxide, and their results match what was actually seen at Mars by NOMAD. These calculations are presented in a companion paper by Yoshida et al., also published in the Planetary Science Journal.
The results are consistent with the idea that the atmosphere of early Mars was rich in carbon monoxide, and that this gas was responsible for forming the organic matter seen on the planet’s surface.
Deeper than ever before
Using isotope ratios is a widely applicable way to explore the Universe; we can study bodies throughout the Solar System and cosmos, such as exoplanets, in this way to unravel their history and properties.
“TGO’s two gas-hunting instruments, NOMAD and the Atmospheric Chemistry Suite (ACS), are doing a great job mapping isotope ratios in the atmosphere of Mars,” says Colin Wilson, ESA's ExoMars Trace Gas Orbiter project scientist.
“A real strength of TGO’s investigations is that we have multiple ways of measuring the same thing. We're measuring carbon isotopes in different molecules using both NOMAD and ACS independently. In fact, the results reported here from NOMAD agree with complementary observations and modelling of CO isotopes by another team using ACS, also published this month – so this gives us a lot of confidence in the results.”
The findings by Shohei and colleagues highlight the collaborative and complementary nature of our missions to explore the Solar System. For instance, the TGO results will help scientists interpret results from Japan’s forthcoming Martian Moon eXploration (MMX) mission, which will return samples of Mars’ moon Phobos.
“By combining observations from multiple missions, we’ll reveal new details about Mars’ history," adds Colin. “Down on the martian surface, ESA’s forthcoming Rosalind Franklin rover will help us understand the planet’s surface and organic matter. The rover has unique drilling capabilities and a science laboratory unrivalled by any other mission in development. We’ll be able to dig deeper into Mars than ever before.”
- “Depletion of 13C in CO in the atmosphere of Mars suggested by ExoMars-TGO/NOMAD” by Aoki et al. is published in the Planetary Science Journal. This research predominantly uses data from the Belgian-led NOMAD instrument.
The modelling and observations are supported by results presented in a companion paper:
- “Strong depletion of 13C in CO induced by photolysis of CO2 in the Martian atmosphere calculated by a photochemical model” by Yoshida et al. This paper is published in the Planetary Science Journal.
And are consistent with complementary observations and modelling of CO isotopes by a team using ACS:
- “Photochemical depletion of heavy CO isotopes in the Martian atmosphere” by Juan Alday et al. This paper is accepted for publication in Nature Astronomy.