Eleven robotic missions are currently exploring Mars, a combination of orbiters, landers, rovers and an aircraft (the Ingenuity helicopter). Like their predecessors, these missions study the atmosphere, surface and subsurface of Mars to learn more about its past and evolution, including the evolution from a once warmer and wetter environment to the freezing, dusty and extremely arid planet we see today . In addition, these missions will search for evidence of past life on Mars and may learn if and where it still exists today.
A particularly interesting topic is how the atmosphere of Mars – composed mostly of carbon dioxide (CO2) – is relatively enriched in carbon-13 (13C). “heavy carbon”. For years scientists have speculated that the ratio of this isotope to “light carbon” (12C) could be responsible for the organic matter found on the surface (a sign of biological processes!). But after analyzing data from ESA’s ExoMars Trace Gas Orbiter (TGO) mission, an international team led by the Open University concluded that these organics may be “abiotic” (ie non-biological) in origin.
The study was led by Juan Alday, a postdoctoral fellow at the Open University (OU), and members of the Atmospheric and Surface Exploration group. They were joined by the Space Research Institute (IKI), the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS) and the Atmospheric, Oceanic, and Planetary Physics (AOPP) group at the University of Oxford. Their findings were presented in a recent article in Nature Astronomy entitled “Photochemical depletion of heavy CO isotopes in the Martian ambient”.
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Artist’s rendering of the ExoMars 2016 mission to the Red Planet. Photo credit: ESA
Carbon dioxide makes up about 96% of the Martian atmosphere, with trace amounts of carbon monoxide (0.0557%). The relative abundance of the heavy carbon isotope in these gases (representing only 1.1% of the carbon isotopes in the Earth’s atmosphere) has been attributed to the preferential flight of “light carbon” (12C) into space over billions of years. This is based in part on recent measurements from NASA’s Curiosity rover, which showed a drop in 13C in surface organic material (methane gas).
By analyzing this enrichment, scientists hope to learn more about the atmospheric processes that contribute to the evolution of the “isotope ratios” between the upper and lower atmosphere. Because atmospheric CO and organic molecules share the same 13C-depleted isotopic signature, scientists hope to find clues as to whether organic processes (a possible clue to life) may have played a role. For their study, the team led by Dr. Alday vertical CO profiles determined with the TGO Atmospheric Chemistry Suite (ACS).
This suite consists of three infrared echelle spectrometers that collect information in the near, mid and far infrared (NIR, MIR, TIRVIM) channels. Since 2016, these instruments have been collecting spectra of the Martian atmosphere, using absorption lines that indicate the presence of different chemical elements to determine their composition. The team then combined this data with a photochemical model that predicts carbon and oxygen degradation in CO molecules in the atmosphere due to interaction with solar radiation.
Their results indicate (contrary to previous assumptions) that carbon monoxide (CO) in the Martian atmosphere is depleted of heavy rather than light carbon. like dr Alday explained in a press release from OU News.
“The key [to] Understanding why CO contains less 13C lies in the chemical relationship between CO2 and CO. When CO2 molecules are destroyed by sunlight to form CO, 12CO2 molecules are destroyed more efficiently than 13CO2, resulting in the degradation of 13C into CO over long periods of time.”
Primitive, simple life may have thrived on early Mars. Then it might have self-extinguished. Is it too late for a memorial? Photo credit: ESO/M. grain knife
These results help address the long-standing debate over whether biological or non-biological processes led to the presence of organic material on the surface of Mars. Despite the trace amounts of CO in the Martian atmosphere, they have important implications for our understanding of how Martian atmosphere and climate have evolved over time. On the one hand, they could provide insights into past conditions that allowed flowing and standing water at the surface.
On the other hand, it is helping to refine the search for past life on Mars, although the results could be viewed as disappointing. The ultimate goal, said Dr. Alday, is to determine whether the conditions of life ever existed and whether they lasted long enough for life to arise:
“We do not know what the atmosphere of early Mars was like and under what conditions liquid water was able to flow on the surface. The carbon isotopes in the Martian atmosphere can help us estimate how much CO2 there was in the past. The new ExoMars TGO measurements suggest less CO2 escaped from the planet than previously thought, and provide new constraints on the composition of this early Martian atmosphere.”
This research was made possible thanks to the support of the British Space Agency, which funded the development of the ACS spectrometers and the research of the Atmospheric Research and Surface Exploration groups. The TGO is part of the larger ExoMars program, a joint initiative between ESA and Roscosmos. This program will send the Rosalind Franklin rover to Mars in the years to come to further support the ongoing search for past (and perhaps even present) life on Mars.
Further reading: Open University News, Nature Astronomy