By Daniel Webster, dWeb.News
Science & Exploration
ESA / Science & Exploration / Exploration / Robotic Exploration / Human and Robotic Exploration / ExoMars
TheESA-Roscosmos Trace Gas Orbiter set new upper limits for how much methane and ethane, as well as ethylene, phosphine, are in the martian atmosphere. These four gases are known to be ‘biomarkers’ gases, which can indicate potential signs of life.
The ExoMars Trace gas Orbiter’s primary goal is to search for biomarkers on Mars. The ExoMars Trace Gas Orbiter’s primary goal is to search for biomarkers on Mars. Methane is a key biomarker. Most methane on Earth is produced by living organisms or geological activity.
Mars Key Methane Measurements
Previous estimates from Mars and ground-based mission ranged from 0.2 to 30 parts per million by volume (ppbv) which indicates up to 30 molecules per billion molecules. Referring to the above, methane can be found in Earth’s atmosphere at approximately 2000 ppbv. However, the Trace Gas Orbiter’s first results, published in April 2019, did not detect methane. Instead, it calculated that the maximum concentration of methane must be 0.05 ppbv. “We have used the Trace Gas Orbiter now to refine the upper limit of methane on Mars even further, this is because we have collected data for over 1.4 Martian years – 2.7 Earth year,” Franck Montmessin, France, Co-Principal investigator of the Trace Gas Orbiter’s Atmospheric Chemistry Suite, (ACS), and lead author of one new paper on martian biomarkers. “We found no evidence of the gas, which suggests that the amount of methane on Mars is likely to be even lower than previously estimated.”
Franck and colleagues say that methane must be present in a concentration of less than 0.05ppbv because the orbiter’s instruments can detect it very well. The scientists also searched for methane signs around Curiosity’s home, Gale Crater. However, the rover reported methane presence there. “Curiosity measures right at Mars’s surface, while the orbiter takes measurements just a few kilometers above it – so the difference in these two findings could be explained if methane is trapped to the lower atmosphere or within the immediate vicinity of Curiosity,” Franck adds. A paper using data from NOMAD (Nadir Occultation MArs Discover) instrument that spans a full year also searches for methane and two additional biomarkers. Elise Wright Knutsen is the paper’s author. She was previously employed at NASA’s Goddard Space Flight Center in USA and at LATMOS in France. “We searched for global methane and also checked for plumes at over 2000 locations around the planet. We didn’t find anything. This suggests that methane released in this manner must be sporadic.
Elise and her colleagues also looked for methane and ethylene as potential biomarkers gases. These molecules will occur after methane has been broken down by sunlight. They are therefore exciting in their own right as well as in the context of our hunt to find methane. Ethane and other ethylene molecules have short lives, so if they are found in a planet atmosphere, they must have been created or released recently. They are excellent tracers for possible biological or geochemical activity. Elise says that these are ExoMars first results in searching for these gases. “We couldn’t detect either of these gases so we set the upper limits for ethane (0.7 ppbv) and ethylene (0.1 ppbv), respectively. These limits are lower than our methane limits.
The orbiter is also looking for phosphine, a gas that was controversially detected at Venus last year. The majority of phosphine found on Earth is biologically produced. This makes it an exciting biomarker for the atmospheres of terrestrial planets. Kevin Olsen, a lead author of the phosphine research at the University of Oxford in the UK, says, “We didn’t find any signs of Phosphine on Mars.” “Our upper limits for ethane or ethylene are similar – between 0.01 and 0.6ppbv. “The ExoMars programme’s central goal is to search for life on Mars or the lingering signs of it. The Trace Gas Orbiter is particularly focused on the hunt for biomarkers. TGO’s search for biomarkers will be complemented by the ExoMars rover Rosalind, which is scheduled to launch in 2022. Underground samples may be more likely than surface samples to retain biomarkers because they are shielded from space’s harsh radiation environment.
“Whether biomarkers have been detected or not, these discoveries are important for our understanding which processes occur in the martian atmosphere – essential information when considering where we should focus our ongoing investigation of Mars,” says Hakan Svedhem (ESA Project Scientist for ExoMars Trace gas Orbiter). There are many key questions that remain. For example, why does Curiosity detect methane at Gale Crater but not in orbit? This methane could have come from somewhere else, or be only found in certain locations on the planet – or perhaps an unexpected process is destroying any methane before we can find it. It will be exciting to continue working with Rosalind Franklin rovers and Curiosity, which have completely different vantage points than an orbiter, in order to understand what is going on in this mysterious planetary environment.
Notes to editorsThis article is based upon three papers: ‘A stringent upper limit for methane on Mars of 20 pptv and constraints on its dispersion beyond Gale crater’ by F. Montmessin and al. Astronomy and Astrophysics published the article. https://doi.org/10.1051/0004-6361/202140389 ‘Comprehensive investigation Mars methane & organics using ExoMars/NOMAD” by E. Knutsen and al. Icarus. https://doi.org/10.1016/j.icarus.2020.114266 ‘Upper limits of phosphine in the atmosphere Mars’ by K. Olsen and al. Astronomy and Astrophysics. https://doi.org/10.1051/0004-6361/202140868.
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