A joint European-Russian spacecraft orbiting Mars has found no signs of gases related to the existence of life in the atmosphere of Mars, according to three new studies.
Scientists were hunting for telltale signs of methane gas in data from a spacecraft called the Trace Gas Orbiter, which arrived at Mars in 2016 as part of the ExoMars mission by the European Space Agency (ESA) and Russia's Roscosmos. Specifically, they used two instruments to look for traces of methane, as well as the byproducts of its chemical reactions triggered by sunlight, ethane and ethylene.
But despite gathering over two and a half years worth of measurements from one of the instruments (called the Atmospheric Chemistry Suite, or ACS) and over a year's worth of data from the other (known as Nadir Occultation MArs Discovery, or NOMAD) the researchers found no trace of their target gases, ESA said in a statement. A separate team, which was looking for another possible signature of life in the atmosphere of the planet, the presence of phosphines, was also unsuccessful.
The findings are in contrast with measurements obtained by NASA's Curiosity rover, which detected occasional bursts of methane in the lower layers of the atmosphere of Mars as well as near its surface. The ground-based measurements estimated there might be up to 30 molecules (parts) of methane in every billion molecules in the atmosphere of the planet. But the newest measurements by Trace Gas Orbiter indicate a maximum of 0.05 molecules per billion, but more likely only 0.02 molecules per billion.
Hunting methane on Mars
The new results follow on estimates released in 2019, which contained Trace Gas Orbiter data from a shorter period of time.
The researchers concluded that since the instruments aboard the spacecraft are extremely sensitive, the concentration of methane in the upper atmosphere of Mars, if the gas is present there at all, must be extremely low in order to prevent detection over such a long period of time. They, however, still believe the gas might be present closer to the surface.
"Curiosity measures right at Mars' surface while the orbiter takes measurements a few kilometres above," Franck Montmessin, a scientist at the Laboratoire Atmosphères, Observations Spatiales (LATMOS), France, and lead author of the paper based on the ACS data, said in the statement. "So the difference between these two findings could be explained by any methane being trapped by the lower atmosphere or the immediate vicinity of the rover."
The scientists, in fact, attempted to detect methane also above Gale Crater, the area explored by the Curiosity rover.
Measurements obtained by the NOMAD instrument in the period of one year, which were published in a separate paper, showed the same result.
"As well as searching for global methane, we also looked for localised plumes at over 2,000 locations on the planet and didn't detect anything," Elise Wright Knutsen, also of LATMOS, said in the statement. "So if methane is released in this way, it must be sporadic."
The NOMAD study also, for the first time, looked for the byproducts of methane's chemistry, ethane and ethylene, in the atmosphere of the planet. The results, however, were equally disappointing.
"We didn't detect either, and so set upper limits for ethane and ethylene at 0.1 and 0.7 parts per billion, respectively — low, but higher than our limits for methane," Knutsen said.
As both of these gases have rather limited lifespan, their detection by the instrument would mean that they must have been produced fairly recently.
Methane and phosphine on Mars?
Methane, which could be found in Earth's atmosphere in quantities of nearly 2,000 molecules (parts) per billion, could be either produced by living organisms or generated in geological processes. The scientists, however, cannot distinguish between the biologically and geologically produced gas.
Another study, looking at the presence of phospines, the same organic compounds that were reportedly detected in the atmosphere of Venus last year, also emerged with disappointing results.
"We didn't find any signs of phosphine at Mars," Kevin Olsen of the University of Oxford, UK, and lead author of the phosphine study said in the statement. "Our upper limits are similar for those of ethane and ethylene — between 0.1 and 0.6 parts per billion."
Phospines on Earth, unlike methane, are usually produced by bacteria and could therefore be considered a more conclusive evidence of life. However, the scientists behind the ExoMars project don't let the findings tamper their excitement about the upcoming second part of the mission, the launch of the Rosalind Franklin rover to the Red Planet, which is scheduled for September next year. Fitted with a two-meter drill, the rover will search for traces of life way deeper underground than the existing rovers, where potential microorganisms might be protected from the harsh environment on the surface.
"Whether biomarkers are detected or not, these findings are important for our understanding of which processes occur, and which do not, in the martian atmosphere — essential information when considering where to focus our continued investigation of Mars," Håkan Svedhem, ESA Project Scientist for the ExoMars Trace Gas Orbiter, said in the statement.
The three papers have been published in the journals Astronomy and Astrophysics, and Icarus.