The ‘methane mystery’ on Mars has been ongoing for many years, with contradictory findings from seveal missions, wuch as ESA’s Mars Express and NASA’s Curiosity rover, capturing sporadic spikes and bursts of the gas in the Martian atmosphere, fluctuations both in orbit and at the surface, signs of the gas varying with the seasons, or not observing any methane at all. Now, ESA’s Trace Gas Orbiter has set new upper limits on how much methane and three other biosignature gases (ethane, ethylene and phosphine) is in the atmosphere of Mars.
Searching for biosignatures on Mars is a primary goal of ESA’s Trace Gas Orbiter (TGO).
A key biosignature gas of interest is methane, as much of the methane found on Earth is produced by living things or geological activity — and so the same may be true for Mars.
Previous estimates from Mars and ground-based missions range from 0.2 up to 30 parts per billion by volume (ppbv), indicating up to 30 molecules of methane per billion molecules. In Earth’s atmosphere, methane is present at nearly 2,000 ppbv.
The first TGO results, reported in April 2019, spotted no methane, instead calculating that, if present, the gas must have a maximum concentration of just 0.05 ppbv.
“We have now used the Atmospheric Chemistry Suite (ACS) on board the TGO spacecraft to refine the upper limit for methane at Mars even further, this time gathering data for over 1.4 Martian years — 2.7 Earth years,” said Dr. Franck Montmessin, a researcher at LATMOS.
“We found no sign of the gas at all, suggesting that the amount of methane at Mars is likely even lower than previous estimates suggest.”
“As the orbiter’s instruments are highly sensitive, if methane is present it must be at an abundance of less than 0.05 ppbv — and more likely less than 0.02 ppbv.”
Dr. Montmessin and colleagues also hunted for signs of methane around Curiosity’s home, Gale crater, and found nothing, despite the rover reporting the presence of methane there.
“Curiosity measures right at Mars’ surface while the orbiter takes measurements a few kilometers above — so the difference between these two findings could be explained by any methane being trapped to the lower atmosphere or the immediate vicinity of the rover,” Dr. Montmessin said.
The apparent lack of Martian methane reported by the team is supported by an analysis of data from the NOMAD (Nadir Occultation MArs Discovery) instrument on board TGO.
“We also found no sign of methane on Mars, and set an upper limit of 0.06 ppbv, which agrees with TGO’s initial findings using the ACS instrument,” said Dr. Elise Wright Knutsen, also from LATMOS.
“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 — so if methane is released in this way, it must be sporadic.”
Methane, ethane, ethylene and phosphine are often associated with biological or geological processes, so understanding if they are present or not on Mars is key to understanding what processes may or may not be active on the planet today. Image credit: ESA.
Alongside methane, the researchers looked for two other gases: ethane and ethylene. These molecules are expected to occur after methane is broken down by sunlight, and so are exciting both in their own right and in the context of our hunt for methane.
The molecules also have short lifetimes, meaning that if they are found in a planetary atmosphere they must have been recently released or created via an ongoing process. This makes them excellent tracers of possible biological or geological activity.
“These are ExoMars’ first results hunting for these two gases. We didn’t detect either, and so set upper limits for ethane and ethylene at 0.1 and 0.7 ppbv, respectively — low, but higher than our limits for methane,” Dr. Knutsen said.
TGO has also been hunting for phosphine — a gas that caused a splash and huge controversy last year when it was allegedly detected at Venus.
“We didn’t find any signs of phosphine at Mars. Our uppe