ESAs ExoMars Orbiter Detects No Methane in Martian Atmosphere | Planetary Science

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The methane mystery on Mars has been ongoing for many years, with contradictory findings from seveal missions, wuch as ESAs Mars Express and NASAs 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, ESAs 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.

ESAs ExoMars Trace Gas Orbiter at Mars. Image credit: D. Ducros / ESA.

Searching forbiosignatures on Mars is a primary goal of ESAs Trace Gas Orbiter (TGO).

A keybiosignature 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 Earths 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 orbiters 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 Curiositys 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 TGOs 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 didnt 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.

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 didnt 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 didnt find any signs of phosphine at Mars. Our upper limits are similar for those of ethane and ethylene — between 0.1 and 0.6 ppbv, said Dr. Kevin Olsen, a researcher at the University of Oxford.

Whetherbiosignatures 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, said TGO project scientist Dr. Hkan Svedhem, a researcher at ESA.

Many key questions remain — for instance, why does Curiosity see methane at Gale Crater, while we find none in orbit? Could this methane have come from elsewhere, or only be found in particular locations across the planet — or could an unexpected process be destroying any methane present before we can detect it?

It will be exciting to continue working collaboratively with missions like the Curiosity and Rosalind Franklin rovers, both of which have totally different vantage points to an orbiter, to really pin down what is happening in this mysterious planetary environment.

The new results appear in three papers in the journal Astronomy & Astrophysics and the journal Icarus.

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F. Montmessin et al. 2021. A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater. A&A 650, A140; doi: 10.1051/0004-6361/202140389

Elise W. Knutsen et al. 2021. Comprehensive investigation of Mars methane and organics with ExoMars/NOMAD. Icarus 357: 114266; doi: 10.1016/j.icarus.2020.114266

K.S. Olsen et al. 2021. Upper limits for phosphine (PH3) in the atmosphere of Mars. A&A 649, L1; doi: 10.1051/0004-6361/202140868

This article is based on text provided by the European Space Agency.

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