One of the keys to scanning the red planet for present or past life is understanding the molecules that make up its thin atmosphere. Various missions have detected methane on Mars, considered to be one of the hallmarks of living organisms. The arrival of the ExoMars mission with its Trace Gas Orbiter (TGO) instrument provided scientists with a chance to take the most accurate measurements of methane concentration yet. However, the first major data release shows almost none of the gas in Mars’ atmosphere. That’s an unexpected result.
The TGO is the orbital portion of the ExoMars mission. It’s also the only part of the ExoMars mission now. Sadly, the Russian-made Schiaparelli lander failed to slow itself with landing thrusters because of a faulty sensor. It crashed into the Martian landscape, leaving the TGO to soldier on alone.
Luckily, the TGO is most sensitive atmospheric tool ever sent to Mars. It carries a variety of infrared and ultraviolet spectrometers that can scan the atmosphere for trace gasses. There’s also a neutron detector for mapping water and hydrated minerals. It sits in a roughly circular orbit 250 miles (400 km) above Mars, sniffing the atmosphere and counting molecules. The first data set from April 2018 to August 2018 shows almost no methane on Mars — a maximum of 0.012 parts per billion (ppb). That’s many times lower than what scientists expected from the rudimentary methane detectors on the Curiosity rover.
There are a few potential explanations for this result. Curiosity and other ground instruments could simply be wrong about methane concentration on Mars, possibly because of short-lived local spikes in methane concentration. Alternatively, methane levels might be higher near the surface, but some process is destroying the molecules before they reach an altitude where the TGO can detect them.
The ExoMars team favors the latter explanation, but the process breaking down methane would have to be about 1,000 times faster than conventional chemistry to explain the discrepancy. The team has proposed methane could be diffusing into Martian rock and soil, or that it’s chemically bonding with eroded quartz deposits.
This result doesn’t necessarily mean that there’s no evidence of life on Mars, but it seems less likely methane will point the way. Missions like the upcoming Mars 2020 rover still have a lot of leads to follow.
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