For many years, planetary scientists have been observing a Martian ghost: methane gas.
It was first seen with ground-based views of Mars back in the 2000s, and then by a spacecraft orbiting the Red Planet. But these comments barely found him, and they were questioned over and over again. There has been a lot of debate, and some of the claims have been contradictory. It has not been closely observed.
So the European Space Agency sent a probe to Mars called the ExoMars Trace Gas Orbiter, equipped with a device called the Nadir and Occultation for MArs Discovery, or NOMAD*. It can look for many gases, including methane, in the Martian atmosphere in a number of ways.
Initial results from the probe did not show that methane was detected, but a more in-depth, long-term study of data was needed.
… And that analysis was done. In a paper that has just come out, a team of planetary scientists announced its result: Nope.
Despite looking very hard for Martian methane over a long period of time, none have been found. They also looked for ethane and ethylene, slightly more complex molecules like methane, and found none. Their best reporting was high levels (meaning, their observations would have seen these molecules if they were more abundant than the numbers reported). These limits are low: For methane, there were no more than 0.06 parts one billion when measured by size (as if a cube contained 60 liters of methane a kilometers on the side). Ethane and ethylene could not be more abundant than 0.1 and 0.7 parts per billion, respectively.
Yikes. That’s low, and it seems to stop the previous measurements, which went as high as 60 ppb. If the expected amounts were seen earlier in the atmosphere, NOMAD would have been seen.
Why is this important? Because of the Earth, most of the methane in the air comes from life. Bacteria that feed on dead plants and animals release it, and some more complex life forms tend to be um, banning it as well.
It can be created by geological processes, too. Lightning can occur, or when hydrogen released by chemical processes (such as some minerals dissolve in water) reacts with carbon dioxide. These are less of a source here than biology, however.
So if methane is seen on Mars, it is either very interesting because it means that there are ongoing geological processes that can take place, or it is TRUE interesting because it means small Martian insects are spreading it.
As you might expect, scientists are keen to see if Mars has methane or not.
NOMAD is cool. It uses the sun as a source of light. When sunlight passes through the Martian atmosphere, specific waves (colors) are captured by different molecules. By marking these waves you can see what is in the air there, and by looking at what has been captured you can also find out how many of those molecules there are.
Among other ways, he uses what is mentioned solar occultations to measure these gases. As the spacecraft moves around Mars it sees the sun pass behind Mars, then a short time later it reappears from behind the planet’s disk – basically, going sunset and sunrise. When one celestial object blocks another object we call it an occultation.
As the sun just begins to set behind Mars, its light passes through the upper atmosphere, and as it gets closer to the edge of the planet ‘s disk NOMAD sees the light. that passes through lower and lower parts of the atmosphere (and the other way when the sun comes back out). By doing this over the entire Earth year (April 2018 to April 2019) it was possible to get a taste of the Martian atmosphere from 6 km above the surface up to 100 km, from 85 ° north to 85 ° south over all longitude – in other words, essentially across the entire planet.
The scientists looked at 240,000 individual global measurements, as well as 2,000 looked at specific sites on the planet for a methane plug. They were also looking for ethane and ethylene as these can be used to determine the source of the methane; biology on Earth mainly produces methane but a geological process does all three. If they had found methane but without ethane or ethylene it would have been very interesting.
But they found nothing.
So does this control methane on Mars? Well, yes and no. This is certainly a strong deterrent. Any methane produced by, say, an open-ended underground pocket would show a strong local signal for a month or so, but then get mixed up around the atmosphere. Since they haven’t seen any, that means a source like this has to be very sporadic.
I note that the year of NOMAD observations covered the northern hemisphere in late summer to early spring (Martian year is two years on Earth), and the end of winter in the southern hemisphere to early fall. So if methane production is seasonal, NOMAD should be seen in one hemisphere or another.
This result is scientifically interesting, as it seems to close the debate on previously observed ideas, even if it is kind of a bummer otherwise. It seems to make life potential on or under the Martian surface much more likely.
It does not stop life from happening billions of years ago, however, and it would still be very interesting to find evidence. Persistence landed on Mars on February 18, 2021, and is designed in part to find that evidence.
Patience. We may have some answers one way or another quickly enough.
Thanks to lead author Elise Knutsen for her help on this.
*Not to be upset with it Tan Ru.