For many years, planetary scientists have been following a Martian ghost: methane gas.
It was first seen through ground observations of Mars in the 2000s, then later by spacecraft orbiting the Red Planet. But these observations barely detected him and have been questioned countless times. There have been many debates, and some of the statements have been contradictory. No truly convincing remarks were made.
So the European Space Agency sent a probe to Mars, called the ExoMars Trace Gas Orbiter, equipped with a device on it called Nadir and Occultation for MArs Discovery, or NOMAD*. It can search for many different gases, including methane, in the Martian atmosphere in a variety of ways.
The initial results of the well did not show any methane found, but a more long-term analysis of the data was needed.
… And that analysis has just been done. In a recent paper, a team of planetary scientists announced the result: No.
Although he searched a lot for Martian methane for a long time, none were found. They also searched for ethane and ethylene, slightly more complex molecules, similar to methane, and found none. The best thing they did was to report upper limits (which means that their observations would have seen these molecules if they were more abundant than the reported number). These limits are low: for methane, there were no more than 0.06 parts per billion when measured in volume (such as having 60 liters of methane in a cube a kilometer on the one hand). Ethane and ethylene could not have been more abundant than 0.1 and 0.7 parts per billion.
Yikes. It is low and seems to exclude previous measurements, which reached up to 60 ppb. If the amounts thought to have been seen earlier had been in the atmosphere, NOMAD would have seen it.
Why is this important? Because on Earth, most of the methane in the air comes from life. Bacteria that celebrate dead plants and animals emit it, and some more complex life forms tend to excrete it.
It can also be created by geological processes. Lightning can occur or when hydrogen released by chemical processes (such as some minerals that dissolve in water) reacts with carbon dioxide. However, they are less of a source than biology.
So if methane is seen on Mars, it’s either very interesting because it means there are geological processes going on that can produce it, or it’s VERY interesting, because it means that some Martian flour mills can handle it.
As you can imagine, scientists intend to see if Mars has methane or not.
NOMAD is cool. Use the sun as a light source. When sunlight passes through the Martian atmosphere, very specific wavelengths (colors) are absorbed by different molecules. By identifying those wavelengths you can see what is in the air there and, looking at how much is absorbed, you can find out how much of that molecule is there.
Among other methods, it uses what is called solar occult to measure these gases. As the spacecraft orbits Mars, it sees the Sun pass behind Mars, then some time later sees it reappear from behind the planet’s disk – essentially the sunset and sunrise. When one astronomical object blocks another, we call it occultation.
As the Sun begins to sit behind Mars, its light passes through the upper atmosphere, and as it approaches the edge of the planet’s disk, NOMAD sees light pass through the lower and lower parts of the atmosphere (and vice versa when the Sun comes out). ). Doing this throughout a terrestrial year (April 2018 – April 2019), he managed to test the Martian atmosphere from 6 km above the surface to 100 km, from 85 ° north to 85 ° south on each longitude – in with other words, essentially all over the planet.
The scientists analyzed 240,000 separate global measurements, as well as 2,000 that analyzed specific sites on the planet to find methane panels. They also looked for ethane and ethylene, as they can be used to determine the source of methane; biology on Earth produces mostly methane, but the geological process does all three. If he had found methane, but no ethane or ethylene, it would have been very interesting.
But they found nothing.
So this excludes methane on Mars? Well, yes and no. This is certainly a very strong constraint. Any methane produced by, say, an underground pocket that opens would show a strong local signal for a month or so, but then it would mix around the atmosphere. Because they haven’t seen any, that means any source like this should be pretty sporadic.
I will note that the year of NOMAD observations covered the northern hemisphere in late summer to early spring (a Martian year is two years Earth) and the southern hemisphere of late winter to early autumn. So, if methane production is seasonal, NOMAD should have seen it in one hemisphere or another.
This finding is scientifically interesting, as it seems to close the debate on previous observations, even if otherwise it is a kind of inconvenience. It seems to make life on or below the Martian surface much less likely.
However, it does not prevent the existence of life there billions of years ago and it would still be incredibly interesting to discover evidence. Perseverance lands on Mars on February 18, 2021 and is partially designed to seek this evidence.
Patience. We may have some answers one way or another soon.
Thanks to lead author Elise Knutsen for her help.
*Not to be confused with Tan Ru.