For many years, planetary scientists have followed a Martian ghost: methane gas.
It was first seen through terrestrial observations of Mars in the 2000s, and later by spacecraft orbiting the red planet. But these observations barely detected it and have been questioned over and over again. There have been many debates and some of the claims have been contradictory. No really convincing observations have been made.
So the European Space Agency sent a probe to Mars called the ExoMars Trace Gas Orbiter, equipped with a device called Nadir and Occultation for Mars Discovery, or NOMAD*. It can search for many different gases, including methane, in the Martian atmosphere in various ways.
The initial results of the probe did not show methane, but a longer-term analysis of the data was needed.
… and this analysis has just been done. In a document that just came out, a team of planetary scientists announced its result: No.
Despite searching for a very hard Martian methane for a long period of time, none was found. They also looked for ethane and ethylene, slightly more complex molecules similar to methane, and found none. The best thing they did was report the upper limits (i.e., their observations would have seen these molecules if they had been more abundant than the figures reported). These limits are low: for methane, there were no more than 0.06 parts per billion when measured by volume (how to have 60 liters of methane in a bucket a kilometer to one side). Ethane and ethylene could not have been more abundant by 0.1 and 0.7 parts per billion.
Yikes. This is low and seems to exclude previous measures, which reached 60 ppb. If the amounts thought to have been seen before were in the atmosphere, NOMAD would have seen it.
Why is it important? Because on Earth, most of the methane in the air comes from life. Bacteria that feed on dead plants and animals emit it, and some more complex life forms tend to excrete them.
It can also be created through geological processes. Lightning can produce it 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.
Therefore, if methane is seen on Mars, it is very interesting because it means that there are ongoing geological processes that can produce it, or it is A LOT interesting because it means that Martian beasts are begging for it.
As you can imagine, scientists intend to see if Mars has methane or not.
NOMAD is great. Use the Sun as a light source. When sunlight passes through the Martian atmosphere, different molecules absorb very specific wavelengths (colors). By identifying these wavelengths, you can see what is in the air, and by looking at the amount absorbed, you can also find out how much of that molecule is there.
Among other methods, it uses what is called solar concealments to measure these gases. As the spacecraft orbits Mars, it sees the Sun pass behind Mars, soon after it rises again from the planet’s disk, essentially the sunset and sunrise. When one astronomical object blocks another, we call it concealment.
As the Sun begins to set 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 rises again). By doing so over an entire Earth year (April 2018 to April 2019), he was able to test the Martian atmosphere from 6 km above the surface to 100 km, from 85 ° north to at 85 ° south at each longitude – in other words, essentially all over the planet.
The scientists examined 240,000 separate global measurements, as well as 2,000 that examined specific places on the planet to find methane feathers. They also looked for ethane and ethylene because they can be used to determine the source of methane; biology on Earth produces predominantly methane, but the geological process produces all three. If they had found methane, but without ethane or ethylene, it would have been very exciting.
But they found nothing.
So does this rule out methane on Mars? Well, yes and no. This is certainly a very strong restriction. Any methane produced by, say, an open underground pocket would show a strong local signal for about a month, but then mix with the atmosphere. Since they didn’t see any, that means any source like this should be pretty sporadic.
I will note that the year of NOMAD observations covered the northern hemisphere from late summer to early spring (a Martian year is two years from Earth), and the southern hemisphere from late winter to early of autumn. Therefore, 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 the above observations, even if it is not a kind of bewilderment. It seems to make the possibility of life above or below the Martian surface much less likely.
However, it doesn’t stop it from existing billions of years ago and it would be incredibly exciting to discover evidence. Perseverance lands on Mars on February 18, 2021 and is designed in part to look for this evidence.
Patience. We may have answers one way or another soon enough.
My thanks to lead author Elise Knutsen for her help in this regard.
*Not to be confused with Tan Ru.