For many years, planetary scientists have been observing a Mars ghost: methane gas.
It was first seen by ground observations of Mars in the 2000s, then by spacecraft orbiting the Red Planet. But these observations hardly detected it and were repeatedly called into question. There was a lot of debate and some of the allegations were contradictory. No truly convincing observation was made of it.
The European Space Agency therefore sent an investigation into 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 different ways.
The initial results of the probe showed that no methane was found, but a more in-depth, long-term analysis of data was needed.
… and the analysis has just been done. In a recent article, a team of planetary scientists announced the result: No.
Despite looking very hard at Mars methane over a long period of time, nothing was found. They also searched for ethane and ethylene, slightly more complex molecules similar to methane, and found none. The best they did was to report upper limits (meaning that their observations would see these molecules if they were more than the numbers reported). These limits are low: for methane there were no more than 0.06 parts per billion measured by volume (such as 60 liters of methane in a cube a kilometer on one side). Ethane and ethylene could also not occur more than 0.1 and 0.7 parts per billion.
Yikes. It is low, and it seems to exclude the previous measurements, which reached up to 60 ppb. If the amounts that would have been seen earlier were in the atmosphere, NOMAD would have seen it.
[UPDATE (Dec. 29, 2020): To be clear, NOMAD was able to make measurements down to about 6 km above the Martian surface. The Curiosity rover is on the surface, and found methane at decent levels at one point, which was confirmed by another orbiter. As I note below something like that may be rare, or for some reason doesn’t get to the height necessary for NOMAD to see it. But that would then add another layer of mystery to all this. That doesn’t mean the observation isn’t real; just that this is pretty complicated.]
Why is this important? Because on earth, most methane in the air comes out of life. Bacteria that feed on dead plants and animals release it, and some more complex life forms tend to excrete as well.
It can also be created by geological processes. Lightning can make it, or when hydrogen released by chemical processes (such as some minerals that dissolve in water) reacts with carbon dioxide. However, it is less a source than biology.
If methane is seen on Mars, it is very interesting because it is continuous geological processes that can produce it, or VERY interesting because it means little Martian animals shoot it out.
As you can imagine, scientists plan 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, very specific wavelengths (colors) are absorbed by different molecules. By identifying the wavelengths, you can see what is in the air, and by looking at how much is being absorbed, you can also find out how many of the molecules are there.
Among other methods use what is called solar culture to measure these gases. As the spacecraft orbits Mars, it sees the sun pass behind Mars, and then rise behind the planet’s disk for a while – essentially sunset and sunrise. When one astronomical object blocks another, we call it an occultation.
As the sun begins to set just 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 pass through the lower and lower parts of the atmosphere (and vice versa as the sun comes out again). By doing so over the course of an entire Earth year (April 2018 to April 2019), it was able to test the Martian atmosphere from 6 km above the surface to 100 km, from 85 ° north to 85 ° south over each length – in other words, actually all over the planet.
The scientists looked at 240,000 separate global measurements, as well as 2,000 that looked at specific places on the planet for plume methane. They also searched for ethane and ethylene because it can be used to determine the source of the methane; biology on earth makes up predominantly methane, but geological process makes up all three. If they had found methane but no ethane or ethylene, it would have been very exciting.
But they found nothing.
So does it exclude methane on Mars? Well, yes and no. This is definitely a very strong limitation on it. Any methane produced by, for example, an underground bag that opens will show a strong local signal for a month or so, but then it will mix through the atmosphere. Since they saw nothing, it means that such a source must be fairly sporadic.
I will note that the year of NOMAD observations covered the northern hemisphere until early spring (a Mars year is two Earth years), and the late winter to early autumn of the southern hemisphere. So if methane production is seasonal, NOMAD should have seen it in some hemisphere.
This finding is scientifically interesting because it seems to close the debate on previous observations, even if it is different. It seems to make the possibility of life on or below the Martian surface much less likely.
However, this does not exclude that life there existed billions of years ago, and it will still be incredibly exciting to find evidence for it. Perseverance lands on Mars on February 18, 2021 and is partly designed to search for the evidence.
Patience. We may have answers fast enough one way or another.
My thanks to lead author Elise Knutsen for her help in this.
*Not to be confused with Tan Ru.