Planetary scientists have been following a Martian spirit: methane gas for years.
It was first seen by ground-based observations of Mars in the 2000s, and then by spacecraft orbiting the Red Planet. But these sightings barely discovered it and were questioned time and time again. There has been much debate and some of the claims have been contradictory. No really convincing comment has been made.
So the European Space Agency sent a probe to Mars called the ExoMars Trace Gas Orbiter, equipped with a device on it called the Nadir and Occultation for MARS Discovery, or NOMAD*. It can search for many different gases, including methane, in the atmosphere of Mars in several ways.
Initial results from the probe showed that no methane was found, but a more in-depth, long-term analysis of the data was needed.
… And that analysis has just been done. In a paper just released, a team of planetary scientists announced the result: No.
Despite the intense search for methane on Mars over 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 they did was report upper bounds (meaning their observations would have seen these molecules had they been more abundant than the reported numbers). These limits are low: for methane there was 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 also could not be more abundant than 0.1 and 0.7 parts per billion.
Yikes. That’s low, and seems to rule out the earlier measurements, which went up to 60 ppb. If the amounts thought to have been previously seen 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 feed on dead plants and animals expel it, and some more complex life forms tend to excrete it too.
It can also be created by geological processes. Lightning can save the day, or when hydrogen released by chemical processes (such as some minerals that dissolve in water) reacts with carbon dioxide. However, these are less of a resource here than biology.
So if methane is seen on Mars, it is either very interesting because it means that there are ongoing geological processes that can produce it, or it is VERY interesting because it means little Martian bugs are letting it out.
As you can imagine, scientists are very determined to see if Mars has methane or not.
NOMAD is cool. It uses the sun as a light source. When sunlight passes through the atmosphere of Mars, very specific wavelengths (colors) are absorbed by different molecules. By identifying those wavelengths you can see what’s in the air, and by looking at how much is absorbed you can find out how much of that molecule is there.
Among other things, it uses what is mentioned solar occultations to measure these gases. As the spacecraft orbits Mars, it sees the sun pass behind Mars, and moments later sees it reappear from behind the planet’s disk – 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 planetary disk, NOMAD sees light pass through lower and lower parts of the atmosphere (and vice versa when the sun comes out again). By doing this over the course of an entire Earth year (April 2018 to April 2019), it was able to sample the atmosphere of Mars from 6 km above the surface to 100 km, from 85 ° north latitude to 85 ° south latitude over any longitude – in with in other words, essentially all over the planet.
The scientists looked at 240,000 individual global measurements, as well as 2,000 looking at specific locations on the planet for methane plumes. They also looked for ethane and ethylene because they can be used to determine the source of the methane; biology on Earth mainly makes methane, but the geological process makes all three. If they had found methane, but not ethane or ethylene, that 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 limitation. Any methane produced by, say, an underground bag that flips open would show a strong local signal for about a month, but would then be released into the atmosphere. Since they haven’t seen any, it means any resource like this must be quite sporadic.
I will note that the year of NOMAD observations was late summer to early spring in the Northern Hemisphere (one Mars year is two Earth years), and late winter to early fall in the Southern Hemisphere. So if methane production is seasonal, NOMAD should have seen it in some hemisphere.
This finding is scientifically interesting, as it seems to close the debate over the previous observations, even if it’s a bit of a bummer otherwise. It seems to make the possibility of life on or below the surface of Mars much less likely.
However, it doesn’t rule out the possibility that life existed there billions of years ago, and it would still be incredibly exciting to discover evidence for it. Perseverance lands on Mars on Feb. 18, 2021, and is in part intended to look for that evidence.
Patience. Maybe we’ll have answers somehow soon enough.
My thanks to lead author Elise Knutsen for her help with this.
*Not to be confused with Tan Ru.