Mystery of methane and water on Mars deepens with first-year results of Trace Gas Orbiter
The ExoMars Trace Gas Orbiter (TGO) has been hovering over the Red Planet for a year now, and the first results of its observations have now been released. In that time, the orbiter has made a map of the distribution of watery minerals in the Martian surface, studied how the global dust storm affected the atmosphere, and intriguingly found very little methane, which calls into question the results of a study just last week.
As a by-product of life, methane is an exciting gas to detect on other planets – especially because it doesn't stick around in the atmosphere for long, so any amount picked up must have been released relatively recently. Just last week, NASA and ESA announced that both the Curiosity rover and Mars Express orbiter detected large methane spikes a day apart in 2013, and were even able to triangulate the most likely source.
But the TGO results paint a different picture. Using two onboard spectrometers, ACS and NOMAD, the orbiter's observations make for the most detailed global analysis produced so far, and this places the upper limit of methane in Mars' atmosphere at 0.05 parts per billion by volume (ppbv). That's far less than the amounts reported by Curiosity and Mars Express of 6 ppbv and 15 ppbv, respectively.
"We have beautiful, high-accuracy data tracing signals of water within the range of where we would expect to see methane, but yet we can only report a modest upper limit that suggests a global absence of methane," says Oleg Korablev, lead author of a study describing the find. "The TGO's high-precision measurements seem to be at odds with previous detections; to reconcile the various datasets and match the fast transition from previously reported plumes to the apparently very low background levels, we need to find a method that efficiently destroys methane close to the surface of the planet."
Dusty Red Planet
In the middle of 2018, a global dust storm swept across Mars, which ultimately claimed the "life" of NASA's Opportunity rover. But looking down from high above the surface, TGO had the perfect view to study how such massive storms affect the atmosphere.
Using ACS and NOMAD, the orbiter examined the chemical makeup of the atmosphere by watching how it absorbs and reflects sunlight. The team found that the dust helps spread water vapor – as well as a "semi-heavy" form of water – higher into the atmosphere than it could normally travel.
"In the northern latitudes we saw features such as dust clouds at altitudes of around 25 to 40 km (15.5 to 25 mi) that were not there before, and in southern latitudes we saw dust layers moving to higher altitudes," says Ann Carine Vandaele, principal investigator of the NOMAD instrument and lead author of a study on the subject. "The enhancement of water vapor in the atmosphere happened remarkably quickly, over just a few days during the onset of the storm, indicating a swift reaction of the atmosphere to the dust storm."
These results can allow future studies to better understand the geological history of water vapor on Mars.
Wet Red Planet
But water isn't just floating around Mars in the air – there's plenty of it underground, too. TGO's FREND instrument has created the first global map of how much water is locked away in the soil, down to a depth of 1 m (3.3 ft).
The instrument can read this by watching how cosmic rays interact with the planet's surface. Specifically, it measures the speed of neutrons bouncing back – hydrogen in the form of water ice or hydrated minerals slows them down, allowing the craft to determine where and how much water is present.
Between May and September last year, TGO scanned Mars over and over to produce a planet-wide map of the subsurface water distribution. The results back up previous observations, showing that the polar regions – particularly the north pole – host the most water, while the rest of the planet is covered in alternating wet and dry regions.
Although FREND hasn't yet finished its job, the map produced so far is already the best one we have.
"In just 131 days the instrument had already produced a map that has a higher resolution than that of the 16 years data from its predecessor onboard NASA's Mars Odyssey – and it is set to continue getting better," says Igor Mitrofanov, lead author of a third study detailing the water distribution. "The data is continually improving and we will eventually have what will become the reference data for mapping shallow subsurface water-rich materials on Mars, crucial for understanding the overall evolution of Mars and where all the present water is now. It is important for the science on Mars, and it is also valuable for future Mars exploration."
These results were published in three separate studies. The methane and water vapor studies were published in the journal Nature, while the subsurface water map study has been accepted for publication in the Proceedings of the Russian Academy of Science, the Branch of Physical Science (PDF).