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First results from the Exomars trace gas orbiter

on 25 April 2019

In its first year on Mars orbit, the ExoMars Trace Gas Orbiter found new evidence of the impact of the recent planet-encompassing dust storm on water in the atmosphere, and a surprising lack of methane, among other scientific highlights.

Two papers were recently published in the journal Nature, describing the new results. A third paper, submitted to the Proceedings of the Russian Academy of Science, presents the most detailed map ever produced of water-ice or hydrated minerals in the shallow subsurface of Mars.

The joint ESA-Roscosmos ExoMars Trace Gas Orbiter, or TGO, arrived at the Red Planet in October 2016. TGO’s main science mission began at the end of April 2018, just a couple of months before the start of the global dust storm that would eventually lead to the demise of NASA’s Opportunity rover after 15 years roving the Martian surface. 

Spacecraft in orbit, however, were able to make unique observations, with TGO following the onset and development of the storm and monitoring how the increase in dust affected the water vapour in the atmosphere – important for understanding the history of water at Mars over time. 

Two spectrometers onboard – NOMAD and ACS –made the first high-resolution solar occultation measurements of the atmosphere, looking at the way sunlight is absorbed in the atmosphere to reveal the chemical fingerprints of its ingredients. The new results track the influence of dust in the atmosphere on water, along with the escape of hydrogen atoms into space.

The observations are consistent with global circulation models. Dust absorbs the Sun’s radiation, heating the surrounding gas and causing it to expand, in turn redistributing other ingredients – like water – over a wider vertical range. A higher temperature contrast between equatorial and polar regions is also set up, strengthening atmospheric circulation. At the same time, thanks to the higher temperatures, fewer water-ice clouds form – normally they would confine water vapour to lower altitudes.

The two complementary instruments also started their measurements of trace gases in the Martian atmosphere. Trace gases occupy less than one percent of the atmosphere by volume, and require highly precise measurement techniques to determine their exact chemical fingerprints in the composition.

Methane is of particular interest for Mars scientists, because it can be a signature of life, as well as geological processes – on Earth, for example, 95% of methane in the atmosphere comes from biological processes. 

The new results from TGO provide the most detailed global analysis yet, finding an upper limit of 0.05 ppbv, that is, 10–100 times less methane than all previous reported detections.

While the lively debate on the nature and presence of methane continues, one sure thing is that water once existed on Mars – and still does in the form of water-ice, or as water-hydrated minerals. And where there was water, there might have been life.

To help understand the location and history of water on Mars, TGO’s neutron detector FREND is mapping the distribution of hydrogen in the uppermost metre of the planet’s surface. Aside from the obviously water-rich permafrost of the polar regions, the new map provides more refined details of localised ‘wet’ and ‘dry’ regions. It also highlights water-rich materials in equatorial regions that may signify the presence of water-rich permafrost in present times, or the former locations of the planet’s poles in the past.

Image credit: ESA/ATG medialab