Nomad spectrometer spots light emission on night side of Mars

14 Nov 2023

Trace Gas Orbiter instruments reveal dynamics of Martian atmosphere.

A team led by the University of Liège (ULiège) has made the first observations of light emission in the visible range of the night side of Mars.

Published in Nature Astronomy, the results should assist a better understanding of the dynamics of the upper atmosphere of the Red Planet and its variations throughout the year.

The observations were made by the Trace Gas Orbiter (TGO) mission launched in 2016 as the ESA's first mission in its ExoMars program, designed to search for signs of possible life on the planet.

TGO carries the Nomad instrument, a platform for one ultraviolet and two infrared spectrometers performing high-sensitivity orbital identification of atmospheric components, including methane and other species, via both solar occultation and direct reflected-light nadir observations.

Since 2019 Nomad and companion infra-red instruments have studied the Martian atmosphere and its absorption of sunlight, making discoveries about the vertical distribution of both water and semi-heavy water - molecules in which one hydrogen atom is replaced by deuterium.

Nomad has usually been oriented towards the center of Mars and observed sunlight reflected by the planetary surface and atmosphere. But based on a proposal from ULiège the instrument was oriented towards the limb of the planet in order to observe its atmosphere from the edge.

Illuminations for future astronauts to watch

"Back in 2020, we were already able to detect the presence of a green emission between 40 and 150 km in altitude, present during the Martian day," commented Jean-Claude Gérard of ULiège Laboratory for Planetary and Atmospheric Physics (LPAP). "This was due to the dissociation of carbon dioxide molecules, the main constituent of the atmosphere, by ultraviolet solar radiation."

The new observations add details to these dynamics, with Nomad detecting a new emission between 40 and 70 km in altitude brought about by oxygen atoms created in the summer atmosphere and carried by winds towards the high winter latitudes. There they meet carbon dioxide and reform an oxygen molecule in an excited state, which then emits light in the visible range.

ULiège has observed that this process seems to be reversed every half Martian year, with the luminosity then changing hemisphere. This parallels emission previously spotted and analysed on Venus using images from the Venus Express satellite, with atoms travelling from the sunlit side to the dark side where they emit a similar glow.

"The study will be continued during the TGO mission and will provide us with valuable information about the dynamics of the Martian upper atmosphere and its variations over the course of the Martian year," said Lauriane Soret from LPAP. "We have noticed that another ultraviolet emission due to nitric oxide molecules is also observed in the same regions. Comparing the two emissions will enable us to refine the diagnosis and identify the processes involved."

The intensity of the night glow in the polar regions is strong enough that relatively simple and inexpensive instruments in Martian orbit on future ESA missions could map and monitor atmospheric flows. And the emission is intense enough to be observable during the polar night by future astronauts in orbit or from the Martian surface.