NOMAD detects green glow on the red planet

The NOMAD instrument, on board the Mars probe EXOMARS Trace Gas Orbiter, has just detected for the first time green glow emissions in the Martian atmosphere. These emissions, so far observed only on Earth, could provide us with additional information about this physical phenomenon. These data, collected and analyzed by researchers from the Laboratory of Atmospheric and Planetary Physics (STAR Institute/Faculty of Sciences) of ULiège have just been published in the Nature Astronomy journal.

Airglow is a natural glow in our atmosphere that is constantly present above the entire surface of the globe. There are two main categories of airglow: daytime and nighttime. They both result from the interaction of sunlight with atoms and molecules with the atmosphere. The dayglow results from the dissociation of molecules broken by solar radiation. It is emitted when the excited atoms or molecules return to their original state. One of the brightest emissions on Earth comes from excited oxygen atoms that emit green radiation. Its nocturnal counterpart is quite weak, so it can only be seen at night by looking "sideway" at the emission layer, as astronauts can do in orbit from the International Space Station. It is this oxygen emission that also gives the Northern Lights its dominant green colour.

 
Optical phenomena captured by­ International Space Station cameras

Until now, this green glow has never been observed in the atmospheres of other planets because their surfaces are too bright in relation to atmospheric emission or because previous missions to planets were not equipped with instruments sensitive to visible light.  

 Four researchers from ULiège's Laboratory of Atmospheric and Planetary Physics (STAR Institute/Faculty of Sciences) are part of the scientific team of the NOMAD instrument, on board the Trace Gas Orbiter mission, ESA's first robotic mission for the study of the Martian atmosphere launched in 2016. They proposed to change the orientation of the probe to try to detect this green light around the red planet. The glow of the planetary atmospheres provides important information, such as the composition of the atmosphere, the energy deposition of solar radiation and solar wind, the dynamics of the atmosphere, the coupling with planetary magnetic fields and the chemical relaxation processes following excitation," explains Jean-Claude Gérard a researcher at LPAP, a specialist in the composition and evolution of the Earth's and planetary atmospheres.

The first observations began in April 2019 and have proven to be successful. "The instrument detected the presence of a bright green luminescence. The main emission peak was located near 80 km and its intensity varied depending on the distance between Mars and the sun, the local time and the latitude of the observations. Two maxima were observed at nearly 80 and 120 km," says Jean-Claude Gérard. "Other emissions from oxygen (O), CO₂⁺ and the CO molecule were also observed in the ultraviolet.

Altitude distribution of the observed Martian green day airglow (green dots)
and model prediction (red line) based on the photochemical model developed by the Uliège team.

A photochemical model has been developed by researchers in Liège to understand its details. It was able to reproduce the altitude and luminosity of the emission layer. It indicates that it is mainly produced by the dissociation of CO₂ molecules, the main constituent of the planet's atmosphere, which creates oxygen atoms in an excited state producing green luminescence. A weaker emission, originating from the same higher level of the oxygen atom, has also been observed in the near ultraviolet. From these simultaneous measurements, a ratio of 16.5 between visible and UV emission can be directly deduced. This is consistent with atomic physics models but contradicts previous observations made in the airglow and the aurora on Earth. This ratio is considered a standard for measurements linking the spectral regions from the ultraviolet to the visible.

The results obtained by the research team are of great importance, as they have implications for the study of auroral glow and atmospheric luminescence processes, and for the spectral calibration of optical instruments. In particular, they allow us to predict the response of the Martian atmosphere to seasonal changes and variations in solar activity, information that will be useful for future ESA and NASA robotic missions.

Scientific reference

"Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations”, J.-C. Gérard et al. , Nature Astronomy.