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First detection of oxygen green line dayglow emission on Mars

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The NOMAD/UVIS spectrometer has detected, for the first time in a planetary atmosphere outside the Earth, the oxygen green line emission around 80 km altitude in the Martian atmosphere. The green line at 557.7 nm is emitted by the de-excitation of oxygen atoms, produced by the photolysis of CO2, the major constituent of the Martian atmosphere. The simultaneous measurement of the green line and another oxygen emission at 297.2 nm also allowed solving an old discrepancy between ab initio calculation and terrestrial measurements.
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The NOMAD instrument, developed at the Royal Belgian Institute for Space Aeronomy and currently in orbit around Mars on board ESA's ExoMars Trace Gas Orbiter, has detected a unique green glow of oxygen in the atmosphere surrounding the red planet (around 80 km altitude).

This emission gives its characteristic colour to the terrestrial polar aurora and airglow, but was never observed before in other planetary atmospheres outside of the Earth. This green light is emitted by the de-excitation of oxygen atoms, produced in the Martian atmosphere by the photolysis of carbon dioxide, which is the major constituent of the Mars atmosphere.

Discovery of the green light at Mars

Even though green dayglow has been predicted to exist at Mars for around 40 years, it had never been observed in the atmospheres of other planets until now. To remedy this, the NOMAD team at BIRA-IASB and ULg decided to reorient the instrument’s UV and visible (UVIS) channel from its typical nadir orientation to point at the dayside limb.

Between 24th April and 1st December 2019, the NOMAD team used UVIS to scan altitudes ranging from 20 to 400 km from the Martian surface twice per orbit. On every one of those limb orbits, a strikingly bright signal was detected at 557.7 nm, demonstrating the omnipresence of the green dayglow. The main peak altitude was located near 80 km, and its intensity varied with the Sun-Mars distance, the local time and latitude of the observations. A second emission peak was observed near 120 km.

Solving a long-standing controversy

Another weaker emission of the oxygen dayglow was also observed at 297.2 nm in the near UV range. Such a simultaneous measurement of two oxygen lines in the visible and ultraviolet spectral range is quite unique. It allowed to directly derive a ratio of 16.5 between the visible and the UV emissions, which is difficult even in the laboratory on the Earth. This result agrees with models of quantum mechanics calculations but contradicts earlier atmospheric measurements made in the Earth’s airglow and aurora.

 

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Gérard, J.-C., Aoki, S., Willame, Y., Gkouvelis, L., Depiesse, C., Thomas, I.R., Ristic, B., Vandaele, A.C., Daerden, F., Hubert, B., Mason, J., Patel, M.R., López-Moreno, J.-J., Bellucci, G., López-Valverde, M.A., and Beeckman, B. (2020). Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations. Nature Astronomy, 4(11), 1049-1052. https://doi.org/10.1038/s41550-020-1123-2

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Figure 2: (Left) Observed (green dots) and modelled (red line) altitude distribution of the green line intensity. Dayglow was brightest at 80 km, reached a second peak around 120 km, and dissipated above 150 km. (Right) Example of a NOMAD UVIS dayside limb spectrum, observed on 28 April 2019.