The NOMAD instrument, developed at the Royal Belgian Institute for Space Aeronomy and currently orbiting Mars aboard ESA’s ExoMars Trace Gas Orbiter, has detected a unique green glow of oxygen in the atmosphere surrounding the red planet at an altitude of about 80 km.
This emission accounts for the characteristic color of Earth’s polar aurora and air glow, but has never before been observed in other planetary atmospheres outside of Earth. This light emission is created by the interaction between solar radiation and carbon dioxide, which is the main constituent of Mars’ atmosphere. On Mars, as in a large natural laboratory, we have also been able to simultaneously measure the two oxygen lines in the visible and ultraviolet, which clears up a long-standing controversy between discordant quantum mechanical calculations and atmospheric measurements on Earth.
The aurora, however, is only one of the ways in which planetary atmospheres are illuminated. The atmospheres of planets, including Earth and Mars, constantly glow both day and night as sunlight interacts with atoms and molecules within the atmosphere, a phenomenon commonly referred to as “airglow.” Daytime and nighttime glow is caused by slightly different mechanisms.
Nighttime glow results from the recombination of molecules that have been destroyed by solar radiation during the day. Light is emitted when excited atoms or molecules return to their original unexcited state during the night. One of the brightest emissions on Earth comes from night glow, specifically from excited oxygen atoms that radiate light at a particular wavelength of light (557.7 nanometers), a green radiation emitted in the spectral region where our eyes are most sensitive.
The green layer of night glow is quite faint, so that it can only be seen at night by looking sideways at the layer of air emitting the glow, as with astronauts in orbit. This optical phenomenon has been captured with cameras aboard the Space Shuttle and the International Space Station.
The daytime glow is created when solar radiation interacts directly with the atmosphere. Atoms and molecules (such as nitrogen and oxygen) in the atmosphere absorb some of the sunlight, which temporarily excites them until they release this additional energy in the form of light, either at the same frequency or at a lower frequency (color) than the absorbed light. This emission is much fainter than the light scattered by the Sun, so we cannot see it with the naked eye.
Although it had been predicted for about 40 years that the green daylight glow existed on Mars, until now, it had never been observed in the atmospheres of other planets, either because their surfaces are too bright compared to atmospheric light emission or because previous missions to the planets were not equipped with instruments sensitive to both visible and ultraviolet light. To remedy this, the NOMAD team at the Royal Belgian Institute for Space Aeronomy and the Université de Liège decided to reorient the instrument’s ultraviolet and visible (UVIS) channel from its typical nadir orientation (looking directly down on the Martian surface) toward the diurnal edge of Mars.
Between April 24 and December 1, 2019, the NOMAD team used UVIS to scan at altitudes ranging from 20 to 400 kilometers above the Martian surface twice per orbit. On each of those “special observation modes of the planet’s edge,” the instrument detected surprisingly bright signals at 557.7 nanometers, demonstrating the ubiquity of the daytime green glow. The main peak altitude was near 80 km and its intensity varied due to the change in distance between Mars and the Sun, local time, and latitude of the observations. A second peak emission was observed near 120 km altitude.
Another weaker emission of the oxygen glow was also observed at 297.2 nanometers in the near ultraviolet range. Such a simultaneous measurement of two oxygen lines in the visible and ultraviolet spectral ranges is quite unique and allowed the direct derivation of a 16.5 ratio of visible to UV emissions, which is difficult even for a laboratory on Earth.
A photochemical model, developed at the University of Liège, was used to better understand this green glow on Mars and to compare it to what we see on our own planet. The model was able to reproduce the altitude and brightness of the air layer. It also indicates that the green daytime glow on Mars is produced primarily when carbon dioxide (CO2) molecules are broken down into oxygen (O) and carbon monoxide (CO) by the far ultraviolet emitted by the Sun. The resulting oxygen atoms glowed in both visible and ultraviolet light.
Our result agrees with models from atomic physics but contradicts previous observations made in Earth’s glows and auroras. The emission ratio of 16.5, found in this study, will be considered a standard for measurements linking the ultraviolet to visible spectral regions. This result has implications for the study of auroral processes, airglow, and the spectral calibration of optical instruments.
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“ExoMars NOMAD spots unique green light at Mars.” https://www.aeronomie.be/en/news/2020/exomars-nomad-spots-unique-green-light-mars
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