The ExoMars orbiter has detected a green glow emanating from excited oxygen molecules in the dayside atmosphere of Mars. The observations, gathered over a six-month period last year, confirm a four-decades-old prediction and represent the first time anyone has documented this phenomenon on a planet besides Earth. Further study of this green glow will help scientists better understand the mechanisms behind how a similar glow is generated in Earth's atmosphere, as well as the composition and dynamics of the Martian atmosphere. See also: Atmosphere; Earth; Mars; Planet
Dubbed the Trace Gas Orbiter (TGO), the ExoMars spacecraft is operated by the European Space Agency. To make their observations, researchers remotely oriented the spectrometers aboard TGO to target the limb, or edge, of Mars as seen from orbit. This observation angle is similar to views of Earth taken by the International Space Station that successfully capture our planet's green atmospheric glow, which is a type of airglow—a ring of emissions of visible, infrared, and ultraviolet light from atoms and molecules in the atmosphere, generally in layers. In Mars' case, the airglow is initially generated as sunlight breaks apart carbon dioxide (CO2) molecules into constituent carbon (C) and oxygen (O) atoms. The sunlight then proceeds to excite, or add energy to, the freed oxygen. As excited oxygen returns to an unexcited level, the atoms emit photons, or light particles. On Earth, a green airglow of this sort can also be seen at night. Another source of green light emissions from atmospheric oxygen above Earth is when energized particles from the Sun interact with oxygen and other elements in the terrestrial atmosphere, generating aurorae, commonly called the Northern and Southern Lights. See also: Airglow; Aurora; Carbon; Carbon dioxide; Excitation; How spectroscopy relates to spectrometry; Oxygen; Space station; Spectroscopy
Researchers think that continued characterization of the Martian airglow could boost our understanding of visible light emissions from Earthly atmospheric oxygen, which, despite a long history of investigation, do not fully agree with theoretical models. Further airglow observations will also provide insight into properties of the Martian atmosphere at altitudes that have been largely unstudied. One benefit of more detailed descriptions of the Red Planet's atmosphere is honing the entry and descent for future exploratory missions, which often rely on parachutes to safely land robotic probes and, perhaps someday, astronauts. See also: Astronautics; Space flight
