Detection of Green Dayglow Around Mars

Image credit: European Space Agency

The ExoMars Trace Gas Orbiter (TGO) orbiting Mars has made the first detection of greenline dayglow around a planet other than the Earth. A thin, green glow is seen by astronauts to surround the Earth at an altitude of approximately 90 kilometers as a result of sunlight interacting with molecules in the atmosphere. A similar green dayglow has been long predicted to surround Mars, but only now has it been verified with the Nadir and Occultation for Mars Discovery Ultraviolet and Visible Spectrometer (NOMAD-UVIS) instrument aboard the European Space Agency’s TGO spacecraft. To detect this faint, green glow, Jean-Claude Gérard of the Université de Liège, Belgium, and his team reoriented the NOMAD-UVIS instrument to point at the edge of the Martian atmosphere, similar to how astronauts observing the green glow around the Earth must point their cameras at the edge of the Earth. The glow around Mars originates from the photodissociation (chemical breakdown of a molecule by solar radiation) of carbon dioxide, CO2, liberating excited oxygen atoms in the atmosphere, which produce the characteristic green hue. Two intensity peaks were identified at 80 kilometers and 120 kilometers altitude above the Martian surface. Continued monitoring of the altitudes of the intensity peaks in the green dayglow on our planetary neighbor will help track variations in atmospheric pressure with latitude, season, and dust concentration. READ MORE