Martian dayglow as seen by the SPICAM UV spectrograph on Mars Express

In this paper we describe measurements of the Martian dayglow obtained by SPICAM UV spectrograph on board Mars Express between October 2004 and March 2005. Typical spectra (of more than 24,000 individual spectra) display the main features of the dayglow already reported more than 30 years ago (Barth et al., 1971, 1972). The variations with altitude, solar zenith angle, and aerocentric longitude of the main emissions, namely, the CO Cameron band emissions (a 3 Π – X 1 Σ + ) from 180–260 nm, the CO 2 + ultraviolet doublet (B 2 Σ + ‐ X 2 Π) emission near 289 nm, the O resonance line at 130.4 nm, and the H Lyman alpha line at 121.6 nm, are detailed. The average temperature deduced from the CO 2 + (B 2 Σ + ‐ X 2 Π) emission scale height between 150 and 190 km is T ∞ = 201 ± 10 K. The altitude of the peak of the dayglow emission varies from 120 km at low solar zenith angle (∼30°) up to 132 km at high solar zenith angle (∼75°). We also find 24 orbits in which the individual derived exospheric temperatures do not show significant variations with respect to solar zenith angle, aerocentric longitude, or longitude. The presence of crustal magnetic fields appears to correlate with a significant (∼100 K) increase in exospheric temperature. We report also the first observations of emissions associated with the N 2 Vegard Kaplan band system in the Martian upper atmosphere.