The density of the upper martian atmosphere measured by Lyman-α absorption with Mars Express SPICAM

Absorption of interplanetary Lyman-α emission by Mars’ nightside lower thermosphere was observed by Mars Express Spectrometer for Investigation of Characteristics of the Atmosphere of Mars (SPICAM), and is analyzed to derive the CO2 density at 110 km during a martian year. The observed density seasonal variability is consistent with recent observations obtained by stellar occultations, proving that this method, though not as accurate as stellar occultations could be used complementary to them to characterize large variations of thermospheric density on Mars and provide a better spatial coverage by Lyman-α imagery.

► Analysis of martian Lyman-α absorption at the nightside by SPICAM-MEX. ► Derivation of the CO2 density and its variability in the martian thermosphere. ► Validation of the Lyman-α absorption method by comparison with stellar occultations.