The EUV Reflectance of Mercury's Surface Measured by BepiColombo/PHEBUS

On 9 and 10 October 2021, Probing the Hermean Exosphere by Ultraviolet Spectroscopy aboard BepiColombo observed the disk-integrated reflected light from the Sun by Mercury in the spectral range 90–160 nm at a phase angle of 70°. These observations are used to derive the EUV reflectance of the surface of Mercury in this wavelength range for the first time since the Mariner 10 observations in 1974–1975. Our observations are in agreement with a surface of Mercury darker by a factor ∼0.5–0.6 compared to the Moon in this wavelength range, with an average reflectance of 0.3% at a phase angle of 70°. This lower reflectance of Mercury could be due to the lower abundance of FeO. The derived reflectance near 160 nm is close to the derived reflectance at 220 nm by MESSENGER/MASCS at a similar phase angle. Possible spectral variations measured between 110 and 140 nm could be attributed to SiO2 glass, but the signal to noise ratio is low and other observations, possible at several periods during the rest of the cruise, are needed to confirm this last result.