Observations of Mercury's exosphere during BepiColombo first Mercury flyby

International audienceThe first flyby of Mercury with BepiColombo (ESA/JAXA joint mission) occurred on October 1, 2021. The closest approach took place at 23:34 UTC at an altitude of ~200 km from the planet's surface. During the flyby, the ultraviolet spectrometer PHEBUS (Probing of Hermean Environment By Ultraviolet Spectroscopy) was able to observe with the Extreme UltraViolet channel (spectral range, 55-155 nm) along with the two visible channels centered at 404 nm (K line) and 422 nm (Ca line). The observation started 30 minutes before Closest Approach and lasted one hour. We report the detection of the exospheric emission lines of H (121.6 nm), Ca (422 nm) and He (58.4 nm). This latter had not been observed since Mariner 10 Mercury flyby in 1973. The results clearly depict the geometry of observation, the spacecraft moving from the nightside of Mercury to its dayside. We also see some bursts on the count rate of the visible channels after the exit of Mercury's shadow, and discuss possible correlations with dust particles or the crossing of magnetospheric structures. We also discuss the potential detection of other neutral species and compare the results with MESSENGER data