Mercury's Exosphere as Seen by BepiColombo/PHEBUS Visible Channels During the First Two Flybys

BepiColombo, the ESA/JAXA joint mission performed its first flyby of Mercury on 1 October 2021 and its second on 23 June 2022. PHEBUS observed the exosphere of Mercury during these flybys notably with its visible channels c404 (centered on the potassium emission line at 404 nm) and c422 (centered on the calcium emission line at 422 nm). The c422 signal shows not only an enhancement of calcium (Ca) near the dawn region but also a very extensive Ca exosphere on the morning side beyond 10,000 km. The e-folding distance deduced from our Ca profiles (2,500–2,800 km) is in agreement with the value reported by MESSENGER at similar true anomaly angles. We use a Chamberlain model to determine the temperature and density at the exobase. Using the morning side low-altitude data, we derived a high temperature at the exobase (>50,000 K), in agreement with the MESSENGER results. We also report a day/night asymmetry in the Ca exosphere that could indicate that the source of Ca is predominantly on the dayside or be the consequence of a shift of the main source of Ca away from the dawn region. The c404 channel detected additional species at low altitudes on the morning side during both flybys. Comparison with previous studies is inconclusive and further analysis will be necessary to identify the species. Nevertheless, we can note that the e-folding distance deduced from our profile is relatively small (135 km) and that the Chamberlain model applied to our profiles seems to indicate a temperature at the exobase <3,000 K.