Study of the exosphere of Mercury with the PHEBUS spectrograph on the BepiColombo mission

Abstract

Mercury is the least explored of the inner planets, only three spacecraft have explored it. Mariner 10 observed the planet during three close encounters in 1974 and 1975, unveiling a very thin atmosphere surrounding the planet, somewhat similar to that of the Moon, including hydrogen (H) and helium (He). Sodium (Na), potassium (K) and calcium (Ca) were later detected in Mercury's exosphere with ground based telescopes. The MESSENGER mission, launched in 2004, not only performed three close encounters of the planet but also orbited it for four years. The mission added magnesium (Mg) and manganese (Mn) to the list of species detected in Mercury's exosphere. Iron (Fe) and aluminum (Al) were also discovered from ground-based observations around the same period.BepiColombo is the third mission to visit Mercury. Launched in 2018, the ESA-JAXA joint mission is set to perform six flybys of Mercury before the insertion in orbit around the planet at the end of 2025. Three flybys of Mercury have already been performed, during which PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) was able to observe the surfacebounded exosphere of Mercury. The instrument consists of two UV detectors (EUV and FUV) and two visible channels (c404 and c422). The c404 channel is dedicated to the K emission line at 404.7 nm while the c422 channel is dedicated to the Ca emission line at 422.8 nm. The geometry of observation was similar during the first two flybys, the spacecraft approaching the planet from its nightside, crossing its shadow before moving to its dayside. The closest approach to the surface occurred in the shadow of Mercury at an altitude of ∼200 km. PHEBUS was pointing northward, slightly antisunward.The count rate registered by both channels depicts the geometry of observation, notably the transit in the shadow of Mercury. The maximum is reached shortly after the spacecraft went out of eclipse. The count rate then decreases as the spacecraft moved away from Mercury. During the flybys, both signals are polluted by sporadic spikes whose origin remains uncertain.Despite these spikes, Ca was clearly detected by the c422 channel. The c422 signal shows not only an enhancement of Ca at dawn but also a very extensive Ca corona on the morning side, which was not reported by MESSENGER. The scale height deduced from our Ca profiles (2,500–2,800 km) is in agreement with the value reported by MESSENGER at similar true anomaly angle. Using the Chamberlain model, I determined a high temperature at the exobase (>50,000 K), in agreement with MESSENGER results. Both the large scale height and temperature at the exobase imply a very energetic release process. I 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 next step consists in using a 3D exospheric model that will include the effects of photoionization and non uniform distribution of Ca. Finally, PHEBUS pointing during the flybys did not allow to observe the potential cold component of Ca as the altitudes <200 km were not explored.The c404 channel detected species at low altitudes on the morning side during the flybys, potentially K or Mn. This K emission line was never detected by MESSENGER. As for Mn, MESSENGER has not observed it continuously, its detection was confined to certain seasons and to certain regions of the exosphere. These detection conditions are very different from those of BepiColombo. It is therefore difficult to identify with certainty the detected species. Further observations are necessary. Note that the scale height deduced from the c404 profile is ∼135 km and that the Chamberlain model applied to the c404 profile seems to indicate a temperature at the exobase <3000 K.