Abstract | Mercury’s exosphere was discovered by the Mariner 10 mission, which found the light elements H and He. Decades of ground-based spectroscopic studies since Mariner 10, and the more recent MESSENGER mission, have detected and characterized the distribution of several additional species in Mercury’s exosphere. Extensive temporal and spatial studies of the species Na, K, and Ca have been performed using ground-based telescopes. During its four Earth years in orbit, the MESSENGER Ultraviolet and Visible Spectrometer (UVVS) regularly observed Na, Mg, Ca, and H. During the last few Mercury years of the MESSENGER mission, Al, Mn, and Ca+ (in emission) were observed by UVVS. Finally, Al and Fe have been observed using the Keck telescope on Mauna Kea. Ground-based observations of Mercury’s sodium exosphere continue, with a particular focus on measurements of the sodium tail that can constrain the loss rate and spatial distribution.
Despite the years of observations, there is much that remains poorly understood regarding Mercury’s exosphere, particularly in terms of the physical processes that generate and maintain the exosphere. The Ca exosphere was initially reported to be of high temperature, roughly 20,000 K, from ground-based data, with values up to 70,000 K reported from models using UVVS data. Non-thermal processes may be at work instead and need to be quantified. Analyses of UVVS data have determined a dawn-centered source for Ca, but ground-based data imply extensive Ca at high northern and southern latitudes as well as anti-sunward of the planet. Apparent differences between ground-based and space-based observations are also seen in the Na data. High-latitude peaks at northern or southern latitudes, or both, are often seen in ground-based data but have not been observed in UVVS data. This is perhaps a result of differences in viewing geometry, but recent work suggests that may not be the case. The species Al, Fe, Mn, and Ca+ were searched for on many occasions both from the ground and with UVVS. Although Ca+ was detected by UVVS during the third flyby of Mercury, these species were otherwise only observed during the last few Mercury years of the MESSENGER mission and only on a few observing runs at Keck. The reasons for the sporadic nature of these elements is unknown but they may be related to comet Encke as shown for Ca.
The origins of the spatial and temporal variations in Mercury’s exosphere require more investigation. The causes behind the high-latitude variability of Na and its possible correlation with the magnetic cusps need to be investigated, as should those responsible for the sporadic variability in weakly emitting and/or less abundant species like Al. Finally the interaction of the plasma environment with the surface and exosphere, and the roles of dust, meteoroid streams and cometary streams in the production of exospheric species should be elucidated to provide a more complete understanding of the source processes for the exospher |
