Abstract | Thanks to MESSENGER, it is now clear that Mercury is more interesting and more perplexing than most would have expected when BepiColombo planning began, over two decades ago. Many new questions need to be answered to meet the mission’s objectives.
We now know that Mercury’s surface, and probably its crust as a whole, is rich in volatiles. Evidence includes high abundances of Na, S, K and Cl measured by X- and gamma-ray spectroscopy. There are also clear visual signs of volatile-driven activity such as explosive volcanic eruptions that have excavated compound vents and showered the adjacent area with diffuse explosive deposits, and the growth of ‘hollows’ where patches of surface are somehow dispersed to space, presumably in atomic form. S and C have been suggested in the gas phase to power explosive eruptions, whereas sulfides and chlorides have been suggested as the active constituent of hollow-forming material. Elemental and mineralogic determinations by BepiColombo with improved spatial resolution and greater sensitivity should enable more robust comparisons between explosive volcanic deposits and their surroundings, and between hollows and hollow-forming material. This should tell us more about which volatiles are lost in explosive eruptions and hollow formation, and what minerals host them. Higher resolution imaging of vents and hollows will help us to better understand the physical processes, their history, and their ages.
Large scale effusive eruptions ended about 3.5 Ga ago, but there is evidence for a long tail of waning activity on smaller scales. Improved spatial and spectroscopic resolution by BepiColombo should enable late stage lava effusions to be better identified, and to be more confidently distinguished from impact melt. Mineralogic and elemental data should enable the compositions of lavas of all ages to be determined well enough to understand source region composition, the depth, extent and causes of partial melting, and whether these changed over time.
The degree to which Mercury remains tectonically active, through secular global cooling or another process, remains in doubt. Understanding will be improved by high-resolution imaging of fault breaks, including small keystone grabens on the crests of lobate scarps.
Volatiles are known to be concentrated in polar cold traps. BepiColombo will study these well at both poles, whereas MESSENGER could do little in the far south. We want to know what volatiles occur other than water-ice, to measure the temperatures where each volatile appears to be stable, and to assess the ages and mobility of volatile deposits.
The ability of BepiColombo’s Xray spectrometer (MIXS) to detect, measure and spatially resolve the surface abundance of more elements than MESSENGER’s XRS, such as Ti, Cr, P and Ni whose partitioning between silicates, sulfides, metals and melts depends on oxygen fugacity may teach us much about Mercury’s differentiation history and the depths and mechanisms of magmagenesis.
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