Source of Mercury’s Hollow-Forming Materials: Preliminary Results from Geological Mapping, Spectra, and Impact Simulations

Mercury has a large core, but paradoxically the surface is also volatile rich. Most core-enlarging scenarios envisaged for Mercury would have heated the silicates and preferentially driven-off these volatiles. Mercury’s hollows, flat floored, rimless depressions tens of metres deep and up to tens of kilometres across, appear to have formed by the loss of some volatile material to space upon its exposure at the surface, often by impact craters. Hollows lack superposing craters, indicating that they may be undergoing active formation today. The subsurface distribution of Mercury’s hollow-forming material is not known. If confined to the upper few kilometres of Mercury’s crust, sampled by the craters up to a few hundred kilometres in diameter in which most hollows are found, then perhaps it was accreted as a late veneer after the core-enlarging event. Alternatively, if the hollow-forming material is present throughout a greater thickness of Mercury’s silicate portion then the timing of any high-temperature core-enlarging event must have taken place very early in the planet’s history to allow time for a volatile-rich silicate fraction to reaccrete. Here, we study the Caloris basin: Mercury’s largest, well-preserved impact structure. We employed a combination of geological mapping, reflectance spectroscopy, and numerical impact simulations to map the present-day distribution of the hollow-forming material in Caloris ejecta, preserved as hummocky plains hosting km-scale knobs, back to its pre-Caloris, subsurface distribution. Our results suggest that Mercury’s hollow forming material comes from the whole thickness of the crust, a deeper constraint than previous studies.