On the asymmetry of Nathair Facula, Mercury

Abstract

Nathair Facula is the largest and most spectrally distinct of nearly 200 ‘bright red’ spots (faculae) on Mercury's surface, most of which are accepted to be deposits from explosive volcanic eruptions. Like most of Mercury's faculae, it hosts a non-circular central pit (in this case nearly 40 km wide and 3 km deep). However, the center of this facula does not coincide with its central pit's midpoint. Quantitative analysis of two sets of spectral data shows that the facula's midpoint is offset by 10–30 km northwards or northeastwards, and probably lies outside the pit. The pit area is almost certainly a ‘compound vent’, within which the locus of eruption has migrated between eruptive episodes. The asymmetry of the facula and the texture of the vent floor are consistent with the most energetic and/or the most recent eruptions having occurred from the northeastern part of the compound vent, but evidence that the center point of the facula lies outside the vent indicates that it may be necessary to invoke an additional factor such as asymmetric eruption fountains.