Hollows on Mercury: Global Classification of Degradation States and Insight Into Hollow Evolution

Abstract

Hollows are small, shallow, irregularly shaped landforms, widespread across Mercury, interpreted to have formed via loss of volatiles. Here, we present the first global analysis of hollow degradation states using a new machine learning-derived global catalog. We define three classes, grading from younger/potentially active “Stage 1” (sharp morphology, high visible reflectance) to older/potentially expired “Stage 3” (softened morphology, reflectance similar to that of surroundings). Most analyzed hollows are Stage 1 (N = 1,545 individual hollows), which are more common than Stage 2 (N = 1,111) or Stage 3 (N = 10) hollows near the equator, consistent with the idea that insolation is a primary driver for hollow initiation/growth. Areas where Stage 2 hollows are more common than Stage 1 hollows may indicate regions of relative volatile depletion. Stage 3 hollows are rare, suggesting they are systematically missed during image review, or that hollows on Mercury are mostly young, have been recently reactivated, or are quickly erased once they become inactive. Temperature may limit hollow growth, given that only small hollows are identified in the coldest terrains. There is no meaningful difference in the distribution of hollow sizes between stages, suggesting that their morphological and reflectance properties are not substantially muted until they are fully grown. Stage 1 hollows are more commonly found on steeper slopes than nearby Stage 2 hollows, suggesting that slopes may be an important control on how long hollows remain active. Our hollow classifications are openly available and can help to inform global-scale studies of hollow evolution, and upcoming targeting efforts by the ESA/JAXA BepiColombo mission.