Mercury’s Hollows: Depths, Estimation of Formation Rates, and the Nature of the Bright Haloes

Abstract:

Mercury's hollows are shallow depressions, often with high-reflectance interiors and haloes. The fresh appearance of hollows indicates that they are relatively young features. Their morphology is suggestive of formation via sublimation-like loss of a volatile-bearing phase through solar heating, destruction by UV photolysis, contact with molten rock, or bombardment by micrometeoroids and/or ions. Hollows are found within the low-reflectance material (LRM) color unit. Following an examination of all MESSENGER images with pixel sizes <20 m and incidence angles <85°, shadow-length measurements made on 905 images yielded the depths of 2608 hollows. The mean depth is 24 ± 16 m. The narrow range of depths, despite formation within LRM units that are of much greater and more variable thickness, could result from development of a protective lag as the volatile-bearing phase is lost. The rate at which hollows form may be estimated as follows. The size–frequency distribution of Mercury rayed craters >4 km in diameter gives absolute model ages of 110 to 689 Ma, depending on the crater production model. The 41-km-diameter rayed crater Balanchine has a density of superposed craters similar to the average for all rayed craters, so we take Balanchine's age to be the population average. Hollows on Balanchine's floor are ~300 m wide. The average rate of hollows formation by horizontal scarp retreat for a 110 Ma model age would be 1 cm per 3700 Earth years. If Balanchine formed 689 Ma ago, then the average growth rate would be 1 cm per 23,000 yr. We also consider the mechanisms by which hollows form bright haloes. Calculations show that comet-style lofting of dust by sublimating gas is not important given Mercury's high surface gravitational acceleration. Instead, the bright haloes may form by condensation of sublimated material or by physical modification or chemical alteration of the surface by re-deposited sublimation products.