Knob heights within circum-Caloris geologic units on Mercury: Interpretations of the geologic history of the region

- Knob analyses aid in the understanding of Caloris basin ejecta.
- Knob heights are consistent with current models of ballistic ejecta emplacement.
- Knob heights are indicative of embayed Caloris ejecta.
- Knob densities imply the knobs were embayed by smooth plains material.

The circum-Caloris geologic units show morphology consistent with ejecta-derived formation, however crater counts suggest the units formed after the basin. To determine if the surrounding units are directly related to basin formation or products of later volcanic resurfacing events, we measured the heights and densities of knobs in 22 study regions to the east of the Caloris basin in three geologic units circumferential to the basin: the Odin Formation, the Van Eyck Formation, and the Smooth Plains Formation. In these study regions the size and concentration of ejecta blocks generally decrease away from the crater rim. The morphology that superposes the knobs and the distribution of knob heights within each study region suggests that the knobs were deposited as ejecta and later embayed by one or multiple volcanic events. The distribution of knob height and concentration indicates that the knobs within the circum-Caloris units are related to the formation of the basin.