Modification of Caloris ejecta blocks by long-lived mass-wasting: A volatile-driven process?

Jack Wright,Susan J Conway,Costanza Morino,David A Rothery,Matthew R Balme,Caleb I Fassett

doi:10.1016/j.epsl.2020.116519

Abstract

The Caloris basin is the largest well-preserved impact basin on Mercury. As such, Caloris ejecta afford us an opportunity to study material from Mercury's deep interior with remote sensing. We have made observations of the geomorphology, colour, distribution, and flank slopes of the circum-Caloris knobs. Our observations suggest that these circum-Caloris knobs are modified ejecta blocks from the Caloris impact. High-resolution MESSENGER images show that knobs are conical and relatively uncratered compared with the surrounding plains, which implies the knobs have undergone resurfacing. We have observed material that has sloughed off knobs superposing impact craters that demonstrably postdate the Caloris impact, which requires some knob modification to have been more recent. We have observed hollows, depressions in Mercury's surface generally believed to have been caused by volatile-loss, on and closely associated with several knobs, which indicates that many knobs contain volatile material and that knob modification could extend into Mercury's recent past. Our measurements show that knob flanks typically have slopes of ∼21°, which is steep for a mound of unconsolidated material that was originally emplaced ∼3.8 Ga. The conical shape of knobs, their steep slopes, the dearth of superposed craters on knobs, and knob superposition relationships with other landforms suggest that Caloris ejecta blocks of arbitrary original shape were modified into their present shapes by long-lived mass-wasting. Mass-wasting must have dominated over impact gardening, which would have produced domal morphologies only. We suggest that mass-wasting was probably driven by volatile-loss, in a manner analogous to terrestrial landforms called ‘molards’. If the circum-Caloris knobs are analogous to molards, then they represent a landform and a process hitherto undocumented on Mercury, with implications for the volatile content of the planet's interior. These knobs therefore are prime targets for BepiColombo, which could search for fresh failures and volatile exposures in the knobs.

Highlights

The Caloris basin (∼1,550 km in diameter) is the largest wellpreserved impact structure on Mercury (Murchie et al, 2008)
We found examples of knobs and knobby plains on the uplifted sides of lobate scarps, and on the rims of pre-Caloris impact craters; the floors of these craters have been buried by smooth plains
While the conical and domal shapes of knobs are generally consistent with volcanic constructs elsewhere in the Solar System, they are inconsistent with observations of volcanic landforms on Mercury (Wright et al, 2018)

Summary

Introduction

The Caloris basin (∼1,550 km in diameter) is the largest wellpreserved impact structure on Mercury (Murchie et al, 2008). Hummocky plains called the Odin Formation (Fig. 1a; McCauley et al, 1981) are unevenly distributed around the basin and are generally believed to be exposed remnants of Caloris’ impact ejecta (Denevi et al, 2013; Ernst et al, 2015; Fassett et al, 2009). An ejecta interpretation of the knobs is not straightforward because the Odin Formation has a resolvably younger crater size-frequency distribution than the Caloris basin itself, which leaves the possibility that the knobs formed by some process unrelated to the impact, such as volcanism (Fassett et al, 2009).

Methods

Discussion

Conclusion