Hollows on Mercury: Materials and mechanisms involved in their formation

Rebecca J Thomas,David A Rothery,Susan J Conway,Mahesh Anand

doi:10.1016/j.icarus.2013.11.018

Abstract

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking.In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles.

Highlights

The presence of morphologically fresh depressions on the surface of Mercury has been one of the most surprising discoveries of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft
We identified the spectral type of the regional substrate by reference to global mapping by Denevi et al (2009, 2013) and our own observations of colour composite images, distinguishing between regional low-reflectance material (LRM), intermediate terrain (IT), high-reflectance plains (HRP) and lowreflectance blue plains (LBP)
We found 445 groups of hollows, covering 57,400 km2, which amounts to 0.08% of the surface area imaged at better than 180 m/px. These ranged in areal extent from 0.07 km2 to 6771 km2, with a mean of 129 km2. 140 of these groups were at locations previously catalogued by Blewett et al (2013)

Summary

Introduction

The presence of morphologically fresh depressions on the surface of Mercury has been one of the most surprising discoveries of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft. These were dubbed ‘hollows’ to distinguish them from deeper ‘pits’ with sloping floors, which are proposed to form through magmatic processes (Gillis-Davis et al, 2009; Kerber et al, 2011). They range from individual hollows tens of meters across to clusters of hollows tens of kilometres across (Blewett et al, 2011) and shadow measurements indicate a consistent depth within a particular host crater in the range of tens of meters (Blewett et al, 2011; Vaughan et al, 2012). The bright deposits that gave BCFDs their name are revealed from orbit to occur both on hollow floors and as surrounding haloes

Methods

Results

Discussion

Conclusion