Correcting Mercury’s Radial Contraction for Obscuration of Shortening Structures by Surface Roughness

Abstract

Estimating Mercury’s radial contraction is crucial to understanding the planet’s geologic evolution as it reflects cooling of the interior and growth of the solid inner core [e.g., 1]. Radial contraction has been estimated by mapping shortening structures, such as wrinkle ridges and lobate scarps [e.g., 2, 3]. Depending on the interpretation of small shortening structures, previous studies have estimated Mercury’s radial contraction to be 1–2 km, or up to 7 km. These estimates are used as constraints to thermos-chemical evolution models [e.g., 1]; however, it is unclear whether all shortening structures have been successfully detected on Mercury.

A key to this question lies in the heterogeneous distribution of shortening structures. The density of contractional landforms mapped by previous works [e.g., 4] shows deficits at some locations (Figure 1). While tectonic mapping could be biased by the geometric conditions of the MESSENGER observations [5], the deficit is not consistent with the expected longitudinal patterns. Mantle dynamic pressure could also affect the distribution of long fault scarps [6], but the deficits exist even where compressional tectonism is anticipated. An alternative hypothesis is the obscuration of landforms by extensive ejecta from large impact craters and basins [5], which needs further investigation.

To identify young ejecta that might blanket tectonic landforms, we characterize surface freshness using topographic roughness. Although crater formation increases roughness due to ejecta deposition and secondary craters, subsequent mass-wasting events triggered by impact-induced seismic shakings generally reduce topographic reliefs. Therefore, roughness can be used as a proxy for surface age in one geologic unit [7]. However, roughness data at kilometric baselines have been limited to the north-polar region due to a lack of global high-resolution topography data.

In this study, we create a global roughness map based on the latest high-resolution digital terrain model (DTM) [8] to quantitatively investigate the obscuration of tectonic landforms by ejecta. We then discuss how this process affects estimates of Mercury’s radial contraction.