Global occurrences of very smooth plains patches on Mercury: geologic settings and implications for effusive volcanism

Abstract

Introduction:  Large scale effusive volcanism, responsible for most of Mercury's 'smooth plains', is accepted to have ended by ca. 3.5 Ga [1]. We present local occurrences of smooth surfaces, often with the evidence for being topographically ponded. These examples are seldom larger than a few 10s of km across and are characterised by extremely smooth surfaces with a paucity of impact craters. Some of these deposits may provide evidence for a protracted phase of waning effusive volcanism post-3.5 Ga. We present a map of the global occurrences of very smooth plains patches and investigate their implications for the effusive volcanic evolution of Mercury. Previously reported occurrences: :  During the Mariner 10 era, the partial geological map of H15 was unique in including small very smooth plains (pvs) deposits commonly associated with craters or tectonic features [2]. Similar patches have recently been identified in H10 [3] and an association with tectonic features was discovered (e.g. Calypso, Soya and Enterprise Rupes). Additionally, some patches of very smooth plains have been mapped in the survey of smooth plains deposits < 105 km of Wang et al. [4]. Previous works propose a range of origins which we will explore, namely: ·        Impact-related origin: Either as impact melt or fluidized impact ejecta ·        Small-volume effusive volcanic origin: where the age of such deposits are somewhat contested Newly Identified occurrences: The global survey so far has identified approximately 500 potential patches, with varying confidence dependent on the degree of textural difference between the patch and surrounding terrain. These occur in a variety of settings, including low-lying areas of both smooth and intercrater plains, associated with craters or tectonic features, or catenae-hosted. Ongoing work involves mapping each patch, exploring emplacement scenarios and analysis of the association between patches and structural weaknesses. Data and method: We map using NAC (single frame) and WAC (global mosaic) images obtained by MESSENDER’s MDIS. Mapping is carried out on a 116 m/p monochrome primary basemap; high-incidence east and west, and low-incidence angle secondary basemaps; a 665 m/p enhanced color mosaic; and a 665 m/p stereo-derived digital elevation model. Mapping is carried out using ArcGIS Pro. Each image is mapped in the projection most suited to that quadrangle.