Pyroclastic Eruption Conditions on the Moon and Mercury

Products of explosive eruptions, pyroclastic deposits, represent important sources of information about the thermal history and volatile content of the planet from which they originate. The areal extent and physical characteristics (grain size, shape, texture, glassy vs. crystalline) of pyroclastic deposits, along with their associated vent sizes and morphologies, can be related to eruption conditions and emplacement styles, informing the types of volcanic eruptions that have occurred across the inner Solar System. Recent imaging by the Lunar Reconnaissance Orbiter Camera (LROC) Wide and Narrow Angle Cameras (WAC; NAC) has enabled the identification and characterization of many new lunar pyroclastic deposits. The MESSENGER mission provided compelling evidence for pyroclastic deposits on Mercury; however, the types of eruptions leading to their emplacement have not been well constrained. Our work seeks to investigate the formation of lunar and mercurian pyroclastic deposits and to determine the range and variation in volcanic eruption styles on airless terrestrial bodies, and will use these results to inform comparative volatile gas fractions on the two bodies.

This work will morphologically and photometrically characterize mercurian and lunar pyroclastics and their associated vents, and will use the results to explore the history of explosive volcanism on the two bodies. We will: (1) evaluate what the photometric properties of pyroclastic deposits reveal about their physical properties and style of emplacement, (2) analyze vent morphologies and their associated volatile gas volume fractions and what these tell us about eruption volumes and conditions, and (3) assess how mercurian and lunar pyroclastic eruption conditions and emplacement styles compare.

Photometric analyses of the deposits will allow us to test if textural differences exist between the pyroclastics and surrounding materials, providing information on the eruption styles that emplaced them. Varying cooling rates of materials produce either quenched glass or crystallized beads, both of which have different light-scattering properties. Photometric data will be extracted from MESSENGER MDIS NAC images (~20-70 mpp) and LROC NAC images (0.5-2 mpp). Hapke models will be used to fit phase curves to the data, extract reflectance values at a common phase angle, and extract values for key photometric parameters. We will also create phase-ratio images, which reveal differences in surface texture and have proven a useful tool for identifying differences in regolith textures (e.g., grain size) on Mercury and the Moon.

We will characterize vent morphologies using MDIS and LROC NAC Digital Terrain Models (DTMs). We will create DTMs for selected areas using the StereoPhotoClinometry (SPC) software written by Co-I Gaskell. SPC can create DTMs at resolutions comparable to that of the highest image resolution, and can accurately represent features 5x that resolution. Morphometric measurements of the source vent (area, size, depth) and area of the associated deposits will be made using NAC images and DTMs. These results will be used to calculate gas volume fraction for each deposit, which, combined with photometric studies, will provide a detailed view of the similarities and differences in mercurian and lunar eruption emplacement styles, thereby answering the fundamental question what is the range of and variation in volcanic eruption styles on airless terrestrial bodies? We will present a case study of pyroclastics in Alphonsus crater on the Moon as a proof-of-concept.