Origin and evolution of mare-bearing regions of the Moon: constraints from integrated analysis of imaging, spectroscopy, topography, and gravity, and numerical modeling

In the last decade, new missions to the Moon have returned a flood of new high- resolution imaging, spectroscopy, topography, and gravity data that have triggered major advances in our knowledge of that body's origin, structure, and evolution. One major development is the identification of several large mare provinces (basalt-covered plains) that lack a clear association with the interiors of large impact basins. These include the broad but narrow Mare Frigoris north of the Imbrium and Serentiatis basins, and Mare Tranquillitatis, which occupies the center of a triangular region delineated by the Serentiatis, Crisium, and Nectaris basins (here termed the "SCN Triangle"). Frigoris and the western margin of Tranquillitatis coincide with the proposed volcano-tectonic boundary structures of the Procellarum region (Andrews-Hanna et al., 2014), but a search for gravitational signals of basins (Neumann et al., 2015) revealed evidence for only one small basin in western Tranquillitatis and none in the remainder of Tranquillitatis or Frigoris. These observations clearly show that the standard paradigm for creating maria, with basaltic melt ascending from an anomalously warm (and presumably impact-heated) mantle region beneath an impact basin to fill the basin, is insufficient to explain these regions.

Here, we propose two distinct but interdependent tasks aimed at 1) characterizing the geochemical, petrologic, and geophysical records of these regions as revealed by remotely sensed data (and informed by the lunar sample collection), and 2) constraining their volcanic and tectonic structure via sophisticated numerical modeling of lithospheric stress, deformation, and gravitational signature.

Task 1 will use imaging and spectroscopic data from missions such as Clementine, Lunar Prospector, Kaguya, Chandrayaan-1, and Lunar Reconnaissance Orbiter (LRO) to characterize the compositions and eruptive histories of volcanic units. For example, FeO and TiO2 abundances derived from Clementine and Kaguya visible and near-infrared data, and mineralogy determined from Moon Mineralogy Mapper data will be used to create maps of compositional/geologic units in the entire SCN Triangle region. Such maps will provide information not only on the surface units but also on deeper ones by analyzing material exposed in the rims and ejecta of small, immature craters. This information will constrain possible scenarios for magma generation and emplacement in Task 2.

Task 2 will apply finite element models (using the COMSOL Multiphysics code) of stress and deformation of the lithosphere and of gravitational signals from topography and internal density contrasts to evaluate scenarios for load emplacement (e.g., extrusive or intrusive basaltic units) that are consistent with geophysical observables (topography from LRO/LOLA and gravity from GRAIL) and the geologic and tectonic records. Model scales will vary as required, from small-scale edifices and magma bodies (10s of km), to regional-scale emplacement of mare units and volcanic edifices (100s of km), to planet-scale models of rifts (> 1000 km, in spherical geometry).

These tasks build on previous work of team members in characterizing Frigoris from the spectroscopy/mapping perspective and the SCN Triangle region from the geophysical/lithospheric perspective. The proposed work offers insights that may reach back to primordial events on the Moon (e.g., magma ocean solidification and establishment of the Procellarum KREEP Terrane) and the subsequent history of the proposed Procellarum boundary rifts and emplacement of basalt units in the Triangle basins and encompassed region.