GEOLOGIC MAPPING AND REASSESSMENT OF THE VOLCANIC HISTORY OF LUNAR WESTERN SOUTH POLE-AITKEN BASIN

We are constructing a 1:2,500,000-scale map of the Planck Quadrangle or Lunar Quadrangle 29. Previous maps of the region were based on oblique, low-resolution images that did not contain mineralogical or elemental information. In this investigation, we document the stratigraphy, extent and characteristics of this area by incorporating regional morphology, composition, topography, elevation and elemental abundance data.

For our mapping we use ~100 m/pixel Lunar Reconnaissance Orbiter Camera (LROC) images to assess morphology, Clementine multispectral data to extract information on FeO content and regolith maturity and data acquired by the Lunar Orbiter Laser Altimeter (LOLA) for topographic data (100 m/pixel).

Volcanic deposits were identified based on albedo, texture, morphology and spectral signature. Smooth, low-albedo surface materials with relatively sharp boundaries and elevated mafic content were mapped as mare deposits. Rougher surface deposits with very low albedo, diffuse or irregular boundaries, and elevated mafic content were mapped as pyroclastics. Areas of heightened mafic content that otherwise showed little or no morphologic indication of volcanic origin were considered potential candidates for cryptomare materials.

Thus far, we have identified 22 smooth, low-albedo, higher-Fe deposits, which we interpret to be mare deposits or ponds. We have also identified 18 rougher, very low-albedo, higher-Fe materials with irregular, diffuse boundaries that we interpret to be pyroclastic materials. Several candidate areas for the presence of cryptomare materials were also identified, including small patches in Planck crater. We confirm the previous identification of most mare units based on morphologic characteristics, though the boundaries of some deposits have been refined. Our mapping indicates that the area covered by maria is 5% greater in comparison to previous maps. This would potentially place new constraints on the global lunar thermal budget. Higher resolution also reveals potential individual flows, wrinkle ridges, and fractures associated with the pyroclastic deposits. We infer from this range of volcanic morphologies that mode of emplacement (an indication of conditions at depth) was more diverse in this area than previously assumed.