Spectral Investigation of the Mare Australe Basalts: A Fresh look at the Atypical Volcanism on the Moon

Mare Australe (47.77°S, 91.99°E) is a distinctive volcanic province (diameter ~1000km) at the eastern nearside and farside boundary of the Moon. The basalts of the region were considered a part of mare filling volcanism inside an ‘Australe Basin’ due to the circular arrangement of its 248 basaltic patches [1]. The proposed Australe Basin, however, lacks any discernible topographic signatures, a ring morphology, and a central positive Bouguer anomaly typically associated with the lunar impact basins. The results from the GRAIL mission and geological investigations revealed the presence of a ~880 km diameter impact structure in the northern part of Mare Australe, naming it the Australe North Basin (35.5°S, 96°E) [2, 3]. The Mare Australe basalts are dominantly emplaced outside this newly discovered Australe North Basin, which is perplexing. In this study, we carry out an extensive compositional investigation of the previously uncharacterized Australe region using hyperspectral data from the Moon Mineralogical Mapper (M³) onboard Chandrayaan-1. We investigate both mare and non-mare units in the region to understand their mineralogy in the given geological context. The spectral investigation reveals that despite widespread volcanism, the region lacks the presence of high-Ca pyroxene. Instead, the basalts are primarily composed of low to intermediate Ca-pyroxene in comparison to the rest of the lunar basalts, displaying their unique mineralogical signature. These findings provide new insights into the nature and origin of the atypical volcanism on the Moon in the Australe Region and highlight the distinct geological environment of Mare Australe responsible for the same. This study offers important implications for understanding lunar volcanic evolution and its relationship with impact processes.

[1] Whitford-Stark, J. L. (1979) LPSC X, 2975- 2994. [2] Neumann G. A. et al. (2015) Sci Adv. 1(9), e1500852. [3] Panwar N. and Srivastava N. (2024) Icarus, 408, 115841