New view of the Balmer-Kapteyn region: Cryptomare distribution and formation

¹ State Key Laboratory of Information Engineering in Surveying, Mapping and Remote sensing, Wuhan, PR China
e-mail: denggaoqiu@whu.edu.cn
² Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, PR China

Received: 25 September 2021
Accepted: 14 January 2022

Abstract

Context. The cryptomare in the Balmer-Kapteyn region is the oldest one on the Moon. Determining the extent and formation of this feature can deepen our understanding of early mare volcanism and help establish temporal and spatial constraints on lunar thermal and volcanic history.

Aims. This paper focuses on the identification of lunar cryptomaria and figuring out their formation processes.

Methods. We used the Global WAC digital terrain model to analyze the terrain. We built a mathematical model using support vector machines and input Kaguya Multiband Imager data to estimate oxide concentrations in the Balmer-Kapteyn region. We used the Chandrayaan-1 Moon Mineralogy Mapper (M³) to analyze the minerals. We improved the cryptomare identification model to increase the accuracy of basalt identification in the cryptomare region. Finally, we used three methods to estimate the ejecta thickness of the target basin to the Balmer-Kapteyn region.

Results. New Al₂O₃, CaO, FeO, MgO, and TiO₂ maps were generated using the Kaguya Multiband Imager and a novel machine-learning model. As a result, the extent of the cryptomare in the Balmer-Kapteyn region was redefined and the formation process of the cryptomare in the Balmer-Kapteyn region was divided into five formation stages: Balmer basin formation, ejecta coverage from the Pre-Nectarian and Nectarian large impact basins, mare basalt filling, ejecta secondary coverage of high-albedo materials, and exposure of mare basalts.

Conclusions. We found that the bottom of the Crater Vendelinus is likely to hide ancient mare basalt. Moreover, the high-aluminum mare basalt of the cryptomare is different from the composition of the exposed mare basalts in Mare Fecunditatis and Crater Vendelinus. The high-albedo material covering the cryptomare in the Balmer-Kapteyn region could have come from the Langrenus, Petavius, Humboldt, La Perouse, and Ansgarius Craters, along with some from the Orientale Basin impact event or potentially from the Imbrium Basin impact event.