Comparison of lunar red spots including the crater Copernicus

Abstract

The lunar red spots, Helmet, Hansteen Alpha, and the NW quadrant of the crater Copernicus, were selected for a complex comparative investigation of their characteristics measured by the spacecraft Clementine, LRO, and Chandrayaan-1. For the analysis we used the following parameters: the reflectance A(750 nm), color-ratio A(750 nm)/A(415 nm), parameter of optical micro-roughness (LRO WAC), parameters deduced from LRO Diviner data, optical maturity OMAT, abundance of FeO and TiO2 (Clementine UVVIS and LRO WAC data), oxygen content determined using Lunar Prospector data, and parameters characterizing the 0.95-µm and 2.2-µm bands of Fe2+ ions (crystal field bands), and 2.8-µm band of H2O/OH and/or Fe2+ ions. The red spots Helmet and Hansteen Alpha are considered to be extrusions of rhyolite composition, which can be attributed to the Nectarian period; we did not find contradictions of this assumption. As for the Copernicus red spot, this, perhaps, is a similar formation that has been destroyed by the impact. We demonstrate that the material of the Copernicus red spot probably has the same composition as the classical red spots Helmet and Hansteen Alpha. The distributions of the parameter of optical micro-roughness and optical maturity OMAT show that the Copernicus red anomaly was not formed during the long evolution of the lunar surface, but results from crater formation. We find several confirmations of the hypothesis that the Copernicus red spot can be a residual of a red material (possibly rhyolite) extrusion that was involved in the impact process. The red material could have been partially melted, crushed, and ejected to the crater's north-western vicinity. The described red asymmetry of the Copernicus ejecta can be related to the eccentricity, relative to the extrusion, of the impact and/or to the inclination of the impactor trajectory. The latter also is confirmed by an analysis of the region, which is based on the geological map shown in this paper.