Lunar dark‐haloed impact craters: Origin and implications for Early Mare volcanism

Abstract

Remote sensing and photogeologic data were used to determine the composition and origin of lunar dark‐haloed craters. Near‐infrared reflectance spectra were obtained for numerous dark‐haloed impact craters located on either the ejecta blankets of large impact craters or Imbrian and Nectarian age light plains deposits. Spectral, thermal, radar, and photogeologic data conclusively demonstrate that Copernicus H and other dark‐haloed craters on the ejecta blanket of Copernicus excavated mare basalts from beneath lighter surface deposits rich in highlands material. Analyses of reflectance spectra of dark‐haloed craters on light plains indicate that in every instance these craters exposed mare basalt which had previously been covered by varying thicknesses of highlands debris. In the Schiller‐Schickard region a relatively thick highlands unit was emplaced as a result of the Orientale impact event. On the interior of Balmer basin, marelike basalt has been excavated from beneath much thinner highlands deposits emplaced by nearby craters. Mafic orbital geochemical anomalies are commonly associated with regions with abundant light plains deposits which exhibit dark‐haloed impact craters. Apparently, some mare material has been incorporated into the regolith either by local mixing during the emplacement of the highlands debris or by subsequent vertical mixing. The results of recent remote‐sensing, photogeologic, and lunar sample studies strongly indicate that mare volcanism was a significant process during much of the pre‐Imbrian and may have been initiated as early as 4.2–4.3 Ga. These very early volcanic episodes contributed materials to the lunar surface which were incorporated into the upper portion of the highlands crust by subsequent impact mixing. Models concerning the extent and duration of mare volcanism as well as those involving the composition and thermal evolution of the lunar interior will have to be revised.