Northern Imbrium Noritic Anomaly

[1] An analysis of the Northern Imbrium Noritic (NIN) region was conducted using Clementine UVVIS multispectral images and derived band parameters. Band parameters were used to map spatial variations in composition and to aid in general separation of compositional signatures from effects such as maturity and topography. Compositional trends in which the abundance of mafic materials relative to feldspathic materials increased were observed with depth in large craters, with distance to the west, and with distance from the center of the Imbrium basin. On average, the region appears to have an anomalously mafic surface composition, with a noritic upper unit overlying and grading into a more feldspathic layer that is exposed in the central peak of the largest crater in the region. Of several hypotheses that could explain these results, we favor a scenario in which feldspathic crustal materials underlie moderately mafic South Pole-Aitken antipodal ejecta, which is covered by and mixed with mafic Imbrium ejecta. Limitations of the Clementine UVVIS data preclude quantitative evaluation of these hypotheses, but compositional information from new high-resolution data sets should allow the direct comparisons needed to test these hypotheses.