REMAPPING VOLCANIC DEPOSITS ON THE MOON'S NORTH RIM USING PHOTOGEOLOGIC AND COMPOSITIONAL DATASETS

Success of the new Lunar Exploration Initiative depends in part upon our ability to accurately assess and map the lunar surface materials, so that appropriate landing sites may be chosen for scientifically important goals. Current geologic maps of the Moon, however, are in some cases over thirty years old and do not integrate the wealth of compositional information that has been produced by more recent missions such as Clementine. We have utilized several disparate datasets to produce a new geologic map of the volcanic deposits at the Moon's north rim, to demonstrate how the blending of multispectral data with photogeologic information may facilitate a re-examination of geologic interpretations of familiar locations.

Using Lunar Orbiter and Clementine 0.750 µm images, we examined the morphology of 25 deposits on the north rim, mapping the area of each deposit, and estimating thickness and volume. Also noted were the age and mode of occurrence of each pond, as well as any features associated with it. Calibrated and corrected Clementine multispectral data were used to determine general pond mineralogy, to judge compositional homogeneity, and to confirm pond boundaries determined by morphological means. FeO and TiO₂ weight percentage estimates derived from Clementine data and Th abundance derived from Lunar Prospector data were also noted.

Deposit areas ranged from 130 km² to 8280 km², with a median value of 1185 km². Volumes ranged from 25 km³ to 6800 km³, with a median value of 340 km³. Such volumes are immense compared to typical historical terrestrial eruptions and are most comparable to large igneous provinces and flood basalts. Pond soil mineralogy conformed to basaltic norms, with an average FeO content of 14-18 wt% and typical TiO₂ content of 1.5-4.5%. This categorizes deposits as low- to very low-TiO₂ basalts. Thorium abundances ranged from 1.19 to 3.43 µg/g, averaging 2.20 µg/g. Such values are typical of soils surrounding the Procellarum and Imbrium regions. Multispectral and elemental datasets also revealed variations in composition that were invisible to previous missions, but have allowed us to redraw the maps of these deposits. Approximately 10% of the deposits previously mapped as single flows have been determined to have characteristics consistent with multiple flows.