Galileo imaging observations of lunar maria and related deposits

Abstract

The Galileo spacecraft imaged parts of the western limb and far side of the Moon in December 1990. Ratios of 0.41/0.56 μm filter images from the Solid State Imaging (SSI) experiment provided information on the titanium content of mare deposits; ratios of the 0.76/0.99 μm images indicated 1 μm absorptions associated with Fe2+ in mafic minerals. Mare ages were derived from crater statistics obtained from Lunar Orbiter images. Results on mare compositions in western Oceanus Procellarum and the Humorum basin are consistent with previous Earth‐based observations, thus providing confidence in the use of Galileo data to extract compositional information. Mare units in the Grimaldi and Riccioli basins range in age from 3.25 to 3.48 Ga and consist of medium‐ to medium‐high titanium (<4 to 7% TiO2) content lavas. The Schiller‐Zucchius basin shows a higher 0.76/0.99 μm ratio than the surrounding highlands, indicating a potentially higher mafic mineral content consistent with previous interpretations that the area includes mare deposits blanketed by highland ejecta and light plains materials. The oldest mare materials in the Orientale basin occur in south‐central Mare Orientale and are 3.7 Ga old; youngest mare materials are in Lacus Autumni and are 2.85 Ga old; these units are medium‐ to medium‐high titanium (<4 to 7% TiO2) basalts. Thus, volcanism was active in Orientale for 0.85 Ga, but lavas were relatively constant in composition. Galileo data suggest that Mendel‐Rydberg mare is similar to Mare Orientale; cryptomare are present as well. Thus, the mare lavas on the western limb and far side (to 178°E) are remarkably uniform in composition, being generally of medium‐ to medium‐high titanium content and having relatively low 0.76/0.99 μm ratios. This region of the Moon is between two postulated large impact structures, the Procellarum and the South Pole‐Aitken basins, and may have a relatively thick crust. In areas underlain by an inferred thinner crust, i.e., zones within large basins (as at Apollo), titanium content is often higher. However, no mare deposits with titanium abundances approaching those of the high‐titanium (9 to 14% TiO2) Apollo 11 and 17 basalts nor of the high‐titanium regions of central Oceanus Procellarum are seen on the western limb or eastern far side. Light plains deposits are generally indistinct from the surrounding highlands in the SSI data and are inferred to be derived primarily from the same material that forms the highlands. Some of the light plains are too young to be related to basin‐forming impacts, suggesting possible volcanic origin. Dark mantle deposit compositions derived from SSI data are consistent with Earth‐based observations of similar near‐side deposits and are interpreted to be pyroclastic materials. However, the moderate albedo and 1 μm absorption of the dark mantle deposit on the southwest margin of the Orientale basin suggest it is a local pyroclastic deposit contaminated with underlying highland materials from the Orientale impact.