The History of Mare Volcanism in the Orientale Basin: Mare Deposit Ages, Compositions and Morphologies

The eruptive history of mare basalts in the Orientale Basin has been studied, using Lunar Orbiter 4 high-resolution photographs, Zond 8 photographs, and recently acquired Galileo EM-1 multispectral images. This work represents a refined set of compositional data incorporating the use of a linear mixing model for mare compositions, crater count data, and a comprehensive morphologic analysis of Orientale Basin mare deposits. Evidence for multiple eruptive episodes has been found, with compositions ranging from medium- to high-Ti basalt (less than 4 to greater than 6 wt. percent TiO2). Eruptive styles included flood, rille-forming, and shield-forming eruptions. Impact crater densities of mare units in the Orientale Basin enable determination of the ages of these deposits, using the method of Neukum et al. Earliest eruptions of mare basalt in the basin occurred at greater than or equal to 3.80 Ga and the latest eruptions occurred at about 2.3-2.5 Ga. Hence, mare volcanism occurred over a period of nearly 1.5 Ga

Multipspectral studies of western limb and farside maria from Galileo Earth-Moon Encounter 1

New visible and near-infrared multispectral images of the Moon obtained by the Galileo solid-state imaging system, along with lunar orbiter images (for crater counts), and spectral mixing analyses wereused to characterize western limb and eastern farside maria to determine compositional and age relationships in selected regions. Results indicate that (1) western limb mare deposits have less variability in titanium content (<2-7 wt%TiO2) and age (2.79-3.86 Ga) than areally extensive maria on the nearside; (2) areally extensive basin-filling maria generally have higher titanium contentsthan smaller, crater-filling mare patches and ponds; (3) ancient maria coveredby highland material (cryptomaria) may be present in the Mendel-Rydberg and South Pole-aitken basins; and (4) maria with compositional and age variations occur in the Grimaldi, Crueger, Mendel-Rydberg, and Apollo regions. No extensive high-titanium (>6 wt% TiO2) mare basalts were observed on the western limb and farside, which may reflect the inability of such ddenser magmas to penetrate the thicker farside crust

Impact Basin Deposits in the Lunar Near Side Northern High Latitudes: Galileo Earth-Moon 2 Encounter Results

Galileo Solid-State Imaging System data provide about the nature and origin of basin deposits, and modes of ejecta emplacement and ejecta mixing, including the formation of light plains deposits. The Humboldtianum basin (61N 84E; 650 km in diamter, middle Nectarian in age) deposits do not show anomalous spectral characteristics relativeto typical highlands, suggesting that the depth of excavation was relatively shallow. Initial analyses show little evidence for a strong 0.76/0.99 signature for the majority of pre-mare Imbrian-aged light plains within the basin. The linearity and angularity of the second and first ring suggest that the inner ring is the most likely candidate for the crater rim crest. One of the highest concentrations of light plains on the Moon is seen north of Imbrium, suggesting the possible presence of cryptomaria. Much of the area north of western and central Mare Frigoris shows highland spectral characteristics; however a small number of the fresh craters in the plains have stronger 0.76/0.99 ratio signatures (indicative of iron-bearing minerals, but not necessarily of basalts). SSI data indicate the extensive younger Imbrian light plains north of eastern Frigoris also show typical mature highland signatures. A small numberof the fresh craters have relatively more intense one-micron signatures, however, and a dark-halo crater with mare basalt affinities occurs in the crater Grtner. This dark-halo crater is themost conclusive evidence for the presence of cryptomare deposits beneath the light plains so far, but it does not provide information on how widespread the deposit might be. These extensive light plain deposits are not as spectrally distinctive as the cryptomaria observed in the Schiller-Schickard area southeast of Orientale, possibly due to the greater thickness of Imbrium ejecta in the northern high latitudes