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

Abstract

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.