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Multiphase eruptions associated_with_lunar_craters_tycho_aristarchus

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  • 1. No. 150 MULTIPHASE ERUPTIONS ASSOCIATED WITH THE LUNAR CRATERS TYCHO AND ARISTARCHUS by Robert G. Strom f and Gilbert Fielder * August 1, 1968 Revised May, 1970 ABSTRACT Numerous lava flows and other volcanic features occur within and in the vicinity of both Tycho and Aristarchus. Mostof the flows associated with Tycho were more viscous and have higher albedos than those associated with Aristarchus indicating differences in composition. Numerous "lakes" occurring on the rims of both craters are thought to consist of lavaerupted from vents primarily associated with the depressions in which they lie. The floors of Tycho and Aristarchus probablyconsist of lava; large volumes of this material probably drained back into subsurface chambers or open fissures. In the case ofAristarchus, late eruptions of lava and/or pyroclastics appear to have been responsible for obscuring certain parts of the floordetail. Widespread deposits surrounding both Tycho and Aristarchus probably resulted from base surges and indicate that ineach case a very large explosion was the principal contributor to the formation of the craters. Counts of probable primary impact craters on the various flow units, the floors and the "lakes" indicate that these featureswere emplaced at widely different times. Counts on the base surges indicate that Aristarchus is about 1.6 times older thanTycho but the floors and flows are roughly the same age. Confirmed observations of red glows on the rim of Aristarchus andvicinity suggest that limited volcanism may be active even at the present time. The volcanism associated with Tycho and Aristarchus may have been triggered by large impacts which tapped a subsurface source of magma. Less likely both craters could be volcanic structures, the volcanism being the natural consequence ofthe development of the craters. 1. Introduction surroundings; and during eclipse or through a luna tion Tycho also follows anomalous thermal curvesTycho, the well-known lunar crater 85 km in diam eter, and Aristarchus, the brightest lunar crater (Saari, 1965) which indicate that the materials of40 km in diameter are two of the most prominent the crater are better thermal conductors than thecraters on the moon. Both craters have clearly sculp surrounding terrain. All these observations may betured rims, are relatively deep, and are strong centers explained on the impact hypothesis.of stratigraphically recent rays: therefore, the craters In 1967 part of Tycho and all of Aristarchushave been widely regarded as recent impact struc were photographed by Orbiter V with a ground resotures. In addition, Tycho presents an anomalously lution of about 5 m. In addition, Orbiter V tookhigh radar reflectivity (Pettengill, 1968) which indi medium resolution photographs of the whole ofcates that the crater is rougher or denser than the both structures and their immediate surroundings, with a ground resolution of about 40 m. It was theret Lunar and Planetary Laboratory, University of Arizona, fore possible to study some parts of the Tycho and Tucson, Arizona. Aristarchus domains in considerable detail and to*Dept. of Environmental Sciences, Univ. of Lancaster, link these studies through a general description of England. the surrounding terrain, in each case. 235

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