HR: 1340h
AN: P23A-0230    [Abstracts]
TI: The formation of the lunar and Martian mascons
AU: ghods, A
EM: aghods@iasbs.ac.ir
AF: Department of Physics, Institute for Advanced Studies in Basin Science, Zanjan, ZN P.O. Box 4 Iran (Islamic Republic of)
AU: shahnas, H
EM: shahnas@eps.mcgill.ca
AF: Earth nad Planetary Sciences, McGill University, Montreal, QC H3A 2A7 Canada
AU: * Arkani-Hamed, J
EM: jafar@eps.mcgill.ca
AF: Earth nad Planetary Sciences, McGill University, Montreal, QC H3A 2A7 Canada
AB: Intermediate size basins of the Moon with diameters of about 700 km have excess mass concentrations (mascons), whereas South Pole Aitken with a diameter of about 2500 km and depth of about 8 km, show no sign of a mascon. Also smaller basins and craters have no mascons either. Similar characteristics are detected for the Martian basins. Isidis and Argyre with diameters of about 1200-1500 km have mascons, whereas larger basins such as Hellas and Chryse with diameters over 2000 km have no masons. Two major scenarios have been proposed for the formation of the lunar mascons: a) Partial melting of deeper parts of the Moon and ascending of melt to flood the basin and b) Partial melting of the upper mantle beneath the surrounding ejecta blanket and lateral motion of melt to flood the basin. The impacts had no effect on partial melting in the first scenario but only produced basins as topographic lowlands prone to be filled, whereas the mascons in the second scenario are the direct result of the basin formation process. Both scenarios have major problems. They cannot explain the absence of mascons in Aitken basin, as well as the prolonged flooding of the basins. The geochemical data on lunar rock samples cannot discriminate one scenario against the other, some minerals are produced at depths greater than 500 km but some at shallower depths, about 100 km. We propose a new scenario for the formation of the mascons of the Moon and Mars that alleviates these difficulties. A large impact that produces a large basin results in melting and evaporation of the impactor as well as the near surface strata of the target. The excavated volume gets appreciably filled through isostatic upwelling of the mantle in a geologically short time and the remaining crust is fluidized enough to pose no appreciable resistance against isostatic uplift. A large or intermediate size basin becomes almost completely compensated in a short time. However, the impact induces mantle convection develops in time and allows hot material from deeper parts to ascend at later times and partially melt at certain depths. In the mean time the ejecta blanket hampers heat loss from the mantle beneath the blanket. Both processes occur concurrently and prevail for a considerable time, well over 100 m.y., and generate partial melting and subsequent flooding of the basins. The giant basins retain almost complete compensation, but the remaining crust and upper mantle beneath the intermediate size craters cool and become strong enough to support excess mass. The impacts that produce small basins and craters do not induce appreciable convection and do not produce thick ejecta blankets, thus cannot result in partial melting and basin flooding
DE: 5417 Gravitational fields (1227)
DE: 5420 Impact phenomena (includes cratering)
DE: 5430 Interiors (8147)
DE: 5499 General or miscellaneous
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting