CRUSTAL AND LITHOSPHERIC STRUCTURE AT ISIDIS PLANITIA, MARS

Isidis Planitia is the site of a large free air gravity anomaly consistent with modification and sedimentary or magmatic filling of an impact basin. Mars Global Surveyor gravity and topography data of the Isidis basin on Mars are analyzed to elucidate the crustal and lithospheric structure near the dichotomy boundary in the Eastern hemisphere. Global gravity and topography datasets are inverted using an extension of Banerdt's [1986] model for deformation of a thin elastic shell and a nominal assumption that the depth of the moho beneath Isidis was correlated with the depth of the basin prior to modification by infilling. Thus far, we have studied models assuming complete isostatic compensation of the pre-fill basin, though other compensation scenarios are under consideration. We attempt to constrain potential variations in (and trade-offs among) local crustal thickness, density and thickness of basin fill material, and lithospheric deformation. The permissible parameter space is limited by assuming the local crustal thickness of the basin cannot be less than zero at the time of infilling. Recent work of Wieczorek and Zuber [2004] indicates that the most likely average crustal thickness of Mars is between 38 and 62 km. On the basis of this range of values for crustal thickness and a nominal difference between crust and mantle densities of 600 kg/m³, we infer that the average density of the fill is more than 3000 kg/m³. A high fill density suggests that the material inside the basin is predominantly of igneous rather than sedimentary origin. A comparison of the faulting observed at Nili Fossae to the predicted zone of normal faulting northwest of Isidis for a nominal case with a 50-km thick crust and a fill density of 3200 kg/m³ indicates that the elastic lithosphere beneath Isidis was between 60 and 130 kilometers thick during the period of loading.