POSSIBLE ORIGINS OF THE UTOPIA BASIN, MARS

Utopia Planitia of Mars is dominated by a large circular depression (> 3000 km in diameter and 1-3 km in depth). The origin has been presumed to be a remnant impact structure formed during the Early Noachian. Although the impact origin hypothesis is plausible, several lines of evidence are absent such as: antipodal effects, mountains/massifs surrounding the basin, radial macrostructures centered about the basin, especially a paucity of potential structures in the southern cratered highlands (macrostructures are large basement structures that form zones of weaknesses in the crust/lithosphere), and ejecta. Other lines of evidence are observed that may result from either impact or endogenic processes (e.g., plume-related modification of surface materials and topography). These include: concentric structures centered about the basin, macrostructures that cut the central basin, a positive gravity mascon in the central basin, distinctive topography, and structural settings such as the spatial relationship of Utopia with the highland-lowland boundary, Elysium rise, Syrtis Major, and the north-trending structurally controlled alignment of Claritas rise, Syria Planum, Ascraeus Mons, Ceraunius rise, and Alba Patera.

The significant age and resurfacing history of Utopia Basin may explain the lack of impact characteristics, but alternative hypothesis such as endogenic processes have not been considered. As with giant impact events during the period of heavy bombardment, endogenic processes during early Mars should also be included in the basin origin assessment. Initial modeling of isostatic adjustment and lithospheric flexure of a large basin structure provides a possible explanation for the broad positive gravity anomaly (100 - 200 mGal) associated with the basin, but alternative explanations will be scrutinized along with comparisons with the Hellas impact structure, a deep (> 10 km) impact basin of comparable lateral extent. In addition, paleotectonic features (fractures, graben, rift systems, and ridges) of the eastern hemisphere (including the adjacent highlands) will be used to determine if Utopia is a center of magma driven tectonic activity.