Magnetic Signature of the Utopia Impact Basin on Mars

We have mapped the amplitude of Mars' crustal magnetic field at 170 km altitude (B170) using electron reflection data from the MAG/ER experiment onboard Mars Global Surveyor. The increased sensitivity and nearly complete sampling of this new map reveal numerous weak magnetic sources in the northern lowlands. A partial ring of enhanced B170 values surrounds the Utopia basin. This magnetic ring lies just outside of the 3200-km-diameter main topographic rim and coincides with an annulus of increased crustal thickness derived from MGS gravity and altimetry [Neumann et al., JGR 109, E08002, 2004]. The southeastern part of the ring is missing, although it could have been thermally demagnetized by the Elysium volcanic region. There is no evidence for magnetic sources inside the ring. The ring could consist of magnetized crust that survived the Utopia impact. The survival of magnetic sources just outside of the main topographic rim would be consistent with the demagnetization signatures of Hellas and Argyre. Alternatively, the annulus of increased crustal thickness could mark the location of an inner ring that was remagnetized after the Utopia impact. A larger 4070-km-diameter ring for the Utopia basin was proposed on the basis that it fits the dichotomy boundary from 70 to 120 degrees east [Frey and Schultz, GRL 15, 229, 1988]. If this ring is interpreted as the main rim, then the magnetic annulus would be at 0.9 basin radii, where part of the crust could have been shock heated above the Curie temperature. An even larger ring, 5250 km in diameter, is defined by a 120-degree arc of magnetic sources to the north and west of the basin that is roughly concentric with the main topographic rim. This largest ring intersects several additional magnetic sources east of the basin. Whether the annulus of increased crustal thickness surrounding Utopia was magnetized before or just after the impact, its existence reveals the presence of magnetized early Noachian crust in the northern lowlands.