Mapping and modeling of magnetic anomalies in the northern polar region of Mars

Vector crustal magnetic field maps of the northern polar zone (60°N to 90°N) are constructed from selected Mars Global Surveyor magnetometer data obtained during the period from May 28 to September 13, 1998. Two medium anomalies (amplitudes >50 nT at 170 km altitude) are mapped in locations consistent with earlier studies. No visible surface features correlate with the anomalies, suggesting that the sources lie beneath the visible veneer of polar deposits and volcanic lava flows. If so, then they formed prior to the immediate end of the heavy bombardment (upper Hesperian) period. Modeling of anomaly vector field components combined with independent constraints on the depth to the Curie isotherm yields lower limits on bulk magnetization intensities (0.4–0.9 A/m) that are significantly greater than those measured for Martian (SNC) meteorite samples. Rocks that contain substantially more titanomagnetite than SNC meteorites, or that contain magnetic phases in addition to titanomagnetite, possibly resulting from hydrothermal alteration, are therefore suggested. Alternatively, remanence acquisition in a field of Earthlike intensity (∼50 μT), rather than in the relatively weak inferred paleointensities for SNC meteorites (∼1–10 μT), would also help to explain the relatively strong inferred remanent magnetizations. The approximate south paleomagnetic pole positions corresponding to these two anomaly sources are located in a region between Olympus Mons and the present north rotational pole. This region is adjacent to the approximate location predicted by Melosh [1980] for the paleopole prior to the formation of the Tharsis gravity anomaly.