Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO₂) environment in the presence of a strong (?10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.