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Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars |
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| Authors: | Lon L Hood Keith P Harrison Robert J Lillis David A Williams |
| Institution: | a Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA b Southwest Research Institute, Boulder, CO 80309, USA c Université de Nantes, Laboratoire de Planétologie et Géodynamique, Nantes F-44000, France d CNRS, UMR 6112, Nantes F-44000, France e Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA f CNRS, Université Paris-Sud, Orsay, France g School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA |
| Abstract: | 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. |
| Keywords: | Mars Magnetic fields Mars Interior Mars Surface |
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