East-west Trending Magnetic Anomalies in the Southern Hemisphere of Mars: Modeling Analysis and Interpretation

Introduct ion: Maps of the martian crustal magnetic field over the strongly magnetized Terra Sirenum region near the 180 meridian in the Southern Hemisphere show extensive east-west trending features in the radial and north components. These features have previously been attributed to quasi-parallel bands of magnetized crust similar to, but much larger in scale, than those associated with sea floor spreading near terrestrial mid-ocean ridges [1,2,3]. Alternatively, dike intrusions generated either by sea floor spreading or by a moving locus of intrusion sources have also been proposed to explain the linear nature of the anomalies [4]. Since the time of initial interpretation of the strong east-west trending anomalies, modeling of relatively isolated, dominantly dipolar anomalies elsewhere on Mars has indicated that the rotational pole in the Northern Hemisphere during the core dynamo epoch was probably located near 34 ± 10N, 202 ± 58E [5; see also ref. 6]. This would imply that the Terra Sirenum region of strong anomalies lies near to the paleoequator. Magnetization vectors in this region should therefore have been oriented nearly in the north or south directions. In this paper, this new constraint is applied to re-examine whether the observed east-west trending anomalies actually require east-west trending bands of magnetization. Specifically, an alternate model in which sources are assumed to consist of circular disks magnetized in a direction determined by the inferred paleopole position (i.e., mainly northward or southward) is considered. Field Mapping and Modeling: Figure 1 is a map of the three vector magnetic field components and the field magnitude for the region centered on 180 E in the S.H. where the strong east-west trending anomalies are located. The map was produced from a subset of MGS magnetometer mapping orbit data using methods described in ref. 5. The effective horizontal resolution is 3.75 in longitude and 0.95 in latitude. As seen in the lower left plot, the altitude of the map ranges from ~360 to 380 km. East-west trending anomalies are evident in the north and radial components but are not present in the east component or in the field magnitude. Figure 2 is a map generated in the same way but assuming artificial sources consisting of three circular disks at the martian surface. The locations, sizes, and magnetization properties were determined using an iterative forward modeling method described in ref. 4. Specifically, initial disk locations were chosen based on the field magnitude map. Directions of magnetization were initially chosen to be consistent with a mean paleomagnetic pole position of 34N, 202E. Disk sizes and magnetic dipole moments were set to produce anomalies consistent with the observed anomaly amplitudes and wavelengths. These initial values were then adjusted iteratively until a minimum variance between model and data was obtained [4].