Magnetic Anomalies within the Elliptical Borealis Basin of Mars

Introduction: The single impact hypothesis for the formation of the Martian hemispheric dichotomy boundary has recently been revived with the advent of hydrodynamic simulations that show that the planet could sustain such an event [1, 2]. For additional evidence , it has been proposed that the shape of dichot-omy boundary is elliptical [3]. This proposed boundary shape is based on carefully chosen martian gravity anomalies and topographic trends that outline an ancient Borealis basin. The elliptical shape suggests formation by a single oblique impact. It has further been proposed that the decline in magnetic field intensity antipodal to the impact is also evidence for a single impact [2]. One issue with this series of hypotheses is that weak but nonetheless significant magnetic anomalies exist within the elliptical Borealis basin itself [4]. These anomalies raise questions about the actual shape of the Borealis basin as well as timing of the core field relative to the purported single impact. Martian Magnetic Anomalies: The most reliable map of martian crustal magnetic anomalies produced to date is the ∆Br map [4]. This map, representing the mean change of the radial component of the field with latitude, is an excellent proxy for the Bφ component at MGS mapping altitude. The ∆Br is thought to be largely free of the effects of external field and allows unprecedented detail of the magnetic anomalies, particularly in areas of Mars where the field is relatively weak Magnetic Anomaly Pattern within the ellipse: The magnetic anomalies of the Borealis basin are most concentrated south of Elysium Mons and south of Chryse (Fig. 1) at the ends of the major axis of the proposed ellipse. The anomalies appear to be completely similar in intensity and pattern to those immediately outside the ellipse. If the major axis of the ellipse were shortened to exclude most of these anomalies , the shape would be circular, not elliptical. Although redrawing the dichotomy boundary in this manner would not rule out a single impact origin for the dichotomy, it takes away the argument that an impact is the simplest explanation for the shape of the basin. The source of the spatial variation in magnetic field intensity on Mars is an ongoing topic of debate. The simplest explanation is a combination of spatially in-homogeneous crustal magnetization on early Mars with subsequent demagnetization in discrete areas by large impacts or rising plumes. Within the proposed