The crust of Mars shows strong remanent magnetization, which was likely acquired during the early phases of planetary evolution when a core dynamo still operated. The direction of the field responsible for magnetizing the crust holds clues to the working of the dynamo and the rotational dynamics of the planet. By analyzing individual crustal magnetic field anomalies and with the aid of additional assumptions, the field orientations can be reconstructed. We have implemented an Equivalent Source Dipole method to determine the main field orientation during magnetization, assuming that the considered anomalies are unidirectionally magnetized without making specific assumptions about the source geometry. The available data are fit in a least squares sense, and the method yields confidence intervals for the admissible paleopole locations. The method was applied to six crustal magnetic field anomalies, two of which require a south pole in the northern hemisphere, while three indicate a south pole in the southern hemisphere. This implies that polar reversals took place at least once in Martian history. Furthermore, one of the investigated anomalies requires a south pole at equatorial to midlatitudes, indicating that a significant amount of true polar wander must have occurred on Mars. Finally, tests with synthetic data indicate that admissible paleopole locations typically spread across at least 25% of the planet, which may partially explain the scatter found in previously published paleopole studies.

Various techniques have been proposed for palaeopole position estimation based on magnetic field measurements. Such estimates can offer insights into the rotational dynamics and the dynamo history of moons and terrestrial planets carrying a crustal magnetic field. Motivated by discrepancies in the estimated palaeopole positions among various studies regarding the Moon and Mars, we examine the limitations of magnetic field measurements as source of information for palaeopole position studies. It is already known that magnetic field measurements cannot constrain the null space of the magnetization nor its full spectral content. However, the extent to which these limitations affect palaeopole estimates has not been previously investigated in a systematic way. In this study, by means of the vector Spherical Harmonics formalism, we show that inferring palaeopole positions from magnetic field measurements necessarily introduces, explicitly or implicitly, assumptions about both the null space and the full spectral content of the magnetization. Moreover, we demonstrate through synthetic tests that if these assumptions are inaccurate, then the resulting palaeopole position estimates are wrong. Based on this finding, we make suggestions that can allow future palaeopole studies to be conducted in a more constructive way.

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