Mars exhibits strong, localized crustal magnetic anomalies concentrated predominantly in the ancient southern highlands. Two influential hypotheses have been advanced to explain them: that they represent thermoremanent magnetization (TRM) acquired during cooling in an ancient global dynamo field, and that their spatial pattern records a history of crustal spreading in which fresh crust was generated at a spreading center, cooled and acquired TRM, and was displaced laterally as the ambient dynamo field reversed. This survey tests both hypotheses against the observational record on four independent lines of evidence. First, we examine the Martian surface and subsurface for the morphological, gravitational, and compositional signatures that active plate tectonics necessarily generates and find none. Second, we demonstrate that the three largest impact basins—each a controlled magnetic reset above the Curie temperature—carry no post-impact TRM, which proves that no planetary field existed at the times of those impacts; the major volcanic provinces are similarly neutral. Third, we examine the topographic and structural character of the anomaly-bearing terrain itself and demonstrate that no ridge systems, tectonic lineaments, or bilateral field symmetry are detectable there. Fourth, we evaluate the anomaly-bearing terrain of Terra Cimmeria and Terra Sirenum directly and find that it is internally inconsistent with both hypotheses: paleomagnetic pole inversions yield no convergent solution, anomaly amplitudes are anomalously high relative to any plausible dynamo field, and the apparent stripe pattern resolves into irregular crustal blocks at higher resolution. Fifth, we show that no TRM record anywhere on Mars is consistent with a global dipole field: the neutral terrains recorded nothing, and the anomaly-bearing terrain recorded something a global dipole cannot explain. The dynamo hypothesis has no positive evidence on this planet.
Thomas Andrew Weitzel (2026) studied this question.