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Seismic measurements from the NASA Mars InSight mission revealed that Mars' core has a relatively low density, implying a larger fraction of lighter elements than previously thought, which further leads to a low melting temperature. Thus, Mars probably never developed a solid inner core during its early history when the dynamo was active. We perform full‐sphere dynamo simulations to eliminate the influence of an inner core on dynamo behaviors and investigate how various magnitudes of heat flux perturbations at the core‐mantle boundary affect the field morphology, comparing results to those from models with small inner cores. We find that a hemispheric magnetic field can result when the heat flux is concentrated in one hemisphere. Moreover, a dynamo model without the presence of an inner core can better explain Mars' crustal magnetic field dichotomy than that in a spherical shell surrounding a solid inner core. Plain Language Summary Recent measurements from the NASA Mars InSight mission show that Mars' core is less dense than we previously believed. This indicates that Mars probably never developed a solid inner core in the earliest times in its history. Here we conducted simulations to understand the effects of this lack of a solid inner core. We find that the lack of an inner core can better explain the north‐south dichotomy of Mars' observed magnetic field. Key Points We perform full‐sphere numerical dynamo simulations to investigate the effects of thermal perturbations on the ancient martian dynamo We find hemispheric thermal perturbations at the core‐mantle boundary lead to magnetic fields compatible with Mars' crustal magnetic hemisphericity Full‐sphere dynamo models produce more hemispheric magnetic fields than dynamo models in spherical shells

A new tectonic model is proposed for the origin of the Tharsis rise on Mars, which occupies ∼25% of the planet. The model invokes initiation of plate subduction by a large impact during the Late Heavy Bombardment at ca. 4.0 Ga. The model explains migration of Tharsis volcanism by slab rollback and the lack of magnetized crust in the bulk of Tharsis by formation of juvenile crust after the Mars dynamo creased to operate. The model also explains (1) the formation of thrust systems as a result of impact-generated crustal thickening (i.e., Thaumasia thrust), retro-arc contraction (i.e., Solis-Lunae fold belt), and plate subduction (Lycus and Ulysses thrusts), (2) the development of dominantly NE-trending grabens and a major east-facing V-shaped conjugate strike-slip system across the Tharsis rise as a result of backarc extension, and (3) crustal thickening of the Tharsis rise as a result of magmatic accretion during protracted construction of arcs above an episodically stalled and thus stationary subducting slab. The model has several implications for the way in which a unified global plate-tectonic network may have been established on early Earth. First, large impacts were common during the Late Heavy Bombardment (ca. 4.2-3.9 Ma), and thus impact-induced plate subduction would have been highly likely in the Hadean period. Such subduction systems must be local in scale and associated only with trench retreat and slab rollback. Localized plate subduction permits other modes of tectonic processes to have occurred simultaneously on early Earth, reconciling conflicting observations for plate-tectonic and non-plate-tectonic processes. Second, the presence of water at the surface of Hadean Earth would have allowed rapid transformation of basaltic crust to eclogite, allowing a sustainable plate subduction process once it started. The Hadean and possibly Archean Earth may only have had localized subduction systems, all characterized by slab rollback and trench retreat. Trench advance and related shallow-angle plate subduction probably did not begin on Earth until Proterozoic time, when a single and united global plate-tectonic network was established. This may have been accomplished by gradual coalescence of formerly independent subduction systems over a significant period of geologic time (>1 b.y.). Incorporation of trench-advance and shallow-angle plate subduction in the Proterozoic may have been induced by complex interactions of multiple subduction systems in a single and kinematically linked global tectonic network. This in turn led to the beginning of the formation of the crustal structures and petrologic assemblages of modern Earth. Based on a simple conductive cooling model, it appears that the most critical factors that control whether plate subduction could have been initiated in a rocky planet during the Late Heavy Bombardment in the inner solar system are its initial crustal thickness and the cooling rate/thickening rate of the lithosphere.