Review of the impact of Martian crustal magnetic fields on its atmospheric ion escape

Mars is the most Earth-like planet in the solar system and one of the most accessible targets for spacecraft exploration. Unlike Earth, Mars lacks a global intrinsic magnetic field and only retains localized crustal magnetic anomalies in the southern hemisphere. This unique characteristic has made Mars a long-standing research focus in planetary science. It is widely believed that early Mars may have hosted a dense atmosphere and stable liquid water that supported habitable environmental conditions. However, as atmospheric constituents were progressively lost to space, Mars eventually evolved into the cold and arid desert-like world observed today. The influence of crustal magnetic fields on atmospheric ion escape has therefore attracted considerable attention. In this study, we review recent advances on the role of Martian crustal fields in regulating ion escape and focus on their distinct effects under different spatial and solar wind conditions. When the crustal magnetic anomalies rotate to the dayside, their dominant effect is to suppress ion escape, reducing the global escape rate by approximately 30%–50%. This suppression arises because the closed magnetic field topology blocks solar wind energy input into the ionosphere and constrains radial ion transport. These closed field structures further result in elevated ionospheric densities above the crustal regions, forming swelling "bulges." When the magnetic anomalies rotate to the dawn–dusk and nightside sectors, they primarily enhance ion escape. In these regions, magnetic reconnection frequently occurs, rapidly converting stored magnetic energy into particle acceleration. Meanwhile, the open magnetic topology facilitates radial ion transport, and the crustal fields expand the cross-sectional area available for day-to-night ion flow, effectively enhancing tailward ion escape. Recent studies have proposed a dual-effect framework: crustal magnetic fields suppress ion escape at low altitudes but enhance escape at higher altitudes. Specifically, at low altitudes, ion fluxes in the southern hemisphere are much lower than those in the north; however, as altitude increases, ion fluxes in the south gradually exceed those in the north. This dual regulation explains why the southern hemisphere exhibits higher fluxes while maintaining a global escape rate comparable to that of the northern hemisphere. In summary, the Martian crustal magnetic field acts simultaneously as a “shield” and an “amplifier” in the ion escape process, and its impact depends on its local position and upstream solar wind conditions. Current interpretations remain inconclusive owing to limitations in single-satellite sampling and uncertainties in numerical modeling. Nonetheless, understanding these mechanisms is crucial for reconstructing the evolutionary history of the Martian atmosphere, evaluating planetary habitability, and predicting atmospheric evolution on terrestrial exoplanets. Future progress requires coordinated multi-satellite observations and advanced numerical models to quantitatively assess the contribution of crustal fields to long-term atmospheric loss and thus better reveal the transformation of Mars from a warm, watery world to the cold and dry planet we observe today.