Global dipole magnetic field: Boon or bane for the Martian atmospheric retention?

The absence of global dipole magnetic fields at terrestrial planets is widely perceived as a major impediment in protecting planetary atmospheres from being eroded by stellar winds. In this work, we re-examine this idea by focusing on the rate of atmospheric ion escape from Mars for nominal solar wind parameters and an extreme ``Carrington-type'' space weather event. We carry out extensive numerical simulations using a sophisticated multi-fluid magnetohydrodynamics (MHD) model, thereby demonstrating that the escape rate is a non-monotonic function of the Martian dipole magnetic field strength, and that it varies by more than an order of magnitude in certain instances. Our work illustrates, for the very first time using the state-of-the-art global simulations, that the lack of dipole fields is not necessarily a disadvantage from the viewpoint of atmospheric losses. We are led to the conclusion that the ion escape rates from Mars may have actually been higher when it possessed stronger magnetic moment in the past. We conclude our analysis by briefly exploring the ensuing implications for Martian and exoplanetary habitability.