Effects of a Weak Intrinsic Magnetic Field on Atmospheric Escape From Mars

Abstract

The magnetization of a planet significantly changes the nature of atmospheric escape, and the question of whether a planetary field shields atmosphere from erosion by the solar wind or not remains open. Ion escape processes from Mars are investigated under two hypothetical conditions, namely, no intrinsic magnetic field and weak intrinsic dipole field cases under present solar wind conditions based on multispecies magnetohydrodynamics simulations. The existence of a weak dipole field results in an enhancement of the tailward flux of planetary ions through the four escape channels. Two channels are associated with the open fields at the poles, and the others are caused by magnetic reconnection between the planetary and solar wind magnetic fields at the flank magnetopause. The ion escape rate with the weak dipole is greater than in the no-dipole case. The enhancement is significant for O 2 + and CO 2 + , suggesting ion escape from the low-altitude ionosphere.Plain Language Summary Present Mars has a thin atmosphere and little water on the surface.Space missions have provided some evidences for the existence of liquid water and a thick atmosphere on ancient Mars. These evidences suggest Mars has experienced significant atmospheric loss up to the present. Ion outflow is one of the important atmospheric loss mechanisms. In the present day, Mars does not have a global magnetic field such as that of Earth, and thus, ions escape by the direct interaction with the solar wind (influenced to some extent by Mars' crustal fields). In contrast, it is expected that ancient Mars had a global magnetic field. The global magnetic field forms the magnetosphere around the planet, perhaps changing the ion escape mechanisms. In this study, a numerical simulation of the Mars-solar wind interaction is used to compare ion escape for the case of no planetary field with a case where a weak global magnetic field is assumed. The weak global magnetic field yields four escape routes associated with open magnetic fields and magnetic reconnection. The escape rate of heavy ions increases by~25% compared to the nondipole case, particularly facilitating the escape of O 2 + and CO 2 + present in the low-altitude ionosphere.

Effects of an Intrinsic Magnetic Field on Ion Loss From Ancient Mars Based on Multispecies MHD Simulations

“…Recent MAVEN observations found that the crustal magnetic field induces the escape of molecular ions from the low‐altitude ionosphere (Inui et al, ). Sakai et al () investigated the effects of an intrinsic magnetic field on ion loss from present Mars based on the same simulation model as that used in this study. Their study concluded that the existence of a weak intrinsic magnetic field increases the ion loss, particularly the molecular ion loss.…”

“…Strangeway () revealed the correlation between the ionospheric outflow and the downward Poynting flux based on measurements from the Fast Auroral Snapshot Small Explorer. MHD simulations of present Mars with a weak dipole field also indicated that the existence of a weak intrinsic global dipole magnetic field increases ion loss (Sakai et al, ). Some hybrid simulation studies reported the same effects of a weak intrinsic magnetic field on ion loss, while they also showed that the existence of a strong intrinsic magnetic field reduces ion loss (Egan et al, ; Kallio & Barabash, ).…”

Ion Escape From Mars Through Time: An Extrapolation of Atmospheric Loss Based on 10 Years of Mars Express Measurements

“…Ion escape from the Earth's polar caps is strongly driven by Poynting flux and electron precipitation in the cusps (Strangeway et al, ) and correlates with solar wind‐driven geomagnetic activity, which can push O + escape rates up to ∼10 26 s −1 during severe storms (Slapak et al, ). In the simulations by Sakai et al () the introduction of a weak Martian dipole (100 nT equator surface) enhanced the heavy ion escape rate by 25 % ; however, the screening properties of a dipole may increase with higher magnetic moment values Cnossen et al (). The total effect of the presence of a dipole on ion escape is therefore likely a function of its strength in relation to the intensity of the solar wind, which may have been much weaker at 4.1 Ga (Wood et al, ).…”

Colloquium: Physical constraints for the evolution of life on exoplanets

“…The latter is dependent on the ambipolar electric field that stems from the difference in the velocities of ions and electrons (Axford, 1968); electrons typically move faster, thus causing charge separation and inducing an electric field, which accelerates the ions and permits their escape (Schunk and Nagy, 2009). The polar wind may become increasingly important for weak dipole magnetic fields with B p ∼ 10 −7 T for Mars-like exoplanets (Sakai et al, 2018); see also Lingam (2019).…”