On the effect of the martian crustal magnetic field on atmospheric erosion

Abstract

Without the shielding of a strong intrinsic magnetic field, the martian atmosphere directly interacts with the impacting solar wind. The neutral constituents of the atmospheric corona can be ionized, and then picked up and accelerated by the magnetic field and convection electric field in the solar wind. A significant fraction of pickup ions escape Mars’ gravitational pull and are lost to space. This non-thermal escape process of heavy species is an important mechanism responsible for atmospheric erosion. While there is a perception that the martian magnetic anomalies are significant for the ionospheric density distribution and the bow shock standoff location, little is known about the quantitative influence of the martian crustal magnetic field on the global distribution of escaping pickup ions. In this paper, we apply a newly developed Monte Carlo ion transport model to resolve the crustal field effect on the pickup oxygen ion distribution around Mars. The background magnetic and electric fields, in which test particles are followed, are calculated using an independent three-dimensional multispecies MHD model. The effects of the crustal magnetic field on particle escape are quantified by varying the crustal field orientation in the model setup and comparing the corresponding test particle simulation results. The comparison is made by turning on or off the crustal field or changing the local time of the strongest field from the dayside to the dawnside. It is found that without the protection of the crustal magnetic field, the total amount of atmospheric escape through the tail region would be enhanced by more than a factor of two. It is shown that the crustal magnetic field not only regionally deflects the solar wind around the martian atmosphere, but also has an important global effect on atmospheric erosion and thus on long-term atmospheric evolution.

Introduction

As a weakly magnetized planet, the martian atmosphere is directly exposed to the impacting solar wind plasma flow. This proximity, and in fact overlap between the solar wind and the planetary neutral environment, create a scenario in which particles of planetary origin can be ionized, accelerated (picked up) by the solar wind, and swept away from Mars (e.g., Fang et al., 2008, and references therein). In addition to the pickup process, there are other non-thermal mechanisms responsible for energy gain and subsequent escape of planetary heavy species, including dissociative recombination of molecular ions (McElroy, 1972, Lammer and Bauer, 1991, Fox, 1993), and sputtering of the atmosphere by reentering energetic pickup ions (Luhmann and Kozyra, 1991, Luhmann et al., 1992, Johnson, 1994). In the present study, we focus on atmospheric loss through the pickup process. The acceleration and related escape of pickup ions are complex processes, requiring an investigation in the global context of the Mars–solar wind interaction.

The interaction of Mars with the solar wind is mainly of the atmospheric type like Venus and comets. However, Mars is not a purely unmagnetized body but one with a crustal remanent field. The findings of the Magnetometer onboard the Mars Global Surveyor (MGS) spacecraft clearly show that no global intrinsic magnetic field is currently present at Mars but magnetic anomalies are preserved in the crust (Acuña et al., 1998, Acuña et al., 1999, Connerney et al., 1999, Ness et al., 1999). The crustal field is mainly concentrated in the southern hemisphere where it is highly localized. The strongest crustal sources exist at latitudes higher than 30° S and at longitudes between 120°–210° W (Acuña et al., 1998). Total magnetic field intensities as high as 1500 nT were detected in the southern hemisphere by MGS. Such fields are strong enough to locally raise the ionosphere well above the MGS mapping phase altitude of around 400 km (Mitchell et al., 2001). As a consequence, the existence of crustal magnetic anomalies and their local time change (with the strongest crustal field facing from the dayside to the nightside) due to the rotation of Mars are expected to be important factors to be taken into account in understanding the Mars–solar wind interaction and thus atmospheric erosion.

The influence of the crustal magnetic field on the shape of the martian obstacle boundary has been identified by MGS measurements. It is found that the position of the magnetic pile-up boundary (MPB) is generally further from Mars in the southern hemisphere (Crider et al., 2002) and is raised as high as 500–1000 km by local crustal magnetic anomalies (Verigin et al., 2004). There is a debate on the influence of the crustal field on the bow shock (BS) location. Vignes et al. (2000) reported that no significant effect was found from 553 MGS BS crossings. However, the latitudinal dependence was neglected in their technique. Edberg et al. (2008) examined the entire MGS data set in the pre-mapping mission phase and concluded that the BS in the southern hemisphere is indeed pushed further outward by the stronger crustal field. This effect of the crustal magnetic field on the BS location was also predicted by numerical magneto-hydrodynamic (MHD) simulations (Ma et al., 2002).

Given the fact that crustal magnetic anomalies have a significant effect on Mars–solar wind interaction processes, the distribution and total amount of escaping planetary particles are expected to vary with the global electromagnetic and plasma environments that are altered by the crustal magnetic field. However, little is known about the extent to which the crustal field has an effect on atmospheric erosion. Considering the limited spatial and field of view coverage of satellite missions, global numerical models provide the best alternative to investigate the crustal field effect and interpret satellite ion flux measurements. The magnetic anomaly pattern has been incorporated into the global MHD model of Ma et al., 2002, Ma et al., 2004. However, due to the lack of a strong intrinsic magnetic field and the weakness of the local interplanetary field at 1.5 AU, the gyroradius of a pickup ion at Mars can be very large, comparable to or even larger than the planetary scale (Fang et al., 2008). The fluid approximation in MHD models therefore cannot resolve the finite heavy ion gyroradius effects of the martian pickup ions. For example, the asymmetric planetary ion distribution observed around Mars (e.g., Fedorov et al., 2006) is not evident in the MHD model results. A test particle model or a hybrid model is thus preferred in the study of the global distribution of escaping planetary particles.

In this study, we employ the newly developed MHD field-based test particle model of Fang et al. (2008) to investigate the crustal magnetic field effects on pickup ion escape. The investigation is carried out by comparing a number of simulation cases, in which the local time of the martian crustal source is changed. By this means, the effects of the martian crustal field on atmospheric erosion are quantified for specific cases, suggesting that the effects play a significant role in long-term atmospheric evolution.