On Energization and Loss of the Ionized Heavy Atom and Molecule in Mars' Atmosphere

TL;DR

This study reveals that Mars' crustal magnetic fields can enhance atmospheric loss by facilitating plasma wave-particle interactions, leading to increased ion escape driven by solar wind-induced ultra-low frequency waves.

Contribution

It demonstrates that crustal magnetic fields, rather than global magnetic fields, can promote atmospheric erosion through plasma wave interactions, a counterintuitive insight supported by MAVEN data and simulations.

Findings

Crustal magnetic fields cause ion resonance with ULF waves.

Resonance leads to energization and escape of O+ ions.

Solar wind drives ULF wave excitation, enhancing atmospheric loss.

Abstract

The absence of global magnetic fields is often cited to explain why Mars lacks a dense atmosphere. This line of thought is based on a prevailing theory that magnetic fields can shield the atmosphere from solar wind erosion. However, we present observations here to demonstrate a counterintuitive understanding: unlike the global intrinsic magnetic field, the remnant crustal magnetic fields can enhance atmosphere loss when considering loss induced by plasma wave-particle interactions. An analysis of MAVEN data, combined with observation-based simulations, reveals that the bulk of O+ ions would be in resonance with ultra-low frequency (ULF) waves when the latter were present. This interaction then results in significant particle energization, thus enhancing ion escaping. A more detailed analysis attributes the occurrence of the resonance to the presence of Mars' crustal magnetic fields, which cause the majority of nearby ions to gyrate at a frequency matching the resonant condition ({\omega}-k_{\parallel} v_{\parallel}={\Omega}_i) of the waves. The ULF waves, fundamental drivers of this entire process, are excited and propelled by the upstream solar wind. Consequently, our findings offer a plausible explanation for the mysterious changes in Mars' climate, suggesting that the ancient solar wind imparted substantially more energy.