Anisotropic solar wind sputtering of the lunar surface induced by crustal magnetic anomalies

Abstract

The lunar exosphere is generated by several processes each of which generates neutral distributions with different spatial and temporal variability. Solar wind sputtering of the lunar surface is a major process for many regolith‐derived species and typically generates neutral distributions with a cosine dependence on solar zenith angle. Complicating this picture are remanent crustal magnetic anomalies on the lunar surface, which decelerate and partially reflect the solar wind before it strikes the surface. We use Kaguya maps of solar wind reflection efficiencies, Lunar Prospector maps of crustal field strengths, and published neutral sputtering yields to calculate anisotropic solar wind sputtering maps. We feed these maps to a Monte Carlo neutral exospheric model to explore three‐dimensional exospheric anisotropies and find that significant anisotropies should be present in the neutral exosphere depending on selenographic location and solar wind conditions. Better understanding of solar wind/crustal anomaly interactions could potentially improve our results.

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies

“…Higher spatial resolution mapping may be possible with improved computational resources; however, the error inherent in the ARTEMIS observations (i.e., finite angle and energy bin widths), the Runge‐Kutta backtracing, and the lack of exact knowledge of the electromagnetic fields within anomalies (especially very close to the surface) prevents higher‐resolution mapping. Nevertheless, we note that the reflection map produced here agrees well with that from Chandrayaan [ Lue et al , ] and Kaguya [ Poppe et al , ].…”

“…The reduction of local space weathering rates due to crustal magnetic anomalies is one of the leading theories for the formation of lunar swirls, which are sinuous, superficial, high‐albedo markings on the lunar surface [e.g., Hood and Schubert , ; Hood and Williams , ]. Crustal magnetic anomalies are also believed to play a role in locally suppressing the formation of the lunar neutral exosphere by decelerating and/or reflecting solar wind protons such that charged‐particle sputtering of the lunar surface is locally diminished [ Poppe et al , ]. Preliminary evidence for this phenomenon has been identified in ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of exospheric pickup ion distributions at the Moon [ Halekas et al , ].…”

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

“…Deca et al, ; Lue et al, ; Zimmerman et al, ). The resulting regions are comparable to maps of LMA‐reflected H + observations (Lue et al, ; Poppe et al, , ).…”

Kinetic simulations of kilometer-scale mini-magnetosphere formation on the Moon

“…Interestingly, many magnetic anomalies are colocated with intricate and contrasting patterns of optical brightness, called “lunar swirls.” One hypothesis for formation of swirls is that the solar wind‐anomaly interaction produces coherent electric fields that divert protons away from the surface and protect the areas below from ion‐driven weathering that would otherwise darken the surface [ McCord et al , ]. If ion deenergization is significant enough, it could quench the local sputtered flux of neutrals feeding the global exosphere [ Poppe et al , ]. Magnetic anomalies thus represent a critical and unique scientific laboratory for studying the connections between space plasma physics, surface charging, surface weathering, and exospheric production.…”