Monte Carlo modeling of north‐south asymmetries in Mercury's sodium exosphere

Abstract

[1] Ground-based observations and MESSENGER's first flyby show persistent asymmetries in Mercury's neutral sodium tail, with brightest emission often seen in the northern lobe. This feature may be associated with the recent finding that the magnetic dipole moment is offset from the planet's center by 0.2 R M to the north, while approximately aligned with the spin axis. Such a configuration produces an asymmetry in the magnetosphere cusp whereby more plasma has direct access to the planet's southern hemisphere than the north. Using 3-D numerical modeling, it is demonstrated that ion precipitation, enhanced in the south, can result in the observed Na emission profiles across the tail that are brighter to the north. However, sources located at high-latitude cusp footprints on the dayside are unable to match the observed width of the brightness profiles across the tail. Instead, these simulations provide evidence for lower latitude sources on the dawnside, resulting from the accumulation of sodium during the long Mercury night. Photodesorption, rather than ion sputtering, is determined to be the responsible mechanism for this population's release and escape from the planet's surface.

High‐Resolution, Ground‐Based Observations of the Lunar Sodium Exosphere During the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission

“…Although our temperatures at large phase angles (greater than 40°) are close to that of a PSD source (~1700 K), PSD itself does not sufficiently account for most of our temperatures, specifically near full Moon period. Even though PSD is a nonthermal source (Yakshinskiy & Madey, , ; Schmidt, ), we see no evidence of a high‐energy wing associated with non‐Maxwellian profiles in our data.…”

A cold‐pole enhancement in Mercury's sodium exosphere

“…[], Mura et al . [], and Schmidt []. Note that the tail visible in Figure is populated by the small fraction of relatively energetic exospheric sodium atoms [ Smyth and Marconi , ; Potter et al ., ; Schmidt et al ., ], but most sodium atoms travel on short ballistic hops following photodesorption, reflecting the small‐scale height of the exosphere (~100 km) [ Schleicher et al ., ; Potter et al ., ; Cassidy et al ., ].…”

“…This suggests a depletion of the surface reservoir, a phenomenon discussed extensively in previous literature. Schmidt [] characterized this depletion with an e ‐fold timescale, which he estimates to be approximately one Earth week for the observations of Baumgardner et al . [], several Earth weeks for the observations of Sprague et al .…”

“…Night side reservoirs were subsequently used in the models described by Mura et al . [], Leblanc and Johnson [], Schmidt [], and Teolis and Waite []. In the following sections we describe MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations of the sodium exosphere's local‐time distribution and compare these with ground‐based observations and model predictions.…”

Evidence Connecting Mercury's Magnesium Exosphere to Its Magnesium‐Rich Surface Terrane

“…Because the spatial and temporal distribution of an exospheric species is influenced by a number of parameters and processes, including ejection energy, gravity, radiation pressure, and photoionization with subsequent electric field transport, it was difficult to correlate the Na observations with the regional surface composition. Furthermore, the compositional distribution of Mercury's crust was not known before MESSENGER; therefore, model estimates of exospheric production routinely assumed that Mercury's surface was composed of a constant, globally uniform distribution of neutral‐bearing minerals, making it difficult to isolate a direct connection between the surface and the exosphere (Burger et al, ; Sarantos et al, ; Schmidt, ).…”