A transient enhancement of Mercury\u2019s exosphere at extremely high altitudes inferred from pickup ions

Jamie M Jasinski,Daniel J Gershman,Jim M Raines,James A Slavin,Andrew J Coates,Leonardo H Regoli,Timothy A Cassidy,Tom A Nordheim,Neil Murphy,Ryan M Dewey

doi:10.1038/s41467-020-18220-2

Abstract

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. Here we report on the inferred enhancement of neutral densities (<102 cm−3) at high altitudes (~5300 km) by the MESSENGER spacecraft. Such high-altitude densities cannot be accounted for by the typical exosphere. This event was observed by the Fast-Imaging Plasma Spectrometer (FIPS), which detected heavy ions of planetary origin that were recently ionized, and “picked up” by the solar wind. We estimate that the neutral density required to produce the observed pickup ion fluxes is similar to typical exospheric densities found at ~700 km altitudes. We suggest that this event was most likely caused by a meteroid impact. Understanding meteoroid impacts is critical to understanding the source processes of the exosphere at Mercury, and the use of plasma spectrometers will be crucial for future observations with the Bepi-Colombo mission.

Highlights

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km
The appearance of the spacecraft inwards of the bow shock is a projection effect, and MESSENGER lies outside the bow shock in the solar wind as can more accurately be visualized in Fig. 1b
There are four main processes that account for the neutral exosphere: thermal desorption, photonstimulated desorption, ion sputtering, and impact vaporization

Summary

Introduction

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. We estimate that the neutral density required to produce the observed pickup ion fluxes is similar to typical exospheric densities found at ~700 km altitudes We suggest that this event was most likely caused by a meteroid impact. This sodium exosphere has small dayside scale heights of up to ~100 km at perihelion, with subsolar densities of 103 cm−3 measured at altitudes of ~450 km[3]. We conclude that the cause of the event is the impact of a meteroid at Mercury, which vaporized Na and Si from the surface These particles were subsequently photoionized in the solar wind and observed by the MESSENGER spacecraft

Methods

Results

Conclusion