MESSENGER detection of electron‐induced X‐ray fluorescence from Mercury's surface

Abstract

[1] The X-Ray Spectrometer (XRS) on the MESSENGER spacecraft measures elemental abundances on the surface of Mercury by detecting fluorescent X-ray emissions induced on the planet's surface by the incident solar X-ray flux. The XRS began orbital observations on 23 March 2011 and has observed X-ray fluorescence (XRF) from the surface of the planet whenever a sunlit portion of Mercury has been within the XRS field of view. Solar flares are generally required to provide sufficient signal to detect elements that fluoresce at energies above $2 keV, but XRF up to the calcium line (3.69 keV) has been detected from Mercury's surface at times when the XRS field of view included only unlit portions of the planet. Many such events have been detected and are identified as electron-induced X-ray emission produced by the interaction of $1-10 keV electrons with Mercury's surface. Electrons in this energy range were detected by the XRS during the three Mercury flybys and have also been observed regularly in orbit about Mercury. Knowledge of the energy spectrum of the electrons precipitating at the planet's surface makes it possible to infer surface composition from the measured fluorescent spectra, providing additional measurement opportunities for the XRS. Abundance results for Mg, Al, and Si are in good agreement with those derived from solar-induced XRF data, providing independent validation of the analysis methodologies. Derived S and Ca abundances are somewhat higher than derived from the solar-induced fluorescence data, possibly reflecting incomplete knowledge of the energy spectra of electrons impacting the planet.

MESSENGER observations of suprathermal electrons in Mercury's magnetosphere

“…These observations suggest that the electrons could drift in longitude along closed field lines, as demonstrated in a few cases at higher energies by echoes [ Baker et al , ], and that there could be quasi‐trapped electrons in orbits close to Mercury's magnetic equator as a more or less standard feature. We refer to these particles as quasi‐trapped because most of the electrons at these energies (~ keV) cannot completely drift around the planet before either striking the surface or hitting the dayside magnetopause, and only a few make a complete orbit around the planet [ Schriver et al , ; Starr et al , ]. However, the fact that we detected these electrons on almost every orbit indicates that they represent a semipermanent feature.…”

Comprehensive survey of energetic electron events in Mercury's magnetosphere with data from the MESSENGER Gamma‐Ray and Neutron Spectrometer

“…Do these measurements provide additional evidence for particle loss at Mercury's surface associated with its asymmetric magnetic field, as suggested by Korth et al . [], and as shown with measurements of electron‐induced X‐ray fluorescence [ Starr et al ., ]?…”

Spatial distribution and spectral characteristics of energetic electrons in Mercury's magnetosphere

“…Closer examination reveals that these events are short, with durations equal to or less than the instrument integration period (3 s). This group of events could be related to the low‐energy, low‐intensity X‐ray fluorescence events observed during MESSENGER's low‐latitude flybys [ Ho et al , 2011a] and seen near the planet from orbital XRS observations [ Starr et al , 2012]. Prompted by the low‐energy electron (1–10 keV) events seen during the flybys, Schriver et al [2011] reported kinetic simulation results that suggested a quasi‐trapped population of low‐energy (1–10 keV) ions and electrons near Mercury's equatorial region.…”