MESSENGER X-Ray Spectrometer Detection of Electron-induced X-ray Fluorescence from Mercury's Surface
Abstract
The X-Ray Spectrometer (XRS) onboard 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 most prominent fluorescent lines are the Kα lines from the elements Mg, Al, Si, S, Ca, Ti, and Fe (1-10 keV). The XRS began orbital observations on 23 March 2011 and has observed X-ray fluorescence from the surface of the planet during both "quiet" Sun and flaring conditions whenever a sunlit portion of Mercury has been within the XRS field of view. XRS can detect the characteristic X-rays of Mg, Al, and Si during quiet-Sun conditions, but solar flares are required to produce measureable signals from the elements of higher atomic number such as S, Ca, Ti, and Fe. Nevertheless, X-ray fluorescence up to the Ca fluorescent 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 or during quiet-Sun illumination. To date, seven such events have been detected and are identified as electron-induced X-ray emission produced by ~1-10 keV electrons interacting with Mercury's surface. Electrons in this energy range were detected by the XRS during the three Mercury flybys, and since the beginning of orbital operations electrons of this same energy range have been detected by XRS during almost every orbit. These electron events last from minutes to tens of minutes. Electron transport models suggest that a large percentage of these quasi-trapped electrons do not complete even a single orbit about Mercury before impacting the surface. Knowledge of the precipitating electron distribution at the planet's surface makes it possible to infer surface composition from the measured fluorescent spectra, providing additional measurement opportunities for the XRS.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.P41A1602S
- Keywords:
-
- 5464 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Remote sensing