MESSENGER X-Ray Spectrometer Detection of Electron-induced X-ray Fluorescence from Mercury's Surface

X-ray emission from solar system bodies has been observed for decades. The surface of planets with no atmosphere may be excited by solar X-rays, solar wind particles (primarily electrons), and ions, producing line emission and bremsstrahlung. Measurement of solar-induced X-ray fluorescence (XRF) from planetary surfaces has been used to infer surface elemental abundances at the Moon and the asteroids 433 Eros and 25143 Itokawa. More recently, the MESSENGER X-Ray Spectrometer (XRS) has reported on Mercury's surface composition derived from measurements of solar-flare-induced XRF. 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 measurable 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. Many 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 drift 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. Elemental compositions for Mg, Al, S, Ca, Ti, and Fe have been derived from these measurements and are in line, to within expected uncertainties, with those inferred from solar-induced XRF. This agreement confirms the value of electron-induced X-ray emission measurements as a tool for geochemical analysis at Mercury.