Abstract | The Mercury Imaging X-ray Spectrometer (MIXS) on board BepiColombo will determine the surface composition of Mercury to a high imaging resolution with improved energy resolution. MIXS uses an analytical technique called x-ray fluorescence (XRF) to observe the ‘fingerprint’ of characteristic x-rays emitted by Mercury’s surface to determine its elemental abundances. Laboratory measurements of XRF typically use a constant x-ray source and detector angle to yield consistent results. However, this is not the case at Mercury, with the viewing geometry of MIXS ever changing and the angle of the Sun above Mercury also changing throughout the Mercurian year. The angles between the detector and x-ray source have profound implications on the x-ray spectra. As the incidence angle of the x-ray source increases the intensity of the spectrum tends to also increase. However, higher energy elements are less affected by the change in incidence angle, and as such results in a hardening of the spectrum.
If unaccounted for, this bias towards higher energy elements may lead to an underestimate of the actual abundances of lower energy elements on Mercury’s surface. Although the effect of changing incidence angle is known and has been studied by several authors (e.g. Weider et al. 2011), the magnitude of the incidence angle effect at different energies is yet to be established. The MIXS Ground Reference Facility has the ability to change the viewing geometry between the x-ray source, detector and sample, and so will be able to provide empirical evidence to determine the magnitude of the incidence angle effect.
We showcase the most recent developments on how spectra are affected by the incidence angle using the MIXS Ground Reference Facility. Using a combination of MIXS data and our empirical results of the incidence angle effect we hope to achieve a more accurate global elemental abundance map of Mercury’s surface upon BepiColombo’s arrival.
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