HERMES: an imaging x-ray fluorescence spectrometer for the BepiColombo mission to Mercury

We describe HERMES (High Energy Remote-Sensing of Mercury's Surface), a novel X-ray imaging spectrometer for potential accommodation in the Mercury Planetary Orbiter (MPO) component of ESA's BepiColombo mission to Mercury. The instrument combines recently developed micro channel plate optics with large-format compound semiconductor imaging arrays. MCP optics offer the distinct advantage of a large collecting area coupled to arcminute angular resolution in a light-weight package and short focal length. Measurements on a prototype optic indicate it should be possible to achieve an angular resolution below 1 arcmin over a fov of 1 degree(s). Energy resolution of 270 eV FWHM at 5.9 keV has been achieved at room temperature for a prototype GaAs array. We estimate that HERMES will detect ~2000 x-ray fluorescent photons s-1 from the surface of Mercury during solar quiet conditions at the pericenter of the orbit. The maximum expected surface spatial resolution from this altitude is ~200m and the fov 40 km2. Over the orbiter's 2 year mission life, HERMES will provide the first very high resolution compositional maps of any planetary surface.