  Whether it is fluorescence emission from asteroids and moons, solar wind
charge exchange from comets, exospheric escape from Mars, pion reactions on
Venus, sprite lighting on Saturn, or the Io plasma torus in the Jovian
magnetosphere, the Solar System is surprisingly rich and diverse in X-ray
emitting objects. The compositions of diverse planetary bodies are of
fundamental interest to planetary science, providing clues to the formation and
evolutionary history of the target bodies and the solar system as a whole.
X-ray fluorescence (XRF) lines, triggered either by solar X-rays or energetic
ions, are intrinsic to atomic energy levels and carry an unambiguous signature
of the elemental composition of the emitting bodies. All remote-sensing XRF
spectrometers used so far on planetary orbiters have been collimated
instruments, with limited achievable spatial resolution, and many have used
archaic X-ray detectors with poor energy resolution. Focusing X-ray optics
provide true spectroscopic imaging and are used widely in astrophysics
missions, but until now their mass and volume have been too large for
resource-limited in-situ planetary missions. Recent advances in X-ray
instrumentation such as the Micro-Pore Optics used on the BepiColombo X-ray
instrument (Fraser et al., 2010), Miniature X-ray Optics (Hong et al., 2016)
and highly radiation tolerant CMOS X-ray sensors (e.g., Kenter et al., 2012)
enable compact, yet powerful, truly focusing X-ray Imaging Spectrometers. Such
instruments will enable compositional measurements of planetary bodies with
much better spatial resolution and thus open a large new discovery space in
planetary science, greatly enhancing our understanding of the nature and origin
of diverse planetary bodies. Here, we discuss many examples of the power of XRF
to address key science questions across the solar system.
