Dunn, William; (2022) X-ray Emissions from the Ice Giants and Kuiper Belt. In: Bambi, C and Santangelo, A, (eds.) Handbook of X-ray and Gamma-ray Astrophysics. (pp. 1-23). Springer: Singapore.

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Abstract

The Ice Giants and Kuiper Belt Objects represent cornerstones in our understanding of planetary formation and evolution. There are many aspects of these systems for which X-ray observations can provide unique insights that are fundamental to our understanding of their dynamics and composition. Here, we outline some of the key X-ray emission processes for the Ice Giants and Kuiper Belt Objects and review the small selection of Uranus and Pluto X-ray observations taken from Earth orbit so far. These observations are challenging due to the low signals detected from planetary bodies at such large distances, but are unified by the puzzling excesses in X-ray brightness observed over prior expectations, potentially highlighting key knowledge gaps. For Uranus, X-ray emissions appear to exceed that produced simply through the scattering of solar photons, hinting at time variability from auroral emissions or fluorescence from the rings. For Pluto, the very soft energy emissions observed (0.3–0.6 keV) suggest fluorescence and solar wind charge exchange (SWCX) processes. However, the brightnesses detected are an order of magnitude larger than expected, potentially enabled by the long lifetimes of outgassed material in the outer solar system. While we have not yet reached the ceiling of exploration with current instrumentation, limitations on sensitivity mean that it is likely that the next generation of X-ray observatories (such as Athena) will be needed to truly characterize these X-ray emissions from Earth orbit. An additional opportunity was presented by the 2022 NASA Planetary Decadal, which supported a dedicated mission to Uranus. Recent advances in X-ray instrumentation miniaturization mean there is the transformative and low-risk, low-weight (approx. 10 kg) possibility of in situ X-ray instrumentation for Uranus. Building on the heritage of instruments such as the SMILE SXI or BepiColombo MIXS, a rich treasure trove of potential science is presented by such instrumentation. This includes magnetopause imaging through SWCX to address the open questions of the nature of the solar wind interaction with Ice Giants; fluorescence mapping to identify composition, formation, and biological context of the Uranian moons; imaging and spectral analysis of the aurora to potentially unambiguously determine if it is indeed caused by the magnetospheric cusp; and radiation belt imaging to study the fundamental physical processes that govern particle acceleration and enable Uranus to produce such high fluxes of energetic particles. The science cases are diverse and potentially profound, but given the field’s current infancy, further Uranus observations and modeling efforts are required to define instrument requirements.

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