Four coupled reasons why Uranus is so interesting

Uranus and its system of rings and moons represents the next frontier in the exploration of the giant planets of our solar system. In addition to being close in mass and size to the most abundant known exoplanets, Uranus has a number of interesting features that distinguish it even from Neptune, so similar in size and mass. (1) Uranus has a regular satellite system of coplanar moons that are large enough, in some cases, to have subsurface liquid water. (2) Uranus has a very large axial tilt which may be the result of an early oblique impact, or not. (3) Uranus appears to have little or no internal heat, beyond thermalized sunlight, distinguishing it from all the other giant planets. (4) Both Uranus and Neptune have super-primordial deuterium abundances measured in HD that suggest most of the heavy element fraction in their interiors to be rock rather than ice (with an admixture of ice…it is not one or the other). Properties (1), (2), (3), and (4) constitute a coupled problem; they are not separate issues. That is, one would like to know if the origin of the moons is the result of a giant impact, and whether such an impact could have produced a layered structure that would then suppress any internal heat flux, as has been suggested (Hofstadter et al). If so, why then is the D/H value in HD the same in Uranus and Neptune, where the latter has a strong internal heat flow and thus would be expected to have allowed equilibration between envelope and core, in contrast to Uranus. Further, the average system mass density of the regular Uranian satellites is consistent with the derived rock-to-ice ratio in the interior of Uranus, which in turn argues for an obliquity-generating impact sourcing deep material for the nascent satellite system.  Whether a self-consistent picture can be assembled depends on the outcome of measurements that can be made by a shallow atmospheric entry probe, such as the ⁴⁰Ar abundance (Nimmo et al), gravity measurements of the deep interior and the regular satellites, and other observations by a capable orbiter. A highly capable Uranus Orbiter and Probe mission has the potential to make profound discoveries about the properties and interior dynamics of this enigmatic world that rival what Cassini, Galileo and Juno have discovered at Jupiter and Saturn. 

Hofstadter, M., et al, 2023. https://kiss.caltech.edu/final_reports/Uranus_final_report.pdf

Nimmo et al, 2024, PSJ 5 109.

Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. © 2025. All rights reserved.