Reference Science Payload for an Uranus Entry Probe

The ice giants Uranus and Neptune are the least understood class of planets in our solar system, while planets of their size, the most frequent among exoplanets, represent a common outcome of planet formation. Presumed to have a small rocky core, a deep interior comprising ~70% heavy elements surrounded by a more dilute outer envelope of H2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the dearth of missions dedicated to their exploration, our knowledge of their composition and atmospheric processes is primarily derived from a single Voyager 2 flyby of each, complemented by subsequent remote sensing from Earth-based observatories, including space telescopes. As a result, Uranus's and Neptune's physical and atmospheric properties remain poorly constrained and their roles in the evolution of the Solar System are not well understood. Exploration of ice giant systems is therefore a high-priority science objective as these systems (which link together the magnetospheres, satellites, rings, atmosphere, and interior of these planets) challenge our understanding of planetary formation and evolution. In this context, the US planetary science decadal survey report recently recommended the launch of a flagship mission towards the Uranian system in the early 2030s. This mission would be composed of an orbiter aiming at exploring the Uranian system as a whole and a descent probe to directly sample the giant’s atmosphere.

Measurements to be made with a probe can be defined as Tier 1, representing threshold science required to justify the probe mission, and Tier 2 representing valuable science that significantly complement and enhance the threshold measurements, but of themselves are not sufficient to justify the mission. Tier 1 measurements comprise atmospheric noble gas abundances including helium, key noble gas isotope ratios, and the thermal structure of the atmosphere. Instrumentation required to achieve the Tier 1 measurements include a mass spectrometer, a helium abundance detector, and an atmospheric structure instrument comprising both sensors for pressure, temperature, a Tunable Laser System and atmospheric acoustic properties (speed of sound). Tier 1 science can be achieved with a probe making measurements near one to several bars. Tier 2 science includes measurements of key isotopic ratios, the abundances of atmospheric condensables and disequilibrium species, atmospheric dynamics, the net radiative flux transfer profile of the atmosphere, and the location, composition, properties, and structure of the clouds. To achieve all the Tier 2 science objectives requires a probe descending through at least ten bars carrying the full Tier 1 suite of instruments as well as a nephelometer, net flux radiometer, and an ultrastable oscillator to enable Doppler wind tracking of the probe throughout descent.