Expected Atmospheric Conditions and Measurement Needs at Uranus for Future Entry Probes

We present a range of possible atmospheric conditions that may be encountered at Uranus in the early 2040s to guide a mission that may be launched in the early 2030s. Our predictions address the needs of atmospheric entry probes in particular. The Voyager 2 flyby of Uranus in 1986 found that negligible heat is transported from the interior to the surface (Pearl et al. 1990). The heat from the interior may be transported radiatively and/or convectively, which also control the vertical atmospheric static stability. The degree of stability remains an open question, and we analyze the uncertainties in the altitudes below those where Voyager 2 measured. High above the clouds, we study the effects of large seasonal variation in the thermal forcing caused by the 98 degree obliquity. The thermospheres of all giant planets are heated by an unknown source, and studying the Uranian winter theremosphere may help resolve the so-called thermospheric energy crisis. We also document the current state of the knowledge of the atmospheric composition to guide the design of in-situ compositional measurements. Uranus' bulk composition reflect materials accreted in the form of ices during planetary formation; however, remote sensing measurements to date are limited to the CH₄/H₂ ratio, which gives a range across a factor of 5 (Baines et al. 1995, Sromovsky et al. 2011). The other major volatile species condense at deep atmospheric levels beyond reach of remote-sensing measurements. Precise measurements of H₂S and/or NH₃ in the 5-10 bar region would anchor remote-sensing findings that the N/S ratio is distinctly non-solar in this pressure range, and this measurement would provide insight into the deeper, unobserved chemistry of the H₂O-NH₃-H₂S system. Other heavy elemental abundances are expected to scale roughly with carbon (Atreya et al. 2018), so their abundances are uncertain to at least an order of magnitude. Future in-situ measurements are needed to constrain the abundance of these heavy elements. Lastly, we analyze the atmospheric opacity in the radio frequencies, which will affect the data return from an in-situ probe. Recent observations of H₂S in the upper troposphere of Uranus (Irwin, P. et al., 2018) suggest significantly lower radio opacity than if that region were ammonia-dominated, making probe data relay a less challenging task.