Scientific rationale for Uranus and Neptune in situ explorations

Abstract

The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ∼70% heavy elements surrounded by a more dilute outer envelope of H 2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the lack of dedicated exploration missions, our knowledge of the composition and atmospheric processes of these distant worlds is primarily derived from remote sensing from Earth-based observatories and 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 not well understood. Exploration of an ice giant system is therefore a highpriority science objective as these systems (including the magnetosphere, satellites, rings, atmosphere, and interior) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by a future in situ exploration of an ice giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale heights beneath the tropopause, would yield insight into two broad themes : i) the formation history of the ice giants and, in a broader extent, that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. In addition, possible mission concepts and partnerships are presented, and a strawman ice-giant probe payload is described. An ice-giant atmospheric probe could represent a significant ESA contribution to a future NASA ice-giant flagship mission.

Herschel map of Saturn’s stratospheric water, delivered by the plumes of Enceladus

“…Saturn_H2O_map_observations A&A proofs: manuscript no. Saturn_H2O_map_observations as well as possibly direct in situ measurements (Mousis et al 2014(Mousis et al , 2016(Mousis et al , 2018, will also help to improve our understanding of the outstanding and more general question of the origin of exogenic species in giant planet atmospheres.…”

Atmospheric Electricity at the Ice Giants

“…Uranus and Neptune have very similar atmospheric structures, which have been inferred from remote sensing observations, radiative transfer and photochemical modelling (Mousis et al, 2018). The most recent interpretation, broadly applying to both ice giants, (Mousis et al, 2018) has a stratosphere (0.1 -30 hPa) of an extended, mainly hydrocarbon haze, generated by gravitational settling of aerosol particles from methane photolysis.…”

“…Uranus and Neptune have very similar atmospheric structures, which have been inferred from remote sensing observations, radiative transfer and photochemical modelling (Mousis et al, 2018). The most recent interpretation, broadly applying to both ice giants, (Mousis et al, 2018) has a stratosphere (0.1 -30 hPa) of an extended, mainly hydrocarbon haze, generated by gravitational settling of aerosol particles from methane photolysis. In the troposphere there are expected to be ice cloud layers of methane (CH4), with its base at 1300 hPa, a physically thin but optically thick hydrogen sulphide (H2S) layer between 2000-4000 hPa, and beneath this ammonium hydrosulphide (NH4SH), followed by water (H2O) down to about 50 x 10 3 hPa.…”

“…Estimated maximum atmospheric conductivity at the tropopause in a cloud-free atmosphere. Atmospheric parameters are from Mousis et al (2018) and charged particle concentrations from Capone et al (1977). Mobilities are estimated using data from Harrison and Tammet (2008) for ions and Pack and Phelps (1961) for electrons.…”

Solar‐Driven Variation in the Atmosphere of Uranus

“…Two suggested mechanisms could cause a solar cycle variation in the atmosphere of Uranus, expressed through reflectivity variations: first, cloud formation through condensation of supersaturated vapor (probably methane or butadiyne (diacetylene)) onto ions or electrons created by GCR (Moses et al, 1992). This mechanism was originally suggested for Neptune, but since the cloud, aerosol and atmospheric structures of Uranus and Neptune are similar (Mousis et al, 2017), the same mechanisms should be considered for Uranus. Baines and Smith (1990) proposed a mechanism to explain the Neptune solar cycle variation by which aerosols were photochemically "tanned," changing the planetary albedo; this could potentially also act on Uranus.…”