In Situ exploration of the giant planets

Abstract

Remote sensing observations suffer significant limitations when used to study the bulk atmospheric composition of the giant planets of our Solar System. This impacts our knowledge of the formation of these planets and the physics of their atmospheres. A remarkable example of the superiority of in situ probe measurements was illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases’ abundances and the precise measurement of the helium mixing ratio were only made available through in situ measurements by the Galileo probe. Here we describe the main scientific goals to be addressed by the future in situ exploration of Saturn, Uranus, and Neptune, placing the Galileo probe exploration of Jupiter in a broader context. An atmospheric entry probe targeting the 10-bar level would yield insight into two broad themes: i) the formation history of the giant planets and 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. An atmospheric probe could represent a significant ESA contribution to a future NASA New Frontiers or flagship mission to be launched toward Saturn, Uranus, and/or Neptune.

Chemical and Isotopic Composition Measurements on Atmospheric Probes Exploring Uranus and Neptune

“…As the descent probe was slowed down by the main parachute, the operations switch from low vertical resolution to high vertical resolution mode, where the integration time is optimized and the vertical resolution corresponds a few 100 m. In this part of the atmosphere the probe collects approximately 141 mass spectra in total. At a pressure of a little over 1 bar the probe is expected to enter the cloud layer [4,60]. The vertical resolution is maximized at this location.…”

“…Therefore sampling of both vapor and condensates is essential for determining the correct mixing ratios of volatile species. Clouds are expected to be encountered during the descent of the probe in Uranus' and Neptune's atmosphere [4,60].…”

Why the European Space Agency should join the US mission to Uranus

“…Measuring the composition of Uranus's atmosphere would constrain the conditions under which the planet formed and show how these have changed with time 5 . The puzzlingly large tilt of Uranus's axis of rotation (almost parallel to the plane of the Solar System) could be examined, along with the planet's internal structure and magnetic field 1,2,6,7 .…”

Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Material during the Nice Model Migration

“…We determined ρ p at the base of the upper envelope as P c s 2 , with P as the pressure of 100 bar. The sound speed c s is calculated as RT M , where R is the gas constant, T is the temperature (350 K (Mousis et al 2021)), and M it the mean molar mass of 2.3 g ml −1 , (a solar H 2 -He mix). We assumed an adiabatic region with γ ≈ 7/5, the adiabatic index of H 2 , as 100 bar is below the generally expected radiative-convective boundary and therefore assumed to be convective.…”