Deep Atmosphere Composition, Structure, Origin, and Exploration, with Particular Focus on Critical in situ Science at the Icy Giants

TL;DR

This paper emphasizes the importance of in situ measurements, especially noble gases, for understanding the formation and composition of Uranus and Neptune, advocating for combined orbiter and probe missions without requiring new technology.

Contribution

It highlights the critical role of noble gas measurements and combined observational strategies in constraining icy giant formation models, proposing feasible mission concepts for the next decade.

Findings

Noble gases and isotopic ratios are key to understanding planet formation.

Orbiter and probe missions can provide complementary data without new technology.

Current observational constraints are insufficient for detailed composition analysis.

Abstract

A comprehensive exploration of Uranus and Neptune is essential to understand the formation and evolution of the giant planets, in particular, solar system, in general, and, by extension, a vast population of exoplanets. Though core accretion is generally favored over gravitational instability as the model of the formation of the gas giants, Jupiter and Saturn, observational constraints are presently lacking to make a compelling case for either in the case of the icy giants. Abundances of the heavy elements with mass exceeding that of helium provide the best constraints to the formation and migration models. For Uranus and Neptune, only the C elemental abundance has been determined from methane measurements, but should be considered as a lower limit considering methane is a condensible gas in the icy giants. Well-mixed water, ammonia and hydrogen sulfide to determine O, N and S elemental abundances, respectively, are too deep to measure by any observation technique. However, a precise measurement of the noble gases, He, Ne, Ar, Kr and Xe, together with their isotopic ratios, would circumvent the need for determining the above elements. Only entry probes are capable of measuring the noble gases, but those measurements can be done at relatively shallow pressure levels of 5-10 bars. Complementary observations from orbiter, especially the interior (gravity and magnetic field) and depth profiles of water and ammonia, would greatly enhance the data set for constraining the formation models. No new technology is required to carry out an orbiter-probe mission to either Uranus or Neptune in the next decade.