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Register a new userDeep Atmosphere Composition, Structure, Origin, and Exploration, with Particular Focus on Critical in situ Science at the Icy Giants
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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.
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The distant ice giants of the Solar System, Uranus and Neptune, have only been visited by one space mission, Voyager 2. The current knowledge on their composition remains very limited despite some recent advances. A better characterization of their composition is however essential to constrain their formation and evolution, as a significant fraction of their mass is made of heavy elements, contrary to the gas giants Jupiter and Saturn. An in situ probe like Galileo would provide us with invaluable direct ground-truth composition measurements. However, some of the condensibles will remain out of the grasp of a shallow probe. While additional constraints could be obtained from a complementary orbiter, thermochemistry and diffusion modeling can further help us to increase the science return of an in situ probe.
In this white paper, we present a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan exosphere to the deep interior, and we detail which instrumentation and mission scenarios should be used to answer them. Our intention is to formulate the science goals for the next generation of planetary missions to Titan in order to prepare the future exploration of the moon. The ESA L-class mission concept that we propose is composed of a Titan orbiter and at least an in situ element (lake lander and/or drone(s)).
Satellites of giant planets thought to form in gaseous circumplanetary disks (CPDs) during the late planet-formation phase, but it was unknown so far whether smaller mass planets, such as the ice giants could form such disks, thus moons there. We combined radiative hydrodynamical simulations with satellite population synthesis to investigate the question in the case of Uranus and Neptune. For both ice giants we found that a gaseous CPD is created at the end of their formation. The population synthesis confirmed that Uranian-like, icy, prograde satellite-system could form in these CPDs within a couple of $10^5$ years. This means that Neptune could have a Uranian-like moon-system originally that was wiped away by the capture of Triton. Furthermore, the current moons of Uranus can be reproduced by our model without the need for planet-planet impact to create a debris disk for the moons to grow. These results highlight that even ice giants -- that among the most common mass-category of exoplanets -- can also form satellites, opening a way to a potentially much larger population of exomoons than previously thought.
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.
Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is 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 have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturns atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturns bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturns upper troposphere would constrain its bulk O/H ratio. We highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturns stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations.
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