In Situ exploration of the giant planets

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.

Atmospheric Entry Probes for in situ Exploration of the Ice Giant Planets

<p>Understanding the formation and evolution of the solar system and the formation of the giant planets is constrained by inherent limitations in the capabilities of remote sensing. In situ exploration of planetary atmospheres provides key measurements not possible from remote observations, remarkably demonstrated at Jupiter by the Galileo probe, where key measurements included the determination of noble gas abundances and the precise measurement of the Jupiter helium mixing ratio. In this paper, we describe the primary scientific goals to be addressed by future in situ exploration of the ice giants Uranus and Neptune, placing in situ explorations of the gas giants, including the Galileo probe and a future Saturn probe, into a broader solar system context. An ice giant atmospheric entry probe reaching 10 bars would provide insight into both the formation history of the solar system and the giant planets, and the structure and composition of, and physical processes at play within ice giant atmospheres. An entry probe as an element of a future ice giant flagship mission would descend under parachute to measure the abundances and isotopic ratios of the noble gases, D/H in H<sub>2</sub> and <sup>13</sup>C/<sup>12</sup>C, and the thermal structure and dynamics from the upper atmosphere down to the deepest region from which the probe is able to return data, perhaps 10-20 bars or more. Probe data would be returned to Earth using a Carrier Relay Spacecraft as a relay station. The relay spacecraft, particularly if it is an orbiter with a suite of remote sensing instruments, can significantly enhance the science return from the probe; remote sensing provides the global context from which to understand the probe's local measurements of weather and cloud properties. One or more small atmospheric probes could represent a significant ESA contribution to a future NASA New Frontiers or Flagship Ice Giant mission.</p> <p> </p>

Entry Probe Radio Science for Measurements of Ice Giant Atmospheric Dynamics and Composition

<p>Planetary atmospheric winds, waves, tides, and turbulence represent a tie-point between planetary structure and processes, including atmospheric thermal and energy structure, cloud location and properties, and atmospheric composition and compositional gradients. The only direct means by which dynamics of an ice giant atmosphere can be measured along the probe descent path is via radiometric tracking of an ice giant entry probe. Additionally, measurements on an orbiter of the strength of a probe telecom signal can be used to provide the abundance of microwave absorbing molecules along the probe relay signal raypath, expected to be primarily ammonia (NH<sub>3</sub>) or hydrogen sulfide (H<sub>2</sub>S).</p> <p> </p> <p>Doppler tracking of a descent probe has been demonstrated with the Galileo probe at Jupiter and the Huygens probe at Titan. By including an ultrastable oscillator on both the transmit and receive sides of the probe telemetry relay signal, the time variation of the measured relay signal frequency provides a measure of wind speeds (via the Doppler effect). and the signatures of atmospheric waves, convection, and turbulence. In addition, other probe dynamical effects such as pendulum motion under the parachute, probe spin, and aerodynamic buffeting can be retrieved from careful analysis of the probe telecom signal Doppler residuals. Measurements made on board the orbiter of the time-varying received signal strength would provide a profile of microwave absorbing molecules along the probe radio signal raypath, complementing composition measurements made the probe mass spectrometer.</p> <p> </p> <p>The scientific objectives, measurement requirements, and expected measurement accuracies of the profile of zonal winds and atmospheric absorption will be discussed in this presentation, with a preliminary attempt to quantify the effect of uncertainties in the reconstruction of the probe descent and carrier overflight trajectories.</p>

Aerothermal Modeling Challenges for Ice Giant Entry Probes

<p>In June 2017, NASA published the Ice Giants Pre-Decadal Survey Mission Study Report which took a fresh look at science priorities and mission concepts for missions to the Uranus and Neptune systems. In addition to science objectives, the team explored the state of required technologies for remote and in-situ science exploration. Notably, three of the four mission architectures considered in the study included an atmospheric probe. More recently, interest has grown within ESA for outer planet exploration. In support of this objective, ESA has performed two CFD studies (January & July 2019) which analyzed the feasibility of stand-alone elements (orbiter and probes) provided by ESA as a part of a NASA led mission to the Uranus or Neptune systems. The first study was carried out by ESA experts with active participation of NASA/JPL. ESA highlighted the necessity to deepen the knowledge characterizing the aerothermal environment of the probes.</p><p> </p><p>Entry environments for the NASA study were estimated using an aeroheating correlation that was calibrated to data returned from the Galileo probe entry to Jupiter. For the ESA concept study, aeroheating estimates were made using correlations employed during the design of the Galileo probe. Importantly, these correlations show large discrepancies in predicted total aeroheating (in some cases more than 100%), largely due to differences in the predicted radiative heat load. The magnitude of the disagreement is disconcerting in and of itself, but the problem is made worse by the fact that both correlations are being extrapolated from the extreme Galileo entry conditions to the (relatively) more benign Uranus and Neptune entry. It is likely that neither correlation is providing an accurate assessment of the true aeroheating loads at this time. Given that current NASA predictions are near the limits of existing TPS test capability, and that ESA predictions are more severe, improving the accuracy and associated margins of the prediction is critical to better assess mission feasibility.</p><p> </p><p>Recent work in NASA by Cruden (AIAA Paper No. 2015-0380) and Erb (AIAA Paper No. 2019-3360) have substantially improved our fundamental understanding of aerothermodynamics in Hydrogen-Helium atmospheres. Similar work is planned in ESA as well. However, these recent data have not been incorporated into updated design models for Outer Planet probes. In addition, this work does not address the problem of trace atmospheric constituents (such as Methane) that are known to be present in Ice Giant atmospheres and may substantially alter the resulting shock layer radiation signal by providing a ready source of free electrons to initiate excitation processes. The proposed presentation will review the current status of aerothermal modeling for Ice Giant entries and propose a path forward to reduce key uncertainties and enable optimized thermal protection system designs.</p>

Synergistic Probe-Orbiter Science and Measurements for Understanding the Formation and Evolution of the Icy Giant Planets

<p>The Galileo Probe was designed to measure the abundances of the heavy elements (mass >helium) and helium in Jupiter since they are key to understanding the planet’s formation and heat balance. Broadly speaking, the same formation scenarios presumably apply also to the Icy Giant Planets (IGP), Uranus and Neptune, so the determination of their heavy elements and He is equally important. We will show that the bulk of C, N, S, and O are sequestered in condensible volatiles whose well-mixed regions in the atmospheres of the IGP’s are extremely deep compared to Jupiter. That poses formidable challenges to their direct in situ measurements. On the other hand, being non-condensible and chemically inert, the noble gases − He, Ne, Ar, Kr and Xe – are expected to be uniformly mixed all over the planet, unlike the condensibles whose distribution is governed by dynamics, convection and purported deep oceans. Thus the noble gases would provide the most critical set of data for constraining the IGP formation models. Although the noble gases should be well-mixed everywhere below the homopause, measurements at and below the 1-bar level are needed considering their low mixing ratios, except for He. That depth also gets around any potential cold trapping of the heavy noble gases at the tropopause or adsorption on methane ice aerosols. Entry probes deployed to relatively shallow pressure levels of 5-10 bars would allow a robust determination of the abundances and isotopic ratios of the noble gases, amongst other things. A measurement of CO from orbit, along with other disequilibrium species has the potential of estimating the O/H ratio. Microwave radiometry from orbiter and the Earth have the potential of measuring the depth profiles of NH<sub>3</sub> and H<sub>2</sub>O, which would be important for understanding the atmospheric dynamics and weather in the deep atmosphere. Combined with the above data and the data on the interior and the magnetic field, the probe results on the noble gases would provide essential constraints to the formation, migration and evolution models of the Icy Giant Planets. </p>

Moist Adiabats with Multiple Condensing Species: A New Theory with Application to Giant-Planet Atmospheres

A new formula is derived for calculating the moist adiabatic temperature profile of an atmosphere consisting of ideal gases with multiple condensing species. This expression unifies various formulas published in the literature and can be generalized to account for chemical reactions. Unlike previous methods, it converges to machine precision independent of mesh size. It accounts for any ratio of condensable vapors to dry gas, from zero to infinity, and for variable heat capacities as a function of temperature. Because the derivation is generic, the new formula is not only applicable to planetary atmospheres in the solar system but also to hot Jupiters and brown dwarfs in which a variety of alkali metals, silicates, and exotic materials condense. It is demonstrated that even though the vapors are ideal gases, they interact in their effects on the moist adiabatic lapse rate. Finally, the authors apply the new thermodynamic model to study the effects of downdrafts on the distribution of minor constituents and the thermal profile in the Galileo probe hot spot. The authors find that the Galileo probe measurements can be interpreted as a strong downdraft that displaces an air parcel from the 1-bar to the 4-bar level (1 bar = 100 000 Pa).

Chemistry of the Solar System Revealed in the Interiors of the Giant Planets

The giant planets of our solar system contain a record of elemental and isotopic ratios of keen interest for what they tell us about the origin of the planets and in particular the volatile compositions of the solid phases. In situ measurements of the Jovian atmosphere performed by the Galileo Probe during its descent in 1995 demonstrate the unique value of such a record, but limited currently by the unknown abundance of oxygen in the interior of Jupiter–a gap planned to be filled by the Juno mission set to arrive at Jupiter in July of 2016. Our lack of knowledge of the oxygen abundance allows for a number of models for the Jovian interior with a range of C/O ratios. The implications for the origin of terrestrial water are briefly discussed. The complementary data sets for Saturn may be obtained by a series of very close, nearly polar orbits, at the end of the Cassini-Huygens mission in 2016-2017, and the proposed Saturn Probe. This set can only obtain what we have for Jupiter if the Saturn Probe mission carries a microwave radiometer.