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NASA NTRSDate not supplied

Parametric Analysis of Entry Vehicles for Giant Planet Missions

Summarize concept designs for entry vehicles for giant planet destinations identified as high priority in the latest decadal survey. Identify viable entry states, aerothermal environments, and heatshield thermal protection systems (TPS) for various vehicle dimensions. Viable TPS options and sizing focus on low-g (< 50g) entry conditions to avoid costly instrumentation requalification. TPS manufacturing capabilities are also taken into consideration while adhering to the manufacturing capabilities of the TPS options. Margined mass estimates for heatshield TPS options are provided for Uranus and Saturn test cases.

Giant Planet↗

NASA NTRS1994

NH(sub 3), H(sub 2)S, and the Radio Brightness Temperature Spectra of the Giant Planets

Recent radio interferometer observations of Neptune enable comparisons of the radio brightness temperature (T(sub b)) spectra of all four giant planets. The data shown are collected from many observers. Spectra of Jupiter, Saturn, and Neptune share certain gross characteristics. Ammonia is known to be an important tropospheric constituent at Jupiter and Saturn. Its signature on the Jovian spectrum is obvious, while at Saturn it is a bit more subdued. Uranus' T(sub b) spectrum does not fit the same pattern. One possible cause for the apparent ammonia depletion has been offered. That is that a superabundance of H(sub 2)S could react out most of the NH(sub 3). The presence of the H(sub 2)S was postulated as there is no direct observational evidence of H(sub 2)S, which has not yet been detected at any of the giant planets. It is merely the most likely candidate to deplete NH(sub 3).Some researchers suggest a significant H(sub 2)S superabundance at Neptune also. Some researchers suggest that this H(sub 2)S may be the major source of cm opacity in Neptune's upper troposphere, rather than the NH(sub 3) that had been suggested earlier.

hydrogen sulfide spectra opacity troposphere giant↗

NASA NTRS1986

Planetesimal dissolution in the envelopes of the forming, giant planets

An evaluation is made of the capacity of planetesimals to penetrate the envelopes of giant planets during their growth phase, by means of a core instability mechanism in which the growing core becomes gradually more adept in the gravitational concentration of gas from its solar nebula environment, until a runaway gas accretion occurs. If most of the accreted mass is contained in planetesimals larger that about 1 km, the critical core mass for runaway accretion will not significantly change when planetesimal dissolution is taken into account; it is accordingly suggested that giant planet envelopes should contain above-solar proportions of virtually all elements, relative to hydrogen.

Pollack, J. B.↗

NASA NTRS2016

Recent Variability Observations of Solar System Giant Planets: Fresh Context for Understanding Exoplanet and Brown Dwarf Weather

Over the past several years a number of high cadence photometric observations of solar system giant planets have been acquired by various platforms. Such observations are of interest as they provide points of comparison to the already expansive set of brown dwarf variability observations and the small, but growing, set of exoplanet variability observations. By measuring how rapidly the integrated light from solar system giant planets can evolve, variability observations of substellar objects that are unlikely to ever be resolved can be placed in a fuller context. Examples of brown dwarf variability observations include extensive work from the ground (e.g., Radigen et al. 2014), Spitzer (e.g., Metchev et al. 2015), Kepler (Gizis et al. 2015), and HST (Yang et al. 2015).Variability has been measured on the planetary mass companion to the brown dwarf 2MASS 1207b (Zhou et al. 2016) and further searches are planned in thermal emission for the known directly imaged planets with ground based telescopes (Apai et al. 2016) and in reflected light with future space based telescopes. Recent solar system variability observations include Kepler monitoring of Neptune (Simon et al. 2016) and Uranus, Spitzer observations of Neptune (Stauffer et al. 2016), and Cassini observations of Jupiter (West et al. in prep). The Cassini observations are of particular interest as they measured the variability of Jupiter at a phase angle of approximately 60 deg, comparable to the viewing geometry expected for space based direct imaging of cool extrasolar Jupiters in reflected light. These solar system analog observations capture many of the characteristics seen in brown dwarf variability, including large amplitudes and rapid light curve evolution on timescales as short as a few rotation periods. Simon et al. (2016) attribute such variations at Neptune to a combination of large scale, stable cloud structures along with smaller, more rapidly varying, cloud patches. The observed brown dwarf and exoplanet variability may well arise from comparable cloud structures. In my presentation I will compare and contrast the nature of the variability observed for the various solar system and other substelar objects and present a wish list for future observations.

giant planets↗

NASA NTRS2016

Giant Planets in Reflected Light: What Science Can We Expect?

Interpreting the reflection spectra of cool giant planets will be a challenge. Spectra of such worlds are expected to be primarily shaped by scattering from clouds and hazes and punctuated by absorption bands of methane, water, and ammonia. While the warmest giants may be cloudless, their atmospheres will almost certainly sport substantial photochemical hazes. Furthermore the masses of most direct imaging targets will be constrained by radial velocity observations, their radii, and thus atmospheric gravity, will be imperfectly known. The uncertainty in planet radius and gravity will compound with uncertain aerosol properties to make estimation of key absorber abundances difficult. To address such concerns our group is developing atmospheric retrieval tools to constrain quantities of interest, particular gas mixing ratios. We have applied our Markov Chain Monte Carlo methods to simulated data of the quality expected from the WFIRST CGI instrument and found that given sufficiently high SNR data we can confidentially identify and constrain the abundance of methane, cloud top pressures, gravity, and the star-planet-observer phase angle. In my presentation I will explain the expected characteristics of cool extrasolar giant planet reflection spectra, discuss these and other challenges in their interpretation, and summarize the science results we can expect from direct imaging observations.

methane↗

NASA NTRS2022

Giant Planet Atmospheres: Dynamics and Variability from UV to Near-IR Hubble and Adaptive Optics Imaging

Abstract: Each of the giant planets, Jupiter, Saturn, Uranus, and Neptune, has been observed by at least one robotic spacecraft mission. However, these missions are infrequent; Uranus and Neptune have only had a single flyby by Voyager 2. The Hubble Space Telescope, particularly the Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) instruments, and large ground-based telescopes with adaptive optics systems have enabled high spatial resolution imaging at a higher cadence, and over a longer time, than can be achieved with targeted missions to these worlds. These facilities offer a powerful combination of high spatial resolution, often <0.05”, and broad wavelength coverage, from the ultraviolet through the near infrared, resulting in compelling studies of the clouds, winds, and atmospheric vertical structure. This coverage allows comparisons of atmospheric properties between the planets, as well as in different regions across each planet. Temporal variations in winds, cloud structure, and color over time scales of days to years, have been measured for all four planets. With several decades of data already obtained, we can now begin to investigate seasonal influences on dynamics and aerosol properties, despite orbital periods ranging from 12 to 165 years. Future facilities will enable even greater spatial resolution and, combined with our existing long record of data, will continue to advance our understanding of atmospheric evolution on the giant planets.

Amy A Simon↗

NASA NTRS2018

Do Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?

The NASA Kepler and K2 Missions have recently revealed a population of transiting giant planets orbiting moderately evolved, low-luminosity red giant branch stars. Here, we present radial velocity (RV) measurements of three of these systems, revealing signiﬁcantly non-zero orbital eccentricities in each case. Comparing these systems with the known planet population suggests that close-in giant planets around evolved stars tend to have more eccentric orbits than those around main sequence stars. We interpret this as tentative evidence that the orbits of these planets pass through a transient, moderately eccentric phase where they shrink faster than they circularize due to tides raised on evolved host stars. Additional RV measurements of currently known systems, along with new systems discovered by the recently launched NASA Transiting Exoplanet Survey Satellite (TESS) mission, may constrain the timescale and mass dependence of this process.

Thomas Barclay↗

NASA NTRS2004

Predicting the Atmospheric Composition of Extrasolar Giant Planets

To date, approximately 120 planet-sized objects have been discovered around other stars, mostly through the radial-velocity technique. This technique can provide information about a planet s minimum mass and its orbital period and distance; however, few other planetary data can be obtained at this point in time unless we are fortunate enough to find an extrasolar giant planet that transits its parent star (i.e., the orbit is edge-on as seen from Earth). In that situation, many physical properties of the planet and its parent star can be determined, including some compositional information. Our prospects of directly obtaining spectra from extrasolar planets may improve in the near future, through missions like NASA's Terrestrial Planet Finder. Most of the extrasolar giant planets (EGPs) discovered so far have masses equal to or greater than Jupiter's mass, and roughly 16% have orbital radii less than 0.1 AU - extremely close to the parent star by our own Solar-System standards (note that Mercury is located at a mean distance of 0.39 AU and Jupiter at 5.2 AU from the Sun). Although all EGPs are expected to have hydrogen-dominated atmospheres similar to Jupiter, the orbital distance can strongly affect the planet's temperature, physical, chemical, and spectral properties, and the abundance of minor, detectable atmospheric constituents. Thermochemical equilibrium models can provide good zero-order predictions for the atmospheric composition of EGPs. However, both the composition and spectral properties will depend in large part on disequilibrium processes like photochemistry, chemical kinetics, atmospheric transport, and haze formation. We have developed a photochemical kinetics, radiative transfer, and 1-D vertical transport model to study the atmospheric composition of EGPs. The chemical reaction list contains H-, C-, O-, and N-bearing species and is designed to be valid for atmospheric temperatures ranging from 100-3000 K and pressures up to 50 bar. Here we examine the effect of stellar distance (e.g., incident ultraviolet flux, atmospheric temperature) on the chemical properties of EGPs. The model is applied to two generic Class II and III intermediate temperature EGPs located at 3.3 and 0.27 AU from a solar-like parent star, and the results are compared with a model for Jupiter at 5.2 AU.

Sharp, A. G.↗

NASA NTRS1974

Physical properties of giant planets

The chemical identification and physical nature of giant planets are discussed. The phase equilibria of H2-He mixture is briefly described for these large planets.

Wildt, R.↗

NASA NTRS1982

Origin and evolution of the giant planets

A discussion is presented of two major giant planet origin hypotheses: (1) protoplanet formation in the solar nebula in the form of a gravitationally unstable, gaseous subcondensation, subsequently evolving as a chemically homogeneous object until a stage at which a solid core may form; and (2) solid core formation by accumulation of planetesimals, followed by the accretion of solar-composition gas onto the core until it becomes unstable to collapse. Under either of the scenarios, evolution is found to comprise an early, cool phase in hydrostatic equilibrium, a hydrodynamic collapse, and a final phase of hydrostatic contraction and cooling to the present state. Attention is given to the physical processes that are most important in the determination of evolutionary characteristics. A concluding note on the cases of Uranus and Neptune is also given.

Bodenheimer, P.↗

NASA NTRSDate not supplied

Parametric Analysis of Entry Vehicles for Giant Planet Missions

The Planetary Science Decadal Survey has identified Uranus and Saturn as high priori-ty destinations for a flagship and New Frontier missions respectively in the decade 2023-2032. The pro-posed presentation will focus on the entry and descent aspects of the entry vehicles design, considered as part of Giant Planet probe mission concepts, and associated trades for viable trajectory options. Giant Planet Entry Vehicle Parametric Study: Launch vehicle capabilities are evolving and provide an opportunity to increase instrumented probe dimensions. To assess the impact of larger aeroshell designs, a parametric study was conducted to understand the impact on aerothermal environments, TPS options, and TPS mass over a range of 1.0m to 2.0m aeroshell diameters for Uranus and Saturn probe concept missions. The 45° sphere-cone geometry is a legacy configuration that has demonstrated static stability and been used successfully in missions to Venus (Pioneer-Venus) and Jupiter (Galileo). A nose radius of 0.4 m was considered primarily to reduce the heat flux at the stagnation point compared to the smaller radii used in the Venus and Jupiter missions. Representative inertial velocities are chosen from a prior NASA Ames study. Viable entry trajectories to meet concept mission and science objectives were developed using the tool POST2. The newly developed thermal protection material called HEEET (Heatshield for Extreme Entry Environment Technology) was considered in the study. This material, which is at a technology readiness level (TRL) of 6, is highly customizable and available in two varieties: (i) a dual-layer version consisting of recession layer on top of an insulative layer, and (ii) a single-layer version consisting of the insulative layer alone, termed 3- dimensional Mid-Density Carbon Phenolic (3MDCP). Both options were considered for the forward heatshield (the sphere-cone part) in the pre-sent study.

Thermal Protection System↗

NASA NTRS2016

On the Composition of Young, Directly Imaged Giant Planets

The past decade has seen significant progress on the direct detection and characterization of young, self-luminous giant planets at wide orbital separations from their host stars. Some of these planets show evidence for disequilibrium processes like transport-induced quenching in their atmospheres; photochemistry may also be important, despite the typically large orbital distances. Disequilibrium chemical processes such as these can alter the expected composition, spectral behavior, thermal structure, and cooling history of the planets, and can potentially confuse determinations of bulk elemental ratios, which provide important insights into planet-formation mechanisms. Using a thermo/photochemical kinetics and transport model, we investigate the extent to which disequilibrium chemical processes affect the composition and spectra of directly imaged giant exoplanets. Results for specific "young Jupiters" such as HR 8799 b and c and 51 Eri b are presented, as are general trends as a function of planetary effective temperature, surface gravity, incident ultraviolet flux, and strength of deep atmospheric convection. We find that quenching is very important on young Jupiters, leading to CO/CH4 and N2/NH3 ratios much greater than; and H2O mixing ratios a factor of a few less than chemical equilibrium predictions. Photochemistry can also be important on such planets, with CO2 and HCN being key photochemical products. Carbon dioxide becomes a particularly major constituent when stratospheric temperatures are low and recycling of water following H2O photolysis becomes stifled. Young Jupiters with effective temperatures less than 700 degrees Kelvin are in a particularly interesting photochemical regime that differs from both transiting hot Jupiters and our own solar-system giant planets.

photochemistry↗

NASA NTRS2015

Giant-Planet Chemistry: Ammonium Hydrosulfide (NH4SH), Its IR Spectra and Thermal and Radiolytic Stabilities

Here we present our recent studies of proton-irradiated and unirradiated ammonium hydrosulfide, NH4SH, a compound predicted to be an important tropospheric cloud component of Jupiter and other giant planets. We irradiated both crystalline and amorphous NH4SH at 10-160 K and used IR spectroscopy to observe and identify reaction products in the ice, specifically NH3 and long-chained sulfur-containing ions. Crystalline NH4SH was amorphized during irradiation at all temperatures studied with the rate being the fastest at the lowest temperatures. Irradiation of amorphous NH4SH at approximately 10-75 K showed that 60-80% of the NH4 + remained when equilibrium was reached, and that NH4SH destruction rates were relatively constant within this temperature range. Irradiations at higher temperatures produced different dose dependence and were accompanied by pressure outbursts that, in some cases, fractured the ice. The thermal stability of irradiated NH4SH was found to be greater than that of unirradiated NH4SH, suggesting that an irradiated giant-planet cloud precipitate can exist at temperatures and altitudes not previously considered.

Jupiter↗

NASA NTRS1986

Calculations of the accretion and evolution of giant planets The effects of solid cores

The present calculation of giant planet evolution proceeds under the hypothesis that the solid cores formed by small particle accretion later attracted their gaseous envelopes from the solar nebula gravitationally. Evolutionary calculations are presented for the beginning of gaseous envelope formation around the core mass; the growth of core and envelope to a critical core mass; the derivation of high luminosity from the envelope mass due to gravitational contraction, and the termination of both core and envelope accretion on a time-scale of 5 Gyr.

Bodenheimer, P.↗

NASA NTRS1988

Cloud microphysics of the giant planets

The characteristic time constants for the atmospheres of the giant planets were compared for the purpose of finding analogs for clouds in the atmospheres of these planets among the several types of cloud systems found on earth. The NH3 clouds on Jupiter and Saturn were found to be weakly precipitating systems, similar to the thicker cirrus clouds on earth, while the H2O clouds were found to be much more massive than the water clouds on earth. The CH4 and H2O clouds on Uranus and Neptune are completely unlike any cloud on earth, being 10 to 100 times more massive than the densest water clouds on earth. It is concluded that, while the microphysical characteristics of the clouds on Jupiter and Saturn are comparable to those of terrestrial stratus and cumulus clouds, the massive clouds on Uranus and Neptune reach conditions unlike anything on earth.

Carlson, Barbara E.↗

NASA NTRS2019

A High Binary Fraction for the Most Massive Close-in Giant Planets and Brown Dwarf Desert Members

Stellar multiplicity is believed to influence planetary formation and evolution, although the precise nature and extent of this role remain ambiguous. We present a study aimed at testing the role of stellar multiplicity in the formation and/or evolution of the most massive, close-in planetary and substellar companions. Using past and new direct imaging observations, as well as the Gaia DR2 catalogue, we searched for wide binary companions to 38 stars hosting massive giant planets or brown dwarfs (M > 7 MJup) on orbits shorter than ∼1 au. We report the discovery of a new component in the WASP-14 system, and present an independent confirmation of a comoving companion to WASP-18. From a robust Bayesian statistical analysis, we derived a binary fraction of per cent between 20 and 10 000 au for our sample, twice as high as for field stars with a 3σ significance. This binary frequency was found to be larger than for lower-mass planets on similar orbits, and we observed a marginally higher binary rate for inner companions with periods shorter than 10 d. These results demonstrate that stellar companions greatly influence the formation and/or evolution of these systems, suggesting that the role played by binary companions becomes more important for higher-mass planets, and that this trend may be enhanced for systems with tighter orbits. Our analysis also revealed a peak in binary separation at 250 au, highlighting a shortfall of close binaries among our sample. This indicates that the mechanisms affecting planet and brown dwarf formation or evolution in binaries must operate from wide separations, although we found that the Kozai–Lidov mechanism is unlikely to be the dominant underlying process. We conclude that binarity plays a crucial role in the existence of very massive short-period giant planets and brown dwarf desert inhabitants, which are almost exclusively observed in multiple systems.

C Fontanive↗

NASA NTRS1987

Hydromagnetic constraints on deep zonal flow in the giant planets

A simple model of the equatorial zonal jet in the giant planets is studied in which the flow is assumed uniform on cylinders concentric with the spin axis, and viscous and magnetic torques on each cylinder are balanced. This 'Taylor constraint' is solved simultaneously with the dynamo equation to obtain the velocity and magnetic field in the equatorial plane. The model is used to reproduce the widely differing jet widths of Jupiter and Saturn eddy viscosity of 2500 sq cm/s. For Saturn, the model has a large magnetic Reynolds number where the Chandrasekhar number Q = 1, and hence exhibits substantial axisymmetrization of the field in the equatorial plane.

Kirk, R. L.↗

NASA NTRS1990

CH4 nonlocal thermodynamic equilibrium in the atmospheres of the giant planets

The present consideration of methane's vibrational relaxation in the Giant Planets' upper stratospheres employs a model for the thermalization of solar energy absorbed under non-LTE conditions, at the 1.7, 2.3. and 3.3 micron-centered CH4 band groups. On this basis, a range of model atmospheres was produced, reflecting current uncertainties concerning CH4 collisional excitation rates. At 0.1 mbar, all of the non-LTE models are within 2 K of the LTE reference models; the extreme non-LTE models steadily depart from reference LTE ones, however, and reach about + or - 20 K at 0,1 mbar.

Appleby, John F.↗