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NASA NTRS1987

The rings of Uranus

The rings of Uranus are not all circular; nor do they all lie in the plane of Uranus' equator. The epsilon ring of Uranus, which is the largest and outermost one, ranges in width from 20 to 96 km in proportion to the distance of the ring material from Uranus. Typical rings can be modeled by assuming that its edges are defined by a combination of smooth elliptical orbits and random 'wiggles'. A transition from a few large moons in outer regions to many small moons just outside ring-filled inner regions is a feature noted to be held in common by the ring and moon systems of Saturn, Uranus, and Neptune.

Cuzzi, Jeffrey N.↗
NASA NTRS1987

Plasma wave measurements in the magnetosphere of Uranus

As Voyager 2 traversed the magnetosphere of Uranus, the plasma-wave instrument detected very significant phenomena related to local wave-particle interactions, radio emissions, and dust impacts. Here the region between the inbound and outbound bow shock traversals (covering a distance of more than 250 Uranus radii) is considered, and it is demonstrated that intense plasma-wave activity developed only in the inner magnetosphere (r less than 12 Uranus radii); this result is similar to that found at Saturn, but it is in marked contrast with the Jupiter case, where very strong wave activity was detected out to distances of 250 Jupiter radii. The Uranus plasma-wave observations in the inner magnetosphere are compared with corresponding results from the Jupiter and Saturn encounters, and it is shown that the Uranus wave measurements are unique in several significant ways. These new aspects include: (1) the detection of a marked inbound-outbound asymmetry and (2) the detection of whistler-mode waves that yield the strongest wave-particle interactions found in outer planet magnetospheres.

Scarf, F. L.↗
NASA NTRS1991

Uranus

The present publication discusses Uranus and its interior, atmosphere, rings, satellites, and magnetosphere. Attention is given to photochemistry and vertical mixing, spectroscopy and chemistry of the Uranian atmosphere, the thermal structure and energy balance of Uranian atmospheric dynamics and circulations, and clouds and aerosols in the Uranian atmosphere. Topics considered include particle properties and processes in Uranus' rings, the origin and properties of the Uranian satellites, the geology and cratering of the Uranian satellites, and the planet's magnetic field and magnetospheric configuration. Also addressed are wave-particle interactions in the Uranian magnetosphere, the magnetic field and magnetospheric configuration of Uranus, the plasma environment of Uranus, and Uranus as a radio source.

Bergstralh, Jay T.↗
NASA NTRS2018

Oceanus: A High Science Return Uranus Orbiter with a Low-Cost Instrument Suite

Ice-giant-sized planets are the most common type of observed exoplanet, yet the two ice giants in our own solar system (Uranus and Neptune) are the least explored class of planet, having only been observed through ground-based observations and a single flyby each by Voyager 2 approximately 30 years ago. These single flybys were unable to characterize the spatial and temporal variability in ice giant magnetospheres, some of the most odd and intriguing magnetospheres in the solar system. They also offered only limited constraints on the internal structure of ice giants; understanding the internal structure of a planet is important for understanding its formation and evolution. The most recent planetary science Decadal Survey by the U.S. National Academy of Sciences, ''Vision and Voyages for Planetary Science in the Decade 2013-2022,'' identified the ice giant Uranus as the third highest priority for a Flagship mission in the decade 2013-2022. However, in the event that NASA or another space agency is unable to fly a Flagship-class mission to an ice giant in the next decade, this paper presents a mission concept for a focused, lower cost Uranus orbiter called OCEANUS (Origins and Composition of the Exoplanet Analog Uranus System). OCEANUS would increase our understanding of the interior structure of Uranus, its magnetosphere, and how its magnetic field is generated. These goals could be achieved with just a magnetometer and the spacecraft's radio system. This study shows that several of the objectives outlined by the Decadal Survey, including one of the two identified as highest priority, are within reach for a New-Frontiers-class mission.

Elder, C. M.↗
NASA NTRSDate not supplied

Addressing the Energy Crisis At Uranus via Magnetic Reconnection

The long-standing 'energy crisis' at Uranus refers to the seemingly anomalous heating of the planet's thermosphere (requires ~200GW) compared to the available input energy from the Sun (~20GW). At Uranus this crisis is further exacerbated by a decades-long thermospheric cooling of H3+ observed from 1980 to present. Input energy from the magnetosphere has been invoked as a possible solution to the energy crisis, though the total available energy and how it may vary with time have not been quantified. We apply a recent scaling of the energy conversion rate of magnetic reconnection developed from recent data from NASA's Magnetospheric Multiscale (MMS) mission to the Uranus system. We find that the energy input is most modulated by solar activity, producing >100GW in solar max and approximately half that during solar minimum. During solar minimum, the remaining energy input required to the Uranus system may be dominated by the Kelvin-Helmholtz instability that is more active for time periods with weaker interplanetary magnetic fields. The declining intensity of solar maxima over the past several decades could account for the observed thermospheric cooling and the ascending phase of the current solar cycle should lead to increases in observed thermospheric H3+ temperatures. Neither process of energy input appears to be significantly impacted by Uranus season or magnetospheric

Daniel Gershman↗
NASA NTRSDate not supplied

Connecting Uranus’ Magnetosphere and Upper Atmosphere via Electron Precipitation

Abstract. The precipitation of electrons is a key process through which significant energy is transferred from Uranus’ magnetosphere to its upper atmosphere. These electrons drive atmospheric ionization, thermospheric heating, and auroral emission at Uranus, and their properties are critical to address Uranus’ energy crisis as well as auroral phenomena. We combine measurements of auroral precipitation at Earth, Saturn, and Jupiter, with relevant Voyager 2 observations to estimate the properties of precipitating electrons at Uranus. In order to produce the measured aurora, energy fluxes of ~0.1mW/m 2 are required with energies of ~5keV.The acceleration of particles between the magnetosphere and thermosphere is predicted to be Earth-like, but with significantly lower overall energy flux and field aligned currents due to Uranus’ sparser magnetosphere. The height-integrated Joule heating rates of ~0.2 mW/m 2 are an order of magnitude lower than Earth

Daniel Gershman↗
NASA NTRS2017

HST WFC3 Observations of Uranus' 2014 Storm Clouds and Comparison with VLT/SINFONI and IRTF/Spex Observations

In November 2014 Uranus was observed with the Wide Field Camera 3 (WFC3) instrument of the Hubble Space Telescope as part of the Hubble 2020: Outer Planet Atmospheres Legacy program, OPAL. OPAL annually maps Jupiter, Uranus and Neptune (and will also map Saturn from 2018) in several visible near- infrared wavelength filters. The Uranus 2014 OPAL observations were made on the 89th November at a time when a huge cloud complex, first observed by de Pater et al. (2015) and subsequently tracked by professional and amateur astronomers (Sayanagi et al., 2016), was present at 30-40deg N. We imaged the entire visible atmosphere, including the storm system, in seven filters spanning 467924 nm, capturing variations in the coloration of Uranus clouds and also vertical distribution due to wavelength dependent changes in Rayleigh scattering and methane absorption optical depth. Here we analyse these new HST observations with the NEMESIS radiative-transfer and retrieval code in multiple-scattering mode to determine the vertical cloud structure in and around the storm cloud system. The same storm system was also observed in the H-band (1.4-1.8 micrometers) with the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) on 31st October and 11th November, reported by Irwin et al. (2016, 10.1016j.icarus.2015.09.010). To constrain better the cloud particle sizes and scattering properties over a wide wavelength range we also conducted a limb-darkening analysis of the background cloud structure in the 30-40deg N latitude band by simultaneously fitting: a) these HSTOPAL observations at a range of zenith angles; b) the VLTSINFONI observations at a range of zenith angles; and c) IRTFSpeX observations of this latitude band made in 2009 at a single zenith angle of 23deg, spanning the wavelength range 0.8-1.8 micrometers (Irwin et al., 2015, 10.1016j.icarus.2014.12.020). We find that the HST observations, and the combined HSTVLTIRTF observations at all locations are well modelled with a three-component cloud comprised of: 1) a vertically thin, but optically thick deep tropospheric cloud at a pressure of approximately 2 bars; 2) a methane-ice cloud based at the methane-condensation level of approximately 1.23 bar, with variable vertical extent; and 3) a vertically extended tropospheric haze, also based at the methane-condensation level of 1.23 bar. We find that modelling both haze and tropospheric cloud with particles having an effective radius of approximately 0.1 micron provides a good fit the observations, although for the tropospheric cloud, particles with an effective radius as large as 1.0 micron provide a similarly good fit. We find that the particles in both the tropospheric cloud and haze are more scattering at short wave- lengths, giving them a blue color, but are more absorbing at longer wavelengths, especially for the tropospheric haze. We find that the spectra of the storm clouds are well modelled by localized thickening and vertical extension of the methane-ice cloud. For the particles in the storm clouds, which we assume to be composed of methane ice particles, we find that their mean radii must lie somewhere in the range 0. 1 1. 0 m. We find that the high clouds have low integrated opacity, and that streamers reminiscent of convective thunderstorm anvils are confined to levels deeper than 1 bar. These results argue against vigorous moist convective origins for the cloud features.

atmospheres↗
NASA NTRS2024

Multiple Probe Measurements at Uranus Motivated By Spatial Variability

Predictably, a major motivation for multiple atmospheric probe measurements at Uranus is the understanding of dynamic processes that create and maintain spatial variation in thermal structure, composition, and horizontal winds. But origin questions---regarding the planet's formation and evolution, and conditions in the protoplanetary disk---are also major science drivers for multiprobe exploration. Spatial variation in thermal structure reveals how the atmosphere transports heat from the interior, and measuring compositional variability in the atmosphere is key to ultimately gaining an understanding of the bulk abundances of several heavy elements. We review the current knowledge of spatial variability in Uranus' atmosphere, and we outline how multiple probe exploration would advance our understanding of this variability. The other giant planets are discussed, both to connect multiprobe exploration of those atmospheres to open questions at Uranus, and to demonstrate how multiprobe exploration of Uranus itself is motivated lessons learned about the spatial variation at Jupiter, Saturn, and Neptune. We outline the measurements of highest value from miniature secondary probes (which would complement more detailed investigation by a larger flagship probe), and present the path toward overcoming current challenges and uncertainties in areas including mission design, cost, trajectory, instrument maturity, power, and timeline.

Uranus↗
DOE OSTI2025

The possibility of hydrogen-water demixing in Uranus, Neptune, K2-18 b and TOI-270 d

Context. The internal structures of Uranus and Neptune remain unknown. In addition, sub-Neptunes are now thought to be the most common type of exoplanets. Improving our understanding of the physical processes that govern the interiors of such planets is therefore essential. Phase separation between planetary constituents may occur, in particular, hydrogen-water immiscibility in cold, water-rich intermediate-mass planets. Aims. We assess whether hydrogen-water demixing could occur in Uranus, Neptune, K2-18 b and TOI-270 d, and investigate its effect on the planetary evolution and inferred internal structure. Methods. We couple planetary evolution models with recent ab initio calculations of the hydrogen-water phase diagram, allowing for temperature shifts to account for uncertainties in miscibility gaps. Results. We find that demixing may occur and could lead to a complete depletion of water in the outermost regions of Uranus and Neptune. Temperature offsets of up to 1100 K lead to a depleted region comprising as much as 16% of the planet’s mass, and an increase in planetary radius by nearly 20%. For K2-18 b, our models suggest that hydrogen-water demixing is ongoing and may explain the absence of water features in its JWST spectrum. A temperature offset of 500 K is required to get a complete depletion of water in the atmosphere of K2-18b. TOI-270d may also have experienced hydrogen-water demixing. When applying a similar temperature offset on the phase diagram as for K2-18 b, we find a partial depletion of water in the atmosphere of TOI-270 d, consistent with JWST’s detection of water. Conclusions. Hydrogen-water immiscibility may play a key role in shaping the structure and evolution of both Solar System giant planets like Uranus and Neptune, and cold/temperate exoplanets such as K2-18 b and TOI-270 d. Accounting for such internal processes is crucial to accurately interpret atmospheric observations from current (e.g., JWST) and upcoming (e.g., ARIEL) missions.

planets and satellites: composition↗
DOE OSTI2024

Uranus Study Report: KISS

Determining the internal structure of Uranus is a key objective for planetary science. Knowledge of Uranus’s bulk composition and the distribution of elements is crucial to understanding its origin and evolutionary path. In addition, Uranus represents a poorly understood class of “intermediate-mass planets" (intermediate in size between the relatively well studied terrestrial and gas giant planets), which appear to be very common in the Galaxy. As a result, a better characterization of Uranus will also help us to better understand exoplanets in this mass and size regime.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Uranus and Neptune orbiter missions via solar electric propulsion

The characteristics and capabilities of solar electric propulsion for performing orbiter missions at the planets Uranus and Neptune are described. An assessment of the scientific objectives and instrumentation requirements, their relation to orbit size selection, and parametric analysis of solar electric propulsion trajectory/payload performance are included. Utilizing the Titan 3D/Centaur launch vehicle, minimum flight times of about 3400 days to Uranus and 5300 days to Neptune are required to place the TOPS spacecraft into the nominal orbits. It has been shown that solar electric propulsion can be used effectively to accomplish elliptical orbiter missions at Uranus and Neptune. However, because of the very long flight time required, these mission profiles are not too attractive. Previous studies have shown that nuclear electric propulsion, if developed, would allow much faster trips; 5 years to Uranus and 8 years to Neptune.

Friedlander, A. L.↗
NASA NTRS1974

Near-infrared spectra of Jupiter, Saturn, and Uranus

Near infrared spectra of Jupiter, Saturn, and Uranus were measured at resolutions higher than previously available in the range from 6,000 to 10,750/cm. The resolution was 0.5/cm for Jupiter and Saturn, and 32/cm for Uranus. The spectra are presented both individually and as ratio spectra, in which the planetary spectra are divided by the solar spectrum. The Uranus spectrum is shown with Saturn, Jupiter, and Sun spectra reduced to the same resolution so that Uranus can be compared with the other outer planets. The high resolution Saturn, Jupiter, and Sun spectra are presented in parallel plots to simplify comparisons between them.

Potter, A. E.↗
NASA NTRS1975

The atmosphere of Uranus

Current knowledge of the atmosphere of Uranus is reviewed and specific objectives are suggested for satellite missions to Uranus. The anomalous composition of Uranus makes determinations of its atmospheric composition particularly valuable for testing theories of solar system evolution. The weakness of its atmospheric heating makes the determination of its atmospheric structure and dynamics particularly valuable for testing theories of atmospheric behavior. The large axial inclination of Uranus implies an anomalous latitudinal variation of temperature and dynamics different from that of the other planets.

Stone, P. H.↗
NASA NTRS1975

Why image Uranus

Ratio picture techniques which enhance color or polarization contrasts are proposed for Uranus imagery on the MJU79 mission. Currently available low-resolution pictures of Uranus are assessed, and specific objectives of the fly-by are described, including exploratory imaging, measurement of fundamental properties (rotation period, rotation axis orientation, diameter), and examination of atmospheric properties. An attempt is made to predict the aspect of Uranus from current knowledge about its atmosphere. It is proposed that the imaging system should emphasize photo-polarimetric observations between 5500 and 10,000 A if the planet alone is to be imaged, or it should be a high-resolution system based on the MJS one if the satellites will also be imaged. Performance of a possible high-resolution system at Uranus is assessed.

Belton, M. J. S.↗
NASA NTRS1975

Particle and field environment of Uranus

In 1985 the spin axis of Uranus points within 10 degrees of the sun and the planet's position is very near the solar apex direction. A Uranus mission with an encounter near 1985 might expect to measure the unusual particle and field configuration of a 'pole-on' magnetosphere and also properties of the interstellar medium. Estimates are given of the particle and field environment of Uranus based on extrapolation of solar-wind data from 1 AU and on scaling relations for an earth-type magnetosphere. Since the magnetic moment of Uranus is unknown, all magnetospheric parameters are derived as a function of the dipole strength. The onsets of special magnetospheric properties are identified as the dipole moment increases from small to large values. A fairly complete set of magnetospheric parameters is given for a specific dipole moment to illustrate the case of a large moment.

Siscoe, G. L.↗
NASA NTRS1978

Limb brightening on Uranus - The visible spectrum. II

A detailed analysis is presented of photoelectric area-scanning photometry of the Uranus disk obtained during the 1976 apparition. Coarse quantitative information on the true radial intensity distribution over the Uranus disk is derived in four selected wavebands. Lack of circular symmetry in the intensity distributions indicates the presence of disk structure, especially polar brightening, in each waveband. For the 6190-A CH4 band, the distribution corresponds to a uniform disk upon which a hint of polar brightening is superimposed. For the 6400-A region, moderate limb darkening is found. It is pointed out that the coarse analytical technique used in the analysis to explore the information content of the photometric scans of Uranus has several basic limitations. Direct deconvolution of the Uranus scans is required to thoroughly investigate the two-dimensional photometric structure of its disk.

Price, M. J.↗
NASA NTRS1978

Comparative thermal evolution of Uranus and Neptune

We extend a Jovian convective-cooling model to Uranus and Neptune. The model assumes that efficient interior convection prevails, so that escape of interior heat is governed by the radiative properties of the atmosphere. A comparison of the thermal evolution of Uranus and Neptune indicates that the large amount of solar radiation absorbed in Uranus' atmosphere tends to differentially suppress the escape of interior heat. The model is shown to be consistent with recent infrared observations of the thermal balance of Uranus and Neptune, and with the presumed age of these planets.

Hubbard, W. B.↗
NASA NTRS1980

Structure and evolution of Uranus and Neptune

Three-layer interior models of Uranus and Neptune with central rocky cores, mantles of water, methane, and ammonia (the 'ices'), and outer envelopes primarily composed of hydrogen and helium are presented. The models incorporate a new H2O equation of state based on experimental data which is considerably 'softer' than previous H2O equations of state. Corrections for interior temperatures approximately 5000 K are included in the models, and the thermal evolution of both planets is investigated using recent heat flow measurements. It is found that the evolutionary considerations are consistent with gravitational field data in supporting models with approximately solar abundances of 'ice' and 'rock'. Evolutionary considerations indicate that initial temperatures and luminosities for Uranus and Neptune were not substantially higher than the present value. Both planets apparently have relatively small approximately 1-2 earth masses) hydrogen-helium envelopes, with Neptune's envelope smaller than Uranus'. A monotonic trend is evident among the Jovian planets: all have central rock-ice cores of approximately 15 earth masses, but with hydrogen-helium envelopes which decrease in mass from Jupiter to Saturn to Uranus to Neptune.

Hubbard, W. B.↗