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Leigh N. Fletcher - One of the best experts on this subject based on the ideXlab platform.
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longitudinal variations in the stratosphere of Uranus from the spitzer infrared spectrometer
Icarus, 2021Co-Authors: Naomi Rowegurney, Glenn S. Orton, Leigh N. Fletcher, Julianne I. Moses, Michael T. Roman, Amy Mainzer, Imke De Pater, P G J IrwinAbstract:Abstract NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5–37 μm) disc-averaged spectra of Uranus very near to its equinox in December 2007. A mean spectrum was constructed from observations of multiple central meridian longitudes, spaced equally around the planet, which has provided the opportunity for the most comprehensive globally-averaged characterisation of Uranus' temperature and composition ever obtained (Orton et al., 2014a,b). In this work we analyse the disc-averaged spectra at four separate central meridian longitudes to reveal significant longitudinal variability in thermal emission occurring in Uranus' stratosphere during the 2007 equinox. We detect a variability of up to 15% at wavelengths sensitive to stratospheric methane, ethane and acetylene at the ~0.1-mbar level. The tropospheric hydrogen‑helium continuum and deuterated methane absorption exhibit a negligible variation (less than 2%), constraining the phenomenon to the stratosphere. Building on the forward-modelling analysis of the global average study, we present full optimal estimation inversions (using the NEMESIS retrieval algorithm, Irwin et al., 2008) of the Uranus-2007 spectra at each longitude to distinguish between thermal and compositional variability. We found that the variations can be explained by a temperature change of less than 3 K in the stratosphere. Near-infrared observations from Keck II NIRC2 in December 2007 (Sromovsky et al., 2009; de Pater et al., 2011), and mid-infrared observations from VLT/VISIR in 2009 (Roman et al., 2020), help to localise the potential sources to either large scale uplift or stratospheric wave phenomena.
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Atmospheric chemistry on Uranus and Neptune.
Philosophical transactions. Series A Mathematical physical and engineering sciences, 2020Co-Authors: Julianne I. Moses, Leigh N. Fletcher, Thibault Cavalié, Michael T. RomanAbstract:Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant Ice Giants are challenging, and each planet has only bee...
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Atmospheric chemistry on Uranus and Neptune
arXiv: Earth and Planetary Astrophysics, 2020Co-Authors: Julianne I. Moses, Leigh N. Fletcher, Thibault Cavalié, Michael T. RomanAbstract:Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant ``Ice Giants'' are challenging, and each planet has only been visited by a single spacecraft during brief flybys in the 1980s. Thermochemical equilibrium is expected to control the composition in the deeper, hotter regions of the atmosphere on both planets, but disequilibrium chemical processes such as transport-induced quenching and photochemistry alter the composition in the upper atmospheric regions that can be probed remotely. Surprising disparities in the abundance of disequilibrium chemical products between the two planets point to significant differences in atmospheric transport. The atmospheric composition of Uranus and Neptune can provide critical clues for unravelling details of planet formation and evolution, but only if it is fully understood how and why atmospheric constituents vary in a three-dimensional sense and how material coming in from outside the planet affects observed abundances. Future mission planning should take into account the key outstanding questions that remain unanswered about atmospheric chemistry on Uranus and Neptune, particularly those questions that pertain to planet formation and evolution, and those that address the complex, coupled atmospheric processes that operate on Ice Giants within our solar system and beyond.
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Ice Giant Systems: The Scientific Potential of Missions to the Uranus and Neptune Systems (ESA Voyage 2050 White Paper)
2019Co-Authors: Leigh N. Fletcher, Nicolas André, David Andrews, Michele Bannister, Emma Bunce, T. Cavalié, Sébastien Charnoz, Francesca Ferri, Jonathan Fortney, Davide GrassiAbstract:Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants are the last remaining class of planet in our system to have a dedicated orbital mission. This white paper describes how such a mission could explore their origins, ice-rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the very heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, Uranus as the oddball. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we propose that an Ice Giant System mission should become a strategic cornerstone spacecraft for ESA in the Voyage 2050 programme.
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ice giant systems the scientific potential of missions to Uranus and neptune esa voyage 2050 white paper
2019Co-Authors: Leigh N. Fletcher, Nicolas André, T. Cavalié, Sébastien Charnoz, Francesca Ferri, Jonathan J Fortney, D J Andrews, Michele T Bannister, E J Bunce, Davide GrassiAbstract:Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants are the last remaining class of planet in our system to have a dedicated orbital mission. This white paper describes how such a mission could explore their origins, ice-rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the very heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, Uranus as the oddity. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we propose that an Ice Giant System mission should become a strategic cornerstone spacecraft for ESA in the Voyage 2050 programme.
Ravit Helled - One of the best experts on this subject based on the ideXlab platform.
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the interiors of Uranus and neptune current understanding and open questions
arXiv: Earth and Planetary Astrophysics, 2020Co-Authors: Ravit Helled, Jonathan J FortneyAbstract:Uranus and Neptune form a distinct class of planets in our solar system. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more open questions regarding these planets than answers. In this review we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets' internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions.
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Uranus and Neptune: Origin, Evolution and Internal Structure
Space Science Reviews, 2020Co-Authors: Ravit Helled, Nadine Nettelmann, Tristan GuillotAbstract:There are still many open questions regarding the nature of Uranus and Neptune, the outermost planets in the Solar System. In this review we summarize the current-knowledge about Uranus and Neptune with a focus on their composition and internal structure, formation including potential subsequent giant impacts, and thermal evolution. We present key open questions and discuss the uncertainty in the internal structures of the planets due to the possibility of non-adiabatic and inhomogeneous interiors. We also provide the reasoning for improved observational constraints on their fundamental physical parameters such as their gravitational and magnetic fields, rotation rates, and deep atmospheric composition and temperature. Only this way will we be able to improve our understating of these planetary objects, and the many similar-sized objects orbiting other stars.
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Explaining the low luminosity of Uranus: a self-consistent thermal and structural evolution
'EDP Sciences', 2020Co-Authors: Allona Vazan, Ravit HelledAbstract:The low luminosity of Uranus is a long-standing challenge in planetary science. Simple adiabatic models are inconsistent with the measured luminosity, which indicates that Uranus is non-adiabatic because it has thermal boundary layers and/or conductive regions. A gradual composition distribution acts as a thermal boundary to suppress convection and slow down the internal cooling. Here we investigate whether composition gradients in the deep interior of Uranus can explain its low luminosity, the required composition gradient, and whether it is stable for convective mixing on a timescale of some billion years. We varied the primordial composition distribution and the initial energy budget of the planet, and chose the models that fit the currently measured properties (radius, luminosity, and moment of inertia) of Uranus. We present several alternative non-adiabatic internal structures that fit the Uranus measurements. We found that convective mixing is limited to the interior of Uranus, and a composition gradient is stable and sufficient to explain its current luminosity. As a result, the interior of Uranus might still be very hot, in spite of its low luminosity. The stable composition gradient also indicates that the current internal structure of Uranus is similar to its primordial structure. Moreover, we suggest that the initial energy content of Uranus cannot be greater than 20% of its formation (accretion) energy. We also find that an interior with a mixture of ice and rock, rather than separated ice and rock shells, is consistent with measurements, suggesting that Uranus might not be “differentiated”. Our models can explain the luminosity of Uranus, and they are also consistent with its metal-rich atmosphere and with the predictions for the location where its magnetic field is generated
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Effect of non-adiabatic thermal profiles on the inferred compositions of Uranus and Neptune
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: Morris Podolak, Ravit Helled, Gerald SchubertAbstract:It has been a common assumption of interior models that the outer planets of our Solar system are convective, and that the internal temperature distributions are therefore adiabatic. This assumption is also often applied to exoplanets. However, if a large portion of the thermal flux can be transferred by conduction, or if convection is inhibited, the thermal profile could be substantially different and would therefore affect the inferred planetary composition. Here we investigate how the assumption of non-adiabatic temperature profiles in Uranus and Neptune affects their internal structures and compositions. We use a set of plausible temperature profiles together with density profiles that match the measured gravitational fields to derive the planets’ compositions. We find that the inferred compositions of both Uranus and Neptune are quite sensitive to the assumed thermal profile in the outer layers, but relatively insensitive to the thermal profile in the central, high-pressure region. The overall value of the heavy element mass fraction, Z, for these planets is between 0.8 and 0.9. Finally, we suggest that large parts of Uranus’ interior might be conductive, a conclusion that is consistent with Uranus dynamo models and a hot central inner region.
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the formation of Uranus and neptune challenges and implications for intermediate mass exoplanets
The Astrophysical Journal, 2014Co-Authors: Ravit Helled, Peter BodenheimerAbstract:In this paper we investigate the formation of Uranus and Neptune, according to the core-nucleated accretion model, considering formation locations ranging from 12 to 30 AU from the Sun, and with various disk solid-surface densities and core accretion rates. It is shown that in order to form Uranus-like and Neptune-like planets in terms of final mass and solid-to-gas ratio, very specific conditions are required. We also show that when recently proposed high solid accretion rates are assumed, along with solid surface densities about 10 times those in the minimum-mass solar nebula, the challenge in forming Uranus and Neptune at large radial distances is no longer the formation timescale, but is rather finding agreement with the final mass and composition of these planets. In fact, these conditions are more likely to lead to gas-giant planets. Scattering of planetesimals by the forming planetary core is found to be an important effect at the larger distances. Our study emphasizes how (even slightly) different conditions in the protoplanetary disk and the birth environment of the planetary embryos can lead to the formation of very different planets in terms of final masses and compositions (solid-to-gas ratios), which naturally explains the large diversity of intermediate-mass exoplanets.
Gerald Schubert - One of the best experts on this subject based on the ideXlab platform.
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Effect of non-adiabatic thermal profiles on the inferred compositions of Uranus and Neptune
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: Morris Podolak, Ravit Helled, Gerald SchubertAbstract:It has been a common assumption of interior models that the outer planets of our Solar system are convective, and that the internal temperature distributions are therefore adiabatic. This assumption is also often applied to exoplanets. However, if a large portion of the thermal flux can be transferred by conduction, or if convection is inhibited, the thermal profile could be substantially different and would therefore affect the inferred planetary composition. Here we investigate how the assumption of non-adiabatic temperature profiles in Uranus and Neptune affects their internal structures and compositions. We use a set of plausible temperature profiles together with density profiles that match the measured gravitational fields to derive the planets’ compositions. We find that the inferred compositions of both Uranus and Neptune are quite sensitive to the assumed thermal profile in the outer layers, but relatively insensitive to the thermal profile in the central, high-pressure region. The overall value of the heavy element mass fraction, Z, for these planets is between 0.8 and 0.9. Finally, we suggest that large parts of Uranus’ interior might be conductive, a conclusion that is consistent with Uranus dynamo models and a hot central inner region.
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interior models of Uranus and neptune
The Astrophysical Journal, 2011Co-Authors: Ravit Helled, Morris Podolak, John D. Anderson, Gerald SchubertAbstract:"Empirical" models (pressure versus density) of Uranus and Neptune interiors constrained by the gravitational coefficients J 2, J 4, the planetary radii and masses, and Voyager solid-body rotation periods are presented. The empirical pressure-density profiles are then interpreted in terms of physical equations of state of hydrogen, helium, ice (H2O), and rock (SiO2) to test the physical plausibility of the models. The compositions of Uranus and Neptune are found to be similar with somewhat different distributions of the high-Z material. The big difference between the two planets is that Neptune requires a non-solar envelope, while Uranus is best matched with a solar composition envelope. Our analysis suggests that the heavier elements in both Uranus' and Neptune's interior might increase gradually toward the planetary centers. Indeed it is possible to fit the gravitational moments without sharp compositional transitions.
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interior models of Uranus and neptune
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Ravit Helled, Morris Podolak, John D. Anderson, Gerald SchubertAbstract:'Empirical' models (pressure vs. density) of Uranus and Neptune interiors constrained by the gravitational coefficients J_2, J_4, the planetary radii and masses, and Voyager solid-body rotation periods are presented. The empirical pressure-density profiles are then interpreted in terms of physical equations of state of hydrogen, helium, ice (H_2O), and rock (SiO_2) to test the physical plausibility of the models. The compositions of Uranus and Neptune are found to be similar with somewhat different distributions of the high-Z material. The big difference between the two planets is that Neptune requires a non-solar envelope while Uranus is best matched with a solar composition envelope. Our analysis suggests that the heavier elements in both Uranus' and Neptune's interior might increase gradually towards the planetary centers. Indeed it is possible to fit the gravitational moments without sharp compositional transitions.
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Uranus and Neptune: Shape and rotation
Icarus, 2010Co-Authors: Ravit Helled, John D. Anderson, Gerald SchubertAbstract:Abstract Both Uranus and Neptune are thought to have strong zonal winds with velocities of several 100 m s−1. These wind velocities, however, assume solid-body rotation periods based on Voyager 2 measurements of periodic variations in the planets’ radio signals and of fits to the planets’ magnetic fields; 17.24 h and 16.11 h for Uranus and Neptune, respectively. The realization that the radio period of Saturn does not represent the planet’s deep interior rotation and the complexity of the magnetic fields of Uranus and Neptune raise the possibility that the Voyager 2 radio and magnetic periods might not represent the deep interior rotation periods of the ice giants. Moreover, if there is deep differential rotation within Uranus and Neptune no single solid-body rotation period could characterize the bulk rotation of the planets. We use wind and shape data to investigate the rotation of Uranus and Neptune. The shapes (flattening) of the ice giants are not measured, but only inferred from atmospheric wind speeds and radio occultation measurements at a single latitude. The inferred oblateness values of Uranus and Neptune do not correspond to bodies rotating with the Voyager rotation periods. Minimization of wind velocities or dynamic heights of the 1 bar isosurfaces, constrained by the single occultation radii and gravitational coefficients of the planets, leads to solid-body rotation periods of ∼16.58 h for Uranus and ∼17.46 h for Neptune. Uranus might be rotating faster and Neptune slower than Voyager rotation speeds. We derive shapes for the planets based on these rotation rates. Wind velocities with respect to these rotation periods are essentially identical on Uranus and Neptune and wind speeds are slower than previously thought. Alternatively, if we interpret wind measurements in terms of differential rotation on cylinders there are essentially no residual atmospheric winds.
Nadine Nettelmann - One of the best experts on this subject based on the ideXlab platform.
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Uranus and Neptune: Origin, Evolution and Internal Structure
Space Science Reviews, 2020Co-Authors: Ravit Helled, Nadine Nettelmann, Tristan GuillotAbstract:There are still many open questions regarding the nature of Uranus and Neptune, the outermost planets in the Solar System. In this review we summarize the current-knowledge about Uranus and Neptune with a focus on their composition and internal structure, formation including potential subsequent giant impacts, and thermal evolution. We present key open questions and discuss the uncertainty in the internal structures of the planets due to the possibility of non-adiabatic and inhomogeneous interiors. We also provide the reasoning for improved observational constraints on their fundamental physical parameters such as their gravitational and magnetic fields, rotation rates, and deep atmospheric composition and temperature. Only this way will we be able to improve our understating of these planetary objects, and the many similar-sized objects orbiting other stars.
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Uranus evolution models with simple thermal boundary layers
Icarus, 2016Co-Authors: Nadine Nettelmann, K Wang, J J Fortney, S Hamel, S Yellamilli, M Bethkenhagen, R RedmerAbstract:Abstract The strikingly low luminosity of Uranus (Teff ≃ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2–3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of 1 × solar. Finally, we allow for an equilibrium evolution (Teff ≃ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a ”special time” in Uranus’ evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ≃ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.
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new indication for a dichotomy in the interior structure of Uranus and neptune from the application of modified shape and rotation data
Planetary and Space Science, 2013Co-Authors: Nadine Nettelmann, Ravit Helled, Jonathan J Fortney, R RedmerAbstract:Abstract Since the Voyager fly-bys of Uranus and Neptune, improved gravity field data have been derived from long-term observations of the planets' satellite motions, and modified shape and solid-body rotation periods were suggested. A faster rotation period (−40 min) for Uranus and a slower rotation period (+1h20) of Neptune compared to the Voyager data were found to minimize the dynamical heights and wind speeds. We apply the improved gravity data, the modified shape and rotation data, and the physical LM-R equation of state to compute adiabatic three-layer structure models, where rocks are confined to the core, and homogeneous thermal evolution models of Uranus and Neptune. We present the full range of structure models for both the Voyager and the modified shape and rotation data. In contrast to previous studies based solely on the Voyager data or on empirical EOS, we find that Uranus and Neptune may differ to an observationally significant level in their atmospheric heavy element mass fraction Z1 and nondimensional moment of inertia, λ . For Uranus, we find Z 1 ≤ 8 % and λ = 0.2224 ( 1 ) , while for Neptune Z 1 ≤ 65 % and λ = 0.2555 ( 2 ) when applying the modified shape and rotation data, while for the unmodified data we compute Z 1 ≤ 17 % and λ = 0.230 ( 1 ) for Uranus and Z 1 ≤ 54 % and λ = 0.2410 ( 8 ) for Neptune. In each of these cases, solar metallicity models ( Z 1 = 0.015 ) are still possible. The cooling times obtained for each planet are similar to recent calculations with the Voyager rotation periods: Neptune's luminosity can be explained by assuming an adiabatic interior while Uranus cools far too slowly. More accurate determinations of these planets' gravity fields, shapes, rotation periods, atmospheric heavy element abundances, and intrinsic luminosities are essential for improving our understanding of the internal structure and evolution of icy planets.
R Redmer - One of the best experts on this subject based on the ideXlab platform.
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Uranus evolution models with simple thermal boundary layers
Icarus, 2016Co-Authors: Nadine Nettelmann, K Wang, J J Fortney, S Hamel, S Yellamilli, M Bethkenhagen, R RedmerAbstract:Abstract The strikingly low luminosity of Uranus (Teff ≃ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2–3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of 1 × solar. Finally, we allow for an equilibrium evolution (Teff ≃ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a ”special time” in Uranus’ evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ≃ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.
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new indication for a dichotomy in the interior structure of Uranus and neptune from the application of modified shape and rotation data
Planetary and Space Science, 2013Co-Authors: Nadine Nettelmann, Ravit Helled, Jonathan J Fortney, R RedmerAbstract:Abstract Since the Voyager fly-bys of Uranus and Neptune, improved gravity field data have been derived from long-term observations of the planets' satellite motions, and modified shape and solid-body rotation periods were suggested. A faster rotation period (−40 min) for Uranus and a slower rotation period (+1h20) of Neptune compared to the Voyager data were found to minimize the dynamical heights and wind speeds. We apply the improved gravity data, the modified shape and rotation data, and the physical LM-R equation of state to compute adiabatic three-layer structure models, where rocks are confined to the core, and homogeneous thermal evolution models of Uranus and Neptune. We present the full range of structure models for both the Voyager and the modified shape and rotation data. In contrast to previous studies based solely on the Voyager data or on empirical EOS, we find that Uranus and Neptune may differ to an observationally significant level in their atmospheric heavy element mass fraction Z1 and nondimensional moment of inertia, λ . For Uranus, we find Z 1 ≤ 8 % and λ = 0.2224 ( 1 ) , while for Neptune Z 1 ≤ 65 % and λ = 0.2555 ( 2 ) when applying the modified shape and rotation data, while for the unmodified data we compute Z 1 ≤ 17 % and λ = 0.230 ( 1 ) for Uranus and Z 1 ≤ 54 % and λ = 0.2410 ( 8 ) for Neptune. In each of these cases, solar metallicity models ( Z 1 = 0.015 ) are still possible. The cooling times obtained for each planet are similar to recent calculations with the Voyager rotation periods: Neptune's luminosity can be explained by assuming an adiabatic interior while Uranus cools far too slowly. More accurate determinations of these planets' gravity fields, shapes, rotation periods, atmospheric heavy element abundances, and intrinsic luminosities are essential for improving our understanding of the internal structure and evolution of icy planets.
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the phase diagram of water and the magnetic fields of Uranus and neptune
Icarus, 2011Co-Authors: R Redmer, Thomas R Mattsson, N Nettelmann, Martin FrenchAbstract:Abstract The interior of giant planets can give valuable information on formation and evolution processes of planetary systems. However, the interior and evolution of Uranus and Neptune is still largely unknown. In this paper, we compare water-rich three-layer structure models of these planets with predictions of shell structures derived from magnetic field models. Uranus and Neptune have unusual non-dipolar magnetic fields contrary to that of the Earth. Extensive three-dimensional simulations of Stanley and Bloxham (Stanley, S., Bloxham, J. [2004]. Nature 428, 151–153) have indicated that such a magnetic field is generated in a rather thin shell of at most 0.3 planetary radii located below the H/He rich outer envelope and a conducting core that is fluid but stably stratified. Interior models rely on equation of state data for the planetary materials which have usually considerable uncertainties in the high-pressure domain. We present interior models for Uranus and Neptune that are based on ab initio equation of state data for hydrogen, helium, and water as the representative of all heavier elements or ices. Based on a detailed high-pressure phase diagram of water we can specify the region where superionic water should occur in the inner envelope. This superionic region correlates well with the location of the stably-stratified region as found in the dynamo models. Hence we suggest a significant impact of the phase diagram of water on the generation of the magnetic fields in Uranus and Neptune.
