Uranus and Neptune are the last unexplored planets of the Solar System. I show that they hold crucial keys to understand the atmospheric dynamics and structure of planets with hydrogen atmospheres. Their atmospheres are active and storms are believed to be fueled by methane condensation which is both extremely abundant and occurs at low optical depth. This means that mapping temperature and methane abundance as a function of position and depth will inform us on how convection organizes in an atmosphere with no surface and condensates that are heavier than the surrounding air, a general feature of gas giants. Using this information will be essential to constrain the interior structure of Uranus and Neptune themselves, but also of Jupiter, Saturn and numerous exoplanets with hydrogen atmospheres. Owing to the spatial and temporal variability of these atmospheres, an orbiter is required. A probe would provide a reference profile to lift ambiguities inherent to remote observations. It would also measure abundances of noble gases which can be used to reconstruct the history of planet formation in the Solar System. Finally, mapping the planets' gravity and magnetic fields will be essential to constrain their global composition, structure and evolution.
The internal structures and compositions of Uranus and Neptune are not well constrained due to the uncertainty in rotation period and flattening, as well as the relatively large error bars on the gravitational coefficients. While Uranus and Neptune are similar in mass and radius, they differ in other physical properties such as thermal emission, obliquity, and inferred atmospheric enrichment. In this letter we consider the uncertainty in the planetary rotation periods, show that rotation periods more consistent with the measured oblateness imply that Uranus and Neptune have different internal …
The origin of Uranus and Neptune is still unknown. In particular, it has been challenging for planet formation models to form the planets in their current radial distances within the expected lifetime of the solar nebula. In this paper, we simulate the in-situ formation of Uranus and Neptune via pebble accretion and show that both planets can form within ~ 3 Myr at their current locations, and have final compositions that are consistent with the heavy-element to H-He ratios predicted by structure models. We find that Uranus and Neptune could have been formed at their current locations. In seve…
'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…
Both Uranus and Neptune are thought to have strong zonal winds with velocities of several hundred meters per second. 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; 17.24h and 16.11h 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 per…
Uranus and Neptune are still poorly understood. Their gravitational fields, rotation periods, atmosphere dynamics, and internal structures are not well determined. In this paper we present empirical structure models of Uranus and Neptune where the density profiles are represented by polytropes. By using these models, that are set to fit the planetary gravity field, we predict the higher order gravitational coefficients $J_6$ and $J_8$ for various assumed rotation periods, wind depths, and uncertainty of the low-order harmonics. We show that faster rotation and/or deep winds favour centrally co…
Despite many similarities, there are significant observed differences between Uranus and Neptune: while Uranus is tilted and has a regular set of satellites, suggesting their accretion from a disk, Neptune's moons are irregular and are captured objects. In addition, Neptune seems to have an internal heat source, while Uranus is in equilibrium with solar insulation. Finally, structure models based on gravity data suggest that Uranus is more centrally condensed than Neptune. We perform a large suite of high resolution SPH simulations to investigate whether these differences can be explained by g…
The internal heat flows of both Uranus and Neptune remain major outstanding problems in planetary science. Uranus' surprisingly cold effective temperature is inconsistent with adiabatic thermal evolution models, while Neptune's substantial internal heat flow is twice its received insolation. In this work we constrain the magnitude of influence condensation, including latent heat and inhibition of convection, can have on the thermal evolution of these bodies. We find that while the effect can be significant, it is insufficient to solve the Uranus faintness problem on its own. Self-consistently …
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 s…
The brightness of Neptune is often found to be in accordance with an adiabatic interior, while the low luminosity of Uranus challenges this assumption. Here we apply revised equation of state data of hydrogen, helium, and water and compute the thermal evolution of Uranus and Neptune assuming an adiabatic interior. For this purpose, we have developed a new planetary model and evolution code. We investigate the influence of albedo, solar energy influx, and equations of state of H and He, and water on the cooling time. Our cooling times of about $τ_\text{U}=5.1\times 10^9\text{ years}$ for Uranus…
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 physic…
The origin of Uranus and Neptune remains a challenge for planet formation models. A potential explanation is that the planets formed from a population of a few planetary embryos with masses of a few Earth masses which formed beyond Saturn's orbit and migrated inwards. These embryos can collide and merge to form Uranus and Neptune. In this work we revisit this formation scenario and study the outcomes of such collisions using 3D hydrodynamical simulations. We investigate under what conditions the perfect-merging assumption is appropriate, and infer the planets' final masses, obliquities and rot…
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 …
Reproducing Uranus and Neptune remains a challenge for simulations of solar system formation. The ice giants' peculiar obliquities suggest that they both suffered giant collisions during their formation. Thus, there must have been an epoch of accretion dominated by collisions among large planetary embryos in the primordial outer solar system. We test this idea using N-body numerical simulations including the effects of a gaseous protoplanetary disk. One strong constraint is that the masses of the ice giants are very similar -- the Neptune/Uranus mass ratio is $\sim1.18$. We show that similar-s…
We assess the prospect of using ring seismology to probe the interiors of the ice giants Uranus and Neptune. We do this by calculating normal mode spectra for different interior models of Uranus and Neptune using the stellar oscillation code GYRE. These spectra provide predictions of where in these planets' ring systems the effects of interior oscillations might be detected. We find that f-mode resonances with azimuthal order $m=2$ or $7 \leq m \leq 19$ fall among the inner rings (6, 5, 4, $α$, and $β$) of Uranus, while f-mode resonances with $2 \leq m \leq 12$ fall in the tenuous $ζ$ ring reg…
Giant planet formation process is still not completely understood. The current most accepted paradigm, the core instability model, explains several observed properties of the solar system's giant planets but, to date, has faced difficulties to account for a formation time shorter than the observational estimates of protoplanetary disks' lifetimes, especially for the cases of Uranus and Neptune. In the context of this model, and considering a recently proposed primordial solar system orbital structure, we performed numerical calculations of giant planet formation. Our results show that if accre…
Interior models of Uranus and Neptune often assume discrete layers, but sharp interfaces are expected only if major constituents are immiscible. Diffuse interfaces could arise if accretion favored a central concentration of the least volatile constituents (also incidentally the most dense); compositional gradients arising in such a structure would likely inhibit convection. Currently, two lines of evidence suggest possible hydrogen-water immiscibility in ice giant interiors. The first arises from crude extrapolation of the experimental hydrogen-water critical curve to $\sim 3$ GPa (Bali et al.…
We present large scale electronic structure based molecular dynamics simulations of liquid methane at planetary conditions. In particular, we address the controversy of whether or not the interior of Uranus and Neptune consists of diamond. In our simulations we find no evidence for the formation of diamond, but rather sp2-bonded polymeric carbon. Furthermore, we predict that at high tem- perature hydrogen may exist in its monoatomic and metallic state. The implications of our finding for the planetary models of Uranus and Neptune are in detail discussed.
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 an…
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 i…
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 …
A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the sola…
Using density functional molecular dynamics free energy calculations, we show that the body-centered-cubic phase of superionic ice previously believed to be the only phase is in fact thermodynamically unstable compared to a novel phase with oxygen positions in fcc lattice sites. The novel phase has a lower proton mobility than the bc phase and may exhibit a higher melting temperature. We predict a transition between the two phases at a pressure of 1 +/- 0.5 Mbar, with potential consequences for the interiors of ice giants such as Uranus and Neptune.
With more than 15 years since the the first radial velocity discovery of a planet orbiting a Sun-like star, the time baseline for radial velocity surveys is now extending out beyond the orbit of Jupiter analogs. The sensitivity to exoplanet orbital periods beyond that of Saturn orbital radii however is still beyond our reach such that very few clues regarding the prevalence of ice giants orbiting solar analogs are available to us. Here we simulate the radial velocity, transit, and photometric phase amplitude signatures of the solar system giant planets, in particular Uranus and Neptune, and as…
Uranus and Neptune exhibit fast surface zonal winds that can reach up to few hundred meters per second. Previous studies on zonal gravitational harmonics and Ohmic dissipation constraints suggest that the wind speeds diminish rapidly in relatively shallow depths within the planets. Through a case-by-case comparison between the missing dynamical gravitational harmonic $J^\prime_4$ from structure models, and with that expected from fluid perturbations, we put constraints on zonal wind decay in Uranus and Neptune. To this end, we generate polytropic empirical structure models of Uranus and Neptun…
The formation mechanisms of the ice giants Uranus and Neptune, and the origin of their elemental and isotopic compositions, have long been debated. The density of solids in the outer protosolar nebula is too low to explain their formation, and spectroscopic observations show that both planets are highly enriched in carbon, very poor in nitrogen, and the ices from which they originally formed might had deuterium-to-hydrogen ratios lower than the predicted cometary value, unexplained properties observed in no other planets. Here we show that all these properties can be explained naturally if Ura…
Voyager 2 observations revealed that the internal luminosity of Neptune is an order of magnitude higher than that of Uranus. If the two planets have similar interior structures and cooling histories, the luminosity of Neptune can only be explained by invoking some energy source beyond gravitational contraction. This paper investigates whether Centaur impacts could provide the energy necessary to produce the luminosity of Neptune. The major findings are (1) that impacts on both Uranus and Neptune are too infrequent to provide luminosities of order the observed value for Neptune, even for optimi…
The Solar System harbors deep unresolved mysteries despite centuries-long study. A highly intriguing case concerns anomalous non-dipolar and non-axisymmetric magnetic fields of Uranus and Neptune that have long eluded explanation by the prevailing theory. A thin-shell dynamo conjecture captures observed phenomena but leaves unexplained fundamental material basis and underlying mechanism. Here, we report the discovery of trihydrogen oxide (H3O) in metallic liquid state stabilized at extreme pressure and temperature conditions inside these icy planets. Calculated stability pressure field compare…
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…
The currently available, detailed properties (e.g., isotopic ratios) of solar system planets may provide guides for constructing better approaches of exoplanet characterization. With this motivation, we explore how the measured values of the deuterium-to-hydrogen (D/H) ratio of Uranus and Neptune can constrain their formation mechanisms. Under the assumption of in-situ formation, we investigate three solid accretion modes; a dominant accretion mode switches from pebble accretion to drag-enhanced three-body accretion and to canonical planetesimal accretion, as the solid radius increases. We con…
Thermal evolution models suggest that the luminosities of both Uranus and Neptune are inconsistent with the classical assumption of an adiabatic interior. Such models commonly predict Uranus to be brighter and, recently, Neptune to be fainter than observed. In this work, we investigate the influence of a thermally conductive boundary layer on the evolution of Uranus- and Neptune-like planets. This thermal boundary layer (TBL) is assumed to be located deep in the planet, and be caused by a steep compositional gradient between a H-He-dominated outer envelope and an ice-rich inner envelope. We in…
We investigate the enrichment patterns of several delivery scenarios of the volatiles to the atmospheres of ice giants, having in mind that the only well constrained determination made remotely, i.e. the carbon abundance measurement, suggests that their envelopes possess highly supersolar metallicities, i.e. close to two orders of magnitude above that of the protosolar nebula. In the framework of the core accretion model, only the delivery of volatiles in solid forms (amorphous ice, clathrates, pure condensates) to these planets can account for the apparent supersolar metallicity of their enve…
Herschel-PACS measurements of the rotational R(0) and R(1) HD lines in the atmospheres of Uranus and Neptune are analyzed in order to derive a D/H ratio with improved precision for both planets. The derivation of the D/H ratio includes also previous measurements of the R(2) line by the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The available spectroscopic line information of the three rotational transitions is discussed and applied in the radiative transfer calculations. The best simultaneous fit of all three lines requires only a minor departure from the Spit…
Determining the depth of atmospheric winds in the outer planets of the Solar System is a key topic in planetary science. We provide constraints on these depths in Uranus and Neptune via the total induced Ohmic dissipation, due to the interaction of the zonal flows and the planetary magnetic fields. An upper bound can be placed on the induced dissipation via energy and entropy flux throughout the interior. The induced Ohmic dissipation is directly linked to the electrical conductivity profile of the materials involved in the flow. We present a method for calculating electrical conductivity prof…
In this work we present the results of a suite of dynamical simulations following the orbital evolution of 8,022 hypothetical Centaur objects. These Centaurs begin our integrations on orbits in the vicinity of the 2:1 mean motion resonance with Neptune, and we follow their dynamical evolution for a period of 3 Myr under the gravitational influence of a motionless Sun and the four Jovian planets. The great majority of the test particles studied rapidly escaped from the vicinity of the 2:1 mean motion resonance of Neptune and diffused throughout the Solar System. The average libration time of Ce…
Laying the Vision and Voyages (V&V, National Research Council 2011) Decadal Survey 2013-2022 objectives against subsequent budget profiles reveals that separate missions to every desirable target in the Solar System are simply not realistic. In fact, very few of these missions will be achievable under current budget realities. Given the cost and difficulty in reaching the outer Solar System, competition between high-value science missions to an Ice Giant system and the Kuiper Belt is counterproductive. A superior approach is to combine the two programs into an integrated strategy that maximize…
We observe similar structures in the orbital radii of satellite and ring systems of Uranus, Jupiter and Neptune. This stimulates an investigation that the evolution of these systems and Saturn's system is initiated by a common mechanism that involves the interaction of radiation with their subnebulae (protosatellite disks). A model is presented that is characterized by resonance created through stimulated radiative molecular association (SRMA) reactions. In this model thermal energy is extracted from a protoplanetary disk at specific distances from the protoplanet wherever there is a match bet…
The present investigation relates the orbital radii of regular satellites of Uranus, Jupiter, Neptune, and Saturn to photon energies in the spectra of atomic and molecular hydrogen. To explain these observations a model is developed involving stimulated radiative molecular association (SRMA) reactions among the photons and atoms in the protosatellite disks of the planets. In this model thermal energy is extracted from each disk due to a resonance at radii where there is a match between the temperature in the disk and a photon energy. Matter accumulates at these radii, and satellites and rings …
The D/H enrichment observed in Saturn's satellite Enceladus is remarkably similar to the values observed in the nearly-isotropic comets. Given the predicted strong variation of D/H with heliocentric distance in the solar nebula, this observation links the primordial source region of the nearly-isotropic comets with the formation location of Enceladus. That is, comets from the nearly-isotropic class were most likely fed into their current reservoir, the Oort cloud, from a source region near the formation location of Enceladus. Dynamical simulations of the formation of the Oort cloud indicate th…
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 o…
The magnetic fields of the Ice Giant Planets Uranus and Neptune (U/N) are unique in the solar system. Based on a substantial database measured on Earth for representative planetary fluids at representative dynamic pressures up to 200 GPa (2 Mbar) and a few 1000 K, the complex magnetic fields of U/N are (i) probably made primarily by degenerate metallic fluid H (MFH) at or near the crossover from the H-He envelopes to Ice cores at ~100 GPa (Mbar) pressures and normalized radii of ~90% of the radii of U/N; (ii) because those magnetic fields are made relatively close to the surfaces of U/N, non-d…
Uranus and Neptune share properties that are distinct from the other giant planets in the solar system, but they are also distinct from one another, particularly in their relative internal heat flux. Not only does Neptune emit about ten times the amount of heat that emitted by Uranus, the relative amount of emitted heat to the energy they absorb from the sun also differs greatly, being comparable at Uranus and the largest of all giant planets at Neptune. As a result, it is questionable whether thermal convection occurs within the interior of Uranus. However, the presence of an intrinsic magnet…
Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?
Aix-Marseille Université（艾克斯-马赛大学）；Southwest Research Institute（西南研究院）；Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux（波尔多大学天体物理实验室）；Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（巴黎天文台、巴黎文理研究大学、法国国家科学研究中心、索邦大学、巴黎西岱大学）；University of Science and Technology of China（中国科学技术大学）；Dublin Institute for Advanced Studies（都柏林高等研究所）；Space Science Institute（空间科学研究所）
Uranus and Neptune are the archetypes of "ice giants", a class of planets that may be among the most common in the Galaxy. They hold the keys to understand the atmospheric dynamics and structure of planets with hydrogen atmospheres inside and outside the solar system; however, they are also the last unexplored planets of the Solar System. Their atmospheres are active and storms are believed to be fueled by methane condensation which is both extremely abundant and occurs at low optical depth. This means that mapping temperature and methane abundance as a function of position and depth will info…
The core accretion theory of planet formation has at least two fundamental problems explaining the origins of Uranus and Neptune: (1) dynamical times in the trans-Saturnian solar nebula are so long that core growth can take > 15 Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10 Earth masses necessitates a sudden gas accretion cutoff just as the ice giant cores reach critical mass. Both problems may be resolved by allowing the ice giants to migrate outward after their formation in solid-rich feeding zones with planetesimal surface densities well above the minimum-m…
We explore planetary migration scenarios for formation of high inclination Neptune Trojans (NTs) and how they are affected by the planetary migration of Neptune and Uranus. If Neptune and Uranus's eccentricity and inclination were damped during planetary migration, then their eccentricities and inclinations were higher prior and during migration than their current values. Using test particle integrations we study the stability of primordial NTs, objects that were initially Trojans with Neptune prior to migration. We also study Trans-Neptunian objects captured into resonance with Neptune and be…
