Icy Giant Planets Research Articles

Carbon, isotopic ratio 12C/13C, and nitrogen in solar twins: constraints for the chemical evolution of the local disc

ABSTRACT Abundances of light elements in dwarf stars of different ages are important constraints for stellar yields, Galactic chemical evolution and exoplanet chemical composition studies. We have measured C and N abundances and 12C/13C ratios for a sample of 63 solar twins spanning a wide range in age, based on spectral synthesis of a comprehensive list of CH A-X and CN B-X features using HARPS spectra. The analysis of 55 thin disc solar twins confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated as a function of [Fe/H] and age for the first time for these stars. Our derived correlation [C/Fe]–age agrees with works for solar-type stars and solar twins, but the [N/Fe]–age correlation does not. The relations [C,N/Fe]–[Fe/H] and [C,N/Fe]–age for the solar twins lay under solar. 12C/13C is found correlated with [Fe/H] and seems to have decreased along the evolution of the local thin disc. Predictions from chemical evolution models for the solar vicinity corroborate the relations [C,N/Fe]–[Fe/H], 12C/13C–age, and [N/O]–[O/H], but do not for the 12C/13C–[Fe/H] and [C/O]–[O/H] relations. The N/O ratio in the Sun is placed at the high end of the homogeneous distribution of solar twins, which suggests uniformity in the N–O budget for the formation of icy planetesimals, watery superearths, and giant planets. C and N had different nucleosynthetic origins along the thin disc evolution, as shown by the relations of [C/N], [C/O], and [N/O] against [O/H] and age. [C/N] and [C/O] are particularly observed increasing in time for solar twins younger than the Sun.

Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature

Both helium and ammonia are main components of icy giant planets. While ammonia is very reactive, helium is the most inert element in the universe. It is of great interest whether ammonia and helium can react with each other under planetary conditions, and if so, what kinds of structures and states of matter can form. Here, using crystal structure prediction methods and first-principles calculations, we report three new stable stoichiometries and eight new stable phases of He−NH3 compounds under pressures up to 500 GPa. These structures may exhibit perovskitelike structures for HeNH3 and He2NH3, and a host-guest crystal structure for He(NH3)2. Superionic states are found in all these He−NH3 compounds under high pressures and temperatures in which the hydrogen atoms are diffusive while the nitrogen and helium atoms remain fixed. Such dynamical behavior in helium ammonia compounds is quite different from that in helium water compounds, where weakly interacting helium is more diffusive than stronger bound hydrogen. The low-density host-guest phase of space group I4cm is found to be stable at very low pressures (about 3 GPa) and it enters into a plastic state, characterized by freely rotating ammonia molecules. The present results suggest that plastic or superionic helium ammonia compounds may exist under planetary conditions, and helium contributes crucially to the exotic physics and chemistry observed under extreme conditions.Received 25 November 2019Revised 25 February 2020Accepted 10 March 2020DOI:https://doi.org/10.1103/PhysRevX.10.021007Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCrystal meltingCrystal structureLiquid-solid phase transitionPressure effectsPhysical SystemsCore of giant planetsMolecular solidsTechniquesFirst-principles calculationsMolecular dynamicsCondensed Matter, Materials & Applied Physics

Measurement of diamond nucleation rates from hydrocarbons at conditions comparable to the interiors of icy giant planets

We present measurements of the nucleation rate into a diamond lattice in dynamically compressed polystyrene obtained in a pump-probe experiment using a high-energy laser system and in situ femtosecond x-ray diffraction. Different temperature-pressure conditions that occur in planetary interiors were probed. For a single shock reaching 70 GPa and 3000 K no diamond formation was observed, while with a double shock driving polystyrene to pressures around 150 GPa and temperatures around 5000 K nucleation rates between ${10}^{29}$ and ${10}^{34}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ were recorded. These nucleation rates do not agree with predictions of the state-of-the-art theoretical models for carbon-hydrogen mixtures by many orders of magnitude. Our data suggest that there is significant diamond formation to be expected inside icy giant planets like Neptune and Uranus.

Accretion of a giant planet onto a white dwarf star.

The detection1 of a dust disk around the white dwarf star G29-38 and transits from debris orbiting the white dwarf WD 1145+017 (ref. 2) confirmed that the photospheric trace metals found in many white dwarfs3 arise from the accretion of tidally disrupted planetesimals4. The composition of these planetesimals is similar to that of rocky bodies in the inner Solar System5. Gravitational scattering of planetesimals towards the white dwarf requires the presence of more massive bodies6, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a hot (about 27,750 kelvin) white dwarf, WDJ091405.30+191412.25, that is accreting from a circumstellar gaseous disk composed of hydrogen, oxygen and sulfur at a rate of about 3.3×109 grams per second. The composition of this disk is unlike all other known planetary debris around white dwarfs7, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H2O and H2S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of around 15 solar radii will undergo substantial evaporation with expected mass loss rates comparable to the accretion rate that we observe onto the white dwarf. The orbit of the planet is most probably the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of approximately 1 in 10,000 for spectroscopically detectable giant planets in close orbits around white dwarfs.

Electronic bandgap of water in the superionic and plasma phases

Water has been proposed to be one of the main compositions of icy giant planets like Neptune and Uranus. Its thermodynamic states and transport properties at extremes are of interest not only to constrain the interior models but also to understand abnormal magnetic fields of planets. The electronic bandgap of water, which significantly influences the ionization ratio and the conductivity, however, is still under debate. In this work, we revisit the shock reflectivity data reported in the literature. By applying a Drude model, the electronic bandgap of water in the superionic and plasma phases is determined to be 4.4 ± 0.2 eV, in contrast to the threshold of 1.25 ± 0.04 eV for free ion generation in the molecular and ionic fluid phases. Interestingly, the bandgap of water does not show a significant tendency of “closure” with the increase in pressure or temperature in the investigated regime, and the bandgap value is consistent with the predicted value of 4–6 eV by the density functional theory assuming a hybrid Heyd-Scuseria-Ernzerhof functional [Millot et al., Nat. Phys. 14, 297–302 (2018)]. The electronic bandgap and the energy threshold determined in this work provide essential parameters for estimating the conductivity along the radius of Neptune and Uranus and will promote our understanding of the origin of the abnormal magnetic fields.

Icy giant planet exploration: Are entry probes essential?

“Flyby, orbit, land” has been the guiding philosophy of planetary exploration. This systematic approach has been highly successful in addressing the most fundamental questions of the origin, evolution, and habitability of the solar system. For the giant planets, entry probes take the place of “landers”, since the “land” of the gas and icy giant planets, their solid core, lies some tens of thousands of kilometers beneath their cloud tops, hence impractical to reach. On the other hand, during a planet's accretionary heating phase the volatiles trapped in the core material would have been released, forming the atmosphere, together with the most volatile of the gases, hydrogen, helium and neon, that were captured gravitationally when the core became massive enough. Those atmospheric volatiles would thus be accessible by entry probes deployed to relatively shallow depths in the upper troposphere, allowing the determination of the abundances and isotopes of at least the most critical of the “heavy elements” (mass greater than helium). The heavy elements are key constraints to planetary formation and evolution models [1]. Entry probes are essential to retrieve their abundances, which are feasible only in situ at probe depths. That was the rationale behind NASA's 1995 Galileo probe mission at Jupiter and the Saturn Probe mission in NASA's New Frontier 4 candidate list of science themes. Saturn probe proposals to ESA's Cosmic Vision Program are similarly inspired. In 2015, NASA commissioned an Ice Giant Planets Study to recommend a comprehensive set of science goals and objectives, and further to develop potential mission architectures for accomplishing them in the 2023–2033 decade. The most highly rated mission from that study is an orbiter with probe to either Uranus or Neptune [2]. While remote sensing observations from the orbiter will yield the composition, structure and the distribution of neutral and charged particles in the magnetosphere and the upper atmosphere, the entry probe will determine the abundances and isotopic ratios of the noble gases (He, Ne, Ar, Kr, Xe), H, C, and possibly N and S. The noble gases, in particular, are key to discriminating among formation models, and their values from the probe entry location would represent global values as they are unaffected by meteorology, dynamics or chemistry. We will elaborate on these issues and then briefly discuss possible scenario/s for a mission to the icy giant planets, with particular focus on entry probes.

Evidence for Crystalline Structure in Dynamically-Compressed Polyethylene up to 200 GPa

We investigated the high-pressure behavior of polyethylene (CH2) by probing dynamically-compressed samples with X-ray diffraction. At pressures up to 200 GPa, comparable to those present inside icy giant planets (Uranus, Neptune), shock-compressed polyethylene retains a polymer crystal structure, from which we infer the presence of significant covalent bonding. The A2/m structure which we observe has previously been seen at significantly lower pressures, and the equation of state measured agrees with our findings. This result appears to contrast with recent data from shock-compressed polystyrene (CH) at higher temperatures, which demonstrated demixing and recrystallization into a diamond lattice, implying the breaking of the original chemical bonds. As such chemical processes have significant implications for the structure and energy transfer within ice giants, our results highlight the need for a deeper understanding of the chemistry of high pressure hydrocarbons, and the importance of better constraining planetary temperature profiles.

Experimental evidence for superionic water ice using shock compression

In stark contrast to common ice, Ih, water ice at planetary interior conditions has been predicted to become superionic with fast-diffusing (that is, liquid-like) hydrogen ions moving within a solid lattice of oxygen. Likely to constitute a large fraction of icy giant planets, this extraordinary phase has not been observed in the laboratory. Here, we report laser-driven shock-compression experiments on water ice VII. Using time-resolved optical pyrometry and laser velocimetry measurements as well as supporting density functional theory–molecular dynamics (DFT-MD) simulations, we document the shock equation of state of H2O to unprecedented extreme conditions and unravel thermodynamic signatures showing that ice melts near 5,000 K at 190 GPa. Optical reflectivity and absorption measurements also demonstrate the low electronic conductivity of ice, which, combined with previous measurements of the total electrical conductivity under reverberating shock compression, provides experimental evidence for superionic conduction in water ice at planetary interior conditions, verifying a 30-year-old prediction.

Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions

The effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades 1 . Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres 2 , are believed to undergo structural transitions 3,4 that release hydrogen from deeper layers and may lead to compact stratified cores 5–7 . Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature–pressure regime in which methane first transforms into a mixture of hydrocarbon polymers 8 , whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene (C8H8) n samples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus 9 . Our findings demonstrate the necessity of high pressures for initiating carbon–hydrogen separation 3 and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments 4,8,10 . Our results will inform mass–radius relationships of carbon-bearing exoplanets 11 , provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon–hydrogen separation could influence the convective heat transport 7 . Diamonds precipitate from methane under the intense pressures of the atmospheres of Neptune and Uranus. Here, a laser shock experiment on a hydrocarbon sample shows that diamonds may require ten times as much pressure to precipitate as was previously thought.

Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids

To better understand Earth's present tectonic style–plate tectonics–and how it may have evolved from single plate (stagnant lid) tectonics, it is instructive to consider how common it is among similar bodies in the Solar System. Plate tectonics is a style of convection for an active planetoid where lid fragment (plate) motions reflect sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and accompanied by focused upwellings, or mantle plumes; any other tectonic style is usefully called “stagnant lid” or “fragmented lid”. In 2015 humanity completed a 50+ year effort to survey the 30 largest planets, asteroids, satellites, and inner Kuiper Belt objects, which we informally call “planetoids” and use especially images of these bodies to infer their tectonic activity. The four largest planetoids are enveloped in gas and ice (Jupiter, Saturn, Uranus, and Neptune) and are not considered. The other 26 planetoids range in mass over 5 orders of magnitude and in diameter over 2 orders of magnitude, from massive Earth down to tiny Proteus; these bodies also range widely in density, from 1000 to 5500 kg/m3. A gap separates 8 silicate planetoids with ρ = 3000 kg/m3 or greater from 20 icy planetoids (including the gaseous and icy giant planets) with ρ = 2200 kg/m3 or less. We define the “Tectonic Activity Index” (TAI), scoring each body from 0 to 3 based on evidence for recent volcanism, deformation, and resurfacing (inferred from impact crater density). Nine planetoids with TAI = 2 or greater are interpreted to be tectonically and convectively active whereas 17 with TAI <2 are inferred to be tectonically dead. We further infer that active planetoids have lithospheres or icy shells overlying asthenosphere or water/weak ice. TAI of silicate (rocky) planetoids positively correlates with their inferred Rayleigh number. We conclude that some type of stagnant lid tectonics is the dominant mode of heat loss and that plate tectonics is unusual. To make progress understanding Earth's tectonic history and the tectonic style of active exoplanets, we need to better understand the range and controls of active stagnant lid tectonics.

Structure of methane and ethane at high pressure

Methane is one of the most abundant hydrocarbon molecules in the universe and is expected to be a significant part of the icy giant planets (Uranus and Neptune) and their satellites. Ethane is one of the most predictable products of chemical reactivity of methane at extreme pressures and temperatures. In spite of numerous experimental and theoretical studies, the structure and relative stability of these materials even at room temperature remains controversial. We have performed a combined experimental and theoretical study of both methane and ethane up at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the ab-initio evolutionary algorithm, respectively. In the case of methane we have successfully solved the structure of phase B by determining the space group and the positional parameters of carbon atoms, and by completing these results for the hydrogen positions using the theoretical calculations. The general structural behavior under pressure and the relation between phase B and phases A and pre-B will be also discussed. For ethane we have determined the crystallization point, for room temperature, at 1.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been indentified, for the first time, at 18 GPa to a monoclinic phase III, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. We have determined the equations of state of methane and ethane, which provides a solid basis for the discussion of their relative stability at high pressures.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa. Hydrogen and elementary carbon appear at temperatures of >1,200 K, whereas heavier alkanes and unsaturated hydrocarbons (>24 GPa) form in melts of >1,500 K. The phase composition of carbon-hydrogen fluid evolves towards heavy hydrocarbons at pressures and temperatures representative of Earth's lower mantle. We argue that reduced mantle fluids precipitate diamond upon re-equilibration to lighter species in the upwelling mantle. Likewise, our findings suggest that geophysical models of Uranus and Neptune require reassessment because chemical reactivity of planetary ices is underestimated.

Atmospheric characterization of cold exoplanets using a 1.5-m coronagraphic space telescope

Context. High-contrast imaging is currently the only available technique for the study of the thermodynamical and compositional properties of exoplanets in long-period orbits. The SPICES project is a coronagraphic space telescope dedicated to the spectro-polarimetric analysis of gaseous and icy giant planets as well as super-Earths at visible wavelengths. So far, studies for high-contrast imaging instruments have mainly focused on technical feasibility because of the challenging planet/star flux ratio of 10-8-10-10 required at short separations (200 mas or so) to image cold exoplanets. However, the analysis of planet atmospheric/surface properties has remained largely unexplored. Aims. The aim of this paper is to determine which planetary properties SPICES or an equivalent direct imaging mission can measure, considering realistic reflected planet spectra and instrument limitation. Methods. We use numerical simulations of the SPICES instrument concept and theoretical planet spectra to carry out this performance study. Results. We find that the characterization of the main planetary properties (identification of molecules, effect of metallicity, presence of clouds and type of surfaces) would require a median signal-to-noise ratio of at least 30. In the case of a solar-type star \leq 10 pc, SPICES will be able to study Jupiters and Neptunes up to ~5 and ~2 AU respectively. It would also analyze cloud and surface coverage of super-Earths of radius 2.5 RE at 1 AU. Finally, we determine the potential targets in terms of planet separation, radius and distance for several stellar types. For a Sun analog, we show that SPICES could characterize Jupiters (M \geq 30 ME) as small as 0.5 Jupiter radii at ~2 AU up to 10 pc, and super-Earths at 1-2 AU for the handful of stars that exist within 4-5 pc. Potentially, SPICES could perform analysis of a hypothetical Earth-size planet around alpha Cen A and B.

Direct detection and spectral characterization of outer exoplanets with the SPICA coronagraph instrument (SCI)

The SPICA coronagraph instrument (SCI) provides high-contrast imaging and moderate resolution (R<200) spectroscopy at the wavelength range from 3.5 to 27μm. Based on the planet evolutional model calculated by Burrows et al. (2003), SCI will search for gas giant planets down to one Jupiter mass around nearby young (1Gyr) stars and two Jupiter masses around nearby old (5Gyr) stars. SCI also allows to characterizing those planets of less than 1Gyr by spectroscopic observations to reveal the nature of planetary formation and evolution.Focusing on the high sensitivity and high contrast at wavelengths longer than 10μm, we show that SCI also allows us to directly image icy giant planets like Uranus and Neptune as well as gas giant planets around nearby early-type stars. In this paper, we compare the capabilities of SCI and the JWST coronagraphs and also discuss a new approach to answering questions concerning the formation and evolution of planetary systems through planet detection with SCI.