Atmosphere Of Uranus Research Articles

Radiative-convective models of the atmospheres of Uranus and Neptune: Heating sources and seasonal effects

Radiative-convective models of the atmospheres of Uranus and Neptune: Heating sources and seasonal effects

Relativistic Chiral Inversion of Non-Zero Nuclear Magnetic Moments During Centrifugal Industrial Fermentative Processes

Fermentative productions of amino acids are thought to retain homochirality. This work can propose rotational motions for accelerations by centrifugations during fermentative processes enrich heavier, stable isotopes of differing nuclear magnetic moments (NMMs) with the consequent possibility of altered chirality and magnitude of rotating polarized light for altering chemical, physical, and biological phenomena of such enantiomers. The author discovered gravity seeding homochirality. Thereby in this work, the author determined that altered gravity can change the magnitude and direction of chirality. In this work, the changes in isotopes alter gravitational effects on the metabolism of enantiomers in hosts eating fermentative produced amino acids. Cancer was determined from eating heavier isotopes from fermentative processes and cancer cells were reasoned in this work to be more affected as the earth’s gravitational field is reduced by travel away from the earth’s surface. Similarly accelerated motional and gravitational effects on isotopes in planetary atmospheres of Earth, Saturn, Jupiter, and Uranus by 14N, 15N, 1H, 3He, 13C and 17O were reasoned in this work to cause lightning. Such solvents of positive NMMs were reasoned by collisional exchange to accumulate and exchange charges in clouds and collisional exchange solute negative NMMs were reasoned to alter charge for the production of leaders and streamers to induce lightning strikes. Lack of sufficient 13C and 17O in the CO2 of Venus’ atmosphere is explained in this work to mediate the lack of charge accumulation and discharge for explaining lack of lightning on Venus. The lower temperature (−214 °C) and raining N2 (boiling temperature of −210 °C) in Neptune were reasoned in this work to strips 15N from the atmosphere of Neptune for explaining its lack of lightning despite its similar atmospheric composition but warmer climate (−193 °C) of Uranus. The charged 1H atmosphere of Neptune is thereby the basis for charging Neptune’s atmosphere and the author thereby explains the disappearance and reappearance of Neptune’s clouds by magnetic coupling of the charged atmosphere of Neptune to Sun’s magnetic field.

The Upper Atmosphere of Uranus from Stellar Occultations. I. Methods and Validation

Measurements made by the Voyager 2 spacecraft during its flybys of Uranus in 1986 found warm stratospheric and hot thermospheric temperatures that cannot be explained by solar energy alone. It contributes to what has become known as the “giant planet energy crisis”; there is a fundamental lack of understanding of the energy balance of giant planets in the solar system. Uranus, in particular, has both the hottest thermospheric temperatures and the weakest internal heat flux of all four giant planets. Moreover, the Voyager 2 UV temperature measurements are at odds with the many contemporaneous Earth-based stellar occultation observations. In this work, we examine the 1977 Uranus stellar occultation (U0) and compare the observed light curve to reported Voyager 2 temperature profiles by simulating the latter into stellar occultation light curves. In this investigation, we find that the observed light curves are in tension with the simulated light curves to a high degree of statistical confidence. Next, we reprocess the U0 light curves using a modern approach, with some significant adjustments described herein, and report updated profiles. We find that the lower thermosphere of Uranus is much cooler than the Voyager 2 profiles suggest but slightly warmer than those originally published from the U0 occultation. In Paper II, we will present the results of applying these methods to many of the dozens of archival Uranus stellar occultations.

One Model to Describe Them All—Well, Ice Giants Anyway

The spectral responses of the atmospheres of Uranus and Neptune can now both be fully characterized with the same model, an achievement that has implications for characterizing the atmospheres of exoplanets.

Assessment of the feasibility of space-based stellar occultation observations of Uranus and Neptune

UV occultation measurements made by Voyager 2 of the upper atmospheres of Uranus and Neptune in the late 1980s found both planets contain warm stratospheres and extremely hot thermospheres. Sunlight reaching the ice giants is far too weak to account for these reported temperatures. This is sometimes called the Giant Planet Energy Crisis. Theorized mechanisms to account for the heating observed at Uranus and Neptune suffer from a dearth of reliable observations. A similar energy crisis was reported from Voyager observations of Jupiter and Saturn, but observations made by the Galileo, Juno, and Cassini-Huygens missions as well as from ground-based facilities have vastly improved efforts to understand upper atmospheric heating. For Uranus and Neptune, Earth-based stellar occultation observations can fill in this significant gap in observations and produce reliable atmospheric measurements. These measurements can also assist in motivating and planning for a flagship mission to either ice giant.A stellar occultation occurs when a solar system body passes across the line of sight from an observer to a star. Refraction of starlight by the upper atmosphere of the occulting body is observed as a light curve and can be used to sample the temperature structure with high vertical resolution. In this paper, we demonstrate that observing stellar occultations by Uranus and Neptune from satellites in low-Earth orbit can make high-quality upper atmospheric measurements of the Ice Giants. Such observations would not be subjected to atmospheric scintillation or disruption due to weather and could be observed at any time of day. We predict that from 2025 through 2035, as many as 56 Uranus and 14 Neptune occultations could be observed from orbit, compared to 20 Uranus and 6 Neptune events from the ground. We find many of these events can be observed with comparable or higher signal-to-noise via a small, Earth-orbiting satellite than a large ground-based observatory and encourage others to explore the launch feasibility of one or more such missions.

Electron Number Density Measurements in a Saturn Entry Condition

The aerodynamic heating experienced by capsules entering into the atmospheres of Saturn, Uranus, and Neptune is greatly affected by chemically nonequilibrium processes occurring in the shock layers. There are several reaction schemes available in numerical predictions for hydrogen dissociation and ionization, and more experimental data would assist verifying these existing models. This paper reports the results of electron number density measurements conducted in the X2 expansion tube at the University of Queensland using a condition representative of a proposed Saturn entry, where significant nonequilibrium effects in the shock layer are expected. Electron number density along the stagnation streamline was obtained from Stark broadening. The data presented here provide independent measurements for evaluating the reaction schemes for the conditions created. It was found that the experimental data were qualitatively modeled by a contemporary kinetic model. Quantitative agreement between experimental and numerical data was found by adjusting the ionization rate coefficients from an existing reaction scheme by a factor of 25. This updated reaction rate set was also cross-validated with electron number density measurements in NASA’s shock tube tests. The adjusted rates had better agreement than using the original rates, and quantitative agreement can be found in the high-density cases.

Ice Giant Atmospheric Science

This white paper, written in support of NASA's 2023-2032 Planetary Decadal Survey, outlines 10 major questions that focus on the origin, evolution, and current processes that shape the atmospheres of Uranus and Neptune. Prioritizing these questions over the next decade will greatly improve our understanding of this unique class of planets, which have remained largely unexplored since the Voyager flybys. Studying the atmospheres of the Ice Giants will greatly inform our understanding of the origin and evolution of the solar system as a whole, in addition to the growing number of exoplanetary systems that contain Neptune-mass planets.

The upper atmospheres of Uranus and Neptune.

We review the current understanding of the upper atmospheres of Uranus and Neptune, and explore the upcoming opportunities available to study these exciting planets. The ice giants are the least understood planets in the solar system, having been only visited by a single spacecraft, in 1986 and 1989, respectively. The upper atmosphere plays a critical role in connecting the atmosphere to the forces and processes contained within the magnetic field. For example, auroral current systems can drive charged particles into the atmosphere, heating it by way of Joule heating. Ground-based observations of H3+ provides a powerful remote diagnostic of the physical properties and processes that occur within the upper atmosphere, and a rich dataset exists for Uranus. These observations span almost three decades and have revealed that the upper atmosphere has continuously cooled between 1992 and 2018 at about 8 K/year, from approximately 750 K to approximately 500 K. The reason for this trend remain unclear, but could be related to seasonally driven changes in the Joule heating rates due to the tilted and offset magnetic field, or could be related to changing vertical distributions of hydrocarbons. H3+ has not yet been detected at Neptune, but this discovery provides low-hanging fruit for upcoming facilities such as the James Webb Space Telescope and the next generation of 30 m telescopes. Detecting H3+ at Neptune would enable the characterization of its upper atmosphere for the first time since 1989. To fully understand the ice giants, we need dedicated orbital missions, in the same way the Cassini spacecraft explored Saturn. Only by combining in situ observations of the magnetic field with in-orbit remote sensing can we get the complete picture of how energy moves between the atmosphere and the magnetic field. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Convective storms and atmospheric vertical structure in Uranus and Neptune.

The ice giants Uranus and Neptune have hydrogen-based atmospheres with several constituents that condense in their cold upper atmospheres. A small number of bright cloud systems observed in both planets are good candidates for moist convective storms, but their observed properties (size, temporal scales and cycles of activity) differ from moist convective storms in the gas giants. These clouds and storms are possibly due to methane condensation and observations also suggest deeper clouds of hydrogen sulfide (H2S) at depths of a few bars. Even deeper, thermochemical models predict clouds of ammonia hydrosulfide (NH4SH) and water at pressures of tens to hundreds of bars, forming extended deep weather layers. Because of hydrogen's low molecular weight and the high abundance of volatiles, their condensation imposes a strongly stabilizing vertical gradient of molecular weight larger than the equivalent one in Jupiter and Saturn. The resulting inhibition of vertical motions should lead to a moist convective regime that differs significantly from the one occurring on nitrogen-based atmospheres like those of Earth or Titan. As a consequence, the thermal structure of the deep atmospheres of Uranus and Neptune is not well understood. Similar processes might occur at the deep water cloud of Jupiter in Saturn, but the ice giants offer the possibility to study these physical aspects in the upper methane cloud layer. A combination of orbital and in situ data will be required to understand convection and its role in atmospheric dynamics in the ice giants, and by extension, in hydrogen atmospheres including Jupiter, Saturn and giant exoplanets. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Constraints on Neptune's haze structure and formation from VLT observations in the H-band

A 1-dimensional microphysics model has been used to constrain the structure and formation of haze in Neptune's atmosphere. These simulations were coupled to a radiative-transfer and retrieval code (NEMESIS) to model spectral observations of Neptune in the H-band performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) in 2013. It was found that observations in the H-band and with emission angles ≤60° are largely unaffected by the imaginary refractive index of haze particles, allowing a notable reduction of the free parameters required to fit the observations. Our analysis shows a total haze production rate of (2.61 ± 0.18) × 10−14 kg m−2 s−1, about 10 times larger than that found in Uranus's atmosphere, and a particle electric charge of q = 8.6 ± 1.1 electrons per μm radius at latitudes between 5 and 15° S. This haze production rate in Neptune results in haze optical depths about 10 times greater than those in Uranus. The effective radius reff was found to be 0.22 ± 0.01 and 0.26 ± 0.02 μm at the 0.1 and 1-bar levels, respectively, with haze number densities of 8.48+1.78−1.31 and 9.31+2.52−1.91 particles per cm3. The fit at weak methane-absorbing wavelengths reveals also the presence of a tropospheric cloud with a total optical depth >10 at 1.46 μm. The tropospheric cloud base altitude was found near the 2.5-bar level, although this estimation may be only representative of the top of a thicker and deeper cloud.Our analysis leads to haze opacities about 3.5 times larger than that derived from Voyager-2 observations (Moses et al., 1995). This larger opacity indicates a haze production rate 2 times larger at least. To study this difference haze opacity or production rate, we performed a timescale analysis with our microphysical model to estimate the time required for haze particles to grow and settle out. Although this analysis shows haze timescales (~15 years) shorter than the time lapsed between Voyager-2 observations and 2013, the solar illumination at the top of the atmosphere has not varied significantly during this period (at the studied latitudes) to explain the increase in haze production. This difference in haze production rate derived for these two periods may arise from: a) the fact that in our analysis we employed spectral observations in the infrared (H-band), while Moses et al. (1995) used photometric images taken at 5 different filters in the visible. While high-phase-angle Voyager observations are more sensitive to small haze particles and at altitudes above the 0.1-bar level, the haze constraints derived from VLT spectra in H-band are limited to pressures greater than 0.1 bar. As a result of the different phase angles of the two set of observations, differences in the estimation of M0 may arise from the use of Mie phase functions as well. b) our 1-dimensional model does not account for latitudinal redistributions of the haze by dynamics. A possible meridional transport of haze with wind velocities greater than ~0.03 m s−1 would result in dynamics timescales shorter than 15 years and thus might explain the observed variations in the haze production rate during this period.Compared with our estimations, photochemical models point to even larger production rates on Neptune (by a factor of 2.4). Assuming that the photochemical simulations are correct, we found that this discrepancy can be explained if haze particles evaporate before reaching the tropospheric-cloud levels. This scenario would decrease the cumulative haze opacity above the 1-bar level, and thus a larger haze production rate would be required to fit our observations. However, to validate this haze vertical structure future microphysical simulations that include the evaporation rates of haze particles are required.

A Numerical Investigation of the Berg Feature on Uranus as a Vortex-Driven System

The Berg cloud feature in the atmosphere of Uranus was first identified as a persistent grouping of clouds located just off the bright South Polar Collar at a latitude of around −34 degrees. Ongoing observations of this feature through the 1990s and 2000s suggested that the feature was oscillating in location by a few degrees in latitude for several years, and then unexpectedly began to drift towards the equator, which continued over the final 4 years until the cloud dissipated. One possible explanation for such a persistent drifting cloud is that it is a cloud-vortex system, in which an unseen vortex drives the creation of the cloud and the motions of the vortex control the cloud location. To explore this possibility, a series of vortices are studied numerically using the Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM). The evolution of these test vortices are simulated to examine their drift rates and the potential for cloud formation. The results indicate that conditions on Uranus could result in an equatorward drifting vortex over a range of latitudes and that significant cloud formation could occur, potentially obscuring observations of the vortex.

Determining the origin of the building blocks of the Ice Giants based on analogue measurements from comets

ABSTRACT The abundances of the heavy elements and isotopic ratios in the present atmospheres of the giant planets can be used to trace the composition of volatiles that were present in the icy solid material that contributed to their formation. The first definitive measurements of noble gas abundances and isotope ratios at comet 67P/Churyumov–Gerasimenko (67P/C–G) were recently published by Marty et al. (2017) and Rubin et al. (2018, 2019). The implications of these abundances for the formation conditions of the 67P/C–G building blocks were then evaluated by Mousis et al. (2018a). We add here an analysis of the implications of these results for understanding the formation conditions of the building blocks of the Ice Giants and discuss how future measurements of Ice Giant atmospheric composition can be interpreted. We first evaluate the best approach for comparing comet observations with giant planet composition, and then determine what would be the current composition of the Ice Giant atmospheres based on four potential sources for their building blocks. We provide four scenarios for the origin of the Ice Giants building blocks based on four primary constraints for building block composition: (1) the bulk abundance of carbon relative to nitrogen, (2) noble gas abundances relative to carbon and nitrogen, (3) abundance ratios Kr/Ar and Xe/Ar, and (4) Xe isotopic ratios. In situ measurements of these quantities by a Galileo-like entry probe in the atmosphere(s) of Uranus and/or Neptune should place important constraints on the formation conditions of the Ice Giants.

Measuring the D/H Ratios of Exoplanets and Brown Dwarfs

Abstract The relative abundance of deuterium and hydrogen is a potent tracer of planet formation and evolution. Jupiter and Saturn have protosolar atmospheric D/H ratios, a relic of substantial gas accretion from the nebula, while the atmospheres of Neptune and Uranus are enhanced in D by accretion of ices into their envelopes. For terrestrial planets, D/H ratios are used to determine the mechanisms of volatile delivery and subsequent atmosphere loss over the lifetime of the planet. Planets and brown dwarfs more massive than ∼13 M J quickly fuse their initial D reservoir. Here, we simulate spectra for giant exoplanets and brown dwarfs (2 M Neptune to ∼10 M Jupiter) from T eff = 200–1800 K including both CH3D and HDO to determine the observability of these dominant deuterium isotopologues in mid-infrared thermal emission spectra. Colder objects have stronger molecular features in their spectra, due to the temperature dependence of molecular cross sections. CH3D is easier to observe than HDO at all temperatures considered, due to the strength of its absorption bands and locations of features at wavelengths with few other strong absorption features. We predict that for nearby cool brown dwarfs, the CH3D abundance can be measured at high signal to noise with the James Webb Space Telescope (JWST); for objects from 200 to 800 K closer than 10 pc, a protosolar D/H ratio would be readily observable in 2.5 hr. Moderately young Jupiter-mass planets (100–300 Myr) and young Neptunes (10 Myr) may be discovered with JWST and provide the best targets for detecting deuterium on an exoplanet in the coming decade. Future telescope designs should consider the importance of isotopes for understanding the formation and evolution of planetary atmospheres.

The H3+ ionosphere of Uranus: decades-long cooling and local-time morphology.

The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H3+ obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H3+ intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H3+ signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset.This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H3+, H5+ and beyond’.

Constraints on Uranus's haze structure, formation and transport

Microphysical simulations have been performed to constrain the formation and structure of haze in Uranus's atmosphere. These simulations were coupled to a radiative-transfer code to fit observations performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) and by the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST) in 2014. Our simulations yield an effective radius of ∼0.2 μm for the haze particles in the tropopause and a density of ∼2.9 particles per cm3. Our simulations also provide an estimate for the haze production rate in the stratosphere of between ∼3.10−16 and 3.10−15 kg m−2 s−1, about 100 times smaller than that found in Titan's atmosphere (e.g. Rannou et al., 2004). This range of values is very similar to that derived by Pollack et al. (1987) from Voyager-2 observations in 1986, suggesting microphysical timescales greater than the elapsed time between these observations (28 years, or 1/3 of a Uranian year). This result is in agreement with analyses performed with our microphysical model that show timescales for haze particles to grow and settle out to be >∼30 years at pressure levels >0.1 bar. However, these timescales are too big to explain the observed variations in the haze structure over Uranus's northern hemisphere after 2007 equinox (e.g. de Pater et al., 2015). This indicates that dynamics may be the main factor controlling the spatial and temporal distribution of the haze over the poles. A meridional stratospheric transport of haze particles with winds velocities >∼0.025 m s−1 would result in dynamics timescales shorter than 30 years and thus may explain the observed variations in the haze structure.

Seasonal change of CH4/H2 ratio in the atmosphere of Uranus

Uranus is mainly composed of elements heavier than hydrogen and helium, such as C, S, even though very little information about these elements has been detected. Methane is one of the most important component of the atmosphere of Uranus. Analysis of the full-disk spectra of Uranus, as observed by the Xinglong 2.16m telescope with BFOSC (Beijing-Faint Object Spectrograph and Camera) on August 26 2016, was performed. A simple empirical model was used to fit the several years of spectrum albedo data to determine the variations of the CH4/H2 ratio in the atmosphere of Uranus. These data include: spectra observed by the 1.52m telescope (1994 and 1995) on La Silla, Chile; spectra observed by the HST-STIS (2002 and 2003); and spectra observed by the Xinglong 2.16m telescope (2016). The interannual change of the CH4/H2 ratio can be derived by comparing the fit results of various spectral albedos. The comparative results indicate that the CH4/H2 ratio in the atmosphere of Uranus declined from 1994 until 2002, and then began to rise thereafter. From the results, we speculate that the variation period of CH4/H2 ratio in the atmosphere of Uranus is longer than twenty years. We were unable to confirm whether the period is 84 or 42 years.

Analysis of the origin of water, carbon monoxide, and carbon dioxide in the Uranus atmosphere

Context. We present here an analysis of the potential sources of oxygen species in the Uranus atmosphere. Aims. Our aim is to explain the current measurements of H2O, CO, and CO2 in the Uranus atmosphere, which would allow us to constrain the influx of oxygen-bearing species and its origin in this planet. Methods. We used a time-dependent photochemical model of the Uranus atmosphere to ascertain the origin of H2O, CO, and CO2. We thoroughly investigated the evolution of material delivered by a cometary impact, together with a combined source, i.e. cometary impact and a steady source of oxygen species from micrometeoroid ablation. Results. We find that an impactor in the size range ~1.2–3.5 km hitting the planet between 450 and 822 yr ago could have delivered the CO currently seen in the Uranus stratosphere. Given the current set of observations, an oxygen-bearing species supply from ice grain ablation cannot be ruled out. Our study also indicates that a cometary impact cannot be the only source for rendering the observed abundances of H2O and CO2. The scenarios in which CO originates by a cometary impact and H2O and CO2 result from ice grain sublimation can explain both the space telescope and ground-based data for H2O, CO, and CO2. Similarly, a steady influx of water, carbon monoxide, and carbon dioxide, and a cometary impact delivering carbon monoxide give rise to abundances matching the observations. The time evolution of HCN also delivered by a cometary impact (as 1% of the CO in mass), when discarding chemical recycling of HCN once it is lost by photolysis and condensation, produces a very low stratospheric abundance which could be likely non-detectable. Consideration of N2-initiated chemistry could represent a source of HCN allowing for a likely observable stratospheric mixing ratio. Conclusions. Our modelling strongly indicates that water in the Uranus atmosphere likely originates from micrometeroid ablation, whereas its cometary origin can be discarded with a very high level of confidence. Also, we cannot firmly constrain the origin of the detected carbon monoxide on Uranus as a cometary impact, ice grain ablation, or a combined source due to both processes can give rise to the atmospheric mixing ratio measured with the Herschel Space Observatory. To establish the origin of oxygen species in the Uranus atmosphere, observations have to allow the retrieval of vertical profiles or H2O, CO, and CO2. Measurements in narrow pressure ranges, i.e. basically one pressure level, can be reproduced by different models because it is not possible to break this degeneracy about these three oxygen species in the Uranian atmosphere.

Detection of hydrogen sulfide above the clouds in Uranus's atmosphere

Detection of hydrogen sulfide above the clouds in Uranus's atmosphere

Solar‐Driven Variation in the Atmosphere of Uranus

AbstractLong‐term measurements (1972–2015) of the reflectivity of Uranus at 472 and 551 nm display variability that is incompletely explained by seasonal effects. Spectral analysis shows that this nonseasonal variability tracks the 11 year solar cycle. Two mechanisms could cause solar modulation: (a) nucleation onto ions or electrons created by galactic cosmic rays (GCR) or (b) UV‐induced aerosol color changes. Ion‐aerosol theory is used to identify expected relationships between reflectivity fluctuations and GCR flux, tested with multiple regression and compared to the linear response predicted between reflectivity and solar UV flux. The statistics show that 24% of the variance in reflectivity fluctuations at 472 nm is explained by GCR ion‐induced nucleation, compared to 22% for a UV‐only mechanism. Similar GCR‐related variability exists in Neptune's atmosphere; hence, the effects found at Uranus provide the first example of common variability in two planetary atmospheres driven through energetic particle modulation by their host star.

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.