Horus Research Articles

H2–H2O demixing in Uranus and Neptune: Adiabatic structure models

Context. Demixing properties of major planetary constituents influence the interior structure and evolution of planets. Comparing experimental and computational data on the miscibility of hydrogen and water to adiabatic profiles suggests that phase separation between these two components occurs in the ice giants Uranus and Neptune. Aims. We aim to predict the atmospheric water abundance and transition pressure between the water-poor outer envelope and the water-rich deep interior in Uranus and Neptune. Methods. We constructed seven H2–H2O phase diagrams from the available experimental and computational data. We computed interior adiabatic structure models and compared these to the phase diagrams to infer whether demixing occurred. Results. We obtain a strong water depletion in the top layer due to the rain-out of water and find upper limits on the atmospheric water-mass fraction Zatm of 0.21 for Uranus and 0.16 for Neptune. The transition from the water-poor to the water-rich layer is sharp and occurs at pressures PZ between 4 and 11 GPa. Using these constraints on Zatm and PZ, we find that the observed gravitational harmonics J2 and J4 can be reproduced if PZ ≳ 10 GPa in Uranus and ≳5 GPa in Neptune, and if the deep interior has a high primordial water-mass fraction of 0.8, unless rocks are also present. The agreement with J4 is improved if rocks are confined deeper than PZ, for instance, below a rock cloud level at 2000 K (20–30 GPa). Conclusions. These findings confirm classical few-layer models and suggest that a layered structure may result from a combination of primordial mass accretion and subsequent phase separation. Reduced observational uncertainty in J4 and its dynamic contribution, atmospheric water abundance measurements from the Uranus Orbiter and Probe (UOP) or a Neptune mission, and better understanding of the mixing behaviour of constituents are needed to constrain the interiors of ice giants.

New Moons of Uranus and Neptune from Ultradeep Pencil-beam Surveys

We have conducted extremely ultradeep pencil-beam observations for new satellites around both Uranus and Neptune. Tens of images on several different nights in 2021, 2022, and 2023 were obtained, shifted, and added together to reach as faint as 26.9 and 27.2 mag in the r band around Uranus and Neptune, respectively. One new moon of Uranus, S/2023 U1, and two new moons of Neptune, S/2021 N1 and S/2002 N5, were found. S/2023 U1 was 26.6 mag, is about 7 km in diameter, and has a distant, eccentric, and inclined retrograde orbit similar to Caliban and Stephano, implying these satellites are fragments from a once larger parent satellite. S/2021 N1 was 26.9 mag, about 14 km in size, and has a retrograde orbit similar to Neso and Psamathe, indicating they are a dynamical family. We find S/2021 N1 is in Kozai–Lidov orbital resonance. S/2002 N5 was 25.9 mag, is about 23 km in size, and it makes a family of distant prograde satellites with Sao and Laomedeia. This survey mostly completes the outer satellites of Uranus to about 8 km and Neptune to about 14 km in diameter. The size distributions of satellite dynamical families around the giant planets shows a strong steepening in the power-law size distribution smaller than 5 km in diameter. The satellites of a family become much more common at diameters smaller than 5 km and their size distribution is consistent with a collisional breakup of a once larger parent satellite.

The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous chondrites.

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu/CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites/Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

Looking for Subsurface Oceans Within the Moons of Uranus Using Librations and Gravity

AbstractSeveral of the icy moons in the Jupiter and Saturn systems appear to possess internal liquid water oceans. Our knowledge of the Uranian moons is more limited but a future tour of the system has the potential to detect subsurface oceans. Planning for this requires an understanding of how the moons' internal structures—with and without oceans—relate to observable quantities. Here, we show that the amplitude of forced physical librations could be diagnostic of the presence or absence of subsurface oceans within the Uranian moons. In the presence of a decoupling global ocean, ice shell libration amplitudes at Miranda, Ariel, and Umbriel will exceed 100 m if the shells are thick. The presence of oceans could also imply significant tidal heating within the last few hundred million years. Combining librations with the quadrupole gravity field could provide comprehensive constraints on the internal structures and histories of the Uranian moons.

The Astrobiological Potential of the Uranian Moon System.

The 2023-2032 Planetary Science and Astrobiology Decadal Survey prioritized the Uranus Orbiter and Probe (UOP) mission concept as the next priority flagship mission. The UOP concept includes scientific studies of the Uranian moon system. Although the Uranian moons differ greatly from the ocean worlds in the Jovian and Saturnian systems, the emerging hypothesis is that some of them could at least sustain thin, potentially concentrated, oceans. Herein, we make a case that these moons are important and interesting targets of astrobiological research. Studying these worlds would provide critical astrobiological data related to their habitability, including origin, evolution, and potential death, as well as the formation and evolution of ocean worlds more broadly. There is a strong need for research that connects astrobiology to modeling and experimentation to better characterize the possible conditions of these worlds, and this will be critical in formulating and maximizing the potential science that could be done by a Uranus flagship mission.

Insights on the Formation Conditions of Uranus and Neptune from Their Deep Elemental Compositions

This study, placed in the context of the preparation for the Uranus Orbiter Probe mission, aims to predict the bulk volatile compositions of Uranus and Neptune. Using a protoplanetary disk model, it examines the evolution of trace species through vapor and solid transport as dust and pebbles. Due to the high carbon abundance found in their envelopes, the two planets are postulated to have formed at the carbon monoxide ice line within the protosolar nebula. The time evolution of the abundances of the major volatile species at the location of the CO ice line is then calculated to derive the abundance ratios of the corresponding key elements, including the heavy noble gases, in the feeding zones of Uranus and Neptune. Supersolar metallicity in their envelopes likely results from accreting solids in these zones. Two types of solids are considered: pure condensates (Case 1) and a mixture of pure condensates and clathrates (Case 2). The model, calibrated to observed carbon enrichments, predicts deep compositions. In Case 1, argon is deeply depleted, while nitrogen, oxygen, krypton, phosphorus, sulfur, and xenon are significantly enriched relative to their protosolar abundances in the two planets. Case 2 predicts significant enrichments for all species, including argon, relative to their protosolar abundances. Consequently, Case 1 predicts near-zero Ar/Kr or Ar/Xe ratios, while Case 2 suggests that these ratios are 0.1 and 0.5–1 times their protosolar ratios, respectively. Both cases predict a bulk sulfur-to-nitrogen ratio consistent with atmospheric measurements.

JWST Spectrophotometry of the Small Satellites of Uranus and Neptune

We use 1.4–4.6 μm multiband photometry of the small inner Uranian and Neptunian satellites obtained with the James Webb Space Telescope’s near-infrared imager NIRCam to characterize their surface compositions. We find that the satellites of the ice giants have, to first order, similar compositions to one another, with a 3.0 μm absorption feature possibly associated with an O-H stretch, indicative of water ice or hydrated minerals. Additionally, the spectrophotometry for the small ice-giant satellites matches spectra of some Neptune Trojans and excited Kuiper Belt objects, suggesting shared properties. Future spectroscopy of these small satellites is necessary to identify and better constrain their specific surface compositions.

Uranus Orbiter and Probe: A Radio Science Investigation to Determine the Planet’s Gravity Field, Depth of the Winds, and Tidal Deformations

The most recent Planetary Science and Astrobiology Decadal Survey has proposed Uranus as the target for NASA’s next large-scale mission. The interior structure and atmosphere of the planet are currently poorly understood, and objectives for investigating Uranus’s deeper regions and composition are highly ranked. Traditionally, gravity science has served as one of the primary means for probing the depths of planetary bodies and inferring their internal density distributions. In this work, we present precise numerical simulations of an onboard radio science experiment designed to determine Uranus’s gravity field and tidal deformations, which would offer a rare view into the planet’s interior. We focus on the mission’s orbital planning, discussing crucial parameters such as the number of pericenter passes, orbital inclination, and periapsis altitude necessary to meet the gravity measurement requirements for a Uranus orbiter. Our findings suggest that eight close encounters may be sufficient to determine the zonal gravity field up to J 8 with a relative accuracy of 10%, if the trajectory is optimized. This would allow for the decoupling of the gravity field components due to interior structure and zonal winds. Additionally, we find that the expected end-of-mission uncertainty on Uranus’s Love number k 22 is of order ∼0.01 (3σ). This level of accuracy may offer crucial information about Uranus’s inner state and allow for discriminating between a liquid and solid core, thus shedding light on crucial aspects of the planet’s formation and evolution.

Radio Science Experiments during a Cruise Phase to Uranus

The exploration of Uranus, a key archetype for ice giant planets and a gateway to understanding distant exoplanets, is acquiring increasing interest in recent years, especially after the Uranus Orbiter and Probe (UOP) mission has been prioritized in the Planetary Science Decadal Survey 2023–2032. This paper presents the results of numerical simulations aimed at providing experimental constraints on the parameterized post-Newtonian (PPN) parameter γ, a measure of space–time curvature in general relativity (GR), during the cruise phase of a spacecraft travelling to Uranus. Leveraging advanced radio tracking systems akin to those aboard the JUICE and BepiColombo missions, we explore the potential of solar conjunction experiments (SCEs) to refine current measurements of γ by exploiting the spacecraft’s long journey in the outer Solar System. We discuss the anticipated enhancements over previous estimates, underscoring the prospect of detecting violations of GR. Our simulations predict that by using an advanced radio tracking system, it is possible to obtain an improvement in the estimation of γ up to more than an order of magnitude with respect to the latest measurement performed by the Cassini–Huygens mission in 2002, contingent on the calibration capabilities against solar plasma noise. The results reveal that a number of SCEs during the mission can substantially strengthen the validation of GR. In tandem with fundamental physics tests, the use of radio links during SCEs presents a valuable opportunity to dissect the solar corona’s plasma dynamics, contributing to solar physics and space weather forecasting. This paper also enumerates methodologies to analyze electron density, localize plasma features, and deduce solar wind velocity, enriching the scientific yield of the experiments beyond the primary objective of testing GR during the cruise phase of a mission to Uranus.

The equilibrium vapor pressures of ammonia and oxygen ices at outer solar system temperatures

Few laboratory studies have investigated the vapor pressures of the volatiles that may be present as ices in the outer solar system; even fewer studies have investigated these species at the temperatures and pressures suitable to the surfaces of icy bodies in the Saturnian and Uranian systems (<100 K, <10−9 bar). This study adds to the work of Grundy et al. (2024) in extending the known equilibrium vapor pressures of outer solar system ices through laboratory investigations at very low temperatures. Our experiments with ammonia and oxygen ices provide new thermodynamic models for these species’ respective enthalpies of sublimation. We find that ammonia ice, and to a lesser degree oxygen ice, are stable at higher temperatures than extrapolations in previous literature have predicted. Our results show that these ices should be retained over longer periods of time than previous extrapolations would predict, and a greater amount of these solids is required to support observation in exospheres of airless bodies in the outer solar system.

Cratering and Tectonic History of the Largest Uranian Satellite, Titania: New Insights Enabled by Image Reprocessing

From heavily cratered Umbriel to extensively tectonized Miranda, Titania is an intermediary of the Uranian system: heavily cratered, yet tectonically modified. An outstanding mystery in Titania's crater population is its apparent relative lack of large (>30 km) craters. However, progress has been limited by the coverage and quality of images available. Here, we present a new map of Titania enabled by reprocessing Voyager images to reduce the effects of motion blur. Of note, we identify a network of fractures, a set of lineaments that may represent a large multi-ring impact structure, and newly identified catenae. These findings suggest Titania's crater population is missing large craters due to viscous relaxation, tectonic resurfacing, and/or planetocentric debris, and does not necessarily require cryovolcanic resurfacing. In preparation for future missions to the Uranian system, this work presents foundations for identifying imaging targets that can contribute to furthering our understanding of the history and evolution of the Uranian system in a broader context of icy satellite evolution.

Unraveling the Geologic History of Miranda’s Inverness Corona

Miranda is the only icy body whose surface is known to contain the enigmatic features called corona—ovoid to trapezoidal areas of deformation. In this work, we seek to constrain potential formation mechanisms for Inverness Corona, the youngest known region on Miranda. To do this, we created the first detailed geologic map of Inverness, enabling the creation of a stratigraphic column of the order of events that formed this region. We employed a previously published Digital Elevation Model of the northern region of Inverness Corona to analyze the spacing of features in the region, which we propose to be extensional in origin. From this, we estimate an approximate brittle ice shell thickness of 2.5–3.8 km at the time of the region’s formation, indicating that Miranda’s brittle ice shell may have been relatively thin in the geologically recent past. We propose that Inverness formed from extension driven by a rising diapir or ice-shell thickening from a recent orbital resonance with Umbriel. The Uranus Orbiter and Probe mission is the highest priority flagship mission recommendation of the 2023–2032 Planetary Science and Astrobiology Decadal Survey. As such, we suggest measurements related to imaging, composition, gravity, and ice-shell thickness to gain an understanding of the geologic and orbital histories of the Uranian satellites, which would have implications for the evolution of the system as a whole.

Dynamical Interactions and Mass Loss within the Uranian System

The origin and evolution of planetary rings and moons remains an active area of study, particularly as they relate to the impact history and volatile inventory of the outer solar system. The Uranian system contains a complex system of rings that are coplanar with the highly inclined planetary equator relative to the orbital plane. Uranus also harbors five primary regular moons that play an important role in the distribution of material that surrounds the planet. Here we present the results of a dynamical simulation suite for the Uranian system, intended to explore the interaction between the five primary regular moons and particles within the system. We identify regions of extreme mass loss within 40 planetary radii of Uranus, including eccentricity excitation of particle orbits at resonance locations that can promote moonlet formation within the rings. We calculate a total dynamical particle mass-loss rate of 35% within 0.5 × 106 yr and 40% mass loss within 107 yr. We discuss the implications for postimpact material, including dynamical truncation of stable ring locations and/or locations of moon formation promoted by dynamical excitation of ring material.

The Origin of Jupiter’s Obliquity

The origin of the 3.°12 obliquity of Jupiter’s spin axis to its orbit normal is unknown. Improved estimates of Jupiter’s moment of inertia rule out a previously proposed explanation involving a resonance with the precession of the inclined orbit of Uranus. We find that a nonadiabatic crossing of the resonance between Jupiter’s spin precession and the −f 5 + f 6 + g 6 mode could have tilted Jupiter to its present-day obliquity starting from a 0° primordial obliquity. This places constraints on the migration rates of the satellites Ganymede and Callisto.

Characterization of Icy Moon Hydrospheres Through Joint Inversion of Gravity and Magnetic Field Measurements

AbstractSeveral bodies in the outer solar system are believed to host liquid water oceans underneath their icy surfaces. Knowledge of the hydrosphere properties is essential for understanding and assessing their habitability. We introduce a methodology based on Bayesian inference that enables a robust characterization of the hydrosphere through the combination of gravity and magnetic induction data. The interior models retrieved are consistent with the geophysical observations, leading to probability distributions for the relevant interior properties. We apply this joint inversion approach to constrain Europa's hydrosphere with gravity and magnetic field measurements acquired by the Galileo mission. Our results indicate that the combination of these datasets allows simultaneous constraints on the ice shell and ocean thickness, enhancing our knowledge of the hydrosphere structure. This methodology is valuable for synergistic interior science investigations of several missions in development or in planning, including Europa Clipper, JUICE and the Uranus Orbiter and Probe.

Prospects for localizing Planet 9 with a future Uranus mission

ABSTRACT Past years have seen various publications attempting to explain the apparent clustering features of trans-Neptunian objects, the most popular explanation being an unconfirmed ‘Planet 9’. The recently proposed Uranus Orbiter and Probe mission by NASA’s Planetary Science and Astrobiology Decadal Survey could offer the opportunity to precisely determine Planet 9’s sky location and mass by carefully monitoring ranging data during the interplanetary cruise. We use Monte Carlo Markov chain methods to reconstruct simulated spacecraft trajectories in a simplified Solar system model containing Planet 9, providing an estimate of the mission’s localization capacity depending on sky location, Earth-spacecraft Doppler link noise level and data collection rate. We characterize the noise via the Allan deviation σA, scaled to the Cassini-era value $\sigma _{\rm A}^{\rm \scriptscriptstyle Cass} = 3 \times 10^{-15}$, finding that daily measurements of the spacecraft position can lead to ∼0.2 deg2 localization of Planet 9 (assuming M9 = 6.3 M⊕, d9 = 460 au). As little as a three-fold improvement in σA drastically decreases the sky localization area size to ∼0.01 deg2. Thus, we showcase that a future Uranus mission carries a significant potential also for non-Uranian science.

Effect of the inclination in the passage through the 5/3 mean motion resonance between Ariel and Umbriel

The orbits of the main satellites of Uranus are expected to slowly drift away owing to tides raised in the planet. As a result, the 5/3 mean motion resonance between Ariel and Umbriel was likely encountered in the past. Previous studies have shown that, in order to prevent entrapment in this resonance, the eccentricities of the satellites must be larger than ~0.01 at the epoch, which is hard to explain. On the other hand, if the satellites experience some temporary capture and then escape, the inclinations rise to high values that are not observed today. We have revisited this problem both analytically and numerically focussing on the inclination, using a secular two-satellite model with circular orbits. We show that if the inclination of Umbriel was around 0.15° at the time of the 5/3 resonance encounter, capture can be avoided in about 60% of the cases. Moreover, after the resonance crossing, the inclination of Umbriel drops to a mean value around 0.08°, which is close to the presently observed one. The final inclination of Ariel is distributed between 0.01° and 0.25° with a nearly equal probability, which includes the present mean value of 0.02°.

Stability maps for the 5/3 mean motion resonance between Ariel and Umbriel with inclination

AbstractThe evolution of the five largest satellites of Uranus during the crossing of the 5/3 mean motion resonance between Ariel and Umbriel is strongly affected by chaotic motion. Studies with numerical integrations of the equations of motion and analysis of Poincaré surface sections provided helpful insights to the role of chaos on the system. However, they lack of a quantification of this chaos in the phase-space. Here, we construct stability maps using the frequency analysis method. We determine that for low energies (small eccentricity and/or inclinations), the phase-space is mainly stable. As the energy increases, the chaotic regions replace the stable motion, until only small, localized libration regions remain stable.

Near-IR photometry of the small Uranian satellites with Keck at phase angles 0–2°

We present photometric measurements of the seven brightest small inner moons of Uranus (Puck, Belinda, Rosalind, Portia, Juliet, Desdemona and Cressida) obtained with the Keck II telescope at 1.6 and 2.2μm at phase angles of 0.026∘ and 0.36∘ in October–November 2019. We also applied our new calibration technique to the August 2015 data at 2.02∘ from Paradis et al. (2019). We show that the phase curves in the near-infrared are very similar to those at visible wavelengths, which suggests that the opposition surge at near-infrared wavelengths is also caused by both shadow hiding and coherent backscatter.All moons are either neutral in color or slightly reddish; Puck and Portia show a roughly 20% increase in their 2.2μm reflectivity compared to that at 1.6μm. This suggests that these moons are not icy bodies like the large classical moons; they are likely covered by a fluffy (∼80% void space) layer of carbonaceous material.

A Flagship-class Uranus Orbiter and Probe mission concept using aerocapture

The conceptual design for a Flagship-class Uranus Orbiter and Probe (UOP) mission using aerocapture is presented. Uranus is historically the least studied destination for aerocapture, primarily attributed to the lack of an engineering-level atmosphere model until UranusGRAM was released by NASA in 2021. The present study is the first detailed end-to-end study of a Uranus aerocapture mission concept taking into account constraints arising from launch vehicle performance, interplanetary trajectory, aerocapture vehicle design, thermal protection system, and probe delivery. The mission concept uses a Falcon Heavy Expendable launcher and a high-energy, fast-arrival V∞ Earth–Earth–Jupiter–Uranus (EEJU) gravity assist trajectory to deliver a 1400 kg orbiter and a 300 kg entry probe to Uranus. The aerocapture vehicle is a derivative of the Mars Science Laboratory entry system with extensive flight heritage, and uses the state-of-the-art HEEET thermal protection system. Compared to the current baseline UOP mission using conventional propulsive orbit insertion with a 13-year flight time and 5-year orbital mission at Uranus, the proposed 8-year aerocapture mission concept enables a 10-year orbital mission at Uranus within the budgetary and schedule constraints of a Flagship-class mission.