Horus Research Articles

Cupid is not Doomed Yet: On the Stability of the Inner Moons of Uranus

Some of the small inner moons of Uranus have very closely-spaced orbits. Multiple numerical studies have found that the moons Cressida and Desdemona, within the Portia subgroup, are likely to collide in less than 100 Myr. The subsequent discovery of three new moons (Cupid, Perdita, and Mab) made the system even more crowded. In particular, it has been suggested that the Belinda group (Cupid, Belinda, and Perdita) will become unstable in as little as 105 yr. Here we revisit the issue of the stability of the inner moons of Uranus using updated orbital elements and considering tidal dissipation. We find that the Belinda group can be stable on 108 yr timescales due to an orbital resonance between Belinda and Perdita. We find that tidal evolution cannot form the Belinda–Perdita resonance, but convergent migration could contribute to the long-term instability of the Portia group. We propose that Belinda captured Perdita into the resonance during the last episode of disruption and reaccretion among the inner moons, possibly hundreds of Myr ago.

Regional Impact Crater Mapping and Analysis on Saturn's Moon Dione and the Relation to Source Impactors

AbstractRecent dynamical modeling of the formation and evolution of the Saturnian satellites suggests that the ages of the mid‐sized inner moons (Mimas, Enceladus, Tethys, Dione, and Rhea) could be as young as 100 Myr. This estimate is in contrast to most previous modeling and observational work that suggest an age more contemporaneous with the formation of Saturn 4.5 Ga ago. Given the heritage of using craters to constrain surface ages, we examine the impact craters of Dione using imagery from NASA's Cassini ISS camera and analyze their size‐frequency distributions (SFDs) to understand impactor populations. We survey four areas across different geologic terrains and compare our crater counts to standard outer solar system production functions. In addition to crater counts, we study several crater types such as elliptical and polygonal to further examine the bombardment source for the craters. We find evidence for a Saturn‐specific planetocentric impactor source, as none of the standard production functions fit the data. We compare our Dione data with our work on Tethys and find similarly shaped SFDs between the satellites. However, Dione's surface has been extensively modified to remove smaller craters (D ∼ <5 km) and has been bombarded by larger impactors, creating more D ∼ >20 km on Dione than Tethys. In contrast, Tethys more generally represents an ancient unmodified surface within the Saturn system. More complete observations and assessment of the cratering records on the satellites of Uranus and Neptune's moon Triton would enable better constraints on the bombardment history of the Saturn system.

Exosphere-mediated migration of volatile species on airless bodies across the solar system

Surface-bound exospheres facilitate volatile migration across the surfaces of nearly airless bodies. However, such transport requires that the body can both form and retain an exosphere. To form a sublimation exosphere requires the surface of a body to be sufficiently warm for surface volatiles to sublime; to retain an exosphere, the ballistic escape and photodestruction rates and other loss mechanisms must be sufficiently low. Here we construct a simple free molecular model of exospheres formed by volatile desorption or sublimation. We consider the conditions for forming and retaining exospheres for common volatile species across the Solar System, and explore how three processes (desorption/sublimation, ballistic loss, and photodestruction) shape exospheric dynamics on airless bodies. Our model finds that the CO2 exosphere of Callisto is much too dense to be sustained by impact-delivered volatiles, but could be maintained by only ~7 ha (~0.07 km2) of exposed CO2 ice distributed across Callisto (and refreshed through mass wasting). We use our model to predict the peak surface locations of Callisto's CO2 exosphere along with other Galilean moons, which could be tested by JUICE observations. Our model finds that to maintain Iapetus' two-tone appearance, its dark Cassini Regio likely has unresolved exposures of water ice, perhaps in sub-resolution impact craters, that amount to up to approximately ~0.06% of its surface. In the Uranian system, we find that the CO2 deposits on Ariel, Umbriel, Titania, and Oberon are unlikely to have been delivered via impacts, but are consistent with both a magnetospheric origin, (as has been previously suggested) or sourced endogenously. We suggest that the leading/trailing CO2 asymmetries on these moons could result from exosphere-mediated volatile transport, and may be a seasonal equinox feature that could be largely erased by pole-to-pole volatile migration during the Uranian solstices. We calculate that ~2.4–6.4 mm thick layer of CO2 (depending the moon) could migrate about the surface of Uranus' large moons during a seasonal cycle. Our model also confirms that water migration to Mercury's polar cold traps is inefficient without self-shield against photodestroying UV light, and that Callisto's bright spires could be formed/maintained by exospherically deposited H2O.

Orbits of the Irregular Satellites of Uranus and Neptune

We report on an ephemerides update for the irregular satellites of Uranus and Neptune, including Nereid. Our data set contains ground-based and Voyager 2 position measurements of the satellites. Some of the new observations were reduced with respect to the GAIA star catalog and represent high-quality, modern astrometry. The orbital fits are based on the numerically integrated equations of motion. We summarized results in terms of state vectors, orbital uncertainties, and mean elements. A large number of the irregulars have not been followed up in over a decade, and our study shows that their orbits still contain significant uncertainties.

The Influence Of Individual Stars On The long-Term Dynamics Of Comets C/2014 UN271 And C/2017 K2

Context. In June 2021, the discovery of an unusual comet C/2014 UN271 (Bernardinelli-Bernstein) was announced. Its cometary activity beyond the orbit of Uranus has also refreshed interest in similar objects, including C/2017 K2 (PANSTARRS). Another peculiarity of these objects is the long interval of positional data, taken at large heliocentric distances. Aims. These two comets are suitable candidates for a detailed investigation of their long-term motion outside the planetary zone. Using the carefully selected orbital solutions, we aim to estimate the orbital parameters of their orbits at the previous perihelion passage. This might allow us to discriminate between dynamically old and new comets. Methods. To follow the dynamical evolution of long-period comets far outside the planetary zone, it is necessary to take into account both the perturbation caused by the overall Galactic gravitational potential and the actions of individual stars appearing in the solar neighborhood. To this aim, we applied the recently published methods based on the ephemerides of stellar perturbers. Results. For C/2014 UN271, we obtained a precise orbital solution that can be propagated into the past and the future. For C/2017 K2, we have to limit ourselves to studying the past motion because some signs of nongravitational effects can be found in recent positional observations. Therefore, we use a specially selected orbital solution suitable for past motion studies. Using these starting orbits, we propagated both comets to their previous perihelia. We also investigated the future motion of C/2014 UN271. Conclusions. The orbital evolution of these two comets appears to be sensitive to perturbations from several stars that closely approach the Sun. To the detriment of our analysis, the errors on the 6D data for some of these stars are too large to obtain definitive results for the studied comets; nevertheless, we deduce that both comets were probably outside the planetary zone in the previous perihelion.

Coaccretion + Giant-impact Origin of the Uranus System: Tilting Impact

Abstract The origin of the Uranian satellite system remains uncertain. The four major satellites have nearly circular, coplanar orbits, and the ratio of the satellite system to planetary mass resembles Jupiter’s satellite system, suggesting the Uranian system was similarly formed within a disk produced by gas coaccretion. However, Uranus is a retrograde rotator with a high obliquity. The satellites orbit in its highly tilted equatorial plane in the same sense as the planet’s retrograde rotation, a configuration that cannot be explained by coaccretion alone. In this work, we investigate the first stages of the coaccretion + giant-impact scenario proposed by Morbidelli et al. (2012) for the origin of the Uranian system. In this model, a satellite system formed by coaccretion is destabilized by a giant impact that tilts the planet. The primordial satellites collide and disrupt, creating an outer debris disk that can reorient to the planet’s new equatorial plane and accrete into Uranus’ four major satellites. The needed reorientation out to distances comparable to outermost Oberon requires that the impact creates an inner disk with ≥1% of Uranus’ mass. We here simulate giant impacts that appropriately tilt the planet and leave the system with an angular momentum comparable to that of the current system. We find that such impacts do not produce inner debris disks massive enough to realign the outer debris disk to the post-impact equatorial plane. Although our results are inconsistent with the apparent requirements of a coaccretion + giant-impact model, we suggest alternatives that merit further exploration.

Mapping Long-Term Natural Orbits about Titania, a Satellite of Uranus

Close polar and circular orbits are of great interest for the exploration of natural satellites. There are still no studies in the literature investigating orbits around Titania, the largest satellite of Uranus. In this work, we present results of a set of numerical simulations carried out to obtain long-duration orbits for a probe around Titania. Through an expansion of the gravitational potential up to second order, the asymmetry of the gravitational field due to Titania’s coefficient C22, the zonal coefficient J2, and the gravitational perturbation of Uranus is considered. The analysis of lifetime sensitivity due to possible errors in the values of J2 and C22 is investigated using multiple regression models. Simulations were performed for different eccentricity values, and lifetime maps were constructed. The results show that low-altitude and near-circular orbits have longer lifetimes due to the balance between the disturbance of Uranus and the gravitational coefficients of Titania. The results also show that non-zero values of the longitude of periapsis (ω) and longitude of the ascending node (Ω) are essential to increase the lifetime up to eight times compared to cases where ω=Ω=0∘. We also show that an orbit with eccentricity 10−3 is the most affected by errors in the values of J2 and C22.

The Case for a New Frontiers–Class Uranus Orbiter: System Science at an Underexplored and Unique World with a Mid-scale Mission

Abstract Current knowledge of the Uranian system is limited to observations from the flyby of Voyager 2 and limited remote observations. However, Uranus remains a highly compelling scientific target due to the unique properties of many aspects of the planet itself and its system. Future exploration of Uranus must focus on cross-disciplinary science that spans the range of research areas from the planet’s interior, atmosphere, and magnetosphere to the its rings and satellites, as well as the interactions between them. Detailed study of Uranus by an orbiter is crucial not only for valuable insights into the formation and evolution of our solar system but also for providing ground truths for the understanding of exoplanets. As such, exploration of Uranus will not only enhance our understanding of the ice giant planets themselves but also extend to planetary dynamics throughout our solar system and beyond. The timeliness of exploring Uranus is great, as the community hopes to return in time to image unseen portions of the satellites and magnetospheric configurations. This urgency motivates evaluation of what science can be achieved with a lower-cost, potentially faster-turnaround mission, such as a New Frontiers–class orbiter mission. This paper outlines the scientific case for and the technological and design considerations that must be addressed by future studies to enable a New Frontiers–class Uranus orbiter with balanced cross-disciplinary science objectives. In particular, studies that trade scientific scope and instrumentation and operational capabilities against simpler and cheaper options must be fundamental to the mission formulation.

Crater Distributions of Uranus's Mid-sized Satellites and Implications for Outer Solar System Bombardment

Abstract Outer solar system impact bombardment is largely unconstrained. Although recent data from the Jupiter, Saturn, and Pluto systems have produced new constraints, analysis is incomplete without inclusion of the Uranus system. We reanalyze Uranus system crater populations with recent improvements in processing of Voyager 2 imaging. No consensus in crater populations on mid-sized Uranian satellites, Miranda, Ariel, Umbriel, Titania, and Oberon, was resolved during the Voyager era. For satellites with available data, we find variability in crater size–frequency distributions (SFDs) for diameters (D) < 10 km. Most terrains on Miranda show a shallower slope (ratio of smaller to larger craters is smaller), while Inverness Corona on Miranda and Ariel's terrains show a steeper slope (ratio increases). For D > 10 km, satellites with available data show a steeper slope. Shallower-sloped SFDs for D < 10 km and steeper slopes for D > 10 km agree with Pluto system data—a proxy for the heliocentric impactor population originating from the Kuiper Belt—implying these SFDs represent heliocentric bombardment in the Uranus system. The shallow-sloped population for smaller diameters is also observed on Jovian satellites, but not on mid-sized, heavily cratered Saturnian satellites or Triton (Neptune), which have steeper slopes. This implies the heliocentric impactor population originating from the Kuiper Belt reaches throughout the outer solar system, but that the Saturnian, Neptunian, and maybe Uranian systems also might have their own planet-specific impactors. Finally, we find Ariel appears overall younger than the other Uranian satellites, supporting relatively recent geologic activity.

A Geoscientific Review on CO and CO2 Ices in the Outer Solar System

Ground-based telescopes and space exploration have provided outstanding observations of the complexity of icy planetary surfaces. This work presents our review of the varying nature of carbon dioxide (CO2) and carbon monoxide (CO) ices from the cold traps on the Moon to Pluto in the Kuiper Belt. This review is organized into five parts. First, we review the mineral physics (e.g., rheology) relevant to these environments. Next, we review the radiation-induced chemical processes and the current interpretation of spectral signatures. The third section discusses the nature and distribution of CO2 in the giant planetary systems of Jupiter and Saturn, which are much better understood than the satellites of Uranus and Neptune, discussed in the subsequent section. The final sections focus on Pluto in comparison to Triton, having mainly CO, and a brief overview of cometary materials. We find that CO2 ices exist on many of these icy bodies by way of magnetospheric influence, while intermixing into solid ices with CH4 (methane) and N2 (nitrogen) out to Triton and Pluto. Such radiative mechanisms or intermixing can provide a wide diversity of icy surfaces, though we conclude where further experimental research of these ices is still needed.

Cometary Activity beyond the Planets

Abstract Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance r H = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.

Planetesimal rings as the cause of the Solar System’s planetary architecture

Astronomical observations reveal that protoplanetary disks around young stars commonly have ring- and gap-like structures in their dust distributions. These features are associated with pressure bumps trapping dust particles at specific locations, which simulations show are ideal sites for planetesimal formation. Here we show that our Solar System may have formed from rings of planetesimals—created by pressure bumps—rather than a continuous disk. We model the gaseous disk phase assuming the existence of pressure bumps near the silicate sublimation line (at T ~ 1,400 K), water snowline (at T ~ 170 K) and CO snowline (at T ~ 30 K). Our simulations show that dust piles up at the bumps and forms up to three rings of planetesimals: a narrow ring near 1 au, a wide ring between ~3–4 au and ~10–20 au and a distant ring between ~20 au and ~45 au. We use a series of simulations to follow the evolution of the innermost ring and show how it can explain the orbital structure of the inner Solar System and provides a framework to explain the origins of isotopic signatures of Earth, Mars and different classes of meteorites. The central ring contains enough mass to explain the rapid growth of the giant planets’ cores. The outermost ring is consistent with dynamical models of Solar System evolution proposing that the early Solar System had a primordial planetesimal disk beyond the current orbit of Uranus. The planetary architecture of the Solar System and its isotopic dichotomy can be reproduced using a protoplanetary disk model structured with rings and gaps, as commonly seen in protoplanetary disks around other stars.

The five largest satellites of Uranus: Astrometric observations spread over 29 years at the Pico dos Dias Observatory

We present the astrometry of the five largest satellites of Uranus from observations spread over almost three decades with photographic plates and CCDs (mainly), taken at the Pico dos Dias Observatory - Brazil. All positions presented here are obtained from the reanalysis of measurements and images used in previous publications. Reference stars are those from the Gaia Early Data Release 3 (Gaia EDR3) allowing, in addition to a higher accuracy, a larger number of positions of the largest satellites as compared to our previous works. From 1982 to 1987, positions were obtained from photographic plates. From 1989 to 2011, CCDs were used. On average, we obtained Δα cos δ ​= ​− 11 (±52) milli-arcseconds and Δδ ​= ​− 14 (±43) milli-arcseconds for the differences in the sense observation minus ephemerides (DE435 ​+ ​ura111). Comparisons with different ephemerides (DE440, INPOP21a, INPOP19a and NOE-7-2013-MAIN) and results from stellar occultations indicate a possible offset in the (Solar System) barycentric position of the Uranian system barycenter. Overall, our results are useful to improve dynamical models of the Uranian largest satellites as well as the orbit of Uranus.

A note on the possibility of subsurface oceans on the uranian satellites

We use an existing thermal evolution model to assess the plausibility of subsurface oceans on the moons of Uranus. We adopt a conservative approach neglecting tidal heating and find that Titania and Oberon could support a subsurface ocean to the present day if conductive heat loss through the ice shell is limited. This could occur due to porosity, an ammonia-rich ocean, or clathrates at the base of the ice shell. This result emphasizes the importance of designing future missions with the capability to detect an ocean at all five of the large Uranian satellites.

C/2014 UN271 (Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets

Abstract Comet C/2014 UN271 (Bernardinelli-Bernstein), incoming from the Oort cloud, is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet and having been discovered at the heliocentric distance r h ≈ 29 au, farther than any Oort cloud comet. In this work, we describe the discovery process and observations and the properties that can be inferred from images recorded until the first reports of activity in 2021 June. The orbit has i = 95°, with a perihelion of 10.97 au to be reached in 2031 and a previous aphelion at 40,400 ± 260 au. Backward integration of the orbit under a standard Galactic tidal model and known stellar encounters suggests a perihelion of q ≈ 18 au on its previous perihelion passage 3.5 Myr ago; hence, the current data could be the first ever obtained of a comet that has not been inside Uranus’s orbit in 4 Gyr. The photometric data show an unresolved nucleus with absolute magnitude H r = 8.0, colors that are typical of comet nuclei or Damocloids, and no secular trend as it traversed the range 34–23 au. For the r-band geometric albedo p r , this implies a diameter of 150 ( p r / 0.04 ) − 0.5 km. There is strong evidence of brightness fluctuations at the ±0.2 mag level, but no rotation period can be discerned. A coma, nominally consistent with a “stationary” 1/ρ surface brightness distribution, grew in scattering cross section at an exponential rate from A f ρ ≈ 1 to ≈150 m as the comet approached from 28 to 20 au. The activity rate is consistent with a very simple model of sublimation of a surface species in radiative equilibrium with the Sun. The inferred enthalpy of sublimation matches those of CO2 and NH3. More volatile species, such as N2, CH4, and CO, must be far less abundant on the sublimating surfaces.

An approximate superposition method to obtain a planet’s orbit

We demonstrate that the deviation produced by one celestial object on the Keplerian orbit of another around the Sun is largely independent of the presence of the remaining ones. Hence, to calculate the net deviation of an object from its Keplerian orbit, we superpose the deviations produced by every other object. We show that this method will be useful when dealing with a system containing a large number of objects. As a demonstration, we apply our method to the Solar System, with a particular focus on the orbit of Uranus.

UMaMI: A New Frontiers-style Mission Concept to Explore the Uranian System

Abstract With the public and scientific community’s growing interest in ocean worlds, the icy moons of Uranus offer an ideal opportunity to explore a native ice giant satellite system. Although it is uncertain whether any of the Uranian moons currently host subsurface oceans, there is tantalizing evidence—including geologically young surface features and volatiles that are not stable—that this could be the case, making these objects possible ocean worlds in their own right. Determining whether subsurface oceans are present in the interiors of these moons would increase our understanding of the conditions under which subsurface oceans are formed and maintained over the history of the solar system. The presence and stability of a subsurface ocean in the interiors of any icy body is key to identifying its potential as a habitable environment. In this work, we describe a midsize (New Frontiers class) mission concept: the Uranian Magnetosphere and Moons Investigator. The magnetosphere and moons are tightly coupled parts of the Uranian system, complementary to study, and best analyzed together in order to investigate the Uranian moons as potential ocean worlds. Additionally, this mission concept includes study of Uranus’s unique rings and magnetosphere–solar wind interaction. With a future, more detailed trade study, there also could be opportunities for studies of Uranus itself.

Complex Water-ice Mixtures on NII Nereid: Constraints from NIR Reflectance

Abstract Nereid, Neptune’s third-largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on the spectral characterization of Nereid using its reflectance spectrum from 0.8 to 2.4 μm, providing the first measurements over the range of 0.8–1.4 μm. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 μm when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for submicron-sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. Comparison to spectra of large trans-Neptunian objects and satellites of Uranus show that Nereid’s low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by an incomplete understanding of spectral variability among ∼100 km-sized icy bodies.

Transfer design between neighborhoods of planetary moons in the circular restricted three-body problem: the moon-to-moon analytical transfer method

Given the interest in future space missions devoted to the exploration of key moons in the solar system and that may involve libration point orbits, an efficient design strategy for transfers between moons is introduced that leverages the dynamics in these multi-body systems. The moon-to-moon analytical transfer (MMAT) method is introduced, comprised of a general methodology for transfer design between the vicinities of the moons in any given system within the context of the circular restricted three-body problem, useful regardless of the orbital planes in which the moons reside. A simplified model enables analytical constraints to efficiently determine the feasibility of a transfer between two different moons moving in the vicinity of a common planet. In particular, connections between the periodic orbits of such two different moons are achieved. The strategy is applicable for any type of direct transfers that satisfy the analytical constraints. Case studies are presented for the Jovian and Uranian systems. The transition of the transfers into higher-fidelity ephemeris models confirms the validity of the MMAT method as a fast tool to provide possible transfer options between two consecutive moons.

The Science Case for Spacecraft Exploration of the Uranian Satellites: Candidate Ocean Worlds in an Ice Giant System

Abstract The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus’s five largest “classical” moons—Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon, Puck—but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved data sets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons’ surface compositions, and the processes that modify them, is limited to disk-integrated data sets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and to investigate Uranus’s ring moons and irregular satellites, close-up observations and measurements made by instruments on board a Uranus orbiter are needed.