Outer Moons Research Articles

Impact Rates in the Outer Solar System

Previous studies of cometary impacts in the outer solar system used the spatial distribution of ecliptic comets (ECs) from dynamical models that assumed ECs began on low-inclination orbits (≲5°) in the Kuiper Belt. In reality, the source population of ECs—the trans-Neptunian scattered disk—has orbital inclinations reaching up to ∼30°. In Nesvorný et al., we developed a new dynamical model of ECs by following comets as they evolved from the scattered disk to the inner solar system. The model was absolutely calibrated from the population of Centaurs and active ECs. Here we use our EC model to determine the steady-state impact flux of cometary/Centaur impactors on Jupiter, Saturn, Uranus, and their moons. Relative to previous work, we find slightly higher impact probabilities on the outer moons and lower impact probabilities on the inner moons. The impact probabilities are smaller when comet disruption is accounted for. The results provide a modern framework for the interpretation of the cratering record in the outer solar system.

Tidal Heating of Exomoons in Resonance and Implications for Detection

The habitability of exoplanets can be strongly influenced by the presence of an exomoon, and in some cases the exomoon itself could be a possible place for life to develop. For moons outside of the habitable zone, significant tidal heating may raise their surface temperatures enough for them to be considered habitable. Tidal heating of a moon depends on numerous factors such as eccentricity, semimajor axis, size of parent planet, and the presence of additional moons. In this work, we explore the degree of tidal heating possible for multimoon systems in resonance using a combination of semianalytic and numerical models. This demonstrates that even for a moon with zero initial eccentricity, when it moves into resonance with an outer moon, it can generate significant eccentricity and associated tidal heating. Depending on the mass ratio of the two moons, this resonance can either be short-lived (≤200 Myr) or continue to be driven by the tidal migration of the moons. This tidal heating can also assist in making the exomoons easier to discover, and we explore two scenarios: secondary eclipses and outgassing of volcanic species. We then consider hypothetical moons orbiting known planetary systems to identify which will be best suited for finding exomoons with these methods. We conclude with a discussion of current and future instrumentation and missions.

Early Stages of Galilean Moon Formation in a Water-depleted Environment

A key feature of the Galilean satellite system is its monotonic decrease in bulk density with growing distance from Jupiter, indicating an ice mass fraction that is zero in the innermost moon Io and about half in the outer moons Ganymede and Callisto. Jupiter-formation models, and perhaps the Juno spacecraft water measurements, are consistent with the possibility that the Jovian system may have formed, at least partly, from ice-poor material. And yet, models of the formation of the Galilean satellites usually assume abundant water ice in the system. Here, we investigate the possibility that the Jovian circumplanetary disk was populated with ice-depleted chondritic minerals, including phyllosilicates. We show that the dehydration of such particles and the outward diffusion of the released water vapor allow condensation of significant amounts of ice in the formation region of Ganymede and Callisto in the Jovian circumplanetary disk. Our model predicts that Europa, Ganymede, and Callisto should have accreted little, if any, volatiles other than water ice, in contrast to the comet-like composition of Saturn’s moon Enceladus. This mechanism allows for the presence of ice-rich moons in water-depleted formation environments around exoplanets as well.

Dynamical history of the Galilean satellites for a fast migration of Callisto

Abstract The dynamics of the innermost Galilean satellites (Io, Europa, and Ganymede) is characterized by a chain of mean motion resonances, called Laplace resonance, and by a strong tidal dissipation that causes wide variations of their semimajor axes over large time-scales. The precise history of energy dissipation in the Jovian system is not known, but several theories have been proposed. Tidal resonance locking states that big outer moons can also migrate fast. If this is the case for Callisto, then it should have crossed the 2:1 mean motion resonance with Ganymede in the past, affecting the motion of all four Galilean satellites. Therefore, we aim to determine whether a fast migration for Callisto is compatible with the current orbital configuration of the system. Due to the chaotic nature of the resonant crossing, different outcomes are possible. A small portion of our simulations shows that Callisto can cross the 2:1 resonance with Ganymede without being captured and preserving the Laplace resonance. However, in most cases, we found that Callisto is captured into resonance, despite its divergent migration. As Callisto continues to migrate fast outwards, the moons depart substantially from the exact 8:4:2:1 commensurability, while still maintaining the resonant chain. Callisto can eventually escape it by crossing a high-order mean motion resonance with Ganymede. Afterwards, the moons’ system is able to relax to its current configuration for suitable dissipation parameters of the satellites. Therefore, it is possible, although challenging, to build a self-consistent picture of the past history of the Galilean satellites for a fast migration of Callisto.

Challenges in Forming Phobos and Deimos Directly from a Splitting of an Ancestral Single Moon

The origin and evolution of Martian moons have been intensively debated in recent years. It is proposed that Phobos and Deimos may originate directly from the splitting of an ancestral moon orbiting at around the Martian synchronous orbit. At this hypothetical splitting, the apocenter of the inner moon (presumed as Phobos) and the pericenter of the outer moon (presumed as Deimos) would coincide, in that, their semimajor axes would reside inside and outside the Martian synchronous orbit with nonzero eccentricities, respectively. However, the successive orbital evolution of the two moons is not studied. Here, we perform direct N-body orbital integrations of the moons, including the Martian oblateness of the J 2 and J 4 terms. We show that the two moons, while they precess, likely collide within ∼104 yr with an impact velocity of v imp ∼ 100–300 m s−1 (∼10–30 times moons’ escape velocity) and with an isotropic impact direction. The impact occurs around the apocenter and the pericenter of the inner and outer moons, respectively, where the timescale of this periodic orbital alignment is regulated by the precession. By performing additional impact simulations, we show that such a high-velocity impact likely results in a disruptive outcome, forming a debris ring at around the Martian synchronous orbit, from which several small moons would accrete. Such an evolutionary path would eventually form a different Martian moon system from the one we see today. Therefore, it seems unlikely that Phobos and Deimos are split directly from a single ancestral moon.

A comprehensive investigation of the Galilean moon, Io, by tracing mass and energy flows

Io is the most volcanically-active object in the solar system. The moon ejects a tonne per second of sulphur-rich gases that fill the vast magnetosphere of Jupiter and drives million-amp electrical currents that excite strong auroral emissions. We present the case for including a detailed study of Io within Voyage 2050 either as a standalone mission or as a contribution to a NASA New Frontiers mission, possibly within a Solar System theme centred around current evolutionary or dynamical processes. A comprehensive investigation will provide answers to many outstanding questions and will simultaneously provide information on processes that have formed the landscapes of several other objects in the past. A mission investigating Io will also study processes that have shaped the Earth, Moon, terrestrial planets, outer planet moons, and potentially extrasolar planets. The aim would be simple – tracing the mass and energy flows in the Io-Jupiter system.

Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance, implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titan's orbital expansion rate. We find Titan migrates away from Saturn at 11.3 $\pm$ 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q $\simeq$ 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory, sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn's moon system. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed.

A Multilateral Regime for Space Resource Exploration and Utilization

The launched of Sputnik marked the beginning of space race. Since then state always tries to develop its technology to conquer outer space, including its natural resources. Outer Space Treaty and Moon Agreement affirm that outer space as the common heritage of mankind. Therefore, any states can not claim sovereignty over the territory nor natural resources. Yet, in 2015 the United States passed the Space Resource Exploration and Utilization Act which authorize its private entities to exploit and entitles them with series of rights, including the right of ownership over space resource. Thus, this paper examines the concept of the “common heritage of mankind” (“CHM”) in the Agreement Governing the Activities of States on the Moon and Other Celestial Bodies (Moon Agreement) and elaborates on the possibility of a multilateral regime for space resource exploration and utilization. Part 2 discusses the concept of the CHM and its application; it then followed by an in-depth analysis of the future multilateral regime in Part 3. Part 4 concludes that a multilateral regime, instead of a unilateral regime, shall be in the best position to balance the needs for space resource exploration and the interests of developing countries.

On the Co-orbital Motion in the Three-Body Problem: Existence of Quasi-periodic Horseshoe-Shaped Orbits

Janus and Epimetheus are two moons of Saturn with very peculiar motions. As they orbit around Saturn on quasi-coplanar and quasi-circular trajectories whose radii are only 50 km apart (less than their respective diameters), every four (terrestrial) years the bodies approach each other and their mutual gravitational influence lead to a swapping of the orbits: the outer moon becomes the inner one and vice-versa. This behavior generates horseshoe-shaped trajectories depicted in an appropriate rotating frame. In spite of analytical theories and numerical investigations developed to describe their long-term dynamics, so far very few rigorous long-time stability results on the "horseshoe motion" have been obtained even in the restricted three-body problem. Adapting the idea of Arnol'd (1963) to a resonant case (the co-orbital motion is associated with trajectories in 1:1 mean motion resonance), we provide a rigorous proof of existence of 2-dimensional elliptic invariant tori on which the trajectories are similar to those followed by Janus and Epimetheus. For this purpose, we apply KAM theory to the planar three-body problem.

Planetary exploration of Saturn moons Enceladus and Dione

Search for habitability in Outer Planets moons, requires presence of proper chemistry, water, and energy. Dissipation from tidal forces is major energy source in evolution of Icy moon systems, generically exhibiting a multi-layer structure, with outer solid (ice) and intermediate liquid layers, and solid (rocky) core. Saturn moon Enceladus has been found to eject plumes of water vapour and ice, and very recently, hydrogen molecules, a tentative sign of chemistry supporting microbial life. Purpose of the present work is a preliminary design of a minor electrodynamic tether mission to visit Enceladus. Free tether capture and maneuvering would take the S/C to a Saturn orbit with periapsis very close to the planet and apoapsis at Enceladus orbit, the S/C proved to be then in a 1:2 resonance with the moon, allowing parallel, conveniently slow flybys. Such mission could also involve visiting Dione, the 4 th largest Saturn moon, Enceladus and Dione being in a natural 1:2 Laplace resonance; it was recently brought to light that Dione exhibits multiple, parallel color lines, hundred kilometers long.

Red material on the large moons of Uranus: Dust from the irregular satellites?

The large and tidally-locked “classical” moons of Uranus display longitudinal and planetocentric trends in their surface compositions. Spectrally red material has been detected primarily on the leading hemispheres of the outer moons, Titania and Oberon. Furthermore, detected H2O ice bands are stronger on the leading hemispheres of the classical satellites, and the leading/trailing asymmetry in H2O ice band strengths decreases with distance from Uranus. We hypothesize that the observed distribution of red material and trends in H2O ice band strengths results from infalling dust from Uranus’ irregular satellites. These dust particles migrate inward on slowly decaying orbits, eventually reaching the classical satellite zone, where they collide primarily with the outer moons. The latitudinal distribution of dust swept up by these moons should be fairly even across their southern and northern hemispheres. However, red material has only been detected over the southern hemispheres of these moons, during the Voyager 2 flyby of the Uranian system (subsolar latitude ∼81°S). Consequently, to test whether irregular satellite dust impacts drive the observed enhancement in reddening, we have gathered new ground-based data of the now observable northern hemispheres of these satellites (sub-observer latitudes ∼17–35°N). Our results and analyses indicate that longitudinal and planetocentric trends in reddening and H2O ice band strengths are broadly consistent across both southern and northern latitudes of these moons, thereby supporting our hypothesis. Utilizing a suite of numerical best fit models, we investigate the composition of the reddening agent, finding that both complex organics and amorphous pyroxene match the spectral slopes of our data. We also present spectra that span L/L’ bands (∼2.9–4.1 µm), a previously unexplored wavelength range in terms of spectroscopy for the Uranian satellites, and we compare the shape and albedo of the spectral continua in these L/L’ band data to other icy moons in the Jovian and Saturnian systems. Additionally, we discuss possible localized enhancement of reddening on Titania, subtle differences in H2O ice band strengths between the southern and northern hemispheres of the classical satellites, the distribution of constituents on Miranda, and the possible presence of NH3-hydrates on these moons. In closing, we explore potential directions for future observational and numerical modeling work in the Uranian system.

A Librational Model for the Propeller Blériot in the Saturnian Ring System

Abstract The reconstruction of the orbital evolution of the propeller structure Blériot orbiting in Saturn’s A ring from recurrent observations in Cassini ISS images yielded a considerable offset motion from the expected Keplerian orbit. This offset motion can be composed by three sinusoidal harmonics with amplitudes and periods of 1845, 152, 58 km and 11.1, 3.7, and 2.2 years, respectively. In this paper we present results from N-body simulations, where we integrated the orbital evolution of a moonlet, which is placed at the radial position of Blériot under the gravitational action of the Saturnian satellites. Our simulations yield that, especially the gravitational interactions with Prometheus, Pandora, and Mimas are forcing the moonlet to librate with the right frequencies, but the libration amplitudes are too small to explain the observations. Thus, further mechanisms are needed to explain the amplitudes of the forced librations—e.g., moonlet–ring interactions. Here, we develop a model, where the moonlet is allowed to be slightly displaced with respect to its created gaps breaking the point symmetry and causing a repulsive force in this way. As a result, the evolution of the moonlet’s longitude can be described by a harmonic oscillator. In the presence of external forcing by the outer moons, the libration amplitudes get the more amplified the closer the forcing frequency is to the eigenfrequency of the disturbed propeller oscillator. Applying our model to Blériot, it is possible to reproduce a libration period of 13 years with an amplitude of about 2000 km.

On the Existence of Regular and Irregular Outer Moons Orbiting the Pluto–Charon System

Abstract The dwarf planet Pluto is known to host an extended system of five co-planar satellites. Previous studies have explored the formation and evolution of the system in isolation, neglecting perturbative effects by the Sun. Here we show that secular evolution due to the Sun can strongly affect the evolution of outer satellites and rings in the system, if such exist. Although precession due to extended gravitational potential from the inner Pluto–Charon binary quench such secular evolution up to a crit ∼ 0.0035 au (∼0.09 R Hill the Hill radius; including all of the currently known satellites), outer orbits can be significantly altered. In particular, we find that co-planar rings and satellites should not exist beyond a crit; rather, satellites and dust particles in these regions secularly evolve on timescales ranging between 104 and 106 years, and quasi-periodically change their inclinations and eccentricities through secular evolution (Lidov–Kozai oscillations). Such oscillations can lead to high inclinations and eccentricities, constraining the range where such satellites (and dust particles) can exist without crossing the orbits of the inner satellites or crossing the outer Hill stability range. Outer satellites, if such exist are therefore likely to be irregular satellites, with orbits limited to be non-circular and/or highly inclined. Current observations, including the recent data from the New-Horizons mission explored only inner regions (<0.0012 au) and excluded the existence of additional satellites; however, the irregular satellites discussed here should reside farther, in the yet uncharted regions around Pluto.

Космическое пространство и общее наследие человечества: вызовы и решения

The purpose of this research is to provide a comprehensive international legal analysis of the concept of common heritage of mankind as it exists in international space law. In this article scientific methodological approach was used which facilitate determination the whole canvas of the research, identify the main aspects and concepts of the study. The author also widely investigated the concept of common heritage of mankind in the practice of States and international organizations and bodies. Since mankind succeeded to access outer space and opened the window of this infinite realm, the effort was made to systematize spatial activities by codification and adoption numerous international treaties and declarations. One of the significant result of these efforts was the Agreement Governing the Activities of States on the Moon and Other Celestial Bodies that adopted by the United Nations Committee on the Peaceful Uses of Outer Space by consensus on 5 December 1979, declares the Moon and other celestial bodies to be the common heritage of mankind. This Agreement has never been ratified by any State that engaged in self-launched manned space exploration or has plans to do so and thus has an insignificant effect on spatial activities. This article analysis the concept of common heritage of mankind that consists of five elements: non-appropriation, international management of resources, sharing of financial and technological benefits, reservation for peaceful purposes and reservation for further generations. It is shown that the Moon Agreement applying the concept of common heritage of mankind to outer space, Moon and other celestial bodies which has been presented by developing countries to preserve essential resources, always been associated with some problems. Also, on the one hand, there is an attempt to scrutinize these obstacles that prevent applying this concept by paying attention to its challenges. On the other hand, author presents some solutions to strengthen this concept in outer space legal system and motivate space powers to join the Agreement Governing the Activities of States on the Moon and Other Celestial Bodies.

Aquarius, a reusable water-based interplanetary human spaceflight transport

Attributes of a reusable interplanetary human spaceflight transport are proposed and applied to example transits between the Earth/Moon system and Deimos, the outer moon of Mars. Because the transport is 54% water by mass at an interplanetary departure, it is christened Aquarius. In addition to supporting crew hydration/hygiene, water aboard Aquarius serves as propellant and as enhanced crew habitat radiation shielding during interplanetary transit. Key infrastructure and technology supporting Aquarius operations include pre-emplaced consumables and subsurface habitat at Deimos with crew radiation shielding equivalent to sea level on Earth, resupply in a selenocentric distant retrograde orbit, and nuclear thermal propulsion.

The Phobos neutral and ionized torus

AbstractCharged particle sputtering, micrometeoroid impact vaporization, and photon‐stimulated desorption are fundamental processes operating at airless surfaces throughout the solar system. At larger bodies, such as Earth's Moon and several of the outer planet moons, these processes generate tenuous surface‐bound exospheres that have been observed by a variety of methods. Phobos and Deimos, in contrast, are too gravitationally weak to keep ejected neutrals bound and, thus, are suspected to generate neutral tori in orbit around Mars. While these tori have not yet been detected, the distribution and density of both the neutral and ionized components are of fundamental interest. We combine a neutral Monte Carlo model and a hybrid plasma model to investigate both the neutral and ionized components of the Phobos torus. We show that the spatial distribution of the neutral torus is highly dependent on each individual species (due to ionization rates that span nearly 4 orders of magnitude) and on the location of Phobos with respect to Mars. Additionally, we present the flux distribution of torus pickup ions throughout the Martian system and estimate typical pickup ion fluxes. We find that the predicted pickup ion fluxes are too low to perturb the ambient plasma, consistent with previous null detections by spacecraft around Mars.

ANALYSIS OF SELF – GRAVITATING PLANETARY SATELLITES IN THE SOLAR SYSTEM

As of today there have been more than 180 planetary satellites discovered in the Solar system, and the number of outer moons found continues to grow. Most of those natural satellites have insufficient mass and are able to retain their shape only due to the strength of the electromagnetic force. The purpose of this paper is to analyze the moons’ physical properties. The analysis of planetary satellites as self – gravitating bodies, i.e. celestial bodies which rely on the weight of their own mass and resulting gravitational force to maintain their shape and tend to bring it closer to the hydrostatic equilibrium, was performed.

Extraterrestrial environmental impact assessments – A foreseeable prerequisite for wise decisions regarding outer space exploration, research and development

Although existing international instruments such as the Outer Space Treaty and Moon Agreement generally express sentiments for minimizing missions' extraterrestrial environmental impacts, they tend to be limited in scope, vague and generally unenforceable. There is no formal structure for assessing how and to what extent we affect those environments, no opportunity for public participation, no uniform protocol for documenting and registering the effects of our actions and no requirement to mitigate adverse impacts or take them into consideration in the decision-making process. Except for precautions limiting forward biological contamination and issues related to Earth satellites, environmental impact analysis, when done at all, remains focused on how missions affect the Earth and near-Earth environments, not how our actions affect the Moon, Mars, Europa, comets and other potential destinations. Extraterrestrial environmental impacts are potentially counterproductive to future space exploration, exploitation and scientific investigations. Clear, consistent and effective international protocols guiding a process for assessing such impacts are warranted. While instruments such as the US National Environmental Policy Act provide legally tested and efficient regulatory models that can guide impact assessment here on Earth, statutory legal frameworks may not work as well in the international environment of outer space. A proposal for industry-driven standards and an environmental code of conduct based, in part, on best management practices are offered for consideration.

Physical and dynamical properties of the main belt triple Asteroid (87) Sylvia

We present the analysis of high angular resolution observations of the triple Asteroid (87) Sylvia collected with three 8–10m class telescopes (Keck, VLT, Gemini North) and the Hubble Space Telescope. The moons’ mutual orbits were derived individually using a purely Keplerian model. We computed the position of Romulus, the outer moon of the system, at the epoch of a recent stellar occultation which was successfully observed at less than 15km from our predicted position, within the uncertainty of our model. The occultation data revealed that the Moon, with a surface-area equivalent diameter DS=23.1±0.7km, is strongly elongated (axes ratio of 2.7±0.3), significantly more than single asteroids of similar size in the main-belt. We concluded that its shape is probably affected by the tides from the primary. A new shape model of the primary was calculated combining adaptive-optics observations with this occultation and 40 archived light-curves recorded since 1978. The difference between the J2=0.024-0.009+0.016 derived from the 3-D shape model assuming an homogeneous distribution of mass for the volume equivalent diameter DV=273±10km primary and the null J2 implied by the Keplerian orbits suggests a non-homogeneous mass distribution in the asteroid’s interior.

Near-infrared spectroscopy of Miranda

Aims. We present new near-infrared spectra of the leading and trailing hemispheres of Uranus’s icy satellite Miranda. This body is probably the most remarkable of all the satellites of Uranus, because it displays series of surface features such as faults, craters, and large-scale upwelling, a remnant of a geologically very active past. Methods. The observations were obtained with PHARO at Palomar and SpeX at the IRTF Observatory. We performed spectral modelings to further constrain the nature and the chemical and physical states of the compounds possibly present on the surface of Miranda. Results. Water ice signatures are clearly visible in the H and K bands, and it appears to be found in its crystalline state over most of the satellite’s surface. Unlike what has been found for Uranus’s outer moons, we did not find any significative differences in the abundances of ices covering the leading and trailing hemispheres of Miranda. The signature of carbon dioxide cannot be seen in our spectra, which could still account for the presence of ammonia hydrate, though in small amounts.