We review the key observations and theories relevant to the origin and evolution of the Galilean satellites. Key observations include: the potentially undifferentiated nature of Callisto; the increasing ice fraction with semi-major axis; the present-day existence of the Laplace resonance; the potential resurfacing of Ganymede mid-way through its evolution; and the metal-enriched nature of Jupiter’s envelope. The most widely accepted theory for the formation of the satellites is the so-called “starved disk” model, although newer alternatives including decretion disks and pebble accretion have also been proposed. Models that allow slow satellite formation in a cold disk are preferred, based on the density progression and Callisto’s apparent differentiation state. Major model uncertainties include the angular momentum distribution of the material infalling to the circumplanetary disk, the source of the solids, and the thermal and viscosity structure of the disk. We identify six outstanding questions, some of which will be answered by JUICE, Europa Clipper and Tianwen-4. A major difficulty in answering some questions is overprinting of primordial characteristics by later events.

Exoplanetary systems that contain multiple planets on short-period orbits appear to be prevalent in the current observed exoplanetary population, yet the processes that give rise to such configurations remain poorly understood. A common prior assumption is that planetary accretion commences after the infall of gas and solids to the circumstellar disk ended. However, observational evidence indicates that accretion may begin earlier. We propose that compact systems are surviving remnants of planet accretion that occurred during the final phases of infall. In regions of the disk experiencing ongoing infall, the planetary mass is set by the balance between accretion of infalling solids and the increasingly rapid inward migration driven by the surrounding gas as the planet grows. This balance selects for similarly-sized planets whose mass is a function of infall and disk conditions. We show that infall-produced planets can survive until the gas disk disperses and migration ends, and that across a broad range of conditions, the mass of surviving systems is regulated to a few 10^-5 to 10^-4 times the host star's mass. This provides an explanation for the similar mass ratios of known compact systems.

The amount of vapor in the impact-generated protolunar disk carries implications for the dynamics, devolatilization, and moderately volatile element isotope fractionation during lunar formation. The equation of state (EoS) used in simulations of the giant impact is required to calculate the vapor mass fraction (VMF) of the modeled protolunar disk. Recently, a new version of M-ANEOS (Stewart M-ANEOS) was released with an improved treatment of heat capacity and expanded experimental Hugoniot. Here, we compare this new M-ANEOS version with a previous version (N-SPH M-ANEOS) and assess the resulting differences in smoothed particle hydrodynamics (SPH) simulations. We find that Stewart M-ANEOS results in cooler disks with smaller values of VMF and in differences in disk mass that are dependent on the initial impact angle. We also assess the implications of the minimum “cutoff” density ( ρ _c ), similar to a maximum smoothing length, that is set as a fast-computing alternative to an iteratively calculated smoothing length. We find that the low particle resolution of the disk typically results in >40% of disk particles falling to ρ _c , influencing the dynamical evolution and VMF of the disk. Our results show that the choice of EoS, ρ _c , and particle resolution can cause the VMF and disk mass to vary by tens of percent. Moreover, small values of ρ _c produce disks that are prone to numerical instability and artificial shocks. We recommend that future giant impact SPH studies review smoothing methods and ensure the thermodynamic stability of the disk over simulated time.

The Earth-Moon system is unusual in several respects. The Moon is roughly 1/4 the radius of the Earth - a larger satellite-to-planet size ratio than all known satellites other than Pluto's Charon. The Moon has a tiny core, perhaps with only ~1% of its mass, in contrast to Earth whose core contains nearly 30% of its mass. The Earth-Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily explain all of these features has been the focus of decades of research.

We present a new orbit and mass solution for the four small satellites of Pluto: Styx, Nix, Kerberos, and Hydra. We have reanalyzed all available observations of the Pluto system obtained by the Hubble Space Telescope from 2005 to 2019 with the ACS, WFPC2, and WFC3 instruments, as well as the New Horizons LORRI images taken on approach to Pluto in 2015. We have used this high-precision astrometry to produce updated orbits and mass estimates with uncertainties for all four of the small satellites. We find that the masses of Nix and Hydra are smaller than previously published estimates, with a dynamical mass of (1.8 ± 0.4) × 10 ^−3 km ^3 s ^−2 ((2.7 ± 0.6) × 10 ^16 kg) for Nix and (2.0 ± 0.2) × 10 ^−3 km ^3 s ^−2 ((3.0 ± 0.3) × 10 ^16 kg) for Hydra. These masses are 60% and 63% of the mean estimates by Brozovic et al., respectively, although still consistent with their 1 σ uncertainties, and correspond to densities of 1.0 ± 0.2 g cm ^−3 for Nix and 1.2 ± 0.2 g cm ^−3 for Hydra given the moon volume estimates from Porter et al. Although these densities are consistent with a range of ice−rock compositions, depending on the unknown bulk porosity in the moon interiors, the moons’ high albedos and predominantly icy surfaces are most easily explained if their interiors are ice-rich. The tiny masses of Kerberos and Sytx remain very poorly constrained; we find 1 σ upper limits for dynamical mass of 3 × 10 ^−5 km ^3 s ^−2 (5 × 10 ^14 kg) for Styx and 5 ×10 ^−5 km ^3 s ^−2 (8 × 10 ^14 kg) for Kerberos, consistent with densities of <2.1 g cm ^−3 for both bodies.

Research Article| December 01, 2023 Origin of the Moon Robin M. Canup; Robin M. Canup Planetary Sciences Directorate, Southwest Research Institute, 1050 Walnut Street, Boulder, CO 80302, U.S.A. robin@boulder.swri.edu Search for other works by this author on: GSW Google Scholar Kevin Righter; Kevin Righter NASA Lyndon B. Johnson Space Center Mail Code X12, 2101 NASA Parkway Houston, TX 77058, U.S.A. kevin.righter-1@nasa.gov Search for other works by this author on: GSW Google Scholar Nicolas Dauphas; Nicolas Dauphas Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, U.S.A. Search for other works by this author on: GSW Google Scholar Kaveh Pahlevan; Kaveh Pahlevan Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, U.S.A. Search for other works by this author on: GSW Google Scholar Matija Ćuk; Matija Ćuk Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, U.S.A. Search for other works by this author on: GSW Google Scholar Simon J. Lock; Simon J. Lock Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, U.S.A. Search for other works by this author on: GSW Google Scholar Sarah T. Stewart; Sarah T. Stewart Earth and Planetary Sciences, University of California, Davis, One Shields Ave., Davis, CA 95616, U.S.A. Search for other works by this author on: GSW Google Scholar Julien Salmon; Julien Salmon Space Studies Department, Southwest Research Institute, 1050 Walnut Street, Boulder, Colorado, 80302 U.S.A. Search for other works by this author on: GSW Google Scholar Raluca Rufu; Raluca Rufu Space Studies Department, Southwest Research Institute, 1050 Walnut Street, Boulder, Colorado, 80302 U.S.A. Search for other works by this author on: GSW Google Scholar Miki Nakajima; Miki Nakajima Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627, U.S.A. Search for other works by this author on: GSW Google Scholar Tomáš Magna Tomáš Magna Czech Geological Survey, Klárov 3 118 21 Prague 1, Czech Republic Search for other works by this author on: GSW Google Scholar Author and Article Information Robin M. Canup Planetary Sciences Directorate, Southwest Research Institute, 1050 Walnut Street, Boulder, CO 80302, U.S.A. Kevin Righter NASA Lyndon B. Johnson Space Center Mail Code X12, 2101 NASA Parkway Houston, TX 77058, U.S.A. Nicolas Dauphas Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, U.S.A. Kaveh Pahlevan Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, U.S.A. Matija Ćuk Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, U.S.A. Simon J. Lock Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, U.S.A. Sarah T. Stewart Earth and Planetary Sciences, University of California, Davis, One Shields Ave., Davis, CA 95616, U.S.A. Julien Salmon Space Studies Department, Southwest Research Institute, 1050 Walnut Street, Boulder, Colorado, 80302 U.S.A. Raluca Rufu Space Studies Department, Southwest Research Institute, 1050 Walnut Street, Boulder, Colorado, 80302 U.S.A. Miki Nakajima Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627, U.S.A. Tomáš Magna Czech Geological Survey, Klárov 3 118 21 Prague 1, Czech Republic robin@boulder.swri.edu kevin.righter-1@nasa.gov Publisher: Mineralogical Society of America First Online: 04 Dec 2023 Copyright © 2023 by the Mineralogical Society of AmericaMineralogical Society of America Reviews in Mineralogy and Geochemistry (2023) 89 (1): 53–102. https://doi.org/10.2138/rmg.2023.89.02 Article history First Online: 04 Dec 2023 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Robin M. Canup, Kevin Righter, Nicolas Dauphas, Kaveh Pahlevan, Matija Ćuk, Simon J. Lock, Sarah T. Stewart, Julien Salmon, Raluca Rufu, Miki Nakajima, Tomáš Magna; Origin of the Moon. Reviews in Mineralogy and Geochemistry 2023;; 89 (1): 53–102. doi: https://doi.org/10.2138/rmg.2023.89.02 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The Earth–Moon system is unusual in several respects. The Moon is roughly ¼ the radius of the Earth—a larger satellite-to-planet size ratio than all known satellites other than Pluto's Charon. The Moon has a tiny core, perhaps with only ~1% of its mass, in contrast to Earth whose core contains nearly 30% of its mass. The Earth–Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily... 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We investigate aspects of the co-accretion + giant impact scenario proposed by Morbidelli et al. (2012) for the origin of the Uranian satellites. In this model, a regular satellite system formed during gas accretion is impulsively destabilized by a Uranus-tipping impact, producing debris that ultimately re-orients to the planet’s new equatorial plane and re-accumulates into Uranus’ current large moons. We first investigate the nodal randomization of a disk of debris resulting from disruptive collisions between the hypothesized prior satellites. Consistent with Morbidelli et al., we find that an impact-generated interior c-disk with mass ≥10 −2 Uranus masses is needed to cause sufficient nodal randomization to appropriately realign the outer debris disk. We then simulate the reaccumulation of the outer debris disk into satellites and find that disks with larger initial radii are needed to produce an outer debris disk that extends to Oberon’s distance, and that Uranus’ obliquity prior to the giant impact must have been substantial, ≥40°, if its original co-accreted satellite system was broadly similar in radial scale to those at Jupiter and Saturn today. Finally, we explore the subsequent evolution of a massive, water-dominated inner c-disk as it condenses, collisionally spreads, and spawns new moons beyond the Roche limit. We find that intense tidal dissipation in Uranus (i.e., ( Q / k 2 ) U ≤ 10 2 ) is needed to prevent large icy moons spawned from the inner disk from expanding beyond the synchronous orbit, where they would be long lived and inconsistent with the lack of massive inner moons at Uranus today. We conclude that while a co-accretion + giant impact is viable it requires rather specific conditions.

Geodetic and geophysical investigations of the Galilean moon Callisto address fundamental questions regarding the formation and evolution of the Jovian system. Callisto's evolution and internal structure appear to significantly differ from the other Jovian satellites. Similarly-sized Ganymede is a highly evolved ice-rock moon with a differentiated interior, intrinsic magnetic field, and abundant surface evidence of internal activity. In contrast, Callisto's surface is ancient, and Galileo spacecraft data suggest its interior is only incompletely differentiated, despite the presumed presence of a sub-surface ocean. These properties make Callisto uniquely able to constrain the timing and nature of the Jovian system formation. The Magnetics, Altimetry, Gravity, and Imaging of Callisto (MAGIC) mission concept is conceived to fully characterize the properties of this enigmatic moon from its deep interior to the icy shell. Three main instruments are included as a scientific payload. Highly accurate measurements of Callisto's topography, magnetic field, and morphology are obtained by the onboard laser altimeter, magnetometer, and camera, respectively. The telecommunication system supports an additional gravity and radio science investigation. Long- and short-wavelength gravity anomalies afford powerful constraints on internal differentiation and the properties of the hydrosphere (water and ice). Comprehensive numerical simulations and covariance analyses of MAGIC mission scenarios presented in this paper show that the gravitational degree-2 normalized coefficients and the pole obliquity enable the determination of the moment of inertia with an accuracy better than 0.015%. The combination of gravity and altimetry measurements acquired by MAGIC are essential to the characterization of Callisto's interior if - as is likely - the degree-2 gravity includes non-hydrostatic terms. MAGIC's radio science data yield the estimation of Callisto's gravity field with spatial resolutions of <100 km. The combination of gravitational and deformation tides that are retrieved by the radio science and altimetry investigations, respectively, leads to the recovery of the rigid ice shell thickness to within similar to 3 km. Together these datasets would resolve ambiguities inherent in Galileo flyby data, revealing Callisto's interior structure as well as the existence and properties of its postulated internal ocean.

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.

Moderately volatile elements (MVEs) are depleted and isotopically fractionated in the Moon relative to Earth. To understand how the composition of the Moon was established, we calculate the equilibrium and kinetic isotopic fractionation factors associated with evaporation and condensation processes. We also reassess the levels of depletions of K and Rb in planetary bodies. Highly incompatible element ratios are often assumed to be minimally affected by magmatic processes, but we show that this view is not fully warranted, and we develop approaches to mitigate this issue. The K/U weight ratios of Earth and the Moon are estimated to be 9704 and 2448, respectively. The 87Rb/86Sr atomic ratios of Earth and the Moon are estimated to be 0.072 5 and 0.015 4, respectively. We show that the depletions and heavy isotopic compositions of most MVEs in the Moon are best explained by evaporation in 99%-saturated vapor. At 99% saturation in the protolunar disk, Na and K would have been depleted to levels like those encountered in the Moon on timescales of ∼40–400 days at 3500–4500 K, which agrees with model expectations. In contrast, at the same saturation but a temperature of 1600–1800 K relevant to hydrodynamic escape from the lunar magma ocean, Na and K depletions would have taken 0.1–103 Myr, which far exceeds the 1000 yr time span until plagioclase flotation hinders evaporation from the magma ocean. We conclude that the protolunar disk is a much more likely setting for the depletion of MVEs than the lunar magma ocean.

Earth’s origins are challenging to elucidate, given the lack of surviving terrestrial geology from the first 500 Myr of the Solar System. In this Review, we discuss breakthroughs in geochemistry and theoretical modelling that have advanced understanding of Earth accretion. Theory holds that solar nebula dust particles stuck together to form pebbles, concentrations of which gravitationally collapsed into ∼100-km-sized planetesimals, which in turn accreted to yield planets. Isotopic variations in meteorites indicate that pebbles formed within the first 100 kyr of the Solar System, planetesimals melted and differentiated within a few 100 kyr, and Mars accreted quickly within 5 Myr. Earth’s growth was more protracted, with >98% of its mass being accreted by the time of the Moon-forming Giant Impact at ∼70–120 Myr. Earth is more enriched in s- process nuclides than chondritic meteorites, with a chemical composition affected by condensation, melting and loss. Early volatiles acquired from the nebula largely escaped, with the remnant volatiles being diluted by main-stage Earth accretion, accompanied by loss of nitrogen to the core and/or space. Areas for further research should include assessing mixing during large collisions and investigating the origin of very early mantle isotopic heterogeneities, which might indicate mass transfer from core to mantle over time.

The Earth-Moon system is unusual in several respects. The Moon is roughly 1/4 the radius of the Earth - a larger satellite-to-planet size ratio than all known satellites other than Pluto's Charon. The Moon has a tiny core, perhaps with only ~1% of its mass, in contrast to Earth whose core contains nearly 30% of its mass. The Earth-Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily explain all of these features has been the focus of decades of research.

The Earth-Moon system is unusual in several respects. The Moon is roughly 1/4 the radius of the Earth - a larger satellite-to-planet size ratio than all known satellites other than Pluto's Charon. The Moon has a tiny core, perhaps with only ~1% of its mass, in contrast to Earth whose core contains nearly 30% of its mass. The Earth-Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily explain all of these features has been the focus of decades of research.

The Earth-Moon system is unusual in several respects. The Moon is roughly 1/4 the radius of the Earth - a larger satellite-to-planet size ratio than all known satellites other than Pluto's Charon. The Moon has a tiny core, perhaps with only 1 of its mass. The Earth-Moon system has a high total angular momentum, implying a rapidly spinning Earth when the Moon formed. In addition, the early Moon was hot and at least partially molten with a deep magma ocean. Identification of a model for lunar origin that can satisfactorily explain all of these features has been the focus of decades of research.

A high‐angular momentum giant impact with the Earth can produce a Moon with a silicate isotopic composition nearly identical to that of Earth's mantle, consistent with observations of terrestrial and lunar rocks. However, such an event requires subsequent angular momentum removal for consistency with the current Earth‐Moon system. The early Moon may have been captured into the evection resonance, occurring when the lunar perigee precession period equals 1 year. It has been proposed that after a high‐angular momentum giant impact, evection removed the angular momentum excess from the Earth‐Moon pair and transferred it to Earth's orbit about the Sun. However, prior N‐body integrations suggest this result depends on the tidal model and chosen tidal parameters. Here, we examine the Moon's encounter with evection using a complementary analytic description and the Mignard tidal model. While the Moon is in resonance, the lunar longitude of perigee librates, and if tidal evolution excites the libration amplitude sufficiently, escape from resonance occurs. The angular momentum drain produced by formal evection depends on how long the resonance is maintained. We estimate that resonant escape occurs early, leading to only a small reduction (~ few to 10%) in the Earth‐Moon system angular momentum. Moon formation from a high‐angular momentum impact would then require other angular momentum removal mechanisms beyond standard libration in evection, as have been suggested previously.

Forming the Moon by a high-angular momentum impact may explain the Earth-Moon isotopic similarities; however, the post-impact angular momentum needs to be reduced by a factor of 2 or more to the current value (1 L-EM) after the Moon forms. Capture into the evection resonance, occurring when the lunar perigee precession period equals 1 year, could remove the angular momentum excess. However the appropriate angular momentum removal appears sensitive to the tidal model and chosen tidal parameters. In this work, we use a constant-time delay tidal model to explore the Moon's orbital evolution through evection. We find that exit from formal evection occurs early and that, subsequently, the Moon enters a quasi-resonance regime, in which evection still regulates the lunar eccentricity even though the resonance angle is no longer librating. Although not in resonance proper, during quasi-resonance angular momentum is continuously removed from the Earth-Moon system and transferred to Earth's heliocentric orbit. The final angular momentum, set by the timing of quasi-resonance escape, is a function of the ratio of tidal strength in the Moon and Earth and the absolute rate of tidal dissipation in the Earth. We consider a physically motivated model for tidal dissipation in the Earth as the mantle cools from a molten to a partially molten state. We find that as the mantle solidifies, increased terrestrial dissipation drives the Moon out of quasi-resonance. For post-impact systems that contain>2L(EM), final angular momentum values after quasi-resonance escape remain significantly higher than the current Earth-Moon value.

The goal of this chapter is to review hypotheses for the origin of the Pluto system in light of observational constraints that have been considerably refined over the 85-year interval between the discovery of Pluto and its exploration by spacecraft. We focus on the giant impact hypothesis currently understood as the likeliest origin for the Pluto-Charon binary, and devote particular attention to new models of planet formation and migration in the outer solar system. We discuss the origins conundrum posed by the system's four small moons. We also elaborate on the implications of these scenarios for the dynamical environment of the early transneptunian disk, the likelihood of finding a Pluto collisional family, and the origin of other binary systems in the Kuiper belt. Finally, we highlight outstanding open issues regarding the origins of the Pluto system and suggest areas of future progress.