The Moon is generally believed to have formed from the debris disk created by a large body colliding with the early Earth. Recent models of this process predict that the orbit of the newly formed Moon should be in, or very near, the... more
The Moon is generally believed to have formed from the debris disk created by a large body colliding with the early Earth. Recent models of this process predict that the orbit of the newly formed Moon should be in, or very near, the Earth's equatorial plane. This prediction, however, is at odds with the known history of the lunar orbit: the orbit is currently expanding, but can be traced back in time to reveal that, when the Moon formed, its orbital inclination relative to the Earth's equator was I approximately = 10 degrees. The cause of this initial inclination has been a mystery for over 30 years, as most dynamical processes (such as those that act to flatten Saturn's rings) will tend to decrease orbital inclinations. Here we show that the Moon's substantial orbital inclination is probably a natural result of its formation from an impact-generated disk. The mechanism involves a gravitational resonance between the Moon and accretion-disk material, which can increase orbital inclinations up to approximately 15 degrees.
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ABSTRACT
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We consider a scenario in which the Galilean satellites form within a circumplanetary accretion disk produced during the end stages of gas accretion onto Jupiter. In Canup and Ward (2002), we identified disk conditions compatible with... more
We consider a scenario in which the Galilean satellites form within a circumplanetary accretion disk produced during the end stages of gas accretion onto Jupiter. In Canup and Ward (2002), we identified disk conditions compatible with three main constraints on satellite formation: 1) disk temperatures low enough for ices in the general region of Ganymede and Callisto, 2) satellite accretion
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To avoid accretion of substantial hydrogen, it is likely that the final stage of Uranus formation post-dated the bulk of the solar nebula. This renders it unlikely that its satellite system formed from a hydrogen-rich circumplanetary... more
To avoid accretion of substantial hydrogen, it is likely that the final stage of Uranus formation post-dated the bulk of the solar nebula. This renders it unlikely that its satellite system formed from a hydrogen-rich circumplanetary disk, and suggests it might instead be a by-product of a giant impact. Indeed, the 97 degree obliquity of Uranus is often cited as
Research Interests: Geology, Physics, Astrobiology, Energy Dissipation, Uranus, and 3 moreDDA, Solar Nebula, and Angular Momentum
The satellite systems of Jupiter and Saturn are thought to have formed from circumplanetary disks surrounding these planets that developed during the accretion of the planets themselves. Early models assumed that the mass of the disk... more
The satellite systems of Jupiter and Saturn are thought to have formed from circumplanetary disks surrounding these planets that developed during the accretion of the planets themselves. Early models assumed that the mass of the disk could be estimated by augmenting the satellite system to roughly solar composition. When translated to a circumplanetary environment, this predicts a disk in which
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The Jovian and Saturnian regular satellites are believed to have accreted within circumplanetary disks of gas and solids. Such disks probably existed during the final stages of gas planet accretion, because gas inflowing from solar orbit... more
The Jovian and Saturnian regular satellites are believed to have accreted within circumplanetary disks of gas and solids. Such disks probably existed during the final stages of gas planet accretion, because gas inflowing from solar orbit that contained too much angular momentum to fall directly onto the planet would have instead flowed into circumplanetary orbit. As a circumplanetary gas disk
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... THE OBLIQUITY OF JUPITER WILLIAM R. WARD AND ROBIN M. CANUP Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302 Received ... This has both a homogeneous so-lution, sx,hp vh cos (Jat + d), sy,hp vh sin (Jat +... more
... THE OBLIQUITY OF JUPITER WILLIAM R. WARD AND ROBIN M. CANUP Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302 Received ... This has both a homogeneous so-lution, sx,hp vh cos (Jat + d), sy,hp vh sin (Jat + d) with constants vh and d ...
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The leading hypothesis for lunar origin is the giant impact theory, which proposes that the Moon formed from debris ejected into bound earth orbit when early Earth collided with a roughly Mars-sized protoplanet (Hartmann & Davis 1975;... more
The leading hypothesis for lunar origin is the giant impact theory, which proposes that the Moon formed from debris ejected into bound earth orbit when early Earth collided with a roughly Mars-sized protoplanet (Hartmann & Davis 1975; Cameron & Ward 1976). Simulations of potential lunar-forming impacts using a 3-D Lagrangian method known as Smooth Particle Hydrodynamics (or SPH; Lucy 1977) have been performed in numerous works (Benz et al 1986, 1987, 1989; Cameron & Benz 1991; Cameron 1997, 2000, 2001). Numerical resolutions now achievable with SPH allow for the material placed into orbit as a result of the impact-typically representing a few percent of the total mass-to be resolved with 102 to 103 SPH particles. Recent works (Cameron 1997, 2000, 2001) have identified only a limited class of impacts capable of placing a sufficient amount of material into orbit to yield the Moon: those that involve a collisional angular momentum twice that of the current Earth-Moon system, or that require that the lunar-forming impact occurred when Earth was only about half formed. Both of these scenarios are restrictive and somewhat problematic. Here we are performing a new series of SPH simulations of potential satellite-forming impacts. In particular, we are exploring the dependence of collisional outcome on the equation of state, the mass ratio of the impactor-to-target, and the pre-impact partitioning of angular momentum into spin vs. impact parameter. One objective is to test the sensitivity of the empirical scaling relationships observed in Cameron's results (Canup et al. 2001) to a broader variation in impact conditions. Interestingly, our initial results suggest a wider range of impacts than those described to date by Cameron may also be successful lunar forming candidates.
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In the leading hypothesis for lunar origin, the Moon forms from debris ejected as a result of the collision of a roughly Mars-sized impactor with early Earth (Hartmann & Davis 1975; Cameron... more
In the leading hypothesis for lunar origin, the Moon forms from debris ejected as a result of the collision of a roughly Mars-sized impactor with early Earth (Hartmann & Davis 1975; Cameron & Ward 1976). The likelihood of giant impact events has been substantiated by over a decade of planetary accretion simulations (e.g., Wetherill 1985, 1992; Agnor et al. 1999;
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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... more
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 evoluti...
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Research Interests: Physics, Medicine, Astronomy, Satellite, Solar System, and 5 moreUranus, Neptune, Saturn, Satellite System, and Astronomical
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Research Interests: Physics and Solar Nebula
Research Interests: Chemistry, Astrobiology, Zinc, Potassium, Moon, and 3 moreSodium, LPI, and Astronomy and Astrophysics
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Impacts that leave the Earth–Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We... more
Impacts that leave the Earth–Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We here investigate the accretion of the Moon from discs generated by such ‘non-canonical’ impacts, which are typically more compact than discs produced by canonical impacts and have a higher fraction of their mass initially located inside the Roche limit. Our model predicts a similar overall accretional history for both canonical and non-canonical discs, with the Moon forming in three consecutive steps over hundreds of years. However, we find that, to yield a lunar-mass Moon, the more compact non-canonical discs must initially be more massive than implied by prior estimates, and only a few of the discs produced by impact simulations to date appear to meet this condition. Non-canonical impacts require that capture of the Moon into the evection resonance w...
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The formation of a protolunar disc by a giant impact with the early Earth is discussed, focusing on two classes of impacts: (i) canonical impacts, in which a Mars-sized impactor produces a planet–disc system whose angular momentum is... more
The formation of a protolunar disc by a giant impact with the early Earth is discussed, focusing on two classes of impacts: (i) canonical impacts, in which a Mars-sized impactor produces a planet–disc system whose angular momentum is comparable to that in the current Earth and Moon, and (ii) high-angular-momentum impacts, which produce a system whose angular momentum is approximately a factor of 2 larger than that in the current Earth and Moon. In (i), the disc originates primarily from impactor-derived material and thus is expected to have an initial composition distinct from that of the Earth's mantle. In (ii), a hotter, more compact initial disc is produced with a silicate composition that can be nearly identical to that of the silicate Earth. Both scenarios require subsequent processes for consistency with the current Earth and Moon: disc–planet compositional equilibration in the case of (i), or large-scale angular momentum loss during capture of the newly formed Moon into t...
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Two small moons of Pluto have been discovered in low-eccentricity orbits exterior to Pluto's large satellite, Charon. All three satellite orbits are nearly coplanar, implying a common origin. It has been argued that Charon formed as a... more
Two small moons of Pluto have been discovered in low-eccentricity orbits exterior to Pluto's large satellite, Charon. All three satellite orbits are nearly coplanar, implying a common origin. It has been argued that Charon formed as a result of a giant impact with primordial Pluto. The orbital periods of the two new moons are nearly integer multiples of Charon's period, suggesting that they were driven outward by resonant interactions with Charon during its tidal orbital expansion. This could have been accomplished if Charon's orbit was eccentric during most of this orbital evolution, with the small moons originating as debris from the collision that produced Charon.
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Increasingly sophisticated computer simulations show how the four solid planets could have emerged through collisions and accretion. One late, giant collision with Earth is the likely origin of the Moon.