Impact-generated Disk Research Articles

Uranian satellite formation by evolution of a water vapour disk generated by a giant impact

The ice-giant planet Uranus likely underwent a giant impact, given that its spin axis is tilted by 98 degrees. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a significant amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin. From the predicted distribution of condensed ices, our N-body simulation is able to reproduce the observed mass-orbit configuration of Uranian satellites. This scenario contrasts with the giant-impact model for the Earth's Moon, in which about half of the compact, impact-generated, solid or liquid disk is immediately incorporated into the Moon on impact.

On the Impact Origin of Phobos and Deimos. IV. Volatile Depletion

Abstract Recent works have shown that the Martian moons Phobos and Deimos may have accreted within a giant impact-generated disk whose composition is about an equal mixture of Martian material and impactor material. Just after the giant impact, the Martian surface heated up to ∼3000–6000 K and the building blocks of moons, including volatile-rich vapor, were heated up to ∼2000 K. In this paper, we investigate the volatile loss from the building blocks of Phobos and Deimos by hydrodynamic escape of vapor and radiation pressure on condensed particles. We show that a non-negligible amount of volatiles (>10% of the vapor with temperature >1000 K via hydrodynamic escape, and moderately volatile dusts that condense at ∼700–2000 K via radiation pressure) could be removed just after the impact during their first single orbit from their pericenters to apocenters. Our results indicate that bulk Phobos and Deimos are depleted in volatile elements. Together with future explorations such as the Japan Aerospace eXploration Agency’s Martian Moons eXploration mission, our results could be used to constrain the origin of Phobos and Deimos.

On the Impact Origin of Phobos and Deimos. III. Resulting Composition from Different Impactors

Abstract The origin of Phobos and Deimos in a giant impact-generated disk is gaining larger attention. Although this scenario has been the subject of many studies, an evaluation of the chemical composition of the Mars’s moons in this framework is missing. The chemical composition of Phobos and Deimos is unconstrained. The large uncertainties about the origin of the mid-infrared features; the lack of absorption bands in the visible and near-infrared spectra; and the effects of secondary processes on the moons’ surfaces make the determination of their composition very difficult using remote sensing data. Simulations suggest a formation of a disk made of gas and melt with their composition linked to the nature of the impactor and Mars. Using thermodynamic equilibrium, we investigate the composition of dust (condensates from gas) and solids (from a cooling melt) that result from different types of Mars impactors (Mars-, CI-, CV-, EH-, and comet-like). Our calculations show a wide range of possible chemical compositions and noticeable differences between dust and solids, depending on the considered impactors. Assuming that Phobos and Deimos resulted from the accretion and mixing of dust and solids, we find that the derived assemblage (dust-rich in metallic iron, sulfides and/or carbon, and quenched solids rich in silicates) can be compatible with the observations. The JAXA’s Martian Moons eXploration (MMX) mission will investigate the physical and chemical properties of Phobos and Deimos, especially sampling from Phobos, before returning to Earth. Our results could be then used to disentangle the origin and chemical composition of the pristine body that hit Mars and suggest guidelines for helping in the analysis of the returned samples.

On the Impact Origin of Phobos and Deimos. I. Thermodynamic and Physical Aspects

Abstract Phobos and Deimos are the two small moons of Mars. Recent works have shown that they can accrete within an impact-generated disk. However, the detailed structure and initial thermodynamic properties of the disk are poorly understood. In this paper, we perform high-resolution SPH simulations of the Martian moon-forming giant impact that can also form the Borealis basin. This giant impact heats up the disk material (around ∼2000 K in temperature) with an entropy increase of ∼1500 J K−1 kg−1. Thus, the disk material should be mostly molten, though a tiny fraction of disk material ( ) would even experience vaporization. Typically, a piece of molten disk material is estimated to be meter sized owing to the fragmentation regulated by their shear velocity and surface tension during the impact process. The disk materials initially have highly eccentric orbits (e ∼ 0.6–0.9), and successive collisions between meter-sized fragments at high impact velocity (∼1–5 km s−1) can grind them down to ∼100 μm sized particles. On the other hand, a tiny amount of vaporized disk material condenses into ∼0.1 μm sized grains. Thus, the building blocks of the Martian moons are expected to be a mixture of these different sized particles from meter-sized down to ∼100 μm sized particles and ∼0.1 μm sized grains. Our simulations also suggest that the building blocks of Phobos and Deimos contain both impactor and Martian materials (at least 35%), most of which come from the Martian mantle (50–150 km in depth; at least 50%). Our results will give useful information for planning a future sample return mission to Martian moons, such as JAXA’s MMX (Martian Moons eXploration) mission.

RECONCILING THE ORBITAL AND PHYSICAL PROPERTIES OF THE MARTIAN MOONS

ABSTRACT The origin of Phobos and Deimos is still an open question. Currently, none of the three proposed scenarios for their origin (intact capture of two distinct outer solar system small bodies, co-accretion with Mars, and accretion within an impact-generated disk) are able to reconcile their orbital and physical properties. Here we investigate the expected mineralogical composition and size of the grains from which the moons once accreted assuming they formed within an impact-generated accretion disk. A comparison of our results with the present-day spectral properties of the moons allows us to conclude that their building blocks cannot originate from a magma phase, thus preventing their formation in the innermost part of the disk. Instead, gas-to-solid condensation of the building blocks in the outer part of an extended gaseous disk is found as a possible formation mechanism as it does allow reproducing both the spectral and physical properties of the moons. Such a scenario may finally reconcile their orbital and physical properties, alleviating the need to invoke an unlikely capture scenario to explain their physical properties.

LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK.

The Moon may have formed from an Earth-orbiting disk of vapor and melt produced by a giant impact.1 The Moon and Earth's mantles have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks2-3, given that an evaporative escape mechanism4 appears inconsistent with expected disk conditions.5 Dynamical models6-7 suggest that the Moon initially accreted from the outermost disk, but later acquired up to 60% of its mass from melt originating from the inner disk. Here we combine dynamical, thermal and chemical models to show that volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt in the inner disk to the Earth, rather than to the growing Moon. Melt in the inner disk is initially hot and volatile-poor, but volatiles condense as the disk cools. In our simulations, the delivery of inner disk melt to the Moon effectively ceases when gravitational interactions cause the Moon's orbit to expand away from the disk, and this termination of lunar accretion occurs prior to condensation of potassium and more volatile elements. Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted. We suggest that this mechanism may explain part or all of the Moon's volatile depletion, depending on the degree of mixing within the lunar interior.

Density of Charon formed from a disk generated by the impact of partially differentiated bodies

Formation of Pluto’s moon Charon from an impact-generated circumplanetary disk has been considered unlikely because of its composition. Such a disk is created by the collision of two similarly sized, differentiated impactors; the disk would contain material from the impactors’ outer portions; and that material has been presumed to be pure water ice. Charon has been predicted to have a density ∼1000kgm-3 if formed from a disk, much lower than its observed density ≈1630kgm-3. Here we reconsider the composition of the circumplanetary disk. We draw on previously presented models to calculate the thermal evolution and internal structure of the impactors, each assumed to have radii ≈972km and mean densities ≈2000kgm-3. We show such bodies retain crusts of rock and ice on their surfaces about 46km thick, comprising about 13% of the impactors’ mass; the outer layers are not pure ice. Provided ≈6% of the total mass of the impactors escapes the system, an escape fraction supported by numerical simulations, the observed densities of Pluto and Charon are reproduced in a scenario involving a circumplanetary disk. Charon’s density does not rule out an origin from an impact-generated disk. We calculate the thermal evolution of Charon if it formed late from an impact-generated disk. While Charon is less heated by radioactive decay than if it formed earlier, the heat of its accretion causes its complete differentiation. Subsurface liquid and cryovolcanism may have existed on Charon, but only until 1.5Gyr ago. These predictions may be tested by data returned by the New Horizons mission.

LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of the inner disk, resulting in a total lunar accretion timescale of ~10^2 years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing < 2.5 lunar masses (ML), we find that a final Moon with mass > 0.8ML contains < 60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.

Ridge formation and de-spinning of Iapetus via an impact-generated satellite

We present a scenario for building the equatorial ridge and de-spinning Iapetus through an impact-generated disk and satellite. This impact puts debris into orbit, forming a ring inside the Roche limit and a satellite outside. This satellite rapidly pushes the ring material down to the surface of Iapetus, and then itself tidally evolves outward, thereby helping to de-spin Iapetus. This scenario can de-spin Iapetus an order of magnitude faster than when tides due to Saturn act alone, almost independently of its interior geophysical evolution. Eventually, the satellite is stripped from its orbit by Saturn. The range of satellite and impactor masses required is compatible with the estimated impact history of Iapetus.

ON A GIANT IMPACT ORIGIN OF CHARON, NIX, AND HYDRA

It is generally believed that Charon was formed as a result of a large, grazing collision with Pluto that supplied the Pluto-Charon system with its high angular momentum. It has also been proposed that Pluto's small outer moons, Nix and Hydra, formed from debris from the Charon-forming impact, although the viability of this scenario remains unclear. Here I use smooth particle hydrodynamics impact simulations to show that it is possible to simultaneously form an intact Charon and an accompanying debris disk from a single impact. The successful cases involve colliding objects that are partially differentiated prior to impact, having thin outer ice mantles overlying a uniform composition rock-ice core. The composition of the resulting debris disks varies from a mixture of rock and ice (similar to the bulk composition of Pluto and Charon) to a pure ice disk. If Nix and Hydra were formed from such an impact-generated disk, their densities should be less than or similar to that of Charon and Pluto, and the small moons could be composed entirely of ice. If they were instead formed from captured material, a mixed rock-ice composition and densities similar to that of Charon and Pluto would be expected. Improved constraints on the properties of Nix and Hydra through occultations and/or the New Horizons encounter may thus help to distinguish between these two modes of origin, particularly if the small moons are found to have ice-like densities.

High‐Resolution Simulations of a Moon‐forming Impact and Postimpact Evolution

In order to examine the impact hypothesis for the formation of the Moon, we run the first grid-based, high-resolution hydrodynamic simulations of an impact between proto-Earth and a protoplanet. The spatial resolution for the impact-generated disk is greatly improved from previous particle-based simulations. This allows us to explore the fine structures of a circumterrestrial debris disk and its long-term evolution. We find that in order to form a debris disk from which a lunar-sized satellite can be accumulated, the impact must result in a disk of mostly liquid or solid debris, where pressure is not effective, well before the accumulation process starts. If the debris is dominated by vapor gas, strong spiral shocks are generated, and therefore the circumterrestrial disk cannot survive more than several days. This suggests that there could be an appropriate mass range for terrestrial planets to harbor a large moon as a result of giant impacts, since vaporization during an impact depends on the impact energy.

Lunar Accretion from an Impact-Generated Disk

We investigate the evolution of a circumterrestrial disk of debris generated by a giant impact on the Earth and the dynamical characteristics of the moon accreted from the disk by using N-body simulation. We find that in most cases the disk evolution results in the formation of a single large moon on a nearly circular orbit close to the equatorial plane of the initial disk just outside the Roche limit. The efficiency of incorporation of disk material into a moon is 10-55%, which increases with the initial specific angular momentum of the disk. These results hardly depend on the initial condition of the disk as long as the disk mass is a few times the present lunar mass and most disk mass exists inside the Roche limit.

Origin of the Moon's orbital inclination from resonant disk interactions

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.

Lunar accretion from an impact-generated disk

Although the mechanism by which the Moon was formed is currently unknown, several lines of evidence point to its accretion from a circumterrestrial disk of debris generated by a giant impact on the Earth. Theoretical simulations show that a single large moon can be produced from such a disk in less than a year, and establish a direct relationship between the size of the accreted moon and the initial configuration of the debris disk.

Accretion of the Moon from an Impact-Generated Disk

We present the first published numerical calculations of accretion of an impact-generated protolunar disk into a single large Moon. Our calculations are based on the model developed by R. M. Canup and L. W. Esposito (Icarus113, 331–352, 1995) to describe accretion in the Roche zones around the giant planets. Previous numerical simulations of a large impact event predict the formation of a disk of material centered near or within the Roche limit (∼2.9R⊕). A natural expectation based on our previous results and comparison with the satellite systems of the outer planets would be for multiple small moons to arise from such a protolunar disk. Multiple moonlets could accrete to form a single Moon if they evolved into crossing orbits due to tidal interaction with the Earth. This would occur if the innermost moonlet in the disk were also the most massive, so that it evolved outward at the relatively fastest rate and swept up all exterior material. Our calculations, which include both moonlet accretion and orbital evolution, demonstrate that forming massive moonlets in the inner disk near the Roche limit is extremely difficult. We conclude that an Earth system with multiple moons is the final result unless some particularly severe constraints on initial conditions in the disk are met. A disk with a lunar mass of material exterior toa∼ 3.5–4R⊕or an extremely steep radial surface density profile at the onset of collisional growth is required for a single, lunar-sized body to result from accretion of silicate density material in a protolunar disk. The former corresponds most closely to disks produced by impactors with nearly twice the mass of Mars and about twice the angular momentum of the current Earth/Moon system. Other processes, such as gravitational instability or primary accretion of an iron core in the inner disk, might be able to “seed” accretional growth and allow for the formation of a single Moon if disk temperature and compositional requirements are met. Our analysis demonstrates the need for more detailed, higher resolution impact simulations.