Heterogeneous delivery of silicate and metal to the Earth by large planetesimals.

After the Moon's formation, Earth experienced a protracted bombardment by leftover planetesimals. The mass delivered during this stage of late accretion has been estimated to be approximately 0.5% of Earth's present mass, based on highly siderophile element concentrations in the Earth's mantle and the assumption that all highly siderophile elements delivered by impacts were retained in the mantle. However, late accretion may have involved mostly large (≥ 1,500 km in diameter)-and therefore differentiated-projectiles in which highly siderophile elements were sequestered primarily in metallic cores. Here we present smoothed-particle hydrodynamics impact simulations that show that substantial portions of a large planetesimal's core may descend to the Earth's core or escape accretion entirely. Both outcomes reduce the delivery of highly siderophile elements to the Earth's mantle and imply a late accretion mass that may be two to five times greater than previously thought. Further, we demonstrate that projectile material can be concentrated within localized domains of Earth's mantle, producing both positive and negative 182W isotopic anomalies of the order of 10 to 100 ppm. In this scenario, some isotopic anomalies observed in terrestrial rocks can be explained as products of collisions after Moon formation.

ACCRETION OF SATURN'S INNER MID-SIZED MOONS FROM A MASSIVE PRIMORDIAL ICE RING.

Saturn's rings are rock-poor, containing 90 to 95% ice by mass. As a group, Saturn's moons interior to and including Tethys are also about 90% ice. Tethys itself contains < 6% rock by mass, in contrast to its similar-mass outer neighbor Dione, which contains > 40% rock. Here we simulate the evolution of a massive primordial ice-rich ring and the production of satellites as ring material spreads beyond the Roche limit. We describe the Roche-interior ring with an analytic model, and use an N-body code to describe material beyond the Roche limit. We track the accretion and interactions of spawned satellites, including tidal interaction with the planet, assuming a tidal dissipation factor for Saturn of Q ~ 104. We find that ring torques and capture of moons into mutual resonances produces a system of ice-rich inner moons that extends outward to approximately Tethys's orbit in 109 years, even with relatively slow orbital expansion due to tides. The resulting mass and semi-major axis distribution of spawned moons resembles that of Mimas, Enceladus and Tethys. We estimate the mass of rock delivered to the moons by external cometary impactors during a late-heavy bombardment. We find that the inner moons receive a mass in rock comparable to their current total rock content, while Dione and Rhea receive an order-of-magnitude less rock than their current rock content. This suggests that external contamination may have been the primary source of rock in the inner moons, and that Dione and Rhea formed from much more rock-rich source material. Reproducing the distribution of rock among the current inner moons is challenging, and appears to require large impactors and stochasticity and/or the presence of some rock in the initial ring.

Lunar-forming impacts: processes and alternatives.

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 the evection resonance with the Sun in the case of (ii).

Accretion of the Moon from non-canonical discs.

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 with the Sun reduced the Earth-Moon angular momentum by a factor of 2 or more. We find that the Moon's semi-major axis at the end of its accretion is approximately 7R⊕, which is comparable to the location of the evection resonance for a post-impact Earth with a 2.5 h rotation period in the absence of a disc. Thus, the dynamics of the Moon's assembly may directly affect its ability to be captured into the resonance.

Origin of the Moon in a giant impact near the end of the Earth's formation.

The Moon is generally believed to have formed from debris ejected by a large off-centre collision with the early Earth. The impact orientation and size are constrained by the angular momentum contained in both the Earth's spin and the Moon's orbit, a quantity that has been nearly conserved over the past 4.5 billion years. Simulations of potential moon-forming impacts now achieve resolutions sufficient to study the production of bound debris. However, identifying impacts capable of yielding the Earth-Moon system has proved difficult. Previous works found that forming the Moon with an appropriate impact angular momentum required the impact to occur when the Earth was only about half formed, a more restrictive and problematic model than that originally envisaged. Here we report a class of impacts that yield an iron-poor Moon, as well as the current masses and angular momentum of the Earth-Moon system. This class of impacts involves a smaller-and thus more likely-object than previously considered viable, and suggests that the Moon formed near the very end of Earth's accumulation.