Sarah T. Stewart

Making the Moon from a Fast-Spinning Earth: A Giant Impact Followed by Resonant Despinning

Forming the Moon from Earth It is thought that the Moon formed after a Mars-sized planet hit Earth about 4.5 billion years ago. Computer simulations of this event predict that the Moon was produced primarily from material from the impacting planet. However, the Moon has a similar composition to that of Earth, and the impacting planet would likely have had a different composition. Prior models assumed that the impact left the Earth-Moon system with the same angular momentum as it has today (see the Perspective by Halliday). Ćuk and Stewart (p. 1047, published online 17 October; see the cover) show that the angular momentum of the Earth-Moon system could have decreased by half after the Moon-forming impact, opening the door to new impact models. For example, simulations suggest that high-velocity impacts onto a fast-spinning early Earth can lead to a Moon formed primarily from Earths mantle. Canup (p. 1052, published online 17 October) considered instead lower-velocity impacts by planets comparable in mass to the proto-Earth, which could generate a Moon and an Earth with similar compositions. Computer simulations show that a giant impact on early Earth could lead to a Moon with a composition similar to Earth’s. A common origin for the Moon and Earth is required by their identical isotopic composition. However, simulations of the current giant impact hypothesis for Moon formation find that most lunar material originated from the impactor, which should have had a different isotopic signature. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to that of the present day; however, Earth-mass planets are expected to have higher spin rates at the end of accretion. Here, we show that typical last giant impacts onto a fast-spinning proto-Earth can produce a Moon-forming disk derived primarily from Earth’s mantle. Furthermore, we find that a faster-spinning early Earth-Moon system can lose angular momentum and reach the present state through an orbital resonance between the Sun and Moon.

Collisions between Gravity-dominated Bodies. I. Outcome Regimes and Scaling Laws

Collisions are the core agent of planet formation. In this work, we derive an analytic description of the dynamical outcome for any collision between gravity-dominated bodies. We conduct high-resolution simulations of collisions between planetesimals; the results are used to isolate the effects of different impact parameters on collision outcome. During growth from planetesimals to planets, collision outcomes span multiple regimes: cratering, merging, disruption, super-catastrophic disruption, and hit-and-run events. We derive equations (scaling laws) to demarcate the transition between collision regimes and to describe the size and velocity distributions of the post-collision bodies. The scaling laws are used to calculate maps of collision outcomes as a function of mass ratio, impact angle, and impact velocity, and we discuss the implications of the probability of each collision regime during planet formation. Collision outcomes are described in terms of the impact conditions and the catastrophic disruption criteria, Q*RD—the specific energy required to disperse half the total colliding mass. All planet formation and collisional evolution studies have assumed that catastrophic disruption follows pure energy scaling; however, we find that catastrophic disruption follows nearly pure momentum scaling. As a result, Q*RD is strongly dependent on the impact velocity and projectile-to-target mass ratio in addition to the total mass and impact angle. To account for the impact angle, we derive the interacting mass fraction of the projectile; the outcome of a collision is dependent on the kinetic energy of the interacting mass rather than the kinetic energy of the total mass. We also introduce a new material parameter, c*, that defines the catastrophic disruption criteria between equal-mass bodies in units of the specific gravitational binding energy. For a diverse range of planetesimal compositions and internal structures, c* has a value of 5 ± 2; whereas for strengthless planets, we find c* = 1.9 ± 0.3. We refer to the catastrophic disruption criteria for equal-mass bodies as the principal disruption curve, which is used as the reference value in the calculation of Q*RD for any collision scenario. The analytic collision model presented in this work will significantly improve the physics of collisions in numerical simulations of planet formation and collisional evolution.Numerical simulations of the stochastic end stage of planet formation typically begin with a population of embryos and planetesimals that grow into planets by merging. We analyzed the impact parameters of collisions leading to the growth of terrestrial planets from recent

Records of an ancient Martian magnetic field in ALH84001

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Velocity-Dependent Catastrophic Disruption Criteria for Planetesimals

-body simulations that assumed perfect merging and calculated more realistic outcomes using a new analytic collision physics model. We find that collision outcomes are diverse and span all possible regimes: hit-and-run, merging, partial accretion, partial erosion, and catastrophic disruption. The primary outcomes of giant impacts between planetary embryos are approximately evenly split between partial accretion, graze-and-merge, and hit-and-run events. To explore the cumulative effects of more realistic collision outcomes, we modeled the growth of individual planets with a Monte Carlo technique using the distribution of impact parameters from

Full numerical simulations of catastrophic small body collisions

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COLLISIONS BETWEEN GRAVITY-DOMINATED BODIES. II. THE DIVERSITY OF IMPACT OUTCOMES DURING THE END STAGE OF PLANET FORMATION

-body simulations. We find that fewer planets reached masses

Evolution and persistence of 5‐μm hot spots at the Galileo probe entry latitude

>0.7 M_{\rm Earth}

Characteristics of the Galileo probe entry site from Earth‐based remote sensing observations

using the collision physics model compared to simulations that assumed every collision results in perfect merging. For final planets with masses

An Impactor Origin for Lunar Magnetic Anomalies

>0.7 M_{\rm Earth}

Shock Properties of H2O Ice

, 60% are enriched in their core-to-mantle mass fraction by >10% compared to the initial embryo composition. Fragmentation during planet formation produces significant debris (