175 research outputs found
Giant impacts stochastically change the internal pressures of terrestrial planets
Pressure is a key parameter in the physics and chemistry of planet formation and evolution. Previous studies have erroneously assumed that internal pressures monotonically increase with the mass of a body. Using smoothed particle hydrodynamics and potential field method calculations, we demonstrate that the hot, rapidly rotating bodies produced by giant impacts can have much lower internal pressures than cool, slowly rotating planets of the same mass. Pressures subsequently increase because of thermal and rotational evolution of the body. Using the Moon-forming impact as an example, we show that the internal pressures after the collision could have been less than half that in present-day Earth. The current pressure profile was not established until Earth cooled and the Moon receded, a process that may take up to tens of millions of years. Our work defines a new paradigm for pressure evolution during accretion of terrestrial planets: stochastic changes driven by impacts
Atmospheric loss in giant impacts depends on pre-impact surface conditions
Earth likely acquired much of its inventory of volatile elements during the
main stage of its formation. Some of Earth's proto-atmosphere must therefore
have survived the giant impacts, collisions between planet-sized bodies, that
dominate the latter phases of accretion. Here we use a suite of 1D hydrodynamic
simulations and impedance match calculations to quantify the effect that
pre-impact surface conditions (such as atmospheric pressure and presence of an
ocean) have on the efficiency of atmospheric and ocean loss from proto-planets
during giant impacts. We find that -- in the absence of an ocean -- lighter,
hotter, and lower-pressure atmospheres are more easily lost. The presence of an
ocean can significantly increase the efficiency of atmospheric loss compared to
the no-ocean case, with a rapid transition between low and high loss regimes as
the mass ratio of atmosphere to ocean decreases. However, contrary to previous
thinking, the presence of an ocean can also reduce atmospheric loss if the
ocean is not sufficiently massive, typically less than a few times the
atmospheric mass. Volatile loss due to giant impacts is thus highly sensitive
to the surface conditions on the colliding bodies. To allow our results to be
combined with 3D impact simulations, we have developed scaling laws that relate
loss to the ground velocity and surface conditions. Our results demonstrate
that the final volatile budgets of planets are critically dependent on the
exact timing and sequence of impacts experienced by their precursor planetary
embryos, making atmospheric properties a highly stochastic outcome of
accretion.Comment: 45 pages, 17 figures, and 5 tables. Accepted to The Planetary Science
Journa
The energy budget and figure of Earth during recovery from the Moon-forming giant impact
Quantifying the energy budget of Earth in the first few million years following the Moon-forming giant impact is vital to understanding Earth's initial thermal state and the dynamics of lunar tidal evolution. After the impact, the body was substantially vaporized and rotating rapidly, very different from the planet we know today. The subsequent evolution of Earth's energy budget, as the body cooled and angular momentum was transferred during lunar tidal recession, has not been accurately calculated with all relevant energy components included. Here, we use giant impact simulations and planetary structure models to calculate the energy budget at stages in Earth's evolution. We show that the figure and internal structure of Earth changed substantially during its post-impact evolution and that changes in kinetic, potential, and internal energy were all significant. These changes have important implications for the dynamics of tidal recession and the thermal structure of early Earth
Giant impacts stochastically change the internal pressures of terrestrial planets
Pressure is a key parameter in the physics and chemistry of planet formation and evolution. Previous studies have erroneously assumed that internal pressures monotonically increase with the mass of a body. Using smoothed particle hydrodynamics and potential field method calculations, we demonstrate that the hot, rapidly rotating bodies produced by giant impacts can have much lower internal pressures than cool, slowly rotating planets of the same mass. Pressures subsequently increase because of thermal and rotational evolution of the body. Using the Moon-forming impact as an example, we show that the internal pressures after the collision could have been less than half that in present-day Earth. The current pressure profile was not established until Earth cooled and the Moon receded, a process that may take up to tens of millions of years. Our work defines a new paradigm for pressure evolution during accretion of terrestrial planets: stochastic changes driven by impacts
The energy budgets of giant impacts
Giant impacts dominate the final stages of terrestrial planet formation and
set the configuration and compositions of the final system of planets. A giant
impact is believed to be responsible for the formation of Earth's Moon, but the
specific impact parameters are under debate. Because the canonical Moon-forming
impact is the most intensely studied scenario, it is often considered the
archetypal giant impact. However, a wide range of impacts with different
outcomes are possible. Here we examine the total energy budgets of giant
impacts that form Earth-mass bodies and find that they differ substantially
across the wide range of possible Moon-forming events. We show that
gravitational potential energy exchange is important, and we determine the
regime in which potential energy has a significant effect on the collision
outcome. Energy is deposited heterogeneously within the colliding planets,
increasing their internal energies, and portions of each body attain sufficient
entropy for vaporization. After gravitational re-equilibration, post-impact
bodies are strongly thermally stratified, with varying amounts of vaporized and
supercritical mantle. The canonical Moon-forming impact is a relatively low
energy event and should not be considered the archetype of accretionary giant
impacts that form Earth-mass planets. After a giant impact, bodies are
significantly inflated in size compared to condensed planets of the same mass,
and there are substantial differences in the magnitudes of their potential,
kinetic and internal energy components. As a result, the conditions for
metal-silicate equilibration and the subsequent evolution of the planet may
vary widely between different impact scenarios.Comment: 16 pages, 11 figures. Accepted for publication in JGR: Planets.
Supplementary material is available from
http://philipjcarter.com/energybudgets/EBSI.pdf . Accompanying animations are
available from http://philipjcarter.com/energybudgets
The Lithophile Element Budget of Earth's Core
Abstract The relative composition of Earth's core and mantle were set during core formation. By determining how elements partition between metal and silicate at high pressures and temperatures, measurements of the mantle composition and geophysical observations of the core can be used to understand the mechanisms by which Earth formed. Here we present the results of metalāsilicate partitioning experiments for a range of nominally lithophile elements (Al, Ca, K, Mg, O, Si, Th, and U) and S to 85Ā GPa and up to 5400Ā K. With our results and a compilation of literature data, we developed a parameterization for partitioning that accounts for compositional dependencies in both the metal and silicate phases. Using this parameterization in a range of planetary growth models, we find that, in general, lithophile element partitioning into the metallic phase is enhanced at high temperatures. The relative abundances of FeO, SiO2, and MgO in the mantle vary significantly between planetary growth models, and the mantle abundances of these elements can be used to provide important constraints on Earth's accretion. To match Earth's core mass and mantle composition, Earth's building blocks must have been enriched in Fe and depleted in Si compared with CI chondrites. Finally, too little Mg, Si, and O are partitioned into the core for precipitation of oxides to be a major source of energy for the geodynamo. In contrast, several ppb of U can be partitioned into the core at high temperatures, and this energy source must be accounted for in thermal evolution models
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