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Stewart, Sarah T.

Publications and source records attributed to Stewart, Sarah T..

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Miscibility of rock and ice in the interiors of water worlds

Abstract Super-Earths and sub-Neptunes are the most common planet types in our galaxy. A subset of these planets is predicted to be water worlds, bodies that are rich in water and poor in hydrogen gas. The interior structures of water worlds have been assumed to consist of water surrounding a rocky mantle and iron core. In small planets, water and rock form distinct layers with limited incorporation of water into silicate phases, but these materials may interact differently during the growth and evolution of water worlds due to greater interior pressures and temperatures. Here, we use density functional molecular dynamics (DFT-MD) simulations to study the miscibility and interactions of enstatite (MgSiO 3 ), a major end-member silicate phase, and water (H 2 O) at extreme conditions in water world interiors. We explore pressures ranging from 30 to 120 GPa and temperatures from 500 to 8000 K. Our results demonstrate that enstatite and water are miscible in all proportions if the temperature exceeds the melting point of MgSiO 3 . Furthermore, we performed smoothed particle hydrodynamics simulations to demonstrate that the conditions necessary for rock-water miscibility are reached during giant impacts between water-rich bodies of 0.7–4.7 Earth masses. Our simulations lead to water worlds that include a mixed layer of rock and water.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Reduced Atmospheres of Post-impact Worlds: The Early Earth

Impacts may have had a significant effect on the atmospheric chemistry of the early Earth. Reduced phases in the impactor (e.g., metallic iron) can reduce the planet's H 2 O inventory to produce massive atmospheres rich in H 2 . While previous studies have focused on the interactions between the impactor and atmosphere in such scenarios, we investigate two further effects: (1) the distribution of the impactor's iron inventory during impact between the target interior, target atmosphere, and escaping the target; and (2) interactions between the post-impact atmosphere and the impact-generated melt phase. We find that these two effects can potentially counterbalance each other, with the melt–atmosphere interactions acting to restore reducing power to the atmosphere that was initially accreted by the melt phase. For a ~10 22 kg impactor, when the iron accreted by the melt phase is fully available to reduce this melt, we find an equilibrium atmosphere with H 2 column density ~10 4 moles cm –2 (pH 2 ~ 120 bars, X H2 ~ 0.77), consistent with previous estimates. However, when the iron is not available to reduce the melt (e.g., sinking out in large diameter blobs), we find significantly less H 2 (7 × 10 2 – 5 × 10 3 moles cm –2 , pH 2 ≲ 60 bars, X H2 ≲ 0.41). These lower H 2 abundances are sufficiently high that species important to prebiotic chemistry can form (e.g., NH 3 , HCN), but sufficiently low that the greenhouse heating effects associated with highly reducing atmospheres, which are problematic to such chemistry, are suppressed. The manner in which iron is accreted by the impact-generated melt phase is critical in determining the reducing power of the atmosphere and resolidified melt pool in the aftermath of impact.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Did Earth Eat Its Leftovers? Impact Ejecta as a Component of the Late Veneer

The presence of highly siderophile elements in Earth's mantle indicates that a small percentage of Earth's mass was delivered after the last giant impact in a stage of "late accretion." There is ongoing debate about the nature of late-accreted material and the sizes of late-accreted bodies. Earth appears isotopically most similar to enstatite chondrites and achondrites. It has been suggested that late accretion must have been dominated by enstatite-like bodies that originated in the inner disk, rather than ordinary or carbonaceous chondrites. Here we examine the provenances of "leftover" planetesimals present in the inner disk in the late stages of accretion simulations. Dynamically excited planet formation produces planets and embryos with similar provenances, suggesting that the Moon-forming impactor may have had a stable isotope composition very similar to the proto-Earth. Commonly, some planetesimal-sized bodies with similar provenances to the Earth-like planets are left at the end of the main stage of growth. The most chemically similar planetesimals are typically fragments of protoplanets ejected millions of years earlier. If these similar-provenance bodies are later accreted by the planet, they will represent late-accreted mass that naturally matches Earth's composition. The planetesimal-sized bodies that exist during the giant impact phase can have large core mass fractions, with core provenances similar to the proto-Earth. These bodies are an important potential source for highly siderophile elements. The range of core fractions in leftover planetesimals complicates simple inferences as to the mass and origin of late accretion based on the highly siderophile elements in the mantle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Measuring the melting curve of iron at super-Earth core conditions

The discovery of more than 4500 extrasolar planets has created a need for modeling their interior structure and dynamics. Given the prominence of iron in planetary interiors, we require accurate and precise physical properties at extreme pressure and temperature. A first-order property of iron is its melting point, which is still debated for the conditions of Earth’s interior. Here, we used high-energy lasers at the National Ignition Facility and in situ x-ray diffraction to determine the melting point of iron up to 1000 gigapascals, three times the pressure of Earth’s inner core. We used this melting curve to determine the length of dynamo action during core solidification to the hexagonal close-packed (hcp) structure. We find that terrestrial exoplanets with four to six times Earth’s mass have the longest dynamos, which provide important shielding against cosmic radiation.

58 GEOSCIENCES↗
DOE OSTI2021

New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State

Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and Neptune (1 R ⊕ ≤ R ≤ 4 R ⊕ ). A low occurrence rate of planets has been identified at around twice the size of Earth (2 × R ⊕ ), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (T–ρ) graph and the entropy–pressure (s–P) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 × R ⊕ ) and larger planets (>2 × R ⊕ ) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous envelopes. In particular, among the larger planets (>2 × R ⊕ ), when viewed from the perspective of planet equilibrium temperature (T eq ), the hot ones (T eq ≳ 900 K) are consistent with ice-dominated composition without significant gaseous envelopes, while the cold ones (T eq ≲ 900 K) have more diverse compositions, including various amounts of gaseous envelopes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

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. In this work, 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.

58 GEOSCIENCES↗
DOE OSTI2019

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.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

The Principal Hugoniot of Forsterite to 950 GPa

Forsterite (Mg 2 SiO 4 ) single crystals were shock compressed to pressures between 200 and 950 GPa using independent plate–impact steady shocks and laser–driven decaying shock compression experiments. Additionally, we performed density functional theory–based molecular dynamics to aid interpretation of the experimental data and to investigate possible phase transformations and phase separations along the Hugoniot. We show that the experimentally obtained Hugoniot cannot distinguish between a pure liquid Mg 2 SiO 4 and an assemblage of solid MgO plus liquid magnesium silicate. In conclusion, the measured reflectivity is nonzero and increases with pressure, which implies that the liquid is a poor electrical conductor at low pressures and that the conductivity increases with pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Origin of the Moon Within a Terrestrial Synestia

The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moon’s composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular-momentum giant impacts can create a post-impact structure that exceeds the corotation limit, which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-corotation-limit body, traditional definitions of mantle, atmosphere, and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic, and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with bulk silicate Earth vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2007

Effect of Shock on the Magnetic Properties of Pyrrhotite, the Martian Crust, and Meteorites

We performed planar shock recovery experiments on natural pyrrhotite at pressures up to 6.9 GPa. We find that high-field isothermal remanent magnetization in pyrrhotite is demagnetized up to 90% by shock due to preferential removal of low coercivity components of magnetization. Contrary to static experiments, we do not observe complete demagnetization. Post shock permanent changes in magnetic properties include increasing saturation isothermal remanent magnetization, bulk coercivity and lowtemperature memory, and changes in squareness of hysteresis. These changes are consistent with an increase in the volume fraction of single domain grains. The lack of magnetic anomalies over large Martian impact basins is not expected to be solely due to shock demagnetization of the crust. We find that pyrrhotite-bearing rocks and meteorites can retain records of Martian magnetic fields even if shocked to pressures approaching 7 GPa. However, some paleointensity techniques may underestimate this field.

Louzad, Karin L.↗
NASA NTRS2001

Chicxulub Ejecta Dynamics

We modeled in detail the ejecta dynamics associated with the Chicxulub impact. We determined: (1) ejecta trajectories, (2) stratigraphic motions, (3) depth of ejecta stages, (4) thermodynamic histories of the ejecta particles, and (5) the final ejecta distribution. Additional information is contained in the original extended abstract.

OKeefe, John D.↗
NASA NTRS2001

Impact on Comets and Asteroids

We characterized the impact physics in collisions on porous bodies by various density projectiles and defined different penetration modes (compression, spreading, or breakup) based on transitions between instability regimes. Additional information is contained in the original extended abstract.

OKeefe, John D.↗