Fluid Dynamics Experiments for Planetary Interiors

Understanding fluid flows in planetary cores and subsurface oceans, as well as their signatures in available observational data (gravity, magnetism, rotation, etc.), is a tremendous interdisciplinary challenge. In particular, it requires understanding the fundamental fluid dynamics involving turbulence and rotation at typical scales well beyond our day-to-day experience. To do so, laboratory experiments are fully complementary to numerical simulations, especially in systematically exploring extreme flow regimes for long duration. In this review article, we present some illustrative examples where experimental approaches, complemented by theoretical and numerical studies, have been key for a better understanding of planetary interior flows driven by some type of mechanical forcing. We successively address the dynamics of flows driven by precession, by libration, by differential rotation, and by boundary topography.

Dynamos in the Inner Solar System

Dynamo magnetic fields are primarily generated by thermochemical convection of electrically conductive liquid metal within planetary cores. Convection can be sustained by secular cooling and may be bolstered by compositional buoyancy associated with core solidification. Additionally, mechanical stirring of core fluids and external perturbations by large impact events, tidal effects, and orbital precession can also contribute to sustaining dynamo fields. Convective dynamos cease when the core-mantle heat flux becomes subadiabatic or if specific crystallization regimes inhibit core fluid flows. Therefore, exploring the histories of magnetic fields across the Solar System provides a window into the thermal and chemical evolution of planetary interiors. Here we review how recent spacecraft-based studies of remanent crustal magnetism, paleomagnetic studies of rock samples, and planetary interior models have revealed the magnetic and evolutionary histories of Mercury, Earth, Mars, the Moon, and several planetesimals, as well as discuss avenues for future exploration and discovery. ▪ Paleomagnetism and remanent crustal magnetism studies elucidate the magnetic histories of rocky planetary bodies. ▪ Records of ancient dynamo fields have been obtained from 3 out of 4 terrestrial planets, the Moon, and several planetesimals. ▪ The geometries, intensities, and longevities of dynamo fields can provide information on core processes and planetary thermal evolution. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Rapid Formation of Gas-giant Planets via Collisional Coagulation from Dust Grains to Planetary Cores

Gas-giant planets, such as Jupiter, Saturn, and massive exoplanets, were formed via the gas accretion onto the solid cores, each with a mass of roughly 10 Earth masses. However, rapid radial migration due to disk–planet interaction prevents the formation of such massive cores via planetesimal accretion. Comparably rapid core growth via pebble accretion requires very massive protoplanetary disks because most pebbles fall into the central star. Although planetesimal formation, planetary migration, and gas-giant core formation have been studied with a lot of effort, the full evolution path from dust to planets is still uncertain. Here we report the result of full simulations for collisional evolution from dust to planets in a whole disk. Dust growth with realistic porosity allows the formation of icy planetesimals in the inner disk (≲10 au), while pebbles formed in the outer disk drift to the inner disk and there grow to planetesimals. The growth of those pebbles to planetesimals suppresses their radial drift and supplies small planetesimals sustainably in the vicinity of cores. This enables rapid formation of sufficiently massive planetary cores within 0.2–0.4 million years, prior to the planetary migration. Our models shows the first gas giants form at 2–7 au in rather common protoplanetary disks, in agreement with the exoplanet and solar systems.

Fe5S2 identified as a host for sulfur in Earth’s core

Planetary habitability, as we experience on Earth, is linked to a functioning geodynamo which is in part driven by the crystallization of the liquid iron-nickel-alloy core as a planet cools over time. Cosmochemical considerations suggest that sulfur is a candidate light alloying element in rocky planetary cores of varying sizes and oxidation states; such that, iron sulfide phase relations at extreme conditions contribute to outer core thermochemical convection and inner core crystallization in a wide range of planetary bodies. Here we experimentally investigate the structural properties of the Fe-S system and report the discovery of the sulfide, Fe5S2, crystallizing in equilibrium with iron at Earth’s outer core pressures and high temperatures. Using single-crystal X-ray diffraction techniques, Fe5S2 was determined to adopt the complex Ni5As2-type structure (P63cm, Z = 6). These results conclude that Fe5S2 is likely to crystallize at the interface of Earth’s core and mantle and will begin to crystallize during the freezing out of Earth and Venus’ core overtime. The increased metal-metal bonding measured in Fe5S2 compared to the other high P-T iron sulfides may contribute to signatures of higher conductivity from regions of Fe5S2 is crystallization. Fe5S2 could serve as a host for Ni and Si as has been observed in the related meteoritic phase, perryite, (Fe, Ni)8(P, Si)3, adding intricacies to elemental partitioning during inner core crystallization. The stability of Fe5S2 presented here is key to understanding the role of sulfur in the multicomponent crystallization sequences that drive the geodynamics and dictate the structures of Earth and rocky planetary cores.

Phase Relations of Earth’s Core-Forming Materials

Recent updates on phase relations of Earth’s core-forming materials, Fe alloys, as a function of pressure (P), temperature (T), and composition (X) are reviewed for the Fe, Fe-Ni, Fe-O, Fe-Si, Fe-S, Fe-C, Fe-H, Fe-Ni-Si, and Fe-Si-O systems. Thermodynamic models for these systems are highlighted where available, starting with 1 bar to high-P-T conditions. For the Fe and binary systems, the longitudinal wave velocity and density of liquid alloys are discussed and compared with the seismological observations on Earth’s outer core. This review may serve as a guide for future research on the planetary cores.

Which planets trigger longer-lived vortices: low-mass or high-mass?

Recent ALMA observations have found many protoplanetary discs with rings that can be explained by gap-opening planets less massive than Jupiter. Meanwhile, recent studies have suggested that protoplanetary discs should have low levels of turbulence. Past computational work on low-viscosity discs has hinted that these two developments might not be self-consistent because even low-mass planets can be accompanied by vortices instead of conventional double rings. We investigate this potential discrepancy by conducting hydrodynamic simulations of growing planetary cores in discs with various aspect ratios (H/r = 0.04, 0.06, 0.08) and viscosities (1.5 × 10−5 ≲ α ≲ 3 × 10−4), having these cores accrete their gas mass directly from the disc. With α < 10−4, we find that sub-Saturn-mass planets in discs with H/r ≤ 0.06 are more likely to be accompanied by dust asymmetries compared to Jupiter-mass planets because they can trigger several generations of vortices in succession. We also find that vortices with H/r = 0.08 survive >6000 planet orbits regardless of the planet mass or disc mass because they are less affected by the planet’s spiral waves. We connect our results to observations and find that the outward migration of vortices with H/r ≥ 0.08 may be able to explain the cavity in Oph IRS 48 or the two clumps in MWC 758. Lastly, we show that the lack of observed asymmetries in the disc population in Taurus is unexpected given the long asymmetry lifetimes in our low viscosity simulations (α ∼ 2 × 10−5), a discrepancy we suggest is due to these discs having higher viscosities.

Inefficient compaction in small planetary cores -- application to the Moon

<div> <p>Growth of the solid inner core is generally considered to power the Earth's present geodynamo. Cristallisation of a solid central inner core has also been proposed to drive the lunar dynamo and to generate a magnetic field in smaller bodies. In a previous work, we estimated the compaction of planetary cores for different scenarios of growth (with or without supercooling) and different sizes of the inner core. Our main results indicated that small inner cores are unlikely to compact efficiently the liquid trapped during the first steps of the growth.</p> <p>This is especially true for small bodies for which the typical size of the core is similar to the compaction length. The light elements are thus trapped during the cristallisation, reducing the release of latent heat and of light elements. We present here a model to include the effect of an inefficient compaction in the energy budget of a planetary core and investigate the implications for the dynamo evolution in small bodies. We apply this model for the evolution of the core of the Moon. </p> </div>

Crystal plasticity in shock-compressed hcp-iron

<p>Iron is a key constituent of planetary core and an important technological material. Here, we combine <em>in situ</em> ultrafast X-ray diffraction at free electron lasers with optical-laser-induced shock compression experiments on polycrystalline Fe to study the plasticity of hexagonal close-packed (hcp)-Fe under extreme loading states. We identifiy the deformation mechanisms that controls the Fe microstructures and  observe a significant time-evolution of stress over the few nanoseconds of the experiments. These observations illustrate how ultrafast plasticity studies can reveal distinctive materials behavior under extreme loading states and will help constraining the pressure, temperature, and strain rate dependence of materials behavior in planetary cores.</p>