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Gravity and shape measurements for Ceres obtained from the Dawn spacecraft mission show that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37, suggesting that Ceres has a rocky chondritic core overlaid by a volatile-rich icy shell. Geophysical observations of dwarf planet Ceres This paper presents geophysical observations of Ceres—the closest dwarf planet to the Sun, in an orbit between those of Mars and Jupiter—based on radio tracking and onboard image data acquired by the Dawn spacecraft. Gravity and shape measurements provide a key parameter that has been unobtainable through remote observations—the moment of inertia. Ceres is shown to be in hydrostatic equilibrium with an inferred normalized mean moment of inertia of 0.37. The Dawn spacecraft data and analysis reported here give the first constraints on the interior structure of a dwarf planet. Ceres emerges as a partially differentiated body, with a rocky core overlaid by a volatile-rich icy shell. Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated 1 , 2 , 3 , 4 , 5 , 6 . A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation 1 , 7 together with either precession rate 8 , 9 or a validated assumption of hydrostatic equilibrium 10 . However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected 9 . Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies 1 , 4 , 6 . Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated 11 , such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta 8 , 12 , this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient 1 , 4 . To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460–2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites 13 ), which yields an outer-shell thickness of 70–190 kilometres. The density of this outer shell is 1,680–1,950 kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts 14 . Although the gravity and shape data confirm that the interior of Ceres evolved thermally 1 , 4 , 6 , its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000 kilometres across) ice-rich rocky bodies.

Mercury’s metallic core is expected to have formed under highly reducing conditions, resulting in the presence of significant quantities of silicon alloyed to iron. Here we present the phase diagram of the Fe-FeSi system, reconstructed from in situ X-ray diffraction measurements at pressure and temperature conditions spanning over those expected for Mercury’s core, and ex situ chemical analysis of recovered samples. Under high pressure, we do not observe a miscibility gap between the cubic fcc and B2 structures, but rather the formation of a re-entrant bcc phase at temperatures close to melting. Upon melting, the investigated alloys are observed to evolve towards two distinct Fe-rich and Fe-poor liquid compositions at pressures below 35-38 GPa. The evolution of the phase diagram with pressure and temperature prescribes a range of possible core crystallization regimes, with strong dependence on the Si abundance of the core. The iron-silicon phase diagram has been established at the conditions of Mercury’s core. The resulting phase diagram is remarkably complex, and presents an array of new mechanisms which may power Mercury’s inner dynamo.

The Juno spacecraft, which is in a polar orbit around Jupiter, is providing direct measurements of the planet’s magnetic field close to its surface 1 . A recent analysis of observations of Jupiter’s magnetic field from eight (of the first nine) Juno orbits has provided a spherical-harmonic reference model (JRM09) 2 of Jupiter’s magnetic field outside the planet. This model is of particular interest for understanding processes in Jupiter’s magnetosphere, but to study the field within the planet and thus the dynamo mechanism that is responsible for generating Jupiter’s main magnetic field, alternative models are preferred. Here we report maps of the magnetic field at a range of depths within Jupiter. We find that Jupiter’s magnetic field is different from all other known planetary magnetic fields. Within Jupiter, most of the flux emerges from the dynamo region in a narrow band in the northern hemisphere, some of which returns through an intense, isolated flux patch near the equator. Elsewhere, the field is much weaker. The non-dipolar part of the field is confined almost entirely to the northern hemisphere, so there the field is strongly non-dipolar and in the southern hemisphere it is predominantly dipolar. We suggest that Jupiter’s dynamo, unlike Earth’s, does not operate in a thick, homogeneous shell, and we propose that this unexpected field morphology arises from radial variations, possibly including layering, in density or electrical conductivity, or both. Maps of Jupiter’s internal magnetic field at a range of depths reveal an unusual morphology, suggesting that Jupiter’s dynamo, unlike Earth’s, does not operate in a thick, homogeneous shell.

X-ray Free Electron Lasers provide femtosecond x-ray pulses with narrow bandwidth and unprecedented peak brightness. Special modes of operation have been developed to deliver double pulses for x-ray pump, x-ray probe experiments. However, the longest delay between the two pulses achieved with existing single bucket methods is less than 1 picosecond, thus preventing the exploration of longer time-scale dynamics. We present a novel two-bucket scheme covering delays from 350 picoseconds to hundreds of nanoseconds in discrete steps of 350 picoseconds. Performance for each pulse can be similar to the one in a single pulse operation. The method has been experimentally tested with the Linac Coherent Light Source (LCLS-I) and the copper linac with LCLS-II hard x-ray undulators.

The discovery of Mercury’s unusually axisymmetric, anomalously axially offset dipolar magnetic field reveals a new regime of planetary magnetic fields. The cause of the offset dipole remains to be resolved, although some exotic models have been proposed. Deciphering why Mercury has such an anomalous field is crucial not only for understanding the internal dynamics, evolutionary history and origin of the planet, but also for establishing the general dynamo theory. Here we present numerical dynamo models, where core convection is driven as thermo-compositional, double-diffusive convection surrounded by a thermally stably stratified layer. We show that the present models produce magnetic fields similar in morphology and strength to that of Mercury. The dynamo-generated fields act on the flow to force interaction between equatorially symmetric and antisymmetric components that results in north-south asymmetric helicity. This symmetry-breaking magnetic feedback causes the flow to generate and maintain Mercury’s axially offset dipolar field. A new regime of planetary magnetic fields was revealed through the MESSENGER spacecraft mission to Mercury. Here, the authors present a numerical dynamo model that can re-produce both the axisymmetric and anomalously axially offset dipolar magnetic field of Mercury.

The Earth’s solid inner core plays a fundamental role in determining the past and present properties and dynamics of the Earth’s deep interior. Inner core growth powers the geodynamo, producing the protective global magnetic field, and provides a record of core evolution spanning geological timescales. However, the origins of the inner core remain enigmatic. Traditional core evolution models assume that the inner core formed when the core first cooled to its melting temperature, but this neglects the physical requirement that liquids must be supercooled to below their melting point before freezing. Prior estimates from mineral physics calculations of the supercooling required to homogeneously nucleate the inner core from candidate binary alloys exceed constraints of  K inferred from geophysical observations, while a plausible scenario for heterogeneous nucleation has yet to be identified. Here we consider a different possibility, that atomic-scale compositional fluctuations can increase the local melting temperature, and hence supercooling, available for homogeneous nucleation. Using molecular dynamic simulations of Fe-O alloys we find that compositional fluctuations producing O-depleted regions are too rare to aid nucleation, while O-enriched regions can reduce the undercooling by 50 K (  K) for a bulk concentration of 20 mol.% O or 400 K (  K) for a bulk concentration of 30 mol.% O. While these results do not explain the nucleation of Earth’s inner core, they do show that compositional fluctuations can aid the process of homogeneous nucleation.

Water is an important component of exoplanets, with its distribution, that is, whether at the surface or deep inside, fundamentally influencing the planetary properties. The distribution of water in most exoplanets is determined by yet-unknown partition coefficients at extreme conditions. Here we first conduct ab initio molecular dynamics simulations to investigate the metal–silicate partition coefficients of water up to 1,000 GPa and then model planet interiors by considering the effects of water content on density, melting temperature and water partitioning. Our calculations reveal that water strongly partitions into iron over silicate at high pressures and, thus, would preferentially stay in a planet’s core. The results of our planet interior model challenge the notion of water worlds as imagined before: the majority of the bulk water budget (even more than 95%) can be stored deep within the core and the mantle, and not at the surface. For planets more massive than ~6 M ⨁ and Earth-size planets (of lower mass and small water budgets), the majority of water resides deep in the cores of planets. Whether water is assumed to be at the surface or at depth can affect the radius up to 15–25% for a given mass. The exoplanets previously believed to be water-poor on the basis of mass–radius data may actually be rich in water. If water exists in super-Earth and sub-Neptune exoplanets, it is expected to be hidden deep in their cores and mantles, rather than at their surfaces. Exoplanets considered to be relatively dry might actually have abundant water sequestered in their interiors.

Models of planetary core formation traditionally involve the fractionation of Fe,Ni-metal melts from silicate mantles after extensive silicate melting. However, in planetary bodies that form farther from their central star, where moderately volatile elements are more abundant, high concentrations of oxygen and sulfur stabilize Fe,Ni-sulfides over metals. Here we show that percolative sulfide melt migration can occur in primitive, oxidized mineral assemblages prior to silicate melting in partial melting experiments with meteorites. Complementary experiments with partially molten synthetic sulfides show that fractionation of liquid sulfide from solid residues yields distinct noble metal (Os, Ru, Ir, Pd, and Pt) trace element proportions that match those manifested in the most oxidized meteoritic residues, the brachinites, as well as their complementary basaltic silicate melts. Our experiments provide robust evidence for percolative sulfide melt fractionation in meteorites and indicate that sulfide-dominated cores would be expected in oxidized planetary bodies, including Mars. Melting experiments with planetary materials show that oxidized core formation occurred via percolation of molten sulfide at low igneous temperatures.

Electrical resistivity measurements of Fe–10wt%Ni–10wt%Si have been performed in a multi-anvil press from 3 to 20 GPa up to 2200 K. The temperature and pressure dependences of electrical resistivity are analyzed in term of changes in the electron mean free path. Similarities in the thermal properties of Fe–Si and Fe–Ni–Si alloys suggest the effect of Ni is negligible. Electrical resistivity is used to calculate thermal conductivity via the Wiedemann–Franz law, which is then used to estimate the adiabatic heat flow. The adiabatic heat flow at the top of Earth’s core is estimated to be 14 TW from the pressure and temperature dependences of thermal conductivity in the liquid state from this study, suggesting thermal convection may still be an active source to power the dynamo depending on the estimated value taken for the heat flow through the core mantle boundary. The calculated adiabatic heat flux density of 22.7–32.1 mW/m 2 at the top of Mercury’s core suggests a chemically driven magnetic field from 0.02 to 0.21 Gyr after formation. A thermal conductivity of 140–148 Wm −1  K −1 is estimated at the center of a Fe–10wt%Ni–10wt%Si Venusian core, suggesting the presence of a solid inner core and an outer core that is at least partially liquid.

Planetary magnetic fields provide a window into the otherwise largely inaccessible dynamics of a planet’s deep interior. In particular, interaction between fluid flow in electrically conducting interior regions and the magnetic field there gives rise to observable secular variation (time dependency) of the externally observed magnetic field. Secular variation of Jupiter’s field has recently been revealed 1 – 3 and been shown to arise, in part, from an axisymmetric, equatorial jet 2 . Whether this jet is time dependent has not previously been addressed, yet it is of critical importance for understanding the dynamics of the planet’s interior. If steady, it would probably be a manifestation of deep dynamo convective flow (and jets are anticipated as part of that flow 4 – 9 ) but if time dependent on a timescale much shorter than the convective turnover timescale of several hundred years, it would probably have a different origin. Here we show that the jet has a wavelike fluctuation with a period of roughly 4 years, strongly suggestive of the presence of a torsional oscillation 10 (a cylindrically symmetric oscillating flow about the rotation axis) or a localized Alfvén wave in Jupiter’s metallic hydrogen interior. This opens a pathway towards revealing otherwise hidden aspects of the magnetic field within the metallic hydrogen region and hence constraining the dynamo that generates Jupiter’s magnetic field. An axisymmetric, equatorial jet in Jupiter’s interior has a wavelike fluctuation with a 4-year period, revealing hidden aspects of the magnetic field within the metallic hydrogen region and constraining the dynamo that generates the magnetic field.