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. 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 together with either precession rate or a validated assumption of hydrostatic equilibrium. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected. 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. 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, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient. 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), 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. Although the gravity and shape data confirm that the interior of Ceres evolved thermally, 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.
The surface elemental composition of dwarf planet Ceres constrains its regolith ice content, aqueous alteration processes, and interior evolution. Using nuclear spectroscopy data acquired by NASA’s ...Dawn mission, we determined the concentrations of elemental hydrogen, iron, and potassium on Ceres. The data show that surface materials were processed by the action of water within the interior. The non-icy portion of Ceres’ carbon-bearing regolith contains similar amounts of hydrogen to those present in aqueously altered carbonaceous chondrites; however, the concentration of iron on Ceres is lower than in the aforementioned chondrites. This allows for the possibility that Ceres experienced modest ice-rock fractionation, resulting in differences between surface and bulk composition. At mid-to-high latitudes, the regolith contains high concentrations of hydrogen, consistent with broad expanses of water ice, confirming theoretical predictions that ice can survive for billions of years just beneath the surface.
Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small ...spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.
Dawn at Vesta: Testing the Protoplanetary Paradigm Russell, C. T.; Raymond, C. A.; Coradini, A. ...
Science (American Association for the Advancement of Science),
05/2012, Letnik:
336, Številka:
6082
Journal Article
Recenzirano
The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites ...that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.
Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by ...electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We show that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process.
Plasma turbulence is investigated using unprecedented high-resolution ion velocity distribution measurements by the Magnetospheric Multiscale mission (MMS) in the Earth's magnetosheath. This novel ...observation of a highly structured particle distribution suggests a cascadelike process in velocity space. Complex velocity space structure is investigated using a three-dimensional Hermite transform, revealing, for the first time in observational data, a power-law distribution of moments. In analogy to hydrodynamics, a Kolmogorov approach leads directly to a range of predictions for this phase-space transport. The scaling theory is found to be in agreement with observations. The combined use of state-of-the-art MMS data sets, novel implementation of a Hermite transform method, and scaling theory of the velocity cascade opens new pathways to the understanding of plasma turbulence and the crucial velocity space features that lead to dissipation in plasmas.
The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory ...and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale observations at the Earth's bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.
We report unambiguous in situ observation of the coalescence of macroscopic flux ropes by the magnetospheric multiscale (MMS) mission. Two coalescing flux ropes with sizes of ∼1 R_{E} were ...identified at the subsolar magnetopause by the occurrence of an asymmetric quadrupolar signature in the normal component of the magnetic field measured by the MMS spacecraft. An electron diffusion region (EDR) with a width of four local electron inertial lengths was embedded within the merging current sheet. The EDR was characterized by an intense parallel electric field, significant energy dissipation, and suprathermal electrons. Although the electrons were organized by a large guide field, the small observed electron pressure nongyrotropy may be sufficient to support a significant fraction of the parallel electric field within the EDR. Since the flux ropes are observed in the exhaust region, we suggest that secondary EDRs are formed further downstream of the primary reconnection line between the magnetosheath and magnetospheric fields.
The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite-eucrite-diogenite class of ...meteorites. Processes active in the space environment produce 'space weathering' products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere, and it has long been a mystery why Vesta's absorption bands are so strong. Analyses of soil samples from both the Moon and the asteroid Itokawa determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.
Organic compounds occur in some chondritic meteorites, and their signatures on solar system bodies have been sought for decades. Spectral signatures of organics have not been unambiguously identified ...on the surfaces of asteroids, whereas they have been detected on cometary nuclei. Data returned by the Visible and InfraRed Mapping Spectrometer on board the Dawn spacecraft show a clear detection of an organic absorption feature at 3.4 micrometers on dwarf planet Ceres. This signature is characteristic of aliphatic organic matter and is mainly localized on a broad region of ~1000 square kilometers close to the ~50-kilometer Ernutet crater. The combined presence on Ceres of ammonia-bearing hydrated minerals, water ice, carbonates, salts, and organic material indicates a very complex chemical environment, suggesting favorable environments to prebiotic chemistry.
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