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.
Cryovolcanism on Ceres Ruesch, O.; Platz, T.; Schenk, P. ...
Science,
09/2016, Letnik:
353, Številka:
6303
Journal Article
Recenzirano
Odprti dostop
Volcanic edifices are abundant on rocky bodies of the inner solar system. In the cold outer solar system, volcanism can occur on solid bodies with a water-ice shell, but derived cryovolcanic ...constructs have proved elusive. We report the discovery, using Dawn Framing Camera images, of a landform on dwarf planet Ceres that we argue represents a viscous cryovolcanic dome. Parent material of the cryomagma is a mixture of secondary minerals, including salts and water ice. Absolute model ages from impact craters reveal that extrusion of the dome has occurred recently. Ceres' evolution must have been able to sustain recent interior activity and associated surface expressions. We propose salts with low eutectic temperatures and thermal conductivities as key drivers for Ceres' long-term internal evolution.
The Violent Collisional History of Asteroid 4 Vesta Marchi, S.; McSween, H. Y.; O'Brien, D. P. ...
Science (American Association for the Advancement of Science),
05/2012, Letnik:
336, Številka:
6082
Journal Article
Recenzirano
Vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids. The Dawn spacecraft ...extensively imaged Vesta's surface, revealing a collision-dominated history. Results show that Vesta's cratering record has a strong north-south dichotomy. Vesta's northern heavily cratered terrains retain much of their earliest history. The southern hemisphere was reset, however, by two major collisions in more recent times. We estimate that the youngest of these impact structures, about 500 kilometers across, formed about 1 billion years ago, in agreement with estimates of Vesta asteroid family age based on dynamical and collisional constraints, supporting the notion that the Vesta asteroid family was formed during this event.
Thermochemical models have predicted that Ceres, is to some extent, differentiated and should have an icy crust with few or no impact craters. We present observations by the Dawn spacecraft that ...reveal a heavily cratered surface, a heterogeneous crater distribution, and an apparent absence of large craters. The morphology of some impact craters is consistent with ice in the subsurface, which might have favored relaxation, yet large unrelaxed craters are also present. Numerous craters exhibit polygonal shapes, terraces, flowlike features, slumping, smooth deposits, and bright spots. Crater morphology and simple-to-complex crater transition diameters indicate that the crust of Ceres is neither purely icy nor rocky. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages (AMAs) of 550 million and 720 million years, depending on the applied chronology model.
Cometary nuclei consist mostly of dust and water ice. Previous observations have found nuclei to be low-density and highly porous bodies, but have only moderately constrained the range of allowed ...densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov-Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100 kilometres. The gravitational point mass is GM = 666.2 ± 0.2 cubic metres per second squared, giving a mass M = (9,982 ± 3) × 10(9) kilograms. Together with the current estimate of the volume of the nucleus, the average bulk density of the nucleus is 533 ± 6 kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72-74 per cent) dusty body, similar to that of comet 9P/Tempel 1. The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material.
We performed a stereo-photogrammetric (SPG) analysis of more than 1500 Rosetta/OSIRIS NAC images of comet 67P/Churyumov-Gerasimenko (67P). The images with pixel scales in the range 0.2−3.0 m/pixel ...were acquired between August 2014 and February 2016. We finally derived a global high-resolution 3D description of 67P’s surface, the SPG SHAP7 shape model. It consists of about 44 million facets (1−1.5 m horizontal sampling) and a typical vertical accuracy at the decimeter scale. Although some images were taken after perihelion, the SPG SHAP7 shape model can be considered a pre-periheliondescription and replaces the previous SPG SHAP4S shape model. From the new shape model, some measures for 67P with very low 3σ uncertainties can be retrieved: 18.56 km3 ± 0.02 km3 for the volume and 537.8 kg/m3 ± 0.7 kg/m3 for the mean density assuming a mass value of 9.982 × 1012 kg.
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.
The near-Earth asteroid (162173) Ryugu, the target of Hayabusa2 space mission, was observed via both orbiter and the lander instruments. The infrared radiometer on the MASCOT lander (MARA) is the ...only instrument providing spectrally resolved mid-infrared (MIR) data, which is crucial for establishing a link between the asteroid material and meteorites found on Earth. Earlier studies revealed that the single boulder investigated by the lander belongs to the most common type found on Ryugu. Here we show the spectral variation of Ryugu's emissivity using the complete set of in-situ MIR data and compare it to those of various carbonaceous chondritic meteorites, revealing similarities to the most aqueously altered ones, as well as to asteroid (101955) Bennu. The results show that Ryugu experienced strong aqueous alteration prior to any dehydration.
The Dawn spacecraft Framing Camera (FC) acquired over 2400 clear filter images of Ceres with a resolution of about 140m/pixel during the six cycles in the High Altitude Mapping Orbit (HAMO) phase ...between August 18 and October 21, 2015. We ortho-rectified the images from the first cycle and produced a global, high-resolution, controlled photomosaic of Ceres. This global mosaic is the basis for a high-resolution Ceres atlas that consists of 15 tiles mapped at a scale of 1:750,000. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The full atlas is available to the public through the Dawn Geographical Information System (GIS) web page http://dawngis.dlr.de/atlas and will become available through the NASA Planetary Data System (PDS) (http://pdssbn.astro.umd.edu/).
•The mosaic of Ceres was created using camera data from HAMO orbit phase.•An atlas of Ceres consisting of fifteen tiles was created.•The nomenclature for surface features was approved by the IAU and applied to the atlas.
We report on the results of the Mars Express High-Resolution Stereo Camera (HRSC) experiment pertaining to one of its major aims, mapping the surface of Mars by high-resolution digital terrain models ...(DTM, up to 50
m grid spacing) and orthoimages (up to 12.5
m resolution). We introduce the specifications and characteristics of these data products and give an overview of the procedures that have been developed and are applied for their derivation. We also address the performance characteristics of the mapping project related to different aspects of internal accuracy, accuracy with respect to the global reference system, and regional aspects. Using adaptive processing techniques for terrain reconstruction and a revised approach to the improvement of orientation data, a mean precision of the resulting 3D points of about 12
m is obtained, exceeding the mean ground resolution of the stereo images. Using Mars Orbiter Laser Altimeter (MOLA) data, the HRSC models are firmly tied to the global reference system at the scale of the HRSC DTM grid spacing in the lateral dimension, and to within few meters vertically. HRSC high-resolution DTMs are typically generated using a grid size of about 2 times the mean ground resolution, but usually not larger than 3 times the mean ground resolution, and not smaller than 3 times the precision of the integrated 3D points derived from stereo image analysis. Statistically, every grid cell is based on at least one measured 3D point. Thus, horizontal DTM resolution is well established with regard to the precision and density of the derived 3D points, while the concurrent aim of a detailed terrain representation at maximum possible resolution is pursued. Comparison with the DTM derived from MOLA data allows us to identify specific advancements related to this updated view of Martian topography. We also address the mapping performance of HRSC in comparison to MOLA with respect to latitude and to different surface types and morphologies. Finally, comparison with MOLA highlights typical complementarities of the two different approaches for mapping planetary surfaces.
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