Geologic constraints on the origin of red organic‐rich material on Ceres

The geologic context of red organic‐rich materials (ROR) found across an elongated 200 km region on Ceres is evaluated with spectral information from the multispectral framing camera (FC) and the visible and near‐infrared mapping spectrometer (VIR) of Dawn. Discrete areas of ROR materials are found to be associated with small fresh craters less than a few hundred meters in diameter. Regions with the highest concentration of discrete ROR areas exhibit a weaker diffuse background of ROR materials. The observed pattern could be consistent with a field of secondary impacts, but no appropriate primary crater has been found. Both endogenic and exogenic sources are being considered for these distinctive organic materials.     

Meteoritic and other constraints on the internal structure and impact history of small asteroids

Studies of the internal structure of asteroids, which are crucial for understanding their impact history and for hazard mitigation, appear to be in conflict for the S-type asteroids, Eros, Gaspra, and Ida. Spacecraft images and geophysical data show that they are fractured, coherent bodies, whereas models of catastrophic asteroidal impacts, family and satellite formation, and studies of asteroid spin rates, and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble. These conflicting views may be reconciled if 10-50 km S-type asteroids formed as rubble piles, but were later consolidated into coherent bodies. Many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration, metamorphism, igneous and impact processes. Ordinary chondrites, which are the best analogs for S asteroids, are commonly breccias. Some may have formed in cratering events, but many appear to have formed during disruption and reaccretion of their parent asteroids. Some breccias were lithified during metamorphism, and a few were lithified by injected impact melt, but most are regolith and fragmental breccias that were lithified by mild or moderate shock, like their lunar analogs. Shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together, and by friction and pressure welding of silicate and metallic Fe,Ni grains. We suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble. Internal voids would be partly filled with regolith by impact-induced seismic shaking. Consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body. Fractures on Ida that were created by antipodal impacts and are concentrated in and near large craters, and small positive gravity anomalies associated with the Psyche and Himeros craters on Eros, are consistent with this concept. Spin data suggest that smaller asteroids 0.6-6 km in size are unconsolidated rubble piles. C-type asteroids, which are more porous than S-types, and their analogs, the volatile-rich carbonaceous chondrites, were probably not lithified by shock.     

Cometary nuclei internal structure from early aggregation simulations

A new model for the aggregation of cometesimals in the primordial solar nebula is proposed. The simulation of the aggregation takes into account disruptive and sticking effects of impacts on the aggregates properties together with the temporal evolution of cohesive strength during accretion due to sintering processes. Different regimes of aggregation are obtained depending on the value of the homogeneity exponent, μ, that indicates the fraction of kinetic energy available for cohesive energy dissipation during an impact. Porous fractal aggregates with different cohesive strength blocks are formed for 0 < μ < 0.4, while they are compact with a layered structure of different strengths for 0.4 < μ < 0.6 and weak ‘rubble piles’ for 0.6 < μ < 1. Cohesive strength estimations of the final cometary nuclei obtained give values generally lower than 10 kPa. The layered aggregates present the highest global cohesive strength, increasing their probability to survive collisions or moderate tidal stress. These results compare well with the structural and cohesive properties of comets deduced from observations and laboratory simulations.     

Photometric analysis of 1 Ceres and surface mapping from HST observations

The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23-29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087±0.003, 0.056±0.002, and 0.039±0.003, respectively. V-band geometric albedo is calculated to be 0.090±0.003, consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090-1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070±0.002, 0.046±0.002, and 0.032±0.003, respectively. The photometric roughness of Ceres' surface is found to be about 44°±5° from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113-133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40-350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites.     

Radar observations of asteroid 1 Ceres

Radar observations of asteroid 1 Ceres were made at a 12.6-cm wavelength from the Arecibo Observatory in March/April 1977. The measurements, made with a received circular polarization orthogonal to that transmitted, yield a radar cross section of (0.04 ± 0.01)πR², for R = 510 km. The corresponding radar reflectivity is less than that measured for any other celestial body. Within the accuracy of measurement, no significant variation of cross section with rotational phase is apparent. The shape of the power spectrum suggests that Ceres is rougher at the scale of the observing wavelength than the Moon and inner planets, but smoother than the outer three Galilean satellites.     

Dawn Mission to Vesta and Ceres

The initial exploration of any planetary object requires a careful mission design guided by our knowledge of that object as gained by terrestrial observers. This process is very evident in the development of the Dawn mission to the minor planets 1 Ceres and 4 Vesta. This mission was designed to verify the basaltic nature of Vesta inferred both from its reflectance spectrum and from the composition of the howardite, eucrite and diogenite meteorites believed to have originated on Vesta. Hubble Space Telescope observations have determined Vesta’s size and shape, which, together with masses inferred from gravitational perturbations, have provided estimates of its density. These investigations have enabled the Dawn team to choose the appropriate instrumentation and to design its orbital operations at Vesta. Until recently Ceres has remained more of an enigma. Adaptive-optics and HST observations now have provided data from which we can begin to confidently plan the mission. These observations reveal a rotationally symmetric body with little surface relief, an ultraviolet bright point that can be used as a control point for determining the pole and anchoring a geographic coordinate system. They also reveal albedo and color variations that provide tantalizing hints of surface processes.     

Ceres lightcurve analysis—Period determination

The sidereal period of Ceres is refined from 9.075 h to 9.074170±0.000002, making use of recent and historical lightcurves spanning almost 50 years. An observed increase in the amplitude of the lightcurve with solar phase angle is consistent with bright, discrete albedo features contributing a greater fraction of light as the defect of illumination increases. Observations near the same phase angle over this time span show no evidence of changes that would indicate active surface processes.     

Insights into the morphology of the Serra da Cangalha impact structure from geophysical modeling

Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5‐D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies/structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short‐wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.5–1 km below the current surface. These depths are also supported by 2.5‐D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock‐induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s^?1.     

On the composition and differentiation of Ceres

The dwarf planet Ceres has a density of 2040-2250 kg m⁻³, and a dark non-icy surface with signs of hydrated minerals. As opposed to a differentiated internal structure with a nonporous rocky core and a water mantle, there are arguments for undifferentiated porous interior structure. Ceres’ mass and dimensions are uncertain and do not exclude undifferentiated interior even if hydrostatic equilibrium is attained. The rocky surface may be inconsistent with a large-scale water-rock differentiation. A differentiated structure with a thick water mantle below a rocky crust is gravitationally unstable and an overturn would have led to abundant surface salt deposits, which are not observed. A formation of hydrated surface minerals caused by internal heating implies a major density increase through devolatilization of the interior. A later accumulation of hydrated materials is inconsistent with anhydrous surfaces of many asteroids and with a low rate of the cosmic dust deposition in the inner Solar System. Ceres’ internal pressures (<140-200 MPa) are insufficient to significantly reduce porosity of chondritic materials and there is no need for abundant water phases to be present to account for the bulk density. Having the porosity of ordinary chondrites (∼10%), Ceres can consist of rocks with the grain density of pervasively hydrated CI carbonaceous chondrites. However, additional low-density phases (e.g., water ice) require to be present in the body with the grain density of CM chondrites. The likely low-density mineralogy of the interior implies Ceres’ accretion from pervasively aqueously altered carbonaceous planetesimals depleted in short-lived radionuclide ²⁶Al. Abundant water ice may not have accreted. Limited heat sources after accretion may not have caused major mineral dehydration leading to formation of water mantle. These inferences can be tested with the Dawn spacecraft in 2015.     

Mineralogy and temperature of crater Haulani on Ceres

We investigate the region of crater Haulani on Ceres with an emphasis on mineralogy as inferred from data obtained by Dawn's Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features. Haulani, which is one of the youngest craters on Ceres, exhibits a peculiar “blue” visible to near‐infrared spectral slope, and has distinct color properties as seen in multispectral composite images. In this paper, we investigate a number of spectral indices: reflectance; spectral slopes; abundance of Mg‐bearing and NH₄‐bearing phyllosilicates; nature and abundance of carbonates, which are diagnostic of the overall crater mineralogy; plus a temperature map that highlights the major thermal anomaly found on Ceres. In addition, for the first time we quantify the abundances of several spectral endmembers by using VIR data obtained at the highest pixel resolution (~0.1 km). The overall picture we get from all these evidences, in particular the abundance of Na‐ and hydrous Na‐carbonates at specific locations, confirms the young age of Haulani from a mineralogical viewpoint, and suggests that the dehydration of Na‐carbonates in the anhydrous form Na₂CO₃ may be still ongoing.     

Ceres - Neither a porous nor salty ball

This study explores the geophysical implications of two compositional models recently proposed for Ceres, which assume that the dwarf planet is a homogeneous mixture of chondritic material devoid with free water. In order to reproduce Ceres’ density, the rock density has to be offset by the presence of porosity and/or an abundance of hydrated salts resulting from the extensive hydration and oxidation of the chondritic material. Thermal modeling shows that a mixture of hydrated minerals is bound to compact and partly dehydrate as a consequence of long-lived radioisotope decay heat. The resulting interior structure is differentiated in a silicate-rich core and water-rich shell, with little porosity. Hence, this study confirms previous suggestion that Ceres contains a large fraction of free water.     

The butterfly diagram internal structure

This work originates from the need of getting a picture of the spot zone that is sharp enough to efficiently help us place tighter and more realistic constraints than we would usually do on dynamo models, in order to improve their predictive performance. This paper questions the confidence in Maunder’s Butterfly Diagram (BD) as the fundamental tool for describing the magnetic flux large-scale distribution and presents a new version of the time-latitude diagram for cycles 21 through 23, where spot groups are given proportional relevance to their area. The diagram presented here confirms the active regions’ well-known tendency to repeatedly appear in a few photospheric regions (“activity nests”) tightly limited in latitude, active for a short time. Activity nests leave their signature in the BD, in the form of small portions (“knots”) characterized by the spotted area high density. The BD may be described as a cluster of knots. A knot may appear at either lower or higher latitudes than previous ones; accordingly, the spot mean latitude abruptly drifts equatorward or even poleward, even though the knot’s prevalent tendency is to appear at lower and lower latitudes. A careful inspection of the BD suggests that its intricate fine structure may be (partially) disentangled by recognizing that, in any hemisphere, the activity is split into two or more distinct “activity waves” (out of phase compared to each other), drifting equatorward at a rate higher than the spot zone as a whole. Preliminary computations confirm this suggestion.     

Search for sulfates on the surface of Ceres

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile‐rich protoplanets like Ceres. Sulfates, present on water‐bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low‐pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H₂O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO₄·6H₂O by X‐ray powder diffraction. Long‐term (>40 h), continuous exposure of MgSO₄·nH₂O (n = 0, 6, 7) to low pressure (10^?3–10^?6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO₄·6H₂O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO₄·6H₂O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.     

On the composition of ices incorporated in Ceres

We use the clathrate hydrate trapping theory and gas drag formalism to calculate the composition of ices incorporated in the interior of Ceres. Utilizing a time-dependent solar nebula model, we show that icy solids can drift from beyond 5 au to the present location of the asteroid and be preserved from vaporization. We argue that volatiles were trapped in the outer solar nebula in the form of clathrate hydrates, hydrates and pure condensates prior to having been incorporated in icy solids and subsequently in Ceres. Under the assumption that most of volatiles were not vaporized during the accretion phase and the thermal evolution of Ceres, we determine the per mass abundances with respect to H₂O of CO₂, CO, CH₄, N₂, NH₃, Ar, Xe and Kr in the interior of the asteroid. The Dawn space mission, scheduled to explore Ceres in August 2014, may have the capacity to test some predictions. We also show that an in situ measurement of the D/H ratio in H₂O in Ceres could constrain the distance range in the solar nebula where its icy planetesimals were produced.     

On the internal structure of stars

A new model of the internal structure of certain types of celestial bodies is proposed. It is based on the concept that some neutron stars might have been formed earlier than all other type of stars, at an early stage of expansion of the universe, directly from continuous cosmic matter. Under such conditions, a neutron star after forming becomes an efficient center for the accretion of cosmic plasma. The plasma streams falling onto the neutron star carry magnetic fields with them that are created in the process (by thermoelectric currents and the dynamo process) and pack the fields tightly around the star. After a certain time, an extended and strongly magnetized plasma layer is formed around the neutron star. As a result, a stellar configuration is formed with an outer layer, mass, radius, and luminosity similar to those of an ordinary star. In the magnetized part of such a configuration, the gravitational attraction of the masses is compensated for by a magnetic pressure gradient, while the plasma is confifned by the magnetic field itself. Numerical estimates corroborate the possibility that magnetized stars exist. The radii and masses of the magnetized spheres of such stars are considerably less than the radii and masses of the corresponding configurations, so in observations they should not differ from ordinary stars: the outer layers (intermediate layer, photosphere, and chromosphere) of the magnetized configuration are the same as for an ordinary star. Structural differences may appear in the inner regions, however, involving magnetic activity and neutrino luminosity, for example.     

Cosmological constraints on the neutrino mass from CMB anisotropy and large-scale structure of the Universe

Using cosmological data on the CMB anisotropy and large-scale structure of the Universe, we have obtained new constraints on the sum of the masses of three generations of active neutrinos: Σm _ν < 1.05 eV (95% confidence level). Data of the third year of the WMAP mission served as the source of CMB anisotropy data. The mass functions of X-ray clusters of galaxies were taken as the data on the large-scale structure of the Universe. The observational properties of the clusters were obtained during the ROSAT mission and the assumption that the baryon fraction is universal in the Universe was used to determine the total cluster mass.     

Io: Observational constraints on internal energy and thermophysics of the surface

Infrared observations of the Io eclipse of 12 April 1980 in five broad bands from 3 to 30 μm define the thermal emission spectrum both during and after eclipse. A substantial fraction of the emitted radiation during eclipse arises from hot spots; the equivalent global average heat flow is 1.5 ± 0.3 W m^?2, corresponding to an internal source of (6 ± 1) × 10¹³ W. The hot spot spectra can be matched by components with color temperatures of 200–600°K covering 1–2% of the surface. Comparison with observations over the past 8 years suggests that, while the flux at the hottest temperatures may be highly variable, there is no evidence for major changes in the total heat flow, which is emitted primarily in the spectral region 10–20 μm. The heating curves of the surface were observed at 10 and 20 μm; when corrected for the hot spot contribution they indicate a typical global thermal inertia for Io of $\text{(0.2 ± 0.1) × 10}{}^{\mathrm{?3}}\text{cal cm}{}^{\mathrm{?2}}\text{sec}\text{?}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{K}{}^{\mathrm{?1}}$, similar to that of the other Galilean satellites.