Radar observations and a physical model of contact binary Asteroid 4486 Mithra

Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) delay-Doppler radar images obtained in July and August of 2000 reveal that 4486 Mithra is an irregular, significantly bifurcated object, with a central valley ∼380 m deep and a long axis potentially exceeding 2 km. With its bimodal appearance, Mithra is a strong candidate for a contact binary asteroid. Sequences of Goldstone images spanning up to 3 h per day show very little rotation and establish that Mithra is an unusually slow rotator. We used Goldstone and Arecibo data to estimate Mithra’s 3D shape and spin state. We obtain prograde (λ = 337°, β = 19°) and retrograde (λ = 154°, β = −19°) models that give comparable fits, have very similar shapes roughly resembling an hourglass, and have a rotation period of 67.5 ± 6.0 h. The dimensions of these two models are very similar; for the prograde solution the maximum dimensions are X = 2.35 ± 0.15 km, Y = 1.65 ± 0.10 km, Z = 1.44 ± 0.10 km. Dynamical analysis of our models suggests that in the past, Mithra most likely went through a period of even slower rotation with its obliquity close to 90°. The spin rate is predicted to be increasing due to thermal torque (YORP), while the obliquity, which is currently +68° and +106° for the prograde and retrograde models, respectively, is predicted to move away from 90°.     

Gem-quality transparent diaspore (zultanite) in bauxite deposits of the İlbir Mountains, Menderes Massif, SW Turkey

Several m-thick, karst-unconformity-type metabauxite horizons in the İlbir Mountains of SW Turkey host open-space mineralization of gem-quality diaspore (trademarked as zultanite), associated with muscovite, hematite, ilmenite, chloritoid, and younger calcite. The hydrothermal-metamorphogenic mineralization occurs in fracture zones (veins and open structures) that crosscut the metabauxite horizons, but does not extend into the marble host rocks. The white to dark gray marble sequence (over 2,000 m in thickness) is of Cretaceous depositional age and was affected by Alpine (Paleogene) tectonometamorphism which caused the hydrothermal remobilization of primary bauxite components into crosscutting structures.     

Europium sorption to sediments in the presence of natural organic matter: A laboratory and modeling study

Cellulosic materials, such as wood, paper products and cardboard that have been co-disposed with low-level nuclear waste have been shown to produce leachate with natural organic matter (NOM) concentrations of hundreds of mg/L C and, as such, have the potential to influence the fate and transport of radionuclides in the subsurface environment. The objective of this study was to examine the influence of NOM on the sorption of Eu (an analogue for trivalent radionuclides) to two coastal plain sediments from the US Department of Energy’s Savannah River Site. Particular attention was directed at quantifying Eu interactions with NOM sorbed to sediments (NOM_sed) in laboratory experiments and developing conditional stability constants for that interaction using the thermodynamic equilibrium speciation model MINTEQA2. Europium sorption to the two sediments systematically increased as pH increased from 3.9 to 6.7. With increasing additions of NOM to the aqueous phase from 0 to 222 mg/L C, Eu sorption initially increased to a maximum at 10 mg/L C NOM_aq and then decreased with increasing NOM_aq concentrations. Increases in Eu sorption at low NOM additions was attributed to the sorption of NOM to the sediment surface increasing the number of sorption sites on the low cation-exchange capacity sediments and/or increasing the association constant (log K) for the Eu-sediment surface reaction. Decreases in Eu sorption at higher NOM levels was attributed to Eu_aq complexation to NOM_aq being more favored than Eu sorption to the solid phase. A component additivity model was developed to describe the Eu–NOM-sediment system by the additive effects of the three binary system models: Eu–NOM, Eu-sediment and NOM-sediment. The model generally captured the data trends in the ternary system. Conditional stability constants developed from the experimental data for the complexation of Eu to NOM_sed were as much as four orders of magnitude greater than Eu complexation with NOM_aq, presumably due to the NOM_sed deriving additional negative (attractive) charge from the sediment surface. At high initial NOM_aq levels, >99 mg/L C, the model captured the trend of reduced Eu sorption but tended to over-estimate Eu sorption. The additivity approach of combining binary models to form a ternary model was only successful when the unique complexation properties of the NOM_sed were properly calculated.     

Geophysical identification of unmarked historic graves

Down‐hole magnetic susceptibility techniques were explored as a means of improving near‐surface geophysical surveys in historic grave detection. These techniques were used to document distinctive magnetic characteristics of grave shafts at three historic cemeteries first surveyed using various near‐surface geophysical methods. Tests revealed a low magnetic susceptibility signature that soil magnetic studies indicated was largely related to differential soil compaction associated with the excavation and refilling of grave shafts. Most apparent at depths beyond those reached by soil penetrometers, this magnetic signature offers a way to assess anomalies identified by near‐surface techniques that potentially represent graves. At one cemetery, magnetic studies of the interments themselves suggested spatially patterned, magnetically enhanced zones that might also aid in burial identification in certain contexts. While down‐hole techniques will not be foolproof, they do provide another geophysical tool that can be used to improve grave detection. © 2010 Wiley Periodicals, Inc.     

The effects of Prandtl number on black hole accretion flows

Recent numerical simulations of MHD turbulence, under very different driving conditions, and by several different investigators, all indicate a sensitivity of the rms fluctuations to the ratio of the microscopic viscosity to resistivity. This dimensionless quantity is known as the magnetic Prandtl number Pm. In general, standard astrophysical accretion disks are characterized by Pm  1 throughout their radial extent, while low luminosity accretors (e.g. Sag A^*) have Pm  1. Here, we show that standard α models of black hole accretion disks have a transition radius, measured in tens of Schwarzschild radii, at which the flow goes from Pm  1 to Pm  1. Moreover, this transition may well be dynamically unstable, leading to a sort of two-phase “Prandtl number medium” We advance the idea that this is the physical reason underlying the change in the accretion properties of the inner regions of Keplerian disks, leading to a truncation of the cool disk (Pm  1) and the onset of hot, low density gas flow (Pm  1).     

Radar observations of E-class Asteroids 44 Nysa and 434 Hungaria

We observed the E-class main-belt Asteroids (MBAs) 44 Nysa and 434 Hungaria with Arecibo Observatory's S-band (12.6 cm) radar. Both asteroids exhibit polarization ratios higher than those measured for any other MBA: Nysa, μc=0.50±0.02 and Hungaria, μc=0.8±0.1. This is consistent with the high polarization ratios measured for every E-class near-Earth asteroid (NEA) observed by Benner et al. [Benner, L.A.M., and 10 collegues, 2008. Icarus, submitted for publication] and suggests a common cause. Our estimates of radar albedo are 0.19±0.06 for Nysa and 0.22±0.06 for Hungaria. These values are higher than those of most MBAs and, when combined with their high polarization ratios, suggest that the surface bulk density of both asteroids is high. We model Nysa as an ellipsoid of dimension 113×67×65 km (±15%) giving an effective diameter Deff=79±10 km, consistent with previous estimates. The echo waveforms are not consistent with a contact binary as suggested by Kaasalainen et al. [Kaasalainen, M., Torppa, J., Piironen, J., 2002. Astron. Astrophys. 383, L19-L22]. We place a constraint on Hungaria's maximum diameter, Dmax?11 km consistent with previous size estimates.     

Despinning plus global contraction and the orientation of lobate scarps on Mercury: Predictions for MESSENGER

Observations by the Mariner 10 spacecraft suggest that the lobate scarps on Mercury, which have been interpreted to record at most 1-2 km of radial contraction of the planet after the end of the Late Heavy Bombardment, possess a global, preferred N-S orientation but lack a strong latitudinal dependence on their surface expression. Here, we reexamine the idea that a decrease in the planetary rotation rate (despinning) coupled with global contraction of at least 3-5.5 km prior to the end of Late Heavy Bombardment resulted in global N-S oriented thrust faults. The surface expression of these faults is assumed to have been erased by the end of the Late Heavy Bombardment, and the faults were subsequently reactivated by later global contraction, producing generally N-S oriented thrust faults from an isotropic stress field. We use the estimate of >3-5.5 km contraction prior to ∼4 Ga as an additional constraint to thermomechanical simulations of the evolution of Mercury, finding that a wide range of models are consistent with this observation. The fact that a wide range of states are consistent with the contraction of Mercury prior to the end of Late Heavy Bombardment but only a restricted set of states are consistent with the at most 1-2 km of subsequent contraction bolsters the idea that there may be hidden strain on Mercury, features unseen by Mariner 10 but likely visible to the MESSENGER spacecraft.     

Reionization and the large-scale 21-cm cosmic microwave background cross-correlation

Of the many probes of reionization, the 21-cm line and the cosmic microwave background (CMB) are among the most effective. We examine how the cross-correlation of the 21-cm brightness and the CMB Doppler fluctuations on large angular scales can be used to study this epoch. We employ a new model of the growth of large-scale fluctuations of the ionized fraction as reionization proceeds. We take into account the peculiar velocity field of baryons and show that its effect on the cross-correlation can be interpreted as a mixing of Fourier modes. We find that the cross-correlation signal is strongly peaked towards the end of reionization and that the sign of the correlation should be positive because of the inhomogeneity inherent to reionization. The signal peaks at degree scales (ℓ∼ 100) and comes almost entirely from large physical scales ( k ∼ 10⁻² Mpc). Since many of the foregrounds and noise that plague low-frequency radio observations will not correlate with CMB measurements, the cross-correlation might appear to provide a robust diagnostic of the cosmological origin of the 21-cm radiation around the epoch of reionization. Unfortunately, we show that these signals are actually only weakly correlated and that cosmic variance dominates the error budget of any attempted detection. We conclude that the detection of a cross-correlation peak at degree-size angular scales is unlikely even with ideal experiments.