Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km.     

The transition from complex crater to peak-ring basin on the Moon: New observations from the Lunar Orbiter Laser Altimeter (LOLA) instrument

Impact craters on planetary bodies transition with increasing size from simple, to complex, to peak-ring basins and finally to multi-ring basins. Important to understanding the relationship between complex craters with central peaks and multi-ring basins is the analysis of protobasins (exhibiting a rim crest and interior ring plus a central peak) and peak-ring basins (exhibiting a rim crest and an interior ring). New data have permitted improved portrayal and classification of these transitional features on the Moon. We used new 128 pixel/degree gridded topographic data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter, combined with image mosaics, to conduct a survey of craters >50 km in diameter on the Moon and to update the existing catalogs of lunar peak-ring basins and protobasins. Our updated catalog includes 17 peak-ring basins (rim-crest diameters range from 207 km to 582 km, geometric mean = 343 km) and 3 protobasins (137-170 km, geometric mean = 157 km). Several basins inferred to be multi-ring basins in prior studies (Apollo, Moscoviense, Grimaldi, Freundlich-Sharonov, Coulomb-Sarton, and Korolev) are now classified as peak-ring basins due to their similarities with lunar peak-ring basin morphologies and absence of definitive topographic ring structures greater than two in number. We also include in our catalog 23 craters exhibiting small ring-like clusters of peaks (50-205 km, geometric mean = 81 km); one (Humboldt) exhibits a rim-crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings. A power-law fit to ring diameters (D_ring) and rim-crest diameters (D_r) of peak-ring basins on the Moon [D_ring = 0.14 ± 0.10(D_r)^1.21±0.13] reveals a trend that is very similar to a power-law fit to peak-ring basin diameters on Mercury [D_ring = 0.25 ± 0.14(D_rim)^1.13±0.10] [Baker, D.M.H. et al. [2011]. Planet. Space Sci., in press]. Plots of ring/rim-crest ratios versus rim-crest diameters for peak-ring basins and protobasins on the Moon also reveal a continuous, nonlinear trend that is similar to trends observed for Mercury and Venus and suggest that protobasins and peak-ring basins are parts of a continuum of basin morphologies. The surface density of peak-ring basins on the Moon (4.5 × 10⁻⁷ per km²) is a factor of two less than Mercury (9.9 × 10⁻⁷ per km²), which may be a function of their widely different mean impact velocities (19.4 km/s and 42.5 km/s, respectively) and differences in peak-ring basin onset diameters. New calculations of the onset diameter for peak-ring basins on the Moon and the terrestrial planets re-affirm previous analyses that the Moon has the largest onset diameter for peak-ring basins in the inner Solar System. Comparisons of the predictions of models for the formation of peak-ring basins with the characteristics of the new basin catalog for the Moon suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. In particular, a power-law model of growth of an interior melt cavity with increasing crater diameter is consistent with power-law fits to the peak-ring basin data for the Moon and Mercury. We suggest that the relationship between the depth of melting and depth of the transient cavity offers a plausible control on the onset diameter and subsequent development of peak-ring basins and also multi-ring basins, which is consistent with both planetary gravitational acceleration and mean impact velocity being important in determining the onset of basin morphological forms on the terrestrial planets.     

Relaxation of protostellar accretion shocks using the smoothed particle hydrodynamics

It is believed that protostellar accretion disks are formed from nearly ballistic infall of molecular matter in rotating core collapse. Collisions of this infalling matter leads to formation of strong supersonic shocks, which if they cool rapidly, result in accumulation of that material in a thin structure in the equatorial plane. Here, we investigate the relaxation time of the protostellar accretion post‐shock gas using the smoothed particle hydrodynamics (SPH). For this purpose, a one‐dimensional head‐on collision of two molecular sheets is considered, and the time evolution of the temperature and density of the post‐shock region simulated. The results show that in strong supersonic shocks, the temperature of the post‐shock gas quickly increases proportional to square of the Mach number, and then gradually decreases according to the cooling processes. Using a suitable cooling function shows that in appropriate time‐scale, the center of the collision, which is at the equatorial plane of the core, is converted to a thin dense molecular disk, together with atomic and ionized gases around it. This structure for accretion disks may justify the suitable conditions for grain growth and formation of proto‐planetary entities (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rest‐frame UV to optical X‐shooter spectrum of the gravitationally lensed galaxy “the 8 o'clock arc”: Dissection of its physical properties

We present the rest‐frame UV to optical spectrum of the gravitationally lensed galaxy, the “8 o'clock arc” at z = 2.735, obtained with the X‐shooter spectrograph. This is one of the rare comprehensive study of a high‐redshift star‐forming galaxy, usually not feasible due to the faintness of these objects. The spectrum of the 8 o'clock arc is rich in stellar and interstellar absorption features and in nebular emission features. Their analysis allowed us to derive most of the physical properties characterizing the galaxy. From HST images, we reconstruct the morphology of the arc in the source plane and confirm that we resolve a clump in rotation around the main core of galaxy in our X‐shooter spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sino‐Kharosthi and Sino‐Brahmi coins from the silk road of western China identified with stylistic and mineralogical evidence

Three coins of the Khotan horse, or Sino‐Kharosthi, style from Xinjiang province of China were recently obtained from a coin dealer. Because the archaeological context of these coins remains unknown, we conducted chemical and stylistic analyses to help determine their authenticity. Energy dispersive X‐ray (EDX) and wavelength dispersive X‐ray (WDX or microprobe) measurements indicate that they are red (pure) copper and bronze coins, with one coin containing 4.2% Ag. Metal content and the presence of secondary minerals formed through weathering indicate that these are authentic ancient coins that were buried in a dry environment. All three coins bear Chinese characters; one shows Kharosthi, and two display Brahmi, a popular language in Khotan from A.D. 260 to 282. This indicates that the Sino‐Brahmi coins were made slightly later than the Sino‐Kharosthi coins, which were made before A.D. 260. These specimens represent the first Sino‐Brahmi coin and 20‐zhu coin ever reported in the literature. © 2011 Wiley Periodicals, Inc.     

Palaeoenvironments and site formation processes at the Neolithic lakeside settlement of Dispilio,Kastoria, Northern Greece

Dispilio is a lakeside settlement by the Orestias Lake, Kastoria, northern Greece. The site was inhabited from the Middle Neolithic to the Chalcolithic, with some surface evidence of Bronze Age occupation. Microfacies analysis of the sediments, supported by a suite of environmental indices, has provided detailed paleoenvironmental data and elucidated the main processes involved in the formation of the site and its history of occupation. The settlement was established on the lakeshore, on a shallow sand ridge and a shore marsh. Initially, houses were built on raised platforms above the water. After a major conflagration, a range of depositional microenvironments were established that caused local changes in the sedimentation rate. Therefore, some areas quickly emerged and became dry land, while some others continued to be flooded as part of the transitional supra‐littoral environment. On the dry land, houses were built directly on the ground, whereas in the transitional areas houses continued to be built on raised platforms. Thus, gradually, a mound was formed and further shaped by subsequent lake‐level fluctuations. One of the lake‐level rises is tentatively related to the abandonment of the mound in the Chalcolithic and the development of a hardpan on its surface. There is also evidence of later occupation during the Bronze Age in the form of a few, mostly surface, archaeological remains. © 2010 Wiley Periodicals, Inc.     

Effect of reproductive state and sex on movement and food consumption of western rock lobster (Panulirus cygnus) in a tank environment

The catchability of western rock lobsters (Panulirus cygnus) during the spawning season (October and November) may be affected by their reproductive state. In this study, movement and food consumption of males and females of different reproductive states held in laboratory tanks were measured as proxies for catchability. Density of lobsters was found to influence both movement and food consumption. Solitary animals and, to a lesser extent, two lobsters per tank, were more sedentary and consumed significantly less food than animals at a density of three or four specimens per tank. Tests using three animals per tank confirmed that mature, unmated, and ovigerous females carrying early stage eggs moved more frequently away from their shelters than males or females carrying late stage eggs (P < 0.0001, d.f. = 3). Further, unmated females and females with early stage eggs recorded significantly higher food consumption (P < 0.0061, d.f. = 3) compared with males or females carrying late stage eggs. The greater time spent away from shelter and also food consumption make unmated females and females with early stage eggs likely to be more catchable than males or females with late stage eggs. As catchability is commonly used to obtain population estimates from survey data, these results have implications for surveys which use catch rates of breeding animals as indicators of egg production.     

Behavior of soil plugs in open‐ended model piles driven into sands

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.