zobov: a parameter-free void-finding algorithm

zobov (ZOnes Bordering On Voidness) is an algorithm that finds density depressions in a set of points, without any free parameters, or assumptions about shape. It uses the Voronoi tessellation to estimate densities, which it uses to find both voids and subvoids. It also measures probabilities that each void or subvoid arises from Poisson fluctuations. This paper describes the zobov algorithm, and the results from its application to the dark matter particles in a region of the Millennium simulation. Additionally, the paper points out an interesting high-density peak in the probability distribution of dark matter particle densities.     

The incidence of mid-infrared excesses in G and K giants

Using photometric data from the Two-Micron All-Sky Survey (2MASS) and GLIMPSE catalogues, I investigate the incidence of mid-infrared (mid-IR) excesses  (∼10 μm)  in G and K stars of luminosity class III. In order to obtain a large sample size, stars are selected using a near-IR colour–magnitude diagram. Sources which are candidates for showing mid-IR excess are carefully examined and modelled to determined whether they are likely to be G/K giants. It is found that mid-IR excesses are present at a level of  (1.8 ± 0.4) × 10⁻³  . While the origin of these excesses remains uncertain, it is plausible that they arise from debris discs around these stars. I note that the measured incidence is consistent with a scenario in which dust lifetimes in debris discs are determined by Poynting–Robertson drag rather than by collisions.     

Experimental impacts into chondritic targets,part I: Disruption of an L6 chondrite by multiple impacts

Abstract— A fragment of an L6 chondrite (Allan Hills [ALH] 85017,13) with an initial mass (M₀) of 464.1 g was the target in a series of experimental impacts in which the largest remaining fragment (M_R) after each shot was impacted by a 3.18 mm ceramic sphere at a nominal speed of 2 km s^?1. This continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot (M_S). Two chunks of Bushveldt gabbro with similar initial masses were also impacted under the same conditions until M_R was less than half M₀. The two gabbro targets required a total of 1.51 × 10⁷ and 1.75 × 10⁷ erg g^?1 to attain 0.27 and 0.33 M_R/M₀, respectively; the chondrite, however, was considerably tougher, reaching 0.40 and 0.21 M_R/M₀ only after receiving 2.37 × 10⁷ and 3.10 × 10⁷ erg g^?1, respectively. The combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series, which was the number of impacts required to disrupt each gabbro target (i.e., M_R/M₀ ≤ 0.5). Unlike the behavior shown in previous regolith‐evolution series, neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock (about 3 mm), an effect possibly related to the width of the shock pulse. The original chondrite was so fine‐grained and fractured, and the variance in its grain‐size distribution so large, that effects related to grain size were relegated to the<63 μm fraction. Impacts into ALH 85017 produced abundant, fine‐grained debris, but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets. The characteristic slopes of the chondrite's size distributions, however, were notably more constant over the entire nine‐impact series than those from any of the terrestrial targets, a testament to the control over comminution apparently exerted by pre‐existing fractures and other, microscopic damage in the meteorite. The enhancement in the finer fraction of debris from ALH 85017 indicates that ordinary chondrites in solar orbit would be very efficient contributors to the cosmic‐dust complex. At the same time, the greater resistance to disruption displayed by ordinary chondrites relative to that exhibited by igneous rocks indicates that a selection effect could be operative between the annealed, ordinary‐chondritic breccias and relatively weaker, differentiated meteorites. Preferential survival from their time in the regoliths of their parent bodies through their transit to Earth and passage through the atmosphere suggests that meteorite collections could be biased in favor of the ordinary chondrites.     

Keck observations of the 2002-2003 jovian ring plane crossing

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×¹⁰⁻⁶, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×¹⁰⁻⁶, is attributed to micron-sized dust, and the remaining τ≈3×¹⁰⁻⁶ to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×¹⁰⁻⁷, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.     

Response of sandstone to atmospheric heating during the STONE 5 experiment: Implications for the palaeofluid record in meteorites

A 1 cm thick sandstone disk exposed to atmospheric re-entry on the heat shield of a spacecraft (the STONE 5 experiment) shows alteration of fluid inclusions compared to a control sample. The sandstone contained inclusions in quartz grains, feldspar grains and calcite cement before flight. After flight, inclusions in the feldspar were all decrepitated, few inclusions in calcite survived intact and they yielded widely varying microthermometric data, and the quartz inclusions also yielded disturbed microthermometric data. The quartz becomes less affected with depth below the surface, and extrapolation suggests would be unaffected at a depth of about 2 cm. These data show that fluid inclusion data from meteorites must be treated with caution, but that a genuine fluid record may survive in the interior portions. The possibility of thermal sterilization to 2 cm depth also implies that small meteorites may be unsuitable vehicles for the transfer of microbial life from one planetary body to another. As the interiors of larger meteorites tend to have very low porosity and permeability, microbial colonization would be difficult, and the potential for panspermia is accordingly low.     

Provenance and microprobe assays of phyllite‐tempered ceramics from the uplands of central Arizona

The ceramics in use across a broad upland zone of central Arizona during the early Classic period (ca. A.D. 1100–1300) were characterized by a lack of mineralogical variability; nearly all of the clay containers were tempered with one rock type, phyllite. Consequently, nearly all of the upland pottery is assigned to a single pottery type, Wingfield Plain. This compositional uniformity has frustrated ceramic provenance studies, and, as a result, little has been learned previously about the organization of ceramic production and exchange in the upland territory. There are, however, considerable and interpretable chemical differences in the phyllite‐tempered wares, as shown with microanalyses of the temper fragments and pottery clay fractions with an electron microprobe. The chemical patterning is useful for investigating issues pertaining to the upland zone, including the organization of ceramic manufacture, community arrangements, and pottery transactions during a time of prevalent hostilities in central Arizona. © 2008 Wiley Periodicals, Inc.     

Long-term hydrology and aquatic biogeochemistry data from H. J. Andrews Experimental Forest,Cascade Mountains,Oregon

The H. J. Andrews Experimental Forest (HJA) encompasses the 6400 ha Lookout Creek watershed in western Oregon, USA. Hydrologic, chemistry and precipitation data have been collected, curated, and archived for up to 70 years. The HJA was established in 1948 to study the effects of harvest of old-growth conifer forest and logging-road construction on water quality, quantity and vegetation succession. Over time, research questions have expanded to include terrestrial and aquatic species, communities and ecosystem dynamics. There are nine small experimental watersheds and 10 gaging stations in the HJA, including both reference and experimentally treated watersheds. Gaged watershed areas range from 8.5 to 6242 ha. All gaging stations record stage height, water conductivity, water temperature and above-stream air temperature. At nine of the gage sites, flow-proportional water samples are collected and composited over 3-week intervals for chemical analysis. Analysis of stream and precipitation chemistry began in 1968. Analytes include dissolved and particulate species of nitrogen and phosphorus, dissolved organic carbon, pH, specific conductance, suspended sediment, alkalinity, and major cations and anions. Supporting climate measurements began in the 1950s in association with the first small watershed experiments. Over time, and following the initiation of the Long Term Ecological Research (LTER) grant in 1980, infrastructure expanded to include a set of benchmark and secondary meteorological stations located in clearings spanning the elevation range within the Lookout Creek watershed, as well as a large number of forest understory temperature stations. Extensive metadata on sensor configurations, changes in methods over time, sensor accuracy and precision, and data quality control flags are associated with the HJA data.     

Geology ‘far from the madding crowd’, along the northern border of Vietnam

To the north of Hanoi, about a day's drive by car, lies Ha Giang Province, the northernmost region of Vietnam. Ha Giang is remote from the hustle and bustle of daily life, and beyond its eponymous provincial capital towards the border with China, mountains rise quickly to Quan Ba, ‘Heaven's Gate’. The mountains form an uneven landscape of steep‐sided karst rising from deep river‐cut gorges and form a formidable barrier on the northern frontier of Vietnam. Beyond ‘Heaven's Gate’ lies the little travelled region of Dong Van, with its majestic mountains of Palaeozoic strata rising precipitously to the sky. Here, a century ago, the French geologists Henri Mansuy and Jacques Deprat documented early finds of fossils from lower Palaeozoic strata on the border with China.     

Post‐orogenic sediment drape in the Northern Pyrenees explained using a box model

The transition to a post‐orogenic state in mountain ranges has been identified by a change from active subsidence to isostatic rebound of the foreland basin. However, the nature of the interplay between isostatic rebound and sediment supply, and their impact on the topographic evolution of a range and foreland basin during this transition, has not been fully investigated. Here, we use a box model to explore the syn‐ to post‐orogenic evolution of foreland basin/thrust wedge systems. Using a set of parameter values that approximate the northern Pyrenees and the neighbouring Aquitaine foreland basin, we evaluate the controls on sediment drape over the frontal parts of the retro‐wedge following cessation of crustal thickening. Conglomerates preserved at approximately 600‐m elevation, which is ~ 300 m above the present mountain front in the northern Pyrenees are ca. 12 Ma, approximately 10 Myrs younger than the last evidence of crustal thickening in the wedge. Using the model, this post‐orogenic sediment drape is explained by the combination of a sustained, high sediment influx from the range into the basin relative to the efflux out of the basin, combined with cessation of the generation of accommodation space through basin subsidence. Post‐orogenic sediment drape is considered a generic process that is likely to be responsible for elevated low‐gradient surfaces and preserved remnants of continental sedimentation draping the outer margins of the northern Pyrenean thrust wedge.