Experimental simulation of atmospheric entry of micrometeorites

Abstract— Depending on their velocity, entry angle and mass, micrometeorites suffer different degrees of heating during their deceleration in the Earth's atmosphere, leading, in most cases, to significant textural, mineralogical and chemical modifications. One of these modifications is the formation of a magnetite shell around most micrometeorites, which until now could not be reproduced, neither theoretically nor experimentally. The present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell. Fragments of the Murchison and Orgueil meteorites were used as analogue material in flash‐heating experiments performed in a high‐temperature furnace; effects of temperature, heating duration, and oxygen fugacity were investigated. These experiments were able to reproduce most of the micrometeorites textures, from the vesicular fine‐grained micrometeorites to the totally melted cosmic spherules. For the first time, the formation of a magnetite shell could be observed on micrometeorite analogues. We suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite. Furthermore, with this study, it is possible to estimate the atmospheric entry conditions of micrometeorites, such as the peak temperature and the duration of flash‐heating.     

Mapping the narrative positions of new political groups under the UNFCCC

Since the mid-1990s, the aim of keeping climate change within 2?°C has become firmly entrenched in policy discourses. In the past few years, the likelihood of achieving it has been increasingly called into question. The debate around what to do with a target that seems less and less achievable is, however, only just beginning. As the UN commences a two-year review of the 2?°C target, this article moves beyond the somewhat binary debates about whether or not it should or will be met, in order to analyse more fully some of the alternative options that have been identified but not fully explored in the existing literature. For the first time, uncertainties, risks, and opportunities associated with four such options are identified and synthesized from the literature. The analysis finds that the significant risks and uncertainties associated with some options may encourage decision makers to recommit to the 2?°C target as the least unattractive course of action.     

Pulsar sensitivity studies of the GLAST large area telescope

In this contribution we present our preliminary investigation on pulsar sensitivity of the Large Area Telescope, the main instrument aboard the GLAST mission. In particular we concentrated our attention to pulsars located at low galactic latitudes. We created a set of simulated pulsars having different fluxes in an array of galactic coordinates separated by a distance greater than the LAT Point Spread Function in order to avoid confusion between adjacent sources. Galactic gamma-ray sky background as used during the second LAT Data Challenge (DC2) is also included. We then run an automatic routine for testing periodicity for all the pulsars considering an opportune timing solution. In this way we can obtain a map of the sensitivity of the periodic searches for different fluxes and for various Galactic latitudes. Some assumptions have been made by simulating the pulsar sources, but this study is a first step toward an estimate for pulsed emission sensitivity of the GLAST LAT. The pulsed flux sensitivity profile we generate could also be input to a population synthesis code of Galactic pulsars in order to obtain more accurate predictions of the number of expected pulsar detections by GLAST. On behalf of the GLAST LAT Collaboration.     

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments.     

Inter-comparison of cosmogenic in-situ He, Ne and Cl at low latitude along an altitude transect on the SE slope of Kilimanjaro volcano (3°S, Tanzania)

Because the intensity and energy spectrum of the cosmic ray flux are affected by atmospheric depth and geomagnetic-field strength, cosmogenic nuclide production rates increase considerably with altitude and to a lesser degree with latitude. The scaling methods used to account for spatial variability in production rates assume that all cosmogenic nuclides have the same altitude dependence. In this study we evaluate whether the production rates of cosmogenic ³⁶Cl, ³He and ²¹Ne change differently with altitude, which is plausible due to the different threshold energies of their production reactions. If so, nuclide-specific scaling factors would be required.Concentrations of the three cosmogenic nuclides were determined in mafic phenocrysts over an altitude transect between 1000 and 4300 m at Kilimanjaro volcano (3°S). Altitude dependence of relative production rates was assessed in two ways: by determination of concentration ratios and by calculation of apparent exposure age ratios for all nuclide pairs. The latter accounts for characteristics of ³⁶Cl that the stable nuclides ³He and ²¹Ne do not possess (radioactive decay, high sensitivity to mineral composition and significant contributions from production reactions other than spallation). All ratios overlap within error over the entire transect, and altitudinal variation in relative production rates is not therefore evident. This suggests that nuclide-specific scaling factors are not required for the studied nuclides at this low-latitude location. However, because previous studies have documented anomalous altitude-dependent variations in ³He production at mid-latitude sites, the effect of latitude on cross-calibrations should be further evaluated.We determined cosmogenic ²¹Ne/³He concentration ratios of 0.1864 ± 0.0085 in pyroxenes and 0.377 ± 0.018 in olivines, agreeing with those reported in previous studies.Despite the absence of independently determined ages for the studied lava surfaces, the consistency in the dataset should enable progress to be made in the determination of the production rates of all three nuclides as soon as the production rate of one of the nuclides has been accurately defined.To our knowledge this is the first time that ³⁶Cl has been measured in pyroxene. The Cl extraction method was validated by measuring ³⁶Cl in co-existing plagioclase phenocrysts in one of the samples.     

Mapping family connectedness across space and time

Understanding the structure and evolution of family networks embedded in space and time is crucial for various fields such as disaster evacuation planning and provision of care to the elderly. Computation and visualization can potentially play a key role in analyzing and understanding such networks. Graph visualization methods are effective in discovering network patterns; however, they have inadequate capability in discovering spatial and temporal patterns of connections in a network especially when the network exists and changes across space and time. We introduce a measure of family connectedness that summarizes the dynamic relationships in a family network by taking into account the distance (how far individuals live apart), time (the duration of individuals’ coexistence within a neighborhood), and the relationship (kinship or kin proximity) between each pair of individuals. By mapping the family connectedness over a series of time intervals, the method facilitates the discovery of hot spots (hubs) where family connectedness is strong and the changing patterns of such spots across space and time. We demonstrate our approach using a data set of nine families from the US North. Our results highlight that family connectedness reflects changing demographic processes such as migration and population growth.     

Sedimentation rates on embanked floodplains determined through quartz optical dating

We investigate the use of quartz optically stimulated luminescence (OSL) dating for determining fluvial overbank sedimentation rates over decades to centuries. For the study we took 11 samples from three cores from an embanked floodplain along the River Waal (Rhine) near Neerijnen (The Netherlands). We propose a measurement protocol for young fluvial quartz based on the single-aliquot regenerative dose procedure. Parameters for the protocol are chosen to isolate the fast OSL component, eliminate an ultrafast OSL component and avoid thermal transfer. The protocol shows excellent dose recovery and recycling ratios. For each sample, a Gaussian is fitted to the lower part of the equivalent dose distribution to obtain an estimate of the burial dose. We discuss the validity of the OSL ages using internal and external controls, and conclude that there is no evidence for large systematic offsets in the OSL ages. OSL based sedimentation rates are between ～3 and 8 mm/a, in line with previous estimates.     

Direct observation of cosmic strings via their strong gravitational lensing effect – I. Predictions for high-resolution imaging surveys

We use present theoretical estimates for the density of long cosmic strings to predict the number of strong gravitational lensing events in astronomical imaging surveys as a function of the angular resolution and survey area. We show that angular resolution is the single most important factor, and that interesting limits on the dimensionless string tension   G μ/ c ²  can be obtained by existing and planned surveys. At the resolution of the Hubble Space Telescope ( HST ) (0.14 arcsec), it is sufficient to survey of the order of a few square degrees – well within reach of the current HST archive – to probe the regime   G μ/ c ²∼ 10⁻⁷  . If lensing by cosmic strings is not detected, such a survey would improve the limit on the string tension by a factor of two over that available from the cosmic microwave background. Future high resolution imaging surveys, covering a few hundred square degrees or more, either from space in the optical or from large-format radio telescopes on the ground, would be able to further lower this limit to   G μ/ c ²∼ 10⁻⁸  . These limits will not be improved significantly by increasing the solid angle of the survey.     

Interactive effects of hypoxia and PBDE on larval settlement of a marine benthic polychaete

Marine benthic polychaete Capitella sp. I is widely known to adapt to polluted habitats; however, its response to xenobiotics under hypoxic conditions has been rarely studied. This research aimed to test the hypothesis that interactive effects of hypoxia and congener BDE-47 of polybrominated diphenyl ethers (PBDE), which is ubiquitous in marine sediments, may alter the settlement of Capitella sp. I. Our results revealed that under hypoxic condition, settlement success and growth in body length of Capitella larvae were significantly reduced compared to those under normoxia of similar BDE-47 concentration. While no significant changes in morphology of settled larvae were noted in both exposure conditions, the presence of BDE-47 could enhance polychaete growth. The present findings demonstrated that the interactive effects of hypoxia and environmentally realistic concentrations of BDE-47 in sediments could affect polychaete settlement, which, in turn, reduce its recruitment and subsequent population size in the marine benthic ecosystem.