SMART‐1 end of life shallow regolith impact simulations

The SMART‐1 end‐of‐life impact with the lunar surface was simulated with impacts in a two stage light‐gas gun onto inclined basalt targets with a shallow surface layer of sand. This simulated the probable impact site, where a loose regolith will have overlaid a well consolidated basaltic layer of rock. The impact angles used were at 5° and 10° from the horizontal. The impact speed was ~2 km s^?1 and the projectiles were 2.03 mm diameter aluminum spheres. The sand depth was between approximately 0.8 and 1.8 times the projectile diameter, implying a loose lunar surface regolith of similar dimensions to the SMART‐1 spacecraft. A crater in the basement rock itself was only observed in the impact at 10° incidence, and where the depth of loose surface material was less than the projectile diameter, in which case the basement rock also contained a small pit‐like crater. In all cases, the projectile ricocheted away from the impact site at a shallow angle. This implies that at the SMART‐1 impact site the crater will have a complicated structure, with exposed basement rock and some excavated rock displaced nearby, and the main spacecraft body itself will not be present at the main crater.     

Periodic ab initio Hartree-Fock study of trigonal and orthorhombic phases of boric oxides

The structure and electronic properties of trigonal and orthorhombic boric oxide (B₂O₃) are studied using periodic ab initio Hartree-Fock method. The optimised structural parameters for two B₂O₃ polymorphs are in good agreement with experimental data. The analyses of their electronic structures provide insights into the chemical nature of the B–O bond and the way in which it changes with the coordination number around boron and oxygen. Our quantum-chemical study suggests that the orthorhombic form is more ionic than the trigonal form and that the coordination number of boron around oxygen plays a more dominant role than that of oxygen around boron in B₂O₃ crystals.     

The effect of magnetic fields on the formation of circumstellar discs around young stars

We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation.     

The behaviour of iodine and bromine in estuarine surface sediments

The distribution of iodine and bromine was examined in sediments which receive inputs of marine and terrigenous organic matter. The I and Br concentrations are directly related to the content of ‘marine’ organic matter defined using carbon/nitrogen ratios. In the Etive sediments both Br and I may be used as an indicator of ‘marine’ organic matter; Br is of general application as the $\text{Br}\text{C}{}_{\mathrm{mar}}$ ratio (180 × 10^?4) is similar to ratios in other sedimentary environments but the use of I is restricted as the $\text{I}\text{C}{}_{\mathrm{mar}}$ ratio is unlike those in other sediments. Experimental study of iodine sorption clearly shows the importance of decaying marine organic matter and oxygenated conditions in the incorporation of iodine by sediments. This suggests that the mechanism of incorporation of iodine by seston previously proposed is probably an important pathway to sediments. The similarity of Br association with marine organic matter suggests that Br sorption as opposed to residual enrichment may be important for sediment Br accumulation.     

A geochemical record of the mining history of the Erme Estuary, south Devon, UK

The concentration of selected trace metals (Cu, Pb and Zn) in salt-marsh sediments from within the Erme Estuary have been measured in order to assess possible historical sources of pollution. The Erme Estuary, south Devon, UK is an Area of Outstanding Natural Beauty and has remained largely unaffected by industrialisation, although a number of small silver–lead mines were in operation in the 1800s. Five cores reveal comparable geochemical profiles. An increase of lead at 40 cm depth is observed, reaching maximum values of 427 ppm. Less distinct trends are revealed by zinc and copper, probably reflecting the lack of widespread mining for ores of these elements within the catchment and possible post-depositional mobility rendering the metal concentrations non-contemporaneous with the chemostratigraphy of lead. The geochemical analysis of the salt-marsh sediments provides a fairly robust chemostratigraphic scheme and the likely sources of mine waste can be pinpointed within the catchment. Based upon reference to the historical mining record of these mines chemostratigraphic dating of the sediments can be achieved in order to provide an estimate of salt-marsh accretion rates and sea-level rise.     

Comparison of Two Closely Located Meteorological Measurement Sites and Consequences for Their Areal Representativity

We compare meteorological data collected at the Met Office Research Unit, Cardington, UK with similar data from a temporary meteorological station located approximately 8.5 km away. Data were examined for a period of 10 months to ascertain differences in mean quantities, and in heat and radiation budgets, at different heights between the two locations, one of which is located in a wide shallow valley, the other on a plateau at the valley edge. Results reveal that screen-level variables at the two sites show the greatest differences in mean quantities, for most conditions, but that at 50 m the differences are negligible, indicating that temperatures had become aggregated and homogeneous at that height. For flux measurements between 10 and 50 m, however, significant differences were observed at certain times of the year, which appear to be related to local vegetation and soil conditions, and these are discussed. The study also presents data for stable conditions that show that temperature differences at screen level are again the most significant difference between the two sites. On average these were not large, but on occasions discrepancies at low levels (up to approximately 10 m) as large as 5°C were observed. It is thought that these greater differences during stable conditions may be caused by cold air pooling at the valley site, despite the very shallow orography there. Data from the two sites have been compared with forecasts from two Met Office mesoscale models. Results show that, during daytime hours, model-predicted values lie outside the range of values observed at the two sites, indicating a possible model bias. However, the opposite was true for most nighttime hours, during which model values fell within the range observed at the two sites, indicating that at night the model predictions are representative of the region. In this case, however, comparison of the model prediction with either one of the available observations could lead to the false conclusion that the model temperatures were either too high or too low, depending on which observational site was used for the comparison.     

Growing season evapotranspiration in boreal fens in the Athabasca Oil Sands Region: Variability and environmental controls

Current efforts to assess changes to the wetland hydrology caused by growing anthropogenic pressures in the Athabasca Oil Sands Region (AOSR) require well-founded spatial and temporal estimates of actual evapotranspiration (ET), which is the dominant component of the water budget in this region. This study assessed growing season (May–September) and peak growing season (July) ET variability at a treed moderate-rich fen and treed poor fen (in 2013–2018), open poor fen (in 2011–2014), and saline fen (in 2015–2018) using eddy covariance technique and a set of complementary environmental data. Seasonal fluctuations in ET were positively related to net radiation, air temperature and vapour pressure deficit and followed trends typical for the Boreal Plains (BP) and AOSR with highest rates in June–July. However, no strong effect of water table position on ET was found. Strong surface control on ET is evident from lower ET values than potential evapotranspiration (PET); the lowest ET/PET was observed at saline fen, followed by open fen, moderately treed fen, and heavily treed fen, suggesting a strong influence of vegetation on water loss. In most years PET exceeded precipitation (P), and positive relations between P/PET and ET were observed with the highest July ET rates occurring under P/PET ~1. However, during months with P/PET > 1, increased P/PET was associated with decreased July ET. With respect to 30-year mean values of air temperature and P in the area, both dry and wet, cool and warm growing seasons (GS) were observed. No clear trends between ET values and GS wetness/coldness were found, but all wet GS were characterized by peak growing seasons with high daily ET variability.     

The scattering mean free path in the Uranian atmosphere

New measurements of the equivalent widths of the 4-0 S(0) and S(1) H₂ quadrupole lines in the Uranian spectrum have been obtained using high dispersion (4.12 Å/mm) image-tube spectrography. The measured equivalent widths are $\text{62 ± 19}\text{m}\text{A}\text{?}\text{and}\text{58 ± 13}\text{m}\text{A}\text{?}$ for the S(0) and S(1) lines, respectively. Curve-of-growth analysis in terms of a reflecting layer model yields an H₂ column-density of 780_?330⁺⁹⁴⁰km amagat and a temperature of 78_?24⁺⁸⁰°K. Interpretation using a semi-infinite, homogeneous, isotropically scattering model for line formation yields a scattering mean free path at λ6400 Å of 550 ± 250 km amagat. Quoted errors for both the H₂ column-density and the scattering mean free path include the effect of uncertainty in the choice of atmospheric temperature. The results are discussed in terms of current models for the Uranian atmosphere.     

Enhanced geochemical gradients in a marine shallow-water hydrothermal system: Unusual arsenic speciation in horizontal and vertical pore water profiles

The shallow marine hydrothermal vents near Ambitle Island in eastern Papua New Guinea discharge hot, slightly acidic, As-rich, chemically reduced fluid into cool, slightly alkaline, oxygenated seawater. Gradients in temperature, pH, and total As (As^T), among others, are established as the two aqueous phases mix. The hydrothermal fluid contained 900 μg/L As^T, almost exclusively present as the reduced As^III, while local seawater measured between 1.2 and 2.4 μg/L As, with approximately equal levels of As^III and As^V. Of particular interest in this study was As speciation and abundance in pore waters as a function of sediment depth and as a function of distance from the area of focused venting. With increasing distance, As^T concentration in the pore water decreased rapidly, but remained elevated up to 300 m from the area of focused venting when compared to a non-hydrothermal control site. As a function of depth (to 100 cm) As^T concentration in the pore water profiles was elevated and generally increased with depth. Surprisingly, aqueous As^V far exceeded aqueous As^III at almost all distances and depths investigated, while at the control site the As^III concentration exceeded that of As^V. In the Tutum Bay hydrothermal system, chemical disequilibria among As species provide potential metabolic energy for arsenite oxidizing microorganisms where hydrothermal fluid mixes with seawater near the vent orifice, and for arsenate reducing microorganisms with increasing distance and depth from the hydrothermal point source.