Radar and photometric observations and shape modeling of contact binary near-Earth Asteroid (8567) 1996 HW1

We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory on six dates in September 2008, obtaining radar images and spectra. By combining these data with an extensive set of new lightcurves taken during 2008-2009 and with previously published lightcurves from 2005, we were able to reconstruct the object’s shape and spin state. 1996 HW1 is an elongated, bifurcated object with maximum diameters of 3.8 × 1.6 × 1.5 km and a contact-binary shape. It is the most bifurcated near-Earth asteroid yet studied and one of the most elongated as well. The sidereal rotation period is 8.76243 ± 0.00004 h and the pole direction is within 5° of ecliptic longitude and latitude (281°, −31°). Radar astrometry has reduced the orbital element uncertainties by 27% relative to the a priori orbit solution that was based on a half-century of optical data. Simple dynamical arguments are used to demonstrate that this asteroid could have originated as a binary system that tidally decayed and merged.     

The 2010 European Venus Explorer (EVE) mission proposal

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55?km, where temperatures and pressures are benign (～25°C and ～0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.     

Experimental determination of F and Cl partitioning between lherzolite and basaltic melt

We experimentally determined F and Cl partition coefficients together with that of 19 trace elements (including REE, U-Th, HFSE and LILE) between basaltic melt and lherzolite minerals: olivine, orthopyroxene, clinopyroxene, plagioclase and garnet. Under conditions from 8 to 25 kbars and from 1,265 to 1,430°C, compatibilities of F and Cl are globally ordered as D ^Cpx/melt > D ^Opx/melt > D ^Grt/melt > D ^Ol/melt > D ^Plag/melt, and D _F ^mineral/melt is larger than D _Cl^mineral/melt. Four other major results were brought to light. (1) Chlorine partition coefficients positively correlate with the jadeite component in orthopyroxene, and increase of the CaTs component promotes Cl incorporation in clinopyroxene. (2) Variations of fluorine partition coefficients correlate strongly with melt viscosity. (3) F and Cl partition coefficients correlate with the Young’s modulus (E ₀) of pyroxene octahedral sites (M sites) and with Raman vibrational modes of pyroxenes. This demonstrates a fundamental interaction between the M site of pyroxenes and the incorporation of F and Cl. (4) We also determined the parameters of the lattice-strain model applied to 3+ cation trace elements for the two M sites in orthopyroxene and clinopyroxene: D ₀^M1, D ₀^M2, r ₀^M1, r ₀^M2, E ₀^M1 and E ₀^M2.     

Pyroclastic flow erosion and bulking processes: comparing field-based vs. modeling results at Tungurahua volcano,Ecuador

Pyroclastic density currents (PDCs) are high-temperature and high-velocity mixtures that threaten populations in the vicinity of many active volcanoes. Deciphering the cause of their remarkable mobility is essential for volcanic hazard analysis, but remains difficult because of the complex processes occurring within the flows. Here, we investigate the effect of bulking on dense PDC mobility by means of a double approach. First, we estimate the amount of material incorporated into scoria flows emplaced during the August 2006 eruption of Tungurahua volcano, Ecuador. For this, we carry out a detailed analysis of 3D-corrected digital images of well-exposed scoria flow deposits. Componentry analysis indicates that PDC bulking occurs principally on the steep (>25°) upper slope of the volcano, and the deposits typically comprise 40–50 wt% of non-juvenile (i.e., accessory and accidental) material. Secondly, we develop a simple stress-related grain-by-grain equation of erosion combined with two simple depth-averaged geophysical mass-flow models that compare the bulking mechanism to a non-fluidized and a fluidized flow. Two behaviors based on Coulomb and plastic rheologies are used to reproduce, on a first order basis, the 2006 Tungurahua PDCs. Cross-check comparisons between these modeled cases and the erosion pattern inferred from field-based data allow us to evaluate the accuracy of our modeling assumptions. Regardless of the rheological regime, the PDC-induced erosion pattern of the 2006 Tungurahua eruption can only be reproduced by fluctuations of the flow’s basal shear stress during emplacement. Such variations are controlled by flow thinning-thickening processes, notably through the formation of a thick non-erosive flow body that pushes a thin frictional erosive front during PDC emplacement. The input volume of juvenile material, as well as the thickness of the erodible layer available prior to the eruption, are additional key parameters. Our work highlights complexities in PDC erosion and bulking processes that deserve further study. In terms of hazard assessment, our findings reveal that incorporation and bulking translate into increased flow mobility, i.e., the augmented flow mass enhances both flow velocity and runout distance (up to 20 %). These outcomes should be considered closely for hazard analysis at many other andesitic volcanoes worldwide where similar PDC events are common.     

Early Ipswichian (last interglacial) sea level rise in the channel region: Stone Point Site of Special Scientific Interest,Hampshire, England

Constraining the speed of sea level rise at the start of an interglacial is important to understanding the size of the ‘window of opportunity’ available for hominin migration. This is particularly important during the last interglacial when there is no evidence for significant hominin occupation anywhere in Britain. There are very few finer grained fossiliferous sequences in the Channel region that can be used to constrain sea level rise and they are preserved only to the north of the Channel, in England. Of these, the sequence at Stone Point SSSI is by far the most complete. Data from this sequence has been previously reported, and discussed at a Quaternary Research Association Field Meeting, where a number of further questions were raised that necessitated further data generation. In this paper, we report new data from this sequence – thin section analysis, isotopic determinations on ostracod shells, new Optical Stimulated Luminescence ages and Amino Acid Recem analyses. These show early sea level rise in this sequence, starting during the pre-temperate vegetation zone IpI, but no early warming. The implications of this almost certainly last interglacial sequence for the human colonisation of Britain and our understanding of the stratigraphic relationship of interglacial estuarine deposits with their related fluvial terrace sequences is explored.     

A mesocosm experiment investigating the effects of substratum quality and wave exposure on the survival of fish eggs

In a mesocosm experiment, the attachment of bream (Abramis brama) eggs to spawning substrata with and without periphytic biofilm coverage and their subsequent survival with and without low-intensity wave exposure were investigated. Egg attachment was reduced by 73% on spawning substrata with a natural periphytic biofilm, compared to clean substrata. Overall, this initial difference in egg numbers persisted until hatching. The difference in egg numbers was even increased in the wave treatment, while it was reduced in the no-wave control treatment. Exposure to a low-intensity wave regime affected egg development between the two biofilm treatments differently. Waves enhanced egg survival on substrata without a biofilm but reduced the survival of eggs on substrata with biofilm coverage. In the treatment combining biofilm-covered substrata and waves, no attached eggs survived until hatching. In all treatments, more than 75% of the eggs became detached from the spawning substrata during the egg incubation period, and <1% of these detached eggs survived within the substratum interstices. Hence, detached eggs contributed little to the reproductive success we observed. The implications of these results on the spawning success of fish species that use freshly inundated spawning substrata in shallow waters are discussed.     

New radiometric and petrological constraints on the evolution of the Pichincha volcanic complex (Ecuador)

Fieldwork, radiometric (⁴⁰Ar/³⁹Ar and ¹⁴C) ages and whole-rock geochemistry allow a reconstruction of eruptive stages at the active, mainly dacitic, Pichincha Volcanic Complex (PVC), whose eruptions have repeatedly threatened Quito, most recently from 1999 to 2001. After the emplacement of basal lavas dated at ∼1100 to 900 ka, the eruptive activity of the old Rucu Pichincha volcano lasted from ∼850 ka to ∼150 ka before present (BP) and resulted in a 15 × 20 km-wide edifice, which comprises three main building stages: (1) A lower stratocone (Lower Rucu, ∼160 km³ in volume) developed from ∼850 to 600 ka; (2) This edifice was capped by a steeper-sided and less voluminous cone (the Upper Rucu, 40–50 km³), the history of which started 450–430 ka ago and ended around 250 ka with a sector collapse; (3) A smaller (8–10 km³) but more explosive edifice grew in the avalanche amphitheatre and ended Rucu Pichincha's history about 150 ka ago. The Guagua Pichincha volcano (GGP) was developed from 60 ka on the western flank of Rucu with four growth stages separated by major catastrophic events. (1) From ∼60 to 47 ka, a basal effusive stratocone developed, terminating with a large ash-and-pumice flow event. (2) This basal volcano was followed by a long-lasting dome building stage and related explosive episodes, the latter occurring between 28–30 and 22–23 ka. These first two stages formed the main GGP (∼30 km³), a large part of which was removed by a major collapse 11 ka BP. (3) Sustained explosive activity and viscous lava extrusions gave rise to a new edifice, Toaza (4–5 km³ in volume), which in turn collapsed around 4 ka BP. (4) The ensuing amphitheatre was partly filled by the ∼1-km³ Cristal dome, which is the historically active centre of the Pichincha complex. The average output rate for the whole PVC is 0.29 km³/ka. Nevertheless, the chronostratigraphic resolution we obtained for Lower Rucu Pichincha and for the two main edifices of Guagua Pichincha (main GGP and Toaza), leads to eruptive rates of 0.60–0.65 km³/ka during these construction stages. These output rates are compared to those of other mainly dacitic volcanoes from continental arcs. Our study also supports an overall SiO₂ and large-ion lithophile elements enrichment as the PVC develops. In particular, distinctive geochemical signatures indicate the involvement of a new magma batch at the transition between Rucu and Guagua. At the GGP, the same phenomenon occurs at each major collapse event marking the onset of the ensuing magmatic stage. Since the 11-ka-BP collapse event, this magmatic behaviour has led to increasingly explosive activity. Four explosive cycles of between 100 and 200 years long have taken place at the Cristal dome in the past 3.7 ka, and repose intervals between these cycles have tended to decrease with time. As a consequence, we suggest that the 1999–2001 eruptive period may have initiated a new eruptive cycle that might pose a future hazard to Quito (∼2 million inhabitants).     

Near-Earth asteroid surface roughness depends on compositional class

Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular polarization ratio with visible-infrared taxonomic class, establishing distinct differences in the centimeter-to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta.     

Locating a point on a spherical surface relative to a spherical polygon of arbitrary shape

An algorithm for determining if any given point,P, on the surface of a sphere is located inside, outside, or along the border of an arbitrary spherical polygon,S, is described. The polygon is described by specifying coordinates of its vertices, and coordinates of some pointX which is known to lie withinS. The algorithm is based on the principle that an arc joiningX andP will cross the border ofS an odd number of times ifP lies outsideS, and an even number of times ifP lies withinS. The algorithm has been implemented as a set of FORTRAN subroutines, and a listing is provided. The algorithm and subroutine package can be used with spherical polygons containing holes, or with composited spherical polygons.     

Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model

An innovative way to take the large-scale circulation influence into account in coastal primitive-equation models is explored by an inverse modelling approach. Restricted to barotropic external forcing, this work is a first step in the development of a four-dimensional variational (4DVAR) data-assimilation approach to estimate the best initial and open-boundary conditions that force a coastal model according to interior observations. This development is founded on the OPA modelling system which representation of barotropic coastal dynamics is restricted to motions of long time scales ( a day) due to its rigid lid approximation. Twin experiments are performed in an academic configuration of the Gulf of Lions (located in the northwestern Mediterranean Sea) to study the sensitivity of a remote barotropic forcing to different observational networks measuring surface currents deployed in this area. Three monitoring designs are tested for a large-scale barotropic perturbation in the hindcast mode. It is shown that the space and time distribution of observations acts on the efficiency of the 4DVAR method and then allows coarser datasets.Responsible Editor: Phil Dyke