Effects of Varying Proportions of Glass on Reflectance Spectra of HED Polymict Breccias

Some meteorites contain significant amounts of glass, which, in most cases, probably results from impact processes on parent bodies.. Yamato 82202 is an example of one of the unequilibrated eucrites that contains significant proportions of impact glass distributed as veins throughout the meteorite. In other cases, fragments of glass are distributed throughout polymict breccias. For example, the polymict eucrite EET 87509 contains rare angular fragments of devitrified glass. Proportions of glass in most of these meteorites and in lithic clasts within these meteorites may vary locally from small amounts (less than one percent) to much larger amounts (subequal proportions of glass and mineral material). For example, some fragments within the South African polymict eucrite Macibini contain approximately 50% glass. The presence of these variable proportions of meteorite glass confirm the increased recognition that impact processes played an important role in the histories of asteroidal bodies. This study attempts to quantify the effects of a glass component on reflectance spectra by analyzing in the laboratory mixtures of varying proportions of a well-characterized HED polymict breccia and glass derived by melting a bulk sample of that breccia

Neutron Absorption Measurements Constrain Eucrite-Diogenite Mixing in Vesta's Regolith

The NASA Dawn Mission s Gamma Ray and Neutron Detector (GRaND) [1] acquired mapping data during 5 months in a polar, low altitude mapping orbit (LAMO) with approx.460-km mean radius around main-belt asteroid Vesta (264-km mean radius) [2]. Neutrons and gamma rays are produced by galactic cosmic ray interactions and by the decay of natural radioelements (K, Th, U), providing information about the elemental composition of Vesta s regolith to depths of a few decimeters beneath the surface. From the data acquired in LAMO, maps of vestan neutron and gamma ray signatures were determined with a spatial resolution of approx.300 km full-width-at-half-maximum (FWHM), comparable in scale to the Rheasilvia impact basin (approx.500 km diameter). The data from Vesta encounter are available from the NASA Planetary Data System. Based on an analysis of gamma-ray spectra, Vesta s global-average regolith composition was found to be consistent with the Howardite, Eucrite, and Diogenite (HED) meteorites, reinforcing the HED-Vesta connection [2-7]. Further, an analysis of epithermal neutrons revealed variations in the abundance of hydrogen on Vesta s surface, reaching values up to 400 micro-g/g [2]. The association of high concentrations of hydrogen with equatorial, low-albedo surface regions indicated exogenic delivery of hydrogen by the infall of carbonaceous chondrite (CC) materials. This finding was buttressed by the presence of minimally-altered CC clasts in howardites, with inferred bulk hydrogen abundances similar to that found by GRaND, and by studies using data from Dawn s Framing Camera (FC) and VIR instruments [8-10]. In addition, from an analysis of neutron absorption, spatial-variations in the abundance of elements other than hydrogen were detected [2]

Nature of the "Orange" Material on Vesta From Dawn

From ground-based observations of Vesta, it is well-known that the vestan surface has a large variation in albedo. Analysis of images acquired by the Hubble Space Telescope allowed production of the first color maps of Vesta and showed a diverse surface in terms of reflectance. Thanks to images collected by the Dawn spacecraft at Vesta, it became obvious that these specific units observed previously can be linked to geological features. The presence of the darkest material mostly around impact craters and scattered in the Western hemisphere has been associated with carbonaceous chondrite contamination [4]; whereas the brightest materials are believed to result from exposure of unaltered material from the subsurface of Vesta (in fresh looking impact crater rims and in Rheasilvia's ejecta and rim remants). Here we focus on a distinct material characterized by a steep slope in the near-IR relative to all other kinds of materials found on Vesta. It was first detected when combining Dawn Framing Camera (FC) color images in Clementine false-color composites [5] during the Approach phase of the mission (100000 to 5200 km from Vesta). We investigate the mineralogical and elemental composition of this material and its relationship with the HEDs (Howardite-Eucrite- Diogenite group of meteorites)

The Impact History of Vesta: New Views from the Dawn Mission

The Dawn mission has completed its Survey and High-Altitude Mapping Orbit (HAMO) phases at Vesta, resulting in 60-70 meter per pixel imaging, high-resolution image-derived topography, and visual and infrared spectral data covering up to approx.50 degrees north latitude (the north pole was in shadow during these mission phases). These data have provided unprecedented views of the south polar impact structure first detected in HST imaging [1], now named Rheasilvia, and in addition hint at the existence of a population of ancient basins. Smaller craters are seen at all stages from fresh to highly-eroded, with some exposing atypically bright or dark material. The morphology of some craters has been strongly influenced by regional slope. Detailed studies of crater morphology are underway. We have begun making crater counts to constrain the relative ages of different regions of the surface, and are working towards developing an absolute cratering chronology for Vesta's surface

F-ratios calculated with and without urea uptake in nitrogen uptake by phytoplankton

International audienceExamination of data from literature showed that non-inclusion of urea uptake in nitrogen uptake by phytoplankton results in statistically significant overestimates of f. The extent to which f was overestimated because of non-inclusion of urea uptake depended upon the ecosystem type and was 6, 17, 24, 42 and 55%, respectively, in upwelling, coastal, polar, oceanic and estuarine waters. Systematic urea uptake measurements thus become necessary, but where such data are lacking, corrections are possible with the ratio between the f values calculated with and without urea uptake for each ecosystem

Nitrogen uptake in permanently well-mixed temperate coastal waters

International audienceUptake rates of ammonium, nitrate, urea and nitrite were measured for 1 year (1988) at a coastal station in the well-mixed waters of the western English Channel. Ammonium was the major form of nitrogen (N) utilized (48%) by phytoplankton, followed by nitrate (32%), urea (13%) and nitrite (7%). Seasonal changes of uptake of ammonium, nitrate and urea showed a broad, intense summer maximum. Nitrite uptake was low throughout the year except for a peak value in June. Uptake rates of ammonium and nitrate were independent of substrate concentrations, whereas those of urea and nitrite were not. The summer maxima of ammonium, nitrate and total N uptake, and the significant relationships of N-uptake index to ambient light, and of chlorophyll-a-specific N uptake to surface-incident light, indicate that light is the major factor controlling N uptake in these waters. This is due to the permanent vertical mixing which reduces the mean light available for N uptake to <15% of the incident light. Mixing also injects regenerated N continuously into the euphotic zone, thus alleviating nitrogen limitation and accounting for the larger proportion of regenerated N uptake in total N uptake. (C) 1996 Academic Press Limited

NITROGEN NUTRITION OF THE PHYTOPLANKTON IN THE WELL-MIXED WATERS OF THE WESTERN ENGLISH-CHANNEL

Channel Symposium on Fluxes and Processes within a Macrotidal Sea, BREST, FRANCE, SEP 02-04, 1992International audienceUptake of nitrogenous nutrients (NO3-, NO2-, NH4+ and urea) by phytoplankton was measured over an annual cycle at a coastal station in the well-mixed waters of the western English Channel. Seasonal changes of nitrogen uptake showed a single, but spread-out, maximum in summer. The phytoplankton took up the various forms of nitrogen simultaneously. Ammonium uptake accounted for 48% of the total nitrogen uptake and was quantitatively important from the beginning of spring until early autumn. Nitrate uptake was relatively less important (32% of total nitrogen uptake) though nitrate was the major component of available inorganic nitrogen. Nitrate uptake rates were comparable to those of ammonium in winter and spring, but were much lower than the latter in summer as a result of inhibition by high ambient ammonium concentrations. Urea and nitrite contributed much less (respectively 13 and 7%) to nitrogen nutrition of phytoplankton. Regenerated-production accounts for about 70% of the annual production; this proportion is much higher than in several other coastal ecosystems

NITROGEN-SOURCE FOR UPTAKE BY GYRODINIUM CF AUREOLUM IN A TIDAL FRONT

International audienceMeasurements of ammonium and nitrate uptake by a Gyrodinium bloom in the Ushant frontal region showed that ammonium is the preferred form and its assimilation provides 90% of the N required. Uptake (3.75 mmol m-2 h-1) and regeneration (3.41 mmol m-2 h-1) of ammonium were tightly coupled. Regenerative flux of ammonium was through a microbial loop of bacteria, nanoflagellates, and ciliates in the size fraction < 30 mum. Comparison of the conditions before and after the bloom formation, and a mass balance of N, shows that the G. aureolum bloom in the frontal region can be maintained almost exclusively by in situ remineralization of remnants of previous diatom blooms

Spring Evolution of nutrients and phytoplankton on the armorican shelf (north-west european shelf)

International audienceThe evolution of nutrient distribution has been followed in spring on the Armorican shelf in association with hydrological features and phytoplankton development. Data were collected on a sampling grid of 20 stations on the continental shelf during four cruises (SATIR 8 to 11) between March and June 1982. Haline fronts (Ushant haline front, coastal haline front) were found in late winter and early spring; their outlines follow closely the outlines of the summer thermal fronts. The origin of these haline fronts comes mainly from the outflow towards the English Channel of the waters of the Loire river on the South Brittany shelf. At this time of the year, a conservative mixing is observed between offshore waters with relatively low nutrient concentrations and coastal waters nutrient-enriched by the river inputs. A particularly early phytoplankton development has been observed in March in the haline frontal zones. Nutrient assimilation and phytoplankton development take place sequentially in spring on the Armorican shelf: a time lag of nearly three months exists between the initiation of the bloom in the South Brittany low-salinity waters and in the central part of the continental shelf. The initiation of phytoplankton development results mainly from the establishment of a haline or thermal stratification. A simple model of critical depth (Sverdrup, 1953; Riley, 1957) taking into account the evolution of the global solar irradiance and the depth of the mixed layer permits the prediction of periods of initiation of phytoplankton development on the Armorican shelf and explains the evolution of the nutrient distributions

Ammonium assimilation and regeneration by size-fractionated plankton in permanently well-mixed temperate waters

International audienceAmmonium assimilation and regeneration by size-fractionated plankton were measured for 1 year at a coastal station in the permanently well-mixed waters of the western English Channel. The lowest assimilation and regeneration rates (25 ng-at N l(-1) h(-1)) in summer. Vertical profiles showed a light-dependent pattern in assimilation and regeneration, with the maximum rates at intermediate depths and the lowest at the base of the euphotic zone. Nanoplankton (1-15 mu m) assimilated and regenerated more ammonium throughout the year than net-(15-200 mu m) and picoplankton (<1 mu m). Assimilation in net- and nanoplankton was regulated by changes in biomass rather than by photosynthetic efficiency. Assimilation in picoplankton was mainly bacterial, but the autotrophic contribution to it became substantial in spring-summer. Ciliates and bacteria were more important for ammonium regeneration than flagellates. Assimilation to regeneration ratios varied as a function of size class: from 1 in picoplankton through 1.8 in nanoplankton to 2.4 in netplankton. Ammonium regenerated in the whole water column exceeded assimilation requirements in the euphotic zone and this may explain the accumulation of ammonium in spring-summer observed in these waters