Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

The European Space Agency’s Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the H i Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet’s center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 Å is detected at altitudes of 400-1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of .     

Benthic marine pollution around McMurdo Station, Antarctica: A summary of findings

McMurdo Station is the United States Antarctic Research Program's (USARP) centre for scientific and logistical support for most of the continent and is the largest settlement in Antarctica. Over 30 years of human presence with shipping, air-transport, scientific, and municipal activities have resulted in severe but localized nearshore marine benthic pollution. This review will summarize the results of 3 years of research which has documented concentrations of chemical contaminants, changes in community patterns, and the toxicity of sediments to invertebrate species and infaunal communities. The primary contaminants are petroleum hydrocarbons in the sediments of Winter Quarters Bay, the site of shipping activities and a former dumpsite. Total hydrocarbon levels in sediments in Winter Quarters Bay were comparable to the most polluted harbours in temperate latitudes. However, a steep gradient of pollution existed: stations less than 1 km away were nearly pristine. Benthic invertebrate communities changed dramatically along this contamination gradient. The results of both field and laboratory bioassay experiments with contaminated sediments found that biological changes observed in benthic communities around McMurdo Station were most likely caused by hydrocarbons, PCBs, and PCTs. A primary goal of polar pollution research is to determine the response and tolerance of Antarctic biota to physical and chemical anthropogenic disturbance, and the mitigation of negative effects. To this end, future work should include continued monitoring of changes in chemical and community patterns, studies of the physiological responses of Antarctic invertebrates to chemical contamination, and further comparisons of anthropogenic and natural disturbances to benthic communities.     

The demise and recovery of seagrass in the northern Indian River Lagoon,Florida

Seagrass both disappeared and recovered within 4 yr in one region of northern Indian River Lagoon (IRL). For the specific area referred to as Turnbull Bay, a relatively pristine area of the IRL, over 100 ha of seagrass completely disappeared from 1996 to 1997 and then recovered by 2000. Based on lagoon-wide mapping from aerial photographs taken every 2–3 years since 1986, coverage of seagrass in Turnbull Bay declined from 124 ha in 1989 to 34 ha by 1999 and increased to 58 ha in 2003. Bi-annual monitoring of fixed seagrass transects tells a more detailed story. Species composition along the Turnbull transect shifted fromHalodule wrightii toRuppia maritima beginning in 1995, and macroalgal abundance increased. By the summer of 1997, seagrass completely disappeared along the transect, as well as in most of the surrounding areas in Turnbull Bay; macroalgae covered much of the sediment surface. No significant water quality changes were detected. Light attenuation and suspended solid values did increase after the seagrass disappeared. Porewater sulfide concentrations, taken after all the grass was gone in 1997, were high (2,000 μM), but did improve by 1998 (1,200 μM). Seagrass recovery was rapid and occurred in the reverse sequence of species loss. Seedlings ofR. maritima were the first colonizers, then patches ofH. wrightii appeared. In 2000,Halophila engelmannii returned in the deeper water (>0.6m). By the summer of 2000, the beds had completely recovered. We conclude that this demise was a natural event caused by a long-term buildup of seagrass biomass and a thick (10–15 cm) layer of organic detritus and ooze. We surmise that such a crash and subsequent recovery may be a natural cycle of decline and recovery within this semirestricted, poorly-flushed area. The frequency of this cycle remains uncertain.     

Shock-heated NH3 in a Molecular Jet Associated with a High-Mass Young Star

We present the discovery of shock-excited NH3 in a well-collimated jet associated with the extremely young high-mass star IRAS 20126+4104. The NH3 (3, 3) and (4, 4) emission is dominated by three clumps along the SiO jet. At the end of the jet, there exists strong and broad (+/-10 km s-1) NH3 (3, 3) emission. With typical brightness temperatures greater than 500 K, the overall emission indicates a weakly inverted population and appears in an arc, consistent with the excitation by bow shocks. There are two bright spots in the NH3 (3, 3) emission with brightness temperatures of approximately 2000 K. The narrow line width (1.5 km s-1 FWHM), the small sizes (<0&farcs;3), and the unusually high brightness temperature of the features are indicative of maser emission. Our observations provide clear evidence that NH3 (3, 3) masers are excited in shock regions in molecular outflows.     

Emplacement,crystallization and alteration of spinifex-textured komatiitic basalt flows in the Archaean Nondweni greenstone belt,southern Kaapvaal Craton,South Africa

A well exposed succession of spinifex-textured komatiite flows is reported from the Archaean Nondweni greenstone belt located near the southern margin of the Kaapvaal Craton. The flows are relatively thin (1–5 m) compared to similar occurrences in other greenstone belts. They are characterised by well developed cone structures of highly elongate amphibole crystals (after augite) which fan downwards from the tops of the flows. Extreme development of coned spinifex has not been reported from other greenstone belts and points to specific thermal conditions prevailing in the Nondweni environment. The zones of bladed spinifex are contained between layers of random spinifex and overlie a lower cumulus layer originally of augite, orthopyroxene and minor olivine. The observed major and trace element distributions through a 1.7 m thick spinifex-textured flow are consistent with a model involving concentration of phenocryst phases resulting in significant fractionation upwards in the flow. Approximately 40% of the spinifex-textured phenocrysts grew in situ after the lithological units were established. Collapse and displacement of the coned crystal networks, originally attached to the top of the flow, are shown to have influenced the distribution of liquid within the flow and accentuated the fractionation. Associated with the spinifex-textured units are massive aphyric and brecciated flows which show distinct chemical cycles through the succession. The brecciated zones have compositions with <18% MgO and are characterised by ovoid bodies that are not pillows and may represent magmatic reworking and movement of a partly congealed flow. Post-solidus alteration is considered to have caused early hydration of the original mineralogy and also introduced SiO₂ and Na₂O into the upper part of the flow by way of microfractures. The observed alteration is different to that of Mid-Ocean Ridge basalts, and a subaerial/shallow water environment is suggested.     

Trace metal distributions across the continental shelf near Otago Peninsula, New Zealand

The distributions of the trace metals iron (Fe), copper (Cu) and cadmium (Cd) along with hydrological parameters (salinity, temperature and reactive phosphate) across the New Zealand continental shelf near Otago Peninsula have been studied. This is a region in which the Subtropical Convergence (STC), a major oceanic front separating subtropical and subantarctic waters, is uniquely located close to land, permitting an examination of the influence of terrestrial sources of Fe and Cu on oceanic waters containing excess micronutrients. Acid-soluble (110 nmol kg⁻¹) and dissolved (6.3 nmol kg⁻¹) Fe concentrations were highest over the central shelf, and decreased rapidly across the mixing zone of the STC to about 5 nmol kg⁻¹ for both forms. The distribution of acid-soluble and dissolved Cu were similar to their counterparts for Fe. Depth-concentration profiles for acid-soluble Fe and Cu suggest resuspension of shelf sediments is the main source. The ratio of oxine-labile to acid-soluble Fe varied from 0.03 to 0.26, with the highest values found in the near surface waters. Oxine-labile Fe and Cu also decreased in concentration in a seawards direction, and with depth, indicating the influence of near surface processes on the reactivity of these elements. Cd concentrations across the continental shelf were very low (<200 pmol kg⁻¹) and exhibited no clear spatial trend and no correlation with phosphate. Comparison of the Cd/P ratio across the shelf indicated that the waters in this region were strongly depleted in Cd relative to P. Phosphate concentrations were lowest in neritic water and increased in the seawards direction because of mixing with nutrient-rich Subantarctic Surface Water.