The dust trail of Comet 67P/Churyumov-Gerasimenko between 2004 and 2006

We report on observations of the dust trail of Comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2 m telescope at 4.7 AU before aphelion, and at with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter β<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust environment and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of −4.1, and emission speeds for a β=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of .     

Relics of eclogite facies assemblages in the Ceará Central Domain, NW Borborema Province, NE Brazil: Implications for the assembly of West Gondwana

The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil–Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Araçuai and Brasilia fold belts located between the West-African/São Luis, Congo/São Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Médio Coreaú and Central Ceará domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaíba Basin in Brazil (northwest of the Médio Coreaú domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quitéria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P–T conditions under eclogite-facies metamorphism (T = 770 °C and P = 17.3 kbar) were estimated using garnet–clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P–T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Médio Coreaú and Central Ceará domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.     

Palaeoecology of well-preserved coral communities in a siliciclastic environment from the Late Pleistocene (MIS 7), Kish Island, Persian Gulf (Iran): the development of low-relief reef frameworks (biostromes) in increasingly restricted environments

Major changes in community structure and depositional relief of high-latitude coral communities in the southern Persian Gulf between marine isotope stage (MIS) 7 and the present day suggest that the area has become increasingly restricted. Corals and bivalves from outcrops on Kish Island, Iran, were identified in order to interpret the Late Pleistocene palaeoenvironmental setting. U/Th disequilibrium dating was used to constrain the ages of the stratigraphic units. During MIS 7, two coral-bearing sequences were deposited on what is now Kish Island. The lower sequence is dated as MIS 7.5 and changes laterally from an assemblage dominated by Cyphastrea sp. and Platygyra daedalea in the west to one characterized by branching Montipora in the east. By contrast, the upper sequence, dated as MIS 7.1, transitions from an assemblage dominated by platy Montipora in the west to a diverse assemblage of Platygyra and other faviids in the east. The assemblages of both sequences are within a marl matrix and bounded by thin lithified mollusc-rich layers. Corals and bivalves indicate that the sequences were deposited on gentle slopes in sheltered environments less than 20 m deep. The MIS 7 deposits may be classified as coral carpets or biostromes that developed a low-relief framework. During MIS 5, coral communities were no longer framework building and are now limited to an Acropora-rich layer of coral rubble that covers large parts of the island, and two small incipient reefs with sparse faviids. Similarities between the MIS 5 and modern nearshore coral communities suggest that the environmental conditions during MIS 5 were comparable to those of today. The late Pleistocene coral carpets and non-framework coral communities of the southern Persian Gulf may serve as models for coral biostromes in the fossil record, which formed under restricted environmental conditions such as elevated terrigenous input, high turbidity, and strong seasonal changes in temperature and/or salinity.     

The oldest azhdarchoid pterosaur from the Late Jurassic Solnhofen Limestone (Early Tithonian) of Southern Germany

Based on an almost complete three-dimensionally preserved skeleton, a new genus and species of an azhdarchoid pterosaur Aurorazhdarcho primordius n.gen. n.sp. from the Late Jurassic Solnhofen limestone (Early Tithonian) of the Eichstätt area (Bavaria, Germany) is described. Furthermore, a new family the Protazhdarchidae is proposed. The specimen is attributed to the Azhdarchoidea based on its glenoid fossa level with the sternum, the shovel-like shape of the sternal plate, the wide furca of the coracoid, the metacarpus being longer than radius and ulna, the femur being 1/3 longer than the humerus, the femorotibial ratio, and the hammer-shaped humerus among other diagnostic features. Under UV-light, soft tissue preservation around the external mould of the head is visible. It consists of tiny flakes possibly remnants of skin. The dorsally curved outline of the external mould of the head suggests the presence of a cranial crest. The new species is the oldest record of the azhdarchoid pterosaurs. It supports the Eurasian origin of this group that includes the largest flying animal ever.     

Coral extension rates in the NW Indian Ocean I: reconstruction of 20th century SST variability and monsoon current strength

To date, only a few coral proxy studies have investigated coral growth as an indicator of climate variability. This study presents the first extension-rate record (Porites lutea) from the Maldives (NW Indian Ocean), inferred from skeletal δ¹⁸O chronology for the lagoon of Rasdhoo Atoll (4°N/73°W) in the central area of the Maldives, influenced by the Indian monsoon. The record spans 90 years over the period 1917–2007. The mean annual extension over this period was 9.9 mm/year, and an increase of annual extension rates until 1990 by 3 mm/year can be explained by a rise of 0.7°C in sea surface temperature (SST) in this region. After 1990, the extension rates do no continue increasing, possibly due to ecological stress caused by progressive ocean warming and acidification. The correlation between annual extension rates and SSTs is thus significant and strong in the lower part of the record until 1955 (r = +0.69, p < 0.0001), but weaker thereafter (r = +0.44, p < 0.001). The extension rates yield a distinct interannual variability of 3–4 years, caused by interannual SST fluctuations driven by the El Ni?o-Southern Oscillation. A variability of 8–9 years is likely driven by SST variations endemic to the Indian Ocean. Spectral peaks between 18–19 years and 6–7 years cannot be explained by SST fluctuations, but by variations in the strength of the SW monsoon currents. It is suggested that during phases of stronger monsoon activity, the coral sacrificed coral extension in favor of a denser, more robust skeleton. The geomorphology of the atoll may strengthen the potential of this new coral archive to track climate variability.     

The lunar dust environment

Each year the Moon is bombarded by about 10⁶ kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013.Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.     

Zr contents of glaucophane-bearing meta-basalts of Western Crete,Greece

Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their Al^VI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.