Fluvial channels on Titan: Initial Cassini RADAR observations

Cassini radar images show a variety of fluvial channels on Titan's surface, often several hundreds of kilometers in length. Some (predominantly at low- and mid-latitude) are radar-bright and braided, resembling desert washes where fines have been removed by energetic surface liquid flow, presumably from methane rainstorms. Others (predominantly at high latitudes) are radar-dark and meandering and drain into or connect polar lakes, suggesting slower-moving flow depositing fine-grained sediments. A third type, seen predominantly at mid- and high latitudes, have radar brightness patterns indicating topographic incision, with valley widths of up to 3 km across and depth of several hundred meters. These observations show that fluvial activity occurs at least occasionally at all latitudes, not only at the Huygens landing site, and can produce channels much larger in scale than those observed there. The areas in which channels are prominent so far amount to about 1% of Titan's surface, of which only a fraction is actually occupied by channels. The corresponding global sediment volume inferred is not enough to account for the extensive sand seas. Channels observed so far have a consistent large-scale flow pattern, tending to flow polewards and eastwards.     

A new probabilistic approach to estimating marine gastropod densities from baited traps

A new probabilistic approach is proposed to assess muricid species population abundances at scales relevant to both Ancient and Modern coastal fisheries. Motivated by the long‐term goal of reconstructing the dynamics of exploited murex populations during Antiquity, the objective was to estimate the population density of the banded dye‐murex, Hexaplex trunculus (Linnaeus, 1758) from successive captures with baited traps, using a method similar to the technique employed in the Mediterranean purple dye industry. The stochastic model developed simulates cumulative captures while accounting for high variability. It was calibrated with data acquired during a field trapping experiment (Crete Island, Greece). Traps’ catchability and Effective Area of Attraction (EAA) were estimated using the individual speed and behavioural response towards bait from laboratory experiments. Average density of H. trunculus was estimated as 2.2 ± 1.4 SE individuals per square metre, with no significant differences between seagrass and rocky habitats. The clearing time of successive capture experiments averaged 84 ± 6 SE hr. Clearing ca. 0.4 ha of subtidal area would be necessary to produce ca. 1.0 g of pure Tyrian purple pigment. The method described is generalizable to making population abundance estimates for similar groups, such as whelks, in modern fisheries.     

Flare asymmetry as seen in offband Hα filtergrams

Narrow-band Hα filtergrams at ±1 Å and ±2 Å from the line center were used to study the asymmetry of flares. Of the 60 flares studied, 92% show red asymmetry while 5% show blue asymmetry. Typically, the filtergrams show a striking dominance of the red wing over the blue wing from onset until late in the decay phase. The difference in intensity of the flare emissions is further augmented by the extensiveness of the emission area in the brighter wing. New kernel-like emissions were often found many minutes after the flare maximum. Spatially they are displaced from the previous kernels. The late emissions show the same asymmetry as that of the earlier part of the flare.     

PARTICLES AROUND BOXHOLE METEORITE CRATER

A sampling program of soil surrounding the Boxhole meteorite crater reveals very much smaller amounts of meteoritic material in the soil than found for other craters, such as Henbury and Canyon Diablo. Microprobe analyses of individual small particles show the presence of four spherules with Ni-Fe ratios in agreement with that for the parent meteorite.     

Flaring arches

We show detailed observations in X-rays, UV lines, and H of an extended arch, about 300000 km long, which developed as a consequence of a compact subflare. This subflare occurred in an included magnetic polarity of relatively low magnetic field strength (compared to that of the sunspots). The apparition of this big arch was preceded by that of a smaller arch, about 30000 km long, which masked the polarity inversion line filament in the early phase of the subflare. The big arch which developed later, around the time of the main X-ray and UV spike of the subflare, connected the included polarity and the main leading sunspot of the region, and became fully developed in a few minutes. The fact that both arches were simultaneously observed in all spectral domains as well as their fine structure in H can only be explained by considering the arch as composed of several unresolved portions of material having widely different temperatures. The H observations can be interpreted as showing the appearance of this cool material as a result of condensation, but a more appealing interpretation is that there was almost simultaneous ejection of superhot (10⁷ K), hot (10⁶ K), mild (10⁵ K), and cool (10⁴ K) material from the subflare site along previously existing magnetic tubes of much lower density. The termination of the subflare was marked by a rather hard X-ray and UV spike which appeared to originate in a different structure than that of the main spike. The material in the arch gradually cooled and drained down after the end of the subflare.Member of Carrera del Investigador, CONICET, Argentina.     

X-ray bright surges

We present evidence of X-ray emission from surges that are bright in H. These surges have many features common to flaring arches of Martin and vestka (1988); the basic difference between the two is that in flaring arches cold and hot plasma are injected into clearly defined closed magnetic loops, while in the surges the injection goes into large-scale magnetic field structures of which the second footpoint is usually unknown. Because of the steep density gradient in such large-scale structures, the X-ray visibility of bright surges is limited to a few tens of seconds only. A series of repetitive surges, some of them bright and emitting X-rays, occurred on 8 July, 1980 from footpoints of two large-scale coronal structures, which might have been the legs of an enormous arch at least 600 Mm long.     

Nannofossils: the smoking gun for the Canarian hotspot

The origin of volcanism in the Canary Islands has been a matter of controversy for several decades. Discussions have hinged on whether the Canaries owe their origin to seafloor fractures associated with the Atlas Mountain range or to an underlying plume or hotspot of superheated mantle material. However, the debate has recently come to a conclusion following the discovery of nannofossils preserved in the products of the 2011–2012 submarine eruption at El Hierro, which tell us about the age and growth history of the western‐most island of the archipelago. Light coloured, pumice‐like ‘floating rocks’ were found on the sea surface during the first days of the eruption and have been shown to contain fragments of pre‐island sedimentary strata. These sedimentary rock fragments were picked up by ascending magma and transported to the surface during the eruption, and remarkably retained specimens of pre‐island Upper Cretaceous to Pliocene calcareous nannofossils (e.g. coccolithophores). These marine microorganisms are well known biostratigraphical markers and now provide crucial evidence that the westernmost and youngest island in the Canaries is underlain by the youngest sediment relative to the other islands in the archipelago. This finding supports an age progression for the onset of volcanism at the individual islands of the archipeligo. Importantly, as fracture‐related volcanism is known to produce non‐systematic age‐distributions within volcanic alignments, the now‐confirmed age progression corroberates to the relative motion of the African plate over an underlying mantle plume or hotspot as the cause for the present‐day Canary volcanism.     

Volcaniclastic habitats for early life on Earth and Mars: A case study from ∼3.5 Ga-old rocks from the Pilbara, Australia

Within the context of present and future in situ missions to Mars to investigate its habitability and to search for traces of life, we studied the habitability and traces of past life in ∼3.5 Ga-old volcanic sands deposited in littoral environments an analogue to Noachian environments on Mars. The environmental conditions on Noachian Mars (4.1-3.7 Ga) and the Early Archaean (4.0-3.3 Ga) Earth were, in many respects, similar: presence of liquid water, dense CO₂ atmosphere, availability of carbon and bio-essential elements, and availability of energy. For this reason, information contained in Early Archaean terrestrial rocks concerning habitable conditions (on a microbial scale) and traces of past life are of relevance in defining strategies to be used to identify past habitats and past life on Mars.One such example is the 3.446 Ga-old Kitty’s Gap Chert in the Pilbara Craton, NW. Australia. This formation consists of volcanic sediments deposited in a coastal mudflat environment and is thus a relevant analogue for sediments deposited in shallow water environments on Noachian Mars. Two main types of habitat are represented, a volcanic (lithic) habitat and planar stabilized sediment surfaces in sunlit shallow waters. The sediments hosted small (<1 μm in size) microorganisms that formed colonies on volcanic particle surfaces and in pore waters within the volcanic sediments, as well as biofilms on stabilised sediment surfaces. The microorganisms included coccoids, filaments and rare rod-shaped organisms associated with microbial polymer (EPS). The preserved microbial community was apparently dominated by chemotrophic organisms but some locally transported filaments and filamentous mat fragments indicate that possibly photosynthetic mats formed nearby. Both microorganisms and sediments were silicified during very early diagenesis.There are no macroscopic traces of fossilised life in these volcanic sediments and sophisticated instrumentation and specialized sample preparation techniques are required to establish the biogenicity and syngenicity of the traces of past life. The fact that the traces of life are cryptic, and the necessity of using sophisticated instrumentation, reinforces the challenges and difficulties of in situ robotic missions to identify past life on Mars. We therefore recommend the return of samples from Mars to Earth for a definitive search for traces of life.     

Zircon Hf isotope evidence for an enriched mantle source for the Bushveld Igneous Complex

We use the Hf isotope composition of zircon from the Bushveld Complex to better understand the source of its parent magmas. The data set, which consists of 141 individual LA-ICP-MS analyses from 11 samples encompassing the entire cumulate stratigraphy, shows that the parent magmas had a Hf isotope composition unlike that of the depleted mantle at 2.06 Ga. Specifically, sample average ε_Hf(present) values range from ?55.3 to ?52.5 (ε_{Hf(2.06 Ga)} = ?9.0 to ?6.8) and are surprisingly homogeneous. This homogeneity is difficult to reconcile with direct assimilation of crustal material by Bushveld parent magmas because it would require that each batch of magma had assimilated just the right amount of material to all acquire the same Hf isotopic composition. Also, calculations suggest that simple mixing of regional crust into a primitive, mantle-derived liquid cannot account for both the presumed Hf and major elemental concentrations and the ¹⁷⁶Hf/¹⁷⁷Hf ratio of the Bushveld magmas. Rather, the Hf data are consistent with generation of these magmas by partial melting in a sub-continental mantle lithospheric source with an unradiogenic Hf isotopic composition equal to that of the Bushveld parent magmas. Several possibilities for the development of such a source are explored using the new Hf isotope data.     

Regime of carbon compounds in carbonatites in Uzbekistan: Evidence from carbon isotopic composition and thermodynamic simulations

Thermodynamic analysis of equilibria between minerals (with regard for their compositions) in carbonatites of the Chagatai complex, Uzbekistan, provides us with the possibility of estimating the oxygen fugacity at which carbonates could occur in equilibrium with elementary carbon. Isotopic studies and thermodynamic simulations show that graphite started to crystallize at 775°C and an oxygen fugacity value approximately one logarithmic unit below the QFM buffer and continued to crystallize with further cooling, simultaneously with a decrease in the Ti concentration in the equilibrium magnetite. Graphite crystallized from carbonatite melt at higher temperatures and likely precipitated from hydrothermal fluid at lower temperatures. The composition of gas in equilibrium with graphite in the C-H-O system was calculated for oxygen fugacity values evaluated for the Chagatai carbonatites. Inasmuch as the values of oxygen potential are almost identical in graphite- and diamond-bearing carbonatites, the presence of graphite in carbonatite dikes and diatremes can be regarded as a prospecting guide in exploration for diamondiferous carbonatites.