Cassini SAR, radiometry, scatterometry and altimetry observations of Titan’s dune fields

Large expanses of linear dunes cover Titan’s equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini’s radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan’s geology and climate. We estimate that dune fields cover ∼12.5% of Titan’s surface, which corresponds to an area of ∼10 million km², roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ∼11°, dune fields tend to become less emissive and brighter as one moves northward. Above ∼11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ∼14°. In general, scattering from the subsurface (volume scattering and surface scattering from buried interfaces) makes interdunal regions brighter than the dunes. The observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north, thus allowing wave penetration in the underlying substrate. Altimetry measurements over dunes have highlighted a region located in the Fensal dune field (∼5° latitude) where the icy bedrock of Titan is likely exposed within smooth interdune areas. The hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and/or an increase in the ground humidity toward the north, which could be related to Titan’s asymmetric seasonal polar insolation. Alternatively, it may indicate that either the wind pattern or the topography is less favorable for dune formation in Titan’s northern tropics.     

Geiger avalanche photodiodes as tentative light detectors for VHE gamma ray astronomy

Due to its sensitivity and speed, the detector still widely used in Cerenkov astrophysics experiments remains the PhotoMultiplier Tube (PMT). For instance, recent pathbreaking experiments in Very High Energy astrophysics (VHE), such as MAGIC and HESS, have used mainstream PMT technology [Aharonian, F. et al Astron. Astrophys. 492(1):L25–L28 (2008)]. Moreover the Cerenkov Telescope Array (CTA) which is now in its design phase, is also planed to be based on PMT’s. However, there are some disadvantages to the PMT technology: the rather poor quantum efficiency, the use of high voltages, the high cost when used in large number in a matrix arrangement and the large weight. Hence, we have investigated the possibility to design future Cerenkov telescopes based on solid state technology, specifically Geiger avalanche photodiodes. In a preliminary development test, we placed HAMAMATSU avalanche photodiodes at the focal plane of a 60 cm diameter telescope at the Pic du Midi in the French Pyrénées, in order to record incident cosmic rays. In this paper, we describe not only the experimental setup but we also put special emphasis to the reduction of the semi-conductor noise. We also show first data that were recorded during two runs in the fall of 2006, and conclude by the presentation of the design of an “integrated, low-cost solid state photodiode arrangement” which might be an alternative to PMT’s for future VHE telescopes.     

A Late Amazonian alteration layer related to local volcanism on Mars

Hydrated minerals on Mars are most commonly found in ancient terrains dating to the first billion years of the planet’s evolution. Here we discuss the identification of a hydrated light-toned rock unit present in one Chasma of the Noctis Labyrinthus region. Stratigraphy and topography show that this alteration layer is part of a thin unit that drapes pre-existing bedrock. CRISM spectral data show that the unit contains hydrated minerals indicative of aqueous alteration. Potential minerals include sulfates such as bassanite (CaSO₄·1/2H₂O) or possibly hydrated chloride salts. The proximity of a smooth volcanic plain and the similar crater model age (Late Amazonian, <100 Myr) of this plain and the draping deposits suggest that the alteration layer may be formed by the interaction of water with ash layers deposited during this geologically recent volcanic activity. The alteration phases may have formed due to the presence of snow in contact with hot ash, or eventually solid-gas interactions due to the volcanic activity. The relatively young age of the volcanic plain implies that recent alteration processes have occurred on Mars in relation with volcanic activity, but such local processes do not require conditions different than the current climate.     

Slow and Fast Rotating Coronal Holes from Geomagnetic Indices

The evolution of the 27-day recurrence in the series of two solar indices (Wolf number WN and 10.7 cm radio flux F) and two geomagnetic indices (Dst and ζ, variance of the geomagnetic field recorded at a magnetic observatory) have been studied over the 1957 – 2007 time span. Spectral energies contained in two period domains (25 – 27.3 and 27.3 – 31 days), designated as E ₁ and E ₂, have been computed. Whereas the evolution of E ₁ is the same for the four indices, that of E ₂ is essentially different for WN and F on the one hand, Dst and ζ on the other hand. Some general conclusions on the dynamics of the solar outer layers are inferred from these results. First the solar activity, as measured by WN, and when averaged over a few years, evolves in the same way whatever the latitude. Second, two families of coronal holes (CHs) are identified; the rapidly and the slowly rotating CHs evolve quite differently.     

Copper speciation survey from UK marinas, harbours and estuaries

The use of copper in antifouling paints has increased in the UK in the last 20 years as TBT and several other organic biocides have been phased out. To assess the probable impact of copper on estuarine systems a survey was undertaken to measure the different fractions of copper present in the water column at current usage. The different fractions measured were; labile copper, (LCu) considered as both the free copper ions and inorganically bound copper, the total dissolved copper (TDCu) present, and the difference between them taken as the organically bound likely non-toxic copper fraction. The survey considered sites with different levels of boat use, namely marinas, harbours and estuaries, differing physical parameters of suspended and dissolved organic matter, different seasons of the year and different depths in the water column all of which control speciation behaviour. Suspended particulate matter (SPM) values were measured at all sites and increased from West to East coast locations (5.7-34.4 mg/l). Dissolved organic matter (DOM) values ranged from 0.58 to 2.2mg/l C. The total dissolved copper concentrations ranged from 0.30 to 6.68 microg/l, with labile fraction ranging from 0.02 to 2.69 microg/l, and most labile copper concentrations below 1 microg/l. None of the yearly mean copper measurements exceeded the 76/464/EEC EQS of 5 microg/l. Of the 306 measurements, only one dissolved copper value in one season was above 5 microg/l. This ratio of labile to total copper was between 10 and 30%. The results from this survey suggest that if toxicity of copper is due to the labile fraction then using the total dissolved copper concentrations as an indicator of impact overestimate the risk by a factor of four times.     

沈阳长白乡-观音阁断裂、抚顺浑河断裂的定位和活动性研究

本文基于对沈阳长白乡-观音阁断裂,以及抚顺浑河断裂开展的1:10000地质地貌填图工作,获得了这两条断裂在最老第四纪地层热释光的测年结果,它们分别为(112±6.7)ka和(150±9.0)ka。长白乡-观音阁断裂为一沿丘陵山地前缘分布的断裂,由2—3条分支断裂组成,最新的断面活动显示为压扭性正断层性质。断裂在晚更新世坡洪积扇堆积形成以来没有活动显示。3条浅层人工地震测线探测结果表明,其上断点均未错断第四系上更新统底界。断裂为中更新世活动断裂,晚更新世以来没有活动。浑河断裂为一沿丘陵山地前缘分布的断裂,由3—4条分支断裂组成,最新的断面活动显示为压扭性正断层性质。断裂在晚更新世坡洪积扇堆积形成以来没有活动显示。5条浅层人工地震测线探测结果表明,其上断点均在第四系上更新统底界以下。断裂为中更新世活动断裂,晚更新世以来没有活动。     

Late Carboniferous-Early Permian Sequence Stratigraphy and Depositional Evolution in the Northeast Ordos Basin,North China

<正>Sequence stratigraphical analysis was applied to the Upper Carboniferous-Lower Permian sedimentary succession of the northeastern Ordos Basin,north China based on data acquired from ten entire logging curves and eight outcrops.The facies framework of the lithostratigraphical unit,the Taiyuan Formation comprises seven facies in two facies associations,varying from fluvio-delta to shelf-barrier islands.The facies are presented within a chronostratigraphical framework,linked by systems tract,which in turn are limited by flooding surfaces and sequence boundaries.Six third-order depositional sequences are recognised,bounded by six type 2 unconformities.An upwards-shallowing epicontinental sea sedimentary model is created,which consists of a sandstone,coal seam and carbonate succession.     

Establishing constraints on groundwater ages with Cl,C, H,and noble gases: A case study in the eastern Paris basin,France

Groundwaters from the Tithonian/Kimmeridgian, Oxfordian and Upper Dogger aquifers, within the eastern part of the Paris basin (France), were characterised using ³H, ¹⁴C and ³⁶Cl, and noble gases tracers, to evaluate their residence times and determine their recharge period. This information is an important prerequisite to evaluating the confinement properties of the Callovo-Oxfordian clay formation sandwiched between the Oxfordian aquifer and the Dogger aquifer, currently being investigated by the French nuclear waste management agency (Andra) for radioactive waste disposal. Data presented in this paper are used to test 4 hypotheses.