Determining Titan surface topography from Cassini SAR data

A technique, referred to as SARTopo, has been developed for obtaining surface height estimates with 10 km horizontal resolution and 75 m vertical resolution of the surface of Titan along each Cassini Synthetic Aperture Radar (SAR) swath. We describe the technique and present maps of the co-located data sets. A global map and regional maps of Xanadu and the northern hemisphere hydrocarbon lakes district are included in the results. A strength of the technique is that it provides topographic information co-located with SAR imagery. Having a topographic context vastly improves the interpretability of the SAR imagery and is essential for understanding Titan.SARTopo is capable of estimating surface heights for most of the SAR-imaged surface of Titan. Currently nearly 30% of the surface is within 100 km of a SARTopo height profile. Other competing techniques provide orders of magnitude less coverage.We validate the SARTopo technique through comparison with known geomorphological features such as mountain ranges and craters, and by comparison with co-located nadir altimetry, including a 3000 km strip that had been observed by SAR a month earlier. In this area, the SARTopo and nadir altimetry data sets are co-located tightly (within 5-10 km for one 500 km section), have similar resolution, and as expected agree closely in surface height. Furthermore the region contains prominent high spatial resolution topography, so it provides an excellent test of the resolution and precision of both techniques.     

近40年内蒙古候平均气温变化趋势近

基于1964-2003年内蒙古有关台站的逐日气温数据,对前20年(1964-1983年)和后20年(1984-2003年)的候平均气温中值进行了对比,并利用k-means方法进行了候平均气温变化趋势的聚类分析,旨在阐明内蒙古气温变化的时空特点和规律.研究结果表明:内蒙古全境年平均气温普遍上升,没有下降的现象.不同地区、不同季节气温变化格局不同,北部变暖比南部更为明显,冬季和夏末秋初变暖出现得更频繁.绝大部分地区大多数候平均气温都有所上升,而且候平均气温变化存在纬向地带性.大多数台站四季里都有一些显著升温的候,显著升温在冬季发生得更为频繁.同时,有少数几个候出现气温下降的现象,但未达到显著下降的程度.内蒙古绝大部分地区候最高、最低气温都有所升高,但二者的差值在缩小.候最低气温出现时间有所提前,候最高气温出现时间有提前也有推迟,但总体上不同地区候最高、最低气温出现时间更加同步.因此,候最高、最低气温的时空均一性有所增强.候最高气温出现时间与候最低气温出现时间的间隔有扩大迹象.总之,全球气候变暖降低了气温的季节和地区差异.     

On the anomalous features of the geomagnetic quiet-day field variations at Nagpur, India

The quiet-day geomagnetic field variation data from the recently commissioned Nagpur geomagnetic observatory, which has augmented the currently active latitudinal chain of Indian magnetic observatories, are analysed for the year 1993- The variations for diurnal frequencies (Sq) recorded at Nagpur do not follow the expected trend with latitude. This is most conspicuous in the northward horizontal ( X ) component. The anomalous behaviour at Nagpur is also seen in the diurnal harmonic amplitudes when compared with those of the neighbouring stations Alibag (south of Nagpur) and Ujjain (north of Nagpur). This behaviour is attributed to the presence of electrically conducting anomalous sources in the vicinity of Nagpur. The anomalous internal source is inferred to be located at relatively shallower depths and is highly localized.     

Organochlorine pollution in sperm whales,Physeter macrocephalus,from the temperate waters of the eastern North Atlantic

Residues of DDE, DDD, DDT and PCB were measured in different tissues of fourteen sperm whales caught in the waters off Spain's north-western coasts. Unlike what seems common in other marine mammal species, females appear to be more contaminated by organochlorine compounds than males. Pollutant levels in this species were found to be intermediate to other cetaceans, probably due to their feeding habits mainly based on squid and bottom fish. Concentrations of total DDT and PCB in blubber were similar to most of those determined in sperm whales from other areas of the northern hemisphere.     

Genetic variation among age-classes of the mangrove Avicennia marina in clean and contaminated sediments

An examination was made of the genetic variation with age in mangrove populations in two estuaries in south-east Australia, one contaminated, the other uncontaminated. Sediment nutrient and metal levels, and isozyme/allozyme complements were measured in three age-classes in four stands in each estuary, to identify site and intergenerational genetic composition, and influences of sediment contamination. Within each river, there were greater genetic distances among the age-classes than among sampling sites. A downstream gradient of genetic distance was found in the uncontaminated river, but not the contaminated one, where genetic distances seemed to reflect site-specific factors, though apparently not the present sediment metal or nutrient levels. It appears that genetic distances in the polluted river may reflect past fluctuations in pollution pressures. In both rivers, genetic distances indicate that the mangroves within one estuary form one population, with 'neighbourhood' differentiation.     

The Role of a PhD Field Exam in Preparing Graduate Students for Academic Careers*

The Department of Geography at Arizona State University implemented a field exam as part of its PhD program requirements. This field exam requires students to develop an independent field‐based research project based on a general question in the student's specialty area. A survey of current and former PhD students and faculty members document how the field exam assists students in developing skills necessary for continuing graduate research and for preparing them for the rigors of academic employment. The outcomes of the exam include both long‐term, process‐related benefits and more immediate tangible rewards. For some students, the preliminary fieldwork and results redirect student interests and form the basis for their eventual dissertation. The field exam is adaptable to a diversity of geography research methods, subject areas, and graduate degree programs, while remaining grounded in the discipline's vibrant, widely respected fieldwork tradition.     

New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts

The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters.For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ε_Nd and ε_Hf both decrease with water depth. Similarly, ε_Nd and ε_Hf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable.The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific.Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of Antarctic glaciation.     

Lunar ranging instrument for Chandrayaan-1

Lunar Laser Ranging Instrument (LLRI) proposed for the first Indian lunar mission Chandrayaan-1 is aimed to study the topography of the Moon’s surface and its gravitational field by precisely measuring the altitude from a polar orbit around the Moon. Altimetry data close to the poles of the Moon would also be available from the instrument, which was not covered by earlier missions. This instrument supplements the terrain mapping camera and hyperspectral imager payloads on Chandrayaan-1. The instrument consists of a diode pumped Nd:YAG pulsed laser transmitter having 10 nsec pulse width and a receiver system. The receiver system features 17 cm diameter Ritchey—Chrétien collecting optics, Si Avalanche Photo Detector (APD), preamplifiers, constant fraction discriminators, time-of-flight measurement unit and spacecraft interface. Altimeter resolution of better than 5 m is targeted. The received signal strength of LLRI depends on laser pulse backscatter from the Moon’s surface. Moon’s surface being a poor reflector, the choice of receiver size and its type and the selection of detector play an important role in getting a good signal-to-noise ratio and in turn achieving the target resolution. At the same time, the spacecraft puts a limitation on payload size and weight. This paper discusses the proposed LLRI system for Chandrayaan-1 and signal-to-noise ratio estimation.