Formation and evolution of the chaotic terrains by subsidence and magmatism: Hydraotes Chaos, Mars

The origin of the martian chaotic terrains is still uncertain; and a variety of geologic scenarios have been proposed. We provide topographic profiles of different chaos landscapes, notably Aureum and Hydraotes Chaos, showing that an initial shallow ground subsidence occurred at the first step of the chaos formation. We infer that the subsidence was caused by intrusion of a volcanic sill; which could have produced consequent melting as well as release of ground water from disrupted aquifer. Signs of a volcanic activity are observed on the floor of Hydraotes Chaos, a complex and deep depression located at the junction of three channels. The volcanic activity is represented by small, 0.5 to 1.5 km diameter, rounded cones with summit pits. The cone's size and morphology, as well as the presence of possible surrounding lava flows, suggest that they are primary volcanic cones similar to terrestrial cinder cones. The identification of volcanic activity on the deepest chaos, where the lower crustal thickness and the faults/fractures system contributed to the magma rising, reveals that magmatic activity, proved by the cones, and possibly help by structural activity, has been a major factor in the formation of chaotic terrains.     

Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

A total of 205 dust devils were detected in 23 High Resolution Stereo Camera (HRSC) images taken between January 2004 and July 2006 with the ESA Mars Express orbiter, in which average dust devil heights were ∼660 m and average diameters were ∼230 m. For the first time, dust devil velocities were directly measured from orbit, and range from 1 to 59 m/s. The observed dust devil directions of motion are consistent with data derived from a General Circulation Model (GCM). In some respects HRSC dust devil properties agree favorably with data from the NASA Mars Exploration Rover Spirit dust devil analyses. The spatial distribution of the active dust devils detected by HRSC supports the conjecture that the ascending branch of the Hadley circulation is responsible for the increase in dust devil activity, especially observed during southern summer between 50° and 60° S latitude. Combining the dust-lifting rate of 19 kg/km²/sol derived from the Spirit observations with the fewer in number but larger in size dust devils from various other locations observed by HRSC, we suggest that dust devils make a significant contribution to the dust entrainment into the atmosphere and to the martian dust cycle.     

中祁连东段绿岩带刍议

中祁连东段绿岩带可区分出绿岩部分和基底部分两套地层。恢复湟源群建群初期的划分观点。化隆超基性“岩带”和零星分布于西宁盆地边部产状不明的“岩体”应归属绿岩带下部镁铁质—超镁铁质岩系中。     

滩间山金矿田构造轮廓及控矿构造特点

滩间山金矿田产出部位及矿体延展严格受断裂构造制约。矿田区主要发育北西向压扭性断裂、北东向张扭性断裂和近东西向扭性断裂。北西向、北东向断裂均为控矿断裂,但北东向断裂的控矿作用更为明显。     

Titan's surface from Cassini RADAR SAR and high resolution radiometry data of the first five flybys

The first five Titan flybys with Cassini's Synthetic Aperture RADAR (SAR) and radiometer are examined with emphasis on the calibration and interpretation of the high-resolution radiometry data acquired during the SAR mode (SAR-radiometry). Maps of the 2-cm wavelength brightness temperature are obtained coincident with the SAR swath imaging, with spatial resolution approaching 6 km. A preliminary calibration shows that brightness temperature in these maps varies from 64 to 89 K. Surface features and physical properties derived from the SAR-radiometry maps and SAR imaging are strongly correlated; in general, we find that surface features with high radar reflectivity are associated with radiometrically cold regions, while surface features with low radar reflectivity correlate with radiometrically warm regions. We examined scatterplots of the normalized radar cross-section σ⁰ versus brightness temperature, finding differing signatures that characterize various terrains and surface features. Implications for the physical and compositional properties of these features are discussed. The results indicate that volume scattering is important in many areas of Titan's surface, particularly Xanadu, while other areas exhibit complex brightness temperature variations consistent with variable slopes or surface material and compositional properties.     

Identifying groups and clusters in the 2dFGRS

The way in which galaxy properties are influenced by environment is an important constraint on models of galaxy formation. Using the complete2dF Galaxy Redshift Survey (2dFGRS, see Colless et al., 2001),containing over 225,000 galaxy redshifts, we construct the largest and most uniform group catalogue to date. Here we present tests of our methodology, which is underpinned by mock 2dFGRS catalogues made from a combination of high resolution N-body simulations and the Cole et al. semi-analytic model of galaxy formation. Some basic characteristics of the 2dFGRS group catalogue are given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe

Cassini 2.2-cm radar and radiometric observations of seven of Saturn's icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy Galilean satellites. Average radar albedos decrease in the order Enceladus/Tethys, Hyperion, Rhea, Dione, Iapetus, and Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, which is intrinsically very transparent at radio wavelengths. Plausible candidates for contaminants include ammonia, silicates, metallic oxides, and polar organics (ranging from nitriles like HCN to complex tholins). There is correlation of our targets' radar and optical albedos, probably due to variations in the concentration of optically dark contaminants in near-surface water ice and the resulting variable attenuation of the high-order multiple scattering responsible for high radar albedos. Our highest radar albedos, for Enceladus and Tethys, probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex (i.e., mature) enough for there to be high-order multiple scattering within it. At the other extreme, Phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering. Iapetus' 2.2-cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side, whereas 13-cm results reported by Black et al. [Black, G.J., Campbell, D.B., Carter, L.M., Ostro, S.J., 2004. Science 304, 553] show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2.2 cm. These Iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths, and if the leading side's optically dark contaminant is present to depths of at least one to several decimeters. As argued by Lanzerotti et al. [Lanzerotti, L.J., Brown, W.L., Marcantonio, K.J., Johnson, R.E., 1984. Nature 312, 139-140], a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of Saturn's icy satellites. Given the hypersensitivity of water ice's absorption length to ammonia concentration, an increase in ammonia with depth could allow efficient 2.2-cm scattering from within the top one to several decimeters while attenuating 13-cm echoes, which would require a six-fold thicker scattering layer. If so, we would expect each of the icy satellites' average radar albedos to be higher at 2.2 cm than at 13 cm, as is the case so far with Rhea [Black, G., Campbell, D., 2004. Bull. Am. Astron. Soc. 36, 1123] as well as Iapetus.     

MERLIN: today and tomorrow

The MERLIN team together with Melvin Hoare and Karen Wills , review MERLIN's revealing role in studying star formation throughout the universe – from nearby stars to the most distant galaxies.     

辽北凡河群时代探讨兼论殷屯组的时代

辽宁北部地区凡河群源于斋藤林次的凡河系,其时代归属尚有争议。目前多数人将凡河群与蓟县地区的中、上元古界对比;殷屯组的时代则以暂置震且纪居主导地位。笔者稂据凡河群地史演化、所处大地构造位置、变质作用、含海相火山岩夹层、同位素年龄资料等综合因素分析;认为凡河群的时代以归属长城纪为宜,殷屯组时代归属蓟县纪较合适。