The icy Galilean satellites: ultraviolet phase curve analysis

Ultraviolet disk-integrated solar phase curves of the icy galilean satellites Europa, Ganymede, and Callisto are presented, using combined data sets from the International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and the Galileo Ultraviolet Spectrometer. Global, disk-integrated solar phase curves for all three satellites, in addition to disk-integrated solar phase curves for Europa's leading, trailing, jovian, and anti-jovian hemispheres, are modeled using Hapke's equations for 7 broadband UV wavelengths between 260 and 320 nm. The sparse coverage in solar phase angle, particularly for Ganymede and Callisto, and the noise in the data sets poorly constrain some of the photometric parameter values in the model. However, the results are sufficient for forming a preliminary relationship between the effects of particle bombardment on icy surfaces and photometric scattering properties at ultraviolet wavelengths. Callisto exhibits a large UV opposition surge and a surface comprised of relatively low-backward scattering particles. Europa's surface displays a dichotomy between the jovian and anti-jovian hemispheres (the anti-jovian hemisphere is more backward scattering), while a less pronounced hemispherical variation was detected between the leading and trailing hemispheres. Europa's surface, with the exception of the trailing hemisphere region, appears to have become less backscattering between the late-1970s-early-1980s and the mid-1990s. These results are commensurate with the bombardment history of these surfaces by magnetospheric charged particles.     

Local topographic effects on photometry and reflectance spectra of planetary surfaces: An example based on lunar photometry

Abstract— The photometric properties of the average lunar surface are characterized using Hapke's equations and whole disk observations ranging from 0.36 to 1.06 μm. Synthetic spectra across a crater topographic profile are created using the modeling results. The synthetic spectra are examined for spectral variations created by changes in lighting conditions induced by the topography. Changes above the modeling uncertainties are seen in both spectral slope and band depths, though the most pronounced change is in band depth. The data have insufficient spectral resolution to determine if there are any changes in band center due to photometric effects. No additional absorption features are introduced by the photometry. These results have serious implications on the interpretation of spectral observations in terms of abundance estimates and alteration processes as a function of location and association with geologic features.     

U-Th systematics and Th ages of carbonate chimneys at the Lost City Hydrothermal Field

The Lost City Hydrothermal Field (LCHF) is a serpentinite-hosted vent field located 15 km west of the spreading axis of the Mid-Atlantic Ridge. In this study, uranium-thorium (U-Th) geochronological techniques have been used to examine the U-Th systematics of hydrothermal fluids and the ²³⁰Th ages of hydrothermally-precipitated carbonate chimneys at the LCHF. Fluid sample analyses indicate that endmember fluids likely contain only 0.0073 ng/g U or less compared to 3.28 ± 0.03 ng/g of U in ambient seawater. For fluid samples containing only 2-21% ambient seawater (1.1-11 mmol/kg Mg), Th concentration is 0.11-0.13 pg/g and surrounding seawater concentrations average 0.133 ± 0.016 pg/g. The ²³⁰Th/²³²Th atomic ratios of the vent fluids range from 1 (±10) × 10⁻⁶ to 11 (±5) × 10⁻⁶, are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic ²³⁰Th to the LCHF carbonate chimney deposits. Chimney ²³⁸U concentrations range from 1 to 10 μg/g and the average chimney corrected initial δ²³⁴U is 147.2 ± 0.8, which is not significantly different from the ambient seawater value of 146.5 ± 0.6. Carbonate ²³²Th concentrations range broadly from 0.0038 ± 0.0003 to 125 ± 16 ng/g and ²³⁰Th/²³²Th atomic ratios vary from near seawater values of 43 (±8) × 10⁻⁶ up to 530 (±25) × 10⁻³. Chimney ages, corrected for initial ²³⁰Th, range from 17 ± 6 yrs to 120 ± 13 kyrs. The youngest chimneys are at the intersection of two active, steeply-dipping normal faults that cut the Atlantis Massif; the oldest chimneys are located in the southwest portion of the field. Vent deposits on a steep, fault-bounded wall on the east side of the field are all <4 kyrs old, indicating that mass wasting in this region is relatively recent. Comparison of results to prior age-dating investigations of submarine hydrothermal systems shows that the LCHF is the most long-lived hydrothermal system known to date. It is likely that seismic activity and active faulting within the Atlantis Massif and the Atlantis Fracture Zone, coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site. The longevity of venting at the LCHF may have implications for ecological succession of microorganisms within serpentinite-hosted vent environments.     

Desert Pavement Disturbance and Artifact Taphonomy: A Case Study from the Eastern Libyan Plateau,Egypt

Accretionary desert pavements on the eastern Libyan Plateau of central Egypt support a rich Middle and Upper Paleolithic artifact assemblage exhibiting intensive blank production and minimal tool production. These assemblages appear to be in primary context with numerous examples of lithic refits showing on‐site lithic production. However, the smallest (length ≤2.5 cm) archaeological fragments are recovered at a much lower rate on this desert pavement surface than expected given comparable data from lithic assemblages in cave and shelter contexts in France. Excavation of archaeological contexts on the Libyan Plateau reveals the loss of small artifact fragments into the subsurface due to aeolian accumulation of silts, whereas geomorphic examination of desert pavement surfaces suggests a potential for relatively isolated bioturbation as a source of lateral and vertical disturbance of desert pavement surfaces over small areas. Archaeologists should be aware of the potential for long‐term assemblage stability as well as small artifact burial in surficial desert pavement contexts.     

Fulgurites: lightning strikes providing unique clues to palaeoenvironments

Strange in appearance and unique in origin, fulgurites are named for the Latin word for lightning. They are formed when lightning strikes rocks or sand that contains silica. They can form remarkably quickly… seconds as opposed to years or millennia. That in itself makes them remarkable, but they have other unusual qualities that make them even more amazing. Some hold ancient air within that can offer a window into past environments.     

High‐P tectono‐metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines,Italy

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly‐deformed basements, the loss of information on pre‐kinematic phases of mylonites hinders large‐scale correlations based on tectono‐metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)–temperature (T)–deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine‐grained K‐white mica and chlorite enveloping mica‐fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg‐rich cores and Mg‐poor rims. The amphibolite contains green amphibole+plagioclase+garnet+quartz+ilmenite defining S₁ with a superposed mylonitic fabric localized in decimetre‐ to centimetre‐scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P–T constraints that are linked to the reconstructed metamorphic‐deformational stages. For the first time an early high‐P stage at >11 kbar and 510°C was constrained, followed by a temperature peak at 550–590°C and 9–10 kbar and a retrograde stage (<475°C, <7 kbar), during which ductile shear zones developed. The inferred clockwise P–T–D path was most likely related to crustal thickening by continent‐continent collision during the Variscan orogeny. A comparison of this P–T–D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono‐metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.     

Religious Belief across ‘Post-secular’ Sydney: the multiple trends in (de)secularisation

Cities were once thought to be crucibles of secularisation, where the retreat from religion and its expression would be most pronounced. This decline in faith was linked to the heightened levels of heterogeneity and cosmopolitanism within cities. However, there is recent evidence of both the continuation and of the recession of the secularisation process in contemporary Western societies, with the mix and pace of these trends being quite unique to different countries. Significantly, this combination of processes has been argued to be part of a ‘post-secularist’ paradigm. Drawing on census data, this paper examines the spatial patterns of religious affiliation and non-belief in the Australian city of Sydney. Sydney has divergent geographies of faith and non-faith indicating the spatial multiplicities of religious belief. The decrease in Christianity in some areas of the city has been matched by an increase in non-Christian faiths, while in other areas there has been little change. Some parts of the city—especially those areas with a strong mix of affluence, diversity and cosmopolitanism—betray the strongest levels of secularisation and the retreat of faith. At least five geographies of faith and non-belief are recognisable within Sydney. While post-secularisation has been recognised as internationally uneven, this research demonstrates that it is also uneven across a world city like Sydney. The Sydney variations in (de)secularisation reflect city-based effects, including the historic and contemporary patterns of immigrant settlement, established and emerging religious communities, the segmented geographies of class and affluence, and the development of zones of cosmopolitanism. The findings point to the need for further research on the micro-geographies of religious belief and non-belief and community relations, and the on links between religious communities and civil society.     

Post-wildfire erosion response in two geologic terrains in the western USA

Volumes of eroded sediment after wildfires vary substantially throughout different geologic terrains across the western United States. These volumes are difficult to compare because they represent the response to rainstorms and runoff with different characteristics. However, by measuring the erosion response as the erodibility efficiency of water to detach and transport sediment on hillslopes and in channels, the erosion response from different geologic terrains can be compared. Specifically, the erodibility efficiency is the percentage of the total available stream power expended to detach, remobilize, or transport a mass of sediment. Erodibility efficiencies were calculated for the (i) initial detachment, and for the (ii) remobilization and transport of sediment on the hillslopes and in the channels after wildfire in two different geological terrains.The initial detachment efficiencies for the main channel and tributary channel in the granitic terrain were 10 ± 9% and 5 ± 4% and were similar to those for the volcanic terrain, which were 5 ± 5% and 1 ± 1%. No initial detachment efficiency could be measured for the hillslopes in the granitic terrain because hillslope measurements were started after the first major rainstorm. The initial detachment efficiency in the volcanic terrain was 1.3 ± 0.41%. The average remobilization and transport efficiencies associated with flash floods in the channels also were similar in the granitic (0.18 ± 0.57%) and volcanic (0.11 ± 0.41%) terrains. On the hillslope the remobilization and transport efficiency was greater in the volcanic terrain (2.4%) than in the granitic terrain (0.65%). However, this may reflect the reduced sediment availability after the first major rainstorm (30-min maximum rainfall intensity  90 mm h^− 1) in the granitic terrain, while easily erodible fine colluvium remained on the hillslope after the first rainstorm (30-min maximum rainfall intensity = 7.2 mm h^− 1) in the volcanic terrain. The erosion response in channels and on hillslopes of the granitic and volcanic terrains was similar when compared using erodibility efficiencies.