Galilean satellites: 1976 radar results

Radar observations of the Galilean satellites, made in late 1976 using the 12.6-cm radar system of the Arecibo Observatory, have yielded mean geometric albedos of 0.04 ± , 0.69 ± 0.17, 0.37 ± 0.09, and 0.15 ± 0.04, for Io, Europa, Ganymede, and Callisto, respectively. The albedo for Io is about 40% smaller than that obtained approximately a year earlier, while the albedos for the outer three satellites average about 70% larger than the values previously reported for late 1975, raising the possibility of temporal variation. Very little dependence on orbital phase is noted; however, some regional scattering inhomogeneities are seen on the outer three satellites. For Europa, Ganymede, and Callisto, the ratios of the echo received in one mode of circular polarization to that received in the other were: 1.61 ± 0.20 1.48 ± 0.27, and 1.24 ± 0.19, respectively, with the dominant component having the same sence of circularity as that transmitted. This behavior has not previously been encountered in radar studies of solar system objects, whereas the corresponding observations with linear polarization are “normal.” Radii determined from the 1976 radar data for Europa and Ganymede are: 1530 ± 30 and 2670 ± 50 km, in fair agreement with the results from the 1975 radar observations and the best recent optical determinations. Doppler shifts of the radar echoes, useful for the improvement of the orbits of Jupiter and some of the Galilean satellites, are given for 12 nights in 1976 and 10 nights in 1975.     

Radar and photoelectric observations of asteroid 2100 Ra-Shalom

Results of 13-cm-wavelength radar observations and V-filter photoelectric observations of Ra- Shalom during its 1981 Aug–Sep apparition are reported. The radar data yid detections of echoes in the same sense of circular polarization as transmitted (i.e., the SC sense) as well as in the opposite (OC) sense. The estimate of the ratio of SC to OC echo power, μ_c = 0.14 ± 0.02, indicates that most, but certainly not all, of the backscattering is due to single reflections from surface elements that are fairly smooth at decimeter scales. The value obtained for the OC radar cross section on Aug 26 (1.2 ± 0.3 km²) is about three times larger than those obtained on Aug 23, 24, and 25. The echo bandwidth appears to be within about 1.5 Hz of 5.0 Hz on each date. The photoelectric data suggest a value, P_syn = 19.79 hr, for the synodic rotation period, and yield a composite lightcurve with two pairs of extrema. Combining this value for P_syn with a firm lower bound (4 Hz) on the maximum echo bandwidth yields a lower bound of 1.4 km on the maximum distance between Ra-Shalom's spin axis and any point on its surface.     

Instruments, Detectors and the Future of Astronomy with Large Ground Based Telescopes

Results of a survey of instrumentation and detector systems, either currently deployed or planned for use at telescopes larger than 3.5 m, in ground based observatories world-wide, are presented. This survey revealed a number of instrumentation design trends at optical, near, and mid-infrared wavelengths. Some of the most prominent trends include the development of vastly larger optical detector systems (> 10⁹ pixels) than anything built to date, and the frequent use of mosaics of near-infrared detectors – something that was quite rare only a decade ago in astronomy. Some future science applications for detectors are then explored, in an attempt to build a bridge between current detectors and what will be needed to support the research ambitions of astronomers in the future.     

A study of the diurnal variation in the thermosphere as derived by satellite drag

An analysis was performed on 29,574 densities derived from the drag of 10 satellites to determine simultaneously the parameters of the solar-activity effect in the thermosphere on the one hand, and the amplitude and shape of the diurnal-variation curve on the other. This paper reports on the study of the diurnal variation only.Although a considerable amount of smoothing is inherent in the drag method, it seemed useful to see whether we could detect any change in the shape of the diurnal-variation curve with height, latitude and solar activity. None was detected: the curve remains remarkably stable, symmetric, with a maximum at 14 hr 20 min L.S.T. and a minimum at 2 hr 20 min L.S.T. A systematic variation of the temperature range with height is observed when static models are used to derive it.     

Dissolved chromium in the St. Lawrence estuary

A study of dissolved chromium in the St. Lawrence estuary and Gulf of St. Lawrence has been carried out. A chromium concentration of 0·7 μg l^?1 was found in the St. Lawrence River. In the turbidity maximum of the upper estuary, chromium is removed from solution onto fine-grained resuspended sediments and internally produced organic-rich floccules. A simple flux calculation shows that these processes remove ～50% of the total dissolved chromium input of the river. At salinities greater than 5‰ dissolved chromium mixes conservatively.     

A skeletal assemblage classification system for non-tropical carbonate deposits based on New Zealand Cenozoic limestones

Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) BARNAMOL = barnacle/bivalve-dominated; (b) BIMOL = bivalve-dominated; (c) BRYOMOL = bryozoan/bivalve-dominated; (d) ECHINOFOR = echinoderm/benthic foraminiferal-dominated; (e) NANNOFOR = nannofossil/planktonic foraminiferal-dominated; (f) RHODALGAL = calcareous red algal-dominated; and (g) RHODECHFOR = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.     

GPS monitoring of vertical ground motion in northern Ardenne–Eifel: five campaigns (1999–2003) of the HARD project

We present the HARD project of GPS monitoring of vertical ground motion in NE Ardenne and Eifel (western Europe). Its main purposes are to get a better insight into the present-day rates of vertical ground motion in intraplate settings and to identify the various causes of these motions. Since 1999, we have carried out yearly campaigns of simultaneous GPS measurements at 12 sites situated so as to sample the different tectonic subunits of the study area and especially to record potential displacements across the seismogenic Hockai fault zone. Five campaigns (1999–2003) have been processed currently. Key issues of the data processing with the Gamit software are discussed and first results are presented. Though temporally consistent in many cases, the obtained vertical motion rates are spatially highly variable. They are also much too high (several mm/year) to support a tectonic interpretation, and a long-term influence of groundwater level variations is proposed to account for the observed motions. This influence should be distinguished from seasonal variations and from inter-survey variations linked to the varying degree of soil and subsoil drying off during the successive spring surveys.     

Radon transfer from groundwater used in showers to indoor air

Estimation of Rn transfer from water to indoor air based on multi-day measurements may underestimate alpha exposure that occurs at short time scales in confined spaces, such as from showering, in houses with high Rn activities in the water supply. In order to examine one such incremental increase in exposure, variations in Rn in water and indoor air in 18 houses with private wells in western North Carolina (USA) were investigated. Radon in well water ranged from 158 to 811 Bq L⁻¹ (median 239 Bq L⁻¹). After 20-min showers in bathrooms with closed doors, peak Rn in air increases (above background) ranged from 71 to 4420 Bq m⁻³ (median 1170 Bq m⁻³). Calculated transfer coefficients at the scale of a 40-min closed bathroom (20-min shower plus 20 min post-shower) are described by a lognormal distribution whose geometric mean exceeds the widely-used ∼10⁻⁴ whole-house transfer coefficient by about one order of magnitude. As short-lived decay products grow from shower-derived Rn, short-term alpha energy exposure occurs in bathrooms in addition to the exposure caused by Rn mixed throughout the volume of the house. Due to the increasing ratio of Rn decay products to Rn, alpha energy exposure is greatest several minutes after the shower is turned off. For a 7.2-min shower with 10 min of additional exposure before opening the door, a geometric mean 5.6% increase in exposure over the ∼10⁻⁴ whole-house transfer coefficient derived from longer measurement periods was estimated. In addition to Rn activity in water, short-term shower exposure to Rn progeny depends on exposure time, ventilation, attachment and deposition, among other variable factors that characterize individual houses and residents.     

A new map of glacigenic features and glacial landsystems in central mainland Nunavut,Canada

A new digital map of glacial geomorphic features and interpreted glacial landsystems was produced for an area covering ~415 000 km² in the Keewatin Sector of the Laurentide Ice Sheet (LIS) in Nunavut. The map integrates information from previous surficial geology maps and >14 000 field stations, and is significantly improved by the detailed inventory of ~152 000 glacigenic features using high-resolution ArcticDEM data and Landsat 8 imagery. From this, we identify and map coherent patterns of landform development (landsystems) between the Manitoba border and the Arctic coast, many of which are entirely new and others that are significantly modified or updated. In particular, we recognize six separate ice streams, including one probable remnant ice stream, and we delineate numerous palimpsest streamlined landscapes with associated ice-flow trends and relative ages. A continuum of relict terrains with varying basal ice thermal conditions is mapped for the first time in the ice divide migration zone between Baker Lake and Wager Bay. In addition, deglacial cold-based retreat terrains and preserved warm-based landscapes unaffected by younger glacial events have been identified. These new georeferenced, multi-scale data sets and interpreted glacial landsystems provide a comprehensive framework to strengthen reconstructions of the glacial history and dynamics of one of the largest ice domes of the LIS, identify distinct glacial sediment transport paths for applications to mineral exploration, and test numerical modelling of the LIS in support of climate change studies and long-term evolution of modern ice sheets.     

Endolithic biodegradation of cool-water skeletal carbonates on Scott shelf, northwestern Vancouver Island, Canada

Biodegradation of shell material is widespread in the cool-water skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island, and is especially evident in the large aragonitic bivalves, Glycymeris and Humilaria, major primary sediment contributors. Ten types of endolithic microborings have been identified in the shells, including representatives of green algae (e.g., Ostreobium quekettii), blue-green algae (e.g., Plectonema terebrans, ?Scytonema sp.), fungi, bacteria and clionid sponges, as well as macroborings of phoronids, polychaetes and naticid gatropods. Microcrystalline carbonate is not precipitated in vacated bores. Boring physically weakens the shells, rendering them more prone to mechanical abrasion during sediment transport and bioturbation, and to biological abrasion by grazing benthos. Tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells, and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains. Boring also increases significantly the porosity and surface area of skeletal grains, and destroys their organic matrix, making them susceptible to maceration and dissolution on cool-water shelves. Fostered by the generally low rates of carbonate production and accumulation, many aragonitic bivalve shells on Scott shelf have become thoroughly degraded through a combination of endolithic microboring, maceration and dissolution within about 1000 years in ambient sea water. In geologic terms, such selective taphonomic loss of skeletal material may be considerable in ancient temperate-shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments.