Radar and optical observations and physical modeling of near-Earth Asteroid 10115 (1992 SK)

We estimate Asteroid 1992 SK's physical properties from delay-Doppler images and Doppler-only echo spectra obtained during March 22-27, 1999, at Goldstone and from optical lightcurves obtained during February-March 1999 at Ond?ejov Observatory. The images span only about 15° of sky motion and are not strong, but they place up to twenty 40 m by 160 m pixels on the asteroid and have complete rotational phase coverage. Our analysis establishes that the radar observations are confined to subradar latitudes between −20° and −40°. The echo spectra and optical lightcurves span ∼80° of sky motion, which provides important geometric leverage on the pole direction. The lightcurves are essential for accurate estimation of the asteroid's shape and spin state. We estimate the asteroid's period to be 7.3182±0.0003 h and its pole direction to be at ecliptic longitude, latitude=(99°±5°,−3°±5°). The asteroid is about 1.4 km in maximum extent and mildly asymmetric, with an elongation of about 1.5 and relatively subdued topography. The OC radar albedo is 0.11±0.02 and the SC/OC ratio is 0.34±0.05. The current orbital solution permits accurate identification of planetary close approaches during 826-2690. We use our model to predict salient characteristics of radar images and optical lightcurves obtainable during the asteroid's March 2006 approach.     

Wind erosion at the dry-up bottom of Aiby Lake

Clay-rich deposits are usually considered as hard materials to be eroded by wind. Data from both surface monitoring and field survey at the dry-up bottom of Aiby Lake present that clay-rich lacustrine deposits are easily broken down and eroded away by wind in the seasonal alternation process under the natural arid environment, and are the significant source of air dust. The surface of the clay-rich deposits is broken and softened by the freezing-and-thawing action in winter season and/or by salt and alkali action with precipitation. Impact of wind-input particles and plow of plant branches with wind force drive the clay-rich sediments moving. Wind picks up the clay pebbles and repeats the impaction further-ward onto the dry-up surface. Tremendous fine materials, including soft salts, are contributed to air dust, and transported in long distance.

     

The role of arsenic-bearing rocks in groundwater pollution at Zimapán Valley, México

 Interaction of groundwater with As-bearing rocks has been proposed as one of three main sources of arsenic at Zimapán valley, México. The complexity of the geology and hydrogeology of the valley make it difficult to identify the natural causes of arsenic poisoning. Samples from the different rock outcrops and water from wells tapping various rock formations were analyzed. The rocks from mineralized areas contained higher concentrations of arsenic with respect to the same formations in non-mineralized areas. The arsenic minerals arsenopyrite, scorodite, and tennantite were identified in some rock samples. Higher temperature and lower Eh values were found for those wells containing more arsenic. The physicochemical characteristics of these naturally polluted well waters could be produced by arsenopyrite oxidation. The geochemical model PHREEQCI was used to perform the inverse modeling of two wells located along the same fault. Arsenopyrite oxidation and scorodite dissolution appear to be the geochemical processes producing the natural pollution according to the model. The release and transport of arsenic mainly occur through fractures within the cretaceous limestones where the most productive wells are drilled. The presence of arsenic should be expected also in other formations near mineralized zones in the Zimapán Valley. Field determinations of Eh and T could be used to detect potentially polluted wells. Received: 29 April 1999 / Accepted: 18 July 2000     

Application of a solute transport model under variable velocity conditions in a conduit flow aquifer: Olalde karst system, Basque Country, Spain

A model based on numerical solutions, which allows for solving the dispersion equation under variable recharge and velocity conditions, is developed to simulate solute transport in conduit flow aquifers during flow recession periods. As an example, the evolution of a tracer in the little known karst conduit that links the sinking stream of Oma valley to the Olalde spring is investigated in the karstic region of Santa Eufemia-Ereñozar (Basque Country, Spain). The model, with different hypothetical structures, allows for obtaining series of tracer breakthrough curves, which are fitted to experimental data using an optimization algorithm. These results, although they can be used to simulate the tracer evolution between the two points considered, do not allow for determining the internal structure and spatial disposition of contributions in the aquifer.     

Luminosity profiles and the contraction of radiating spheres in general relativity

A semi-numeric method by Herrera, Jiménez and Ruggeri is used to study the evolution of general relativistic spheres in the free streaming radiation approximation when specific contraction schemes are imposed and for particular oscillatory radiation profiles. On one hand, the emerging radiation is studied for oscillating contracting spheres, on the other, the variation of the boundary surface is analyzed when a pulsating profile is provided. We considered three particular equations of state previously studied. It is found that the oscillatory frequency of the surface for the equations of state considered coincide with the frequency of the oscillation for the radiation profile.     

On the Thermochemical Instability of Optically Thin Plasmas

The effects of the thermal conduction on the linear stability analysis of an optically thin plasma with metallicity Z = 1 and including chemical relaxation time effects, is carried out. Generally, a further stabilizing effect on the unstable modes and an increasing of the damping of stable modes appear for disturbances with values of wave number large enough. However, the oscillating mode appearing at low temperature remain unaffected by the thermal conduction.     

Astrometric positions of comets Halley and Giacobini-Zinner obtained at Fabra Observatory

We present 190 astrometric positions of comet Halley and 46 positions of comet Giacobini-Zinner obtained at Fabra Observatory during the 1985/1986 campaign by the astrometric network of the International Halley Watch.