CCD imaging polarimetry of Comet 2P/Encke

We present results and analysis of imaging polarimetric observations of Comet 2P/Encke. The observations were carried out at the 2-m RCC telescope of the Bulgarian National Astronomical Observatory on December 13, 1993 and on January 14, 1994, at phase angles 51.1° and 80.5°, respectively. A wide-band red filter 6940/790 Å was used. This filter is transparent for the continuum and the weak emission bands of NH₂ and H₂O⁺. There is a sunward dust fan with well-defined polarization, which peaks at≈13% in the image obtained on January 14, 1994. Along the sunward fan the degree of polarization decreases progressively. Outside of the fan the coma displays a low polarization of ≈3%. We suggest that this low polarization is caused by the NH₂ emission in the pass-band of the red wide-band filter. Assuming a spherically symmetric NH₂ coma we are able to correct the observed polarization for this effect. The correction leads to an increase of the observed polarization by 1 to 4% at distances 10,000 and 1500 km from the nucleus. A rough estimate shows that the polarization in the near nucleus region of Comet Encke is similar to that for the dusty comets. Even after correction the polarization of Comet Encke's dust fan is significantly less that the polarization observed in dusty comets. The reasons influencing the distribution of dust polarization in the coma are discussed. More polarimetric and colorimetric observations of the dust in Comet Encke on its return in 2003 are needed.     

A distinct class of avalanche scars on Mars

We report observations of a set of surface features on Mars that form a distinct class of avalanche scars. These features have a horizontal scale of hundreds of meters, but a depth scale of meters distinguishes them from the shallower features known as slope streaks. The meters-thick avalanche scars have escaped previous attention because of weak contrast between the interiors of the scarred regions and their surroundings. Often the most visible feature is a shadow cast by the trough wall, a band 1-3 pixels wide in Mars Orbiter Camera narrow angle images, indicating maximum depths of 4-10 m. We investigate the morphology of more than 500 such features. Slopes upon which the avalanches occur average about 27°. Impact craters are seen at the heads of some avalanche scars; this subset exhibits statistically wider opening angles. The scars span an estimated several Ma in age. Those found so far occurred mainly in the Olympus Mons lower aureole. We compare shapes of slope streaks to shapes of meters-thick avalanches, and the results support the notion that the two classes are distinct. The newly-discovered avalanches resemble some terrestrial flows of loose, dry material such as dry snow and glass beads. On the basis of these analogs, we suggest a physical model.     

Impact-generated dust clouds surrounding the Galilean moons

Tenuous dust clouds of Jupiter's Galilean moons Io, Europa, Ganymede and Callisto have been detected with the in-situ dust detector on board the Galileo spacecraft. The majority of the dust particles have been sensed at altitudes below five radii of these lunar-sized satellites. We identify the particles in the duut clouds surrounding the moons by their impact direction, impact velocity, and mass distribution. Average particle sizes are between 0.5 and 1 μm, just above the detector threshold, indicating a size distribution with decreasing numbers towards bigger particles. Our results imply that the particles have been kicked up by hypervelocity impacts of micrometeoroids onto the satellites' surfaces. The measured radial dust density profiles are consistent with predictions by dynamical modeling for satellite ejecta produced by interplanetary impactors (Krivov et al., 2003, Planet. Space Sci. 51, 251-269), assuming yield, mass and velocity distributions of the ejecta from laboratory measurements. A comparison of all four Galilean moons (data for Ganymede published earlier; Krüger et al., 2000, Planet. Space Sci. 48, 1457-1471) shows that the dust clouds of the three outer Galilean moons have very similar properties and are in good agreement with the model predictions for solid ice-silicate surfaces. The dust density in the vicinity of Io, however, is more than an order of magnitude lower than expected from theory. This may be due to a softer, fluffier surface of Io (volcanic deposits) as compared to the other moons. The log-log slope of the dust number density in the clouds vs. distance from the satellite center ranges between −1.6 and −2.8. Appreciable variations of number densities obtained from individual flybys with varying geometry, especially at Callisto, are found. These might be indicative of leading-trailing asymmetries of the clouds due to the motion of the moons with respect to the field of impactors.     

Preliminary analysis of the MER magnetic properties experiment using a computational fluid dynamics model

Motivated by questions raised by the magnetic properties experiments on the NASA Mars Pathfinder and Mars Exploration Rover (MER) missions, we have studied in detail the capture of airborne magnetic dust by permanent magnets using a computational fluid dynamics (CFD) model supported by laboratory simulations. The magnets studied are identical to the capture magnet and filter magnet on MER, though results are more generally applicable.The dust capture process is found to be dependent upon wind speed, dust magnetization, dust grain size and dust grain mass density. Here we develop an understanding of how these parameters affect dust capture rates and patterns on the magnets and set bounds for these parameters based on MER data and results from the numerical model. This results in a consistent picture of the dust as containing varying amounts of at least two separate components with different physical properties.     

FT-IR microspectroscopy of extraterrestrial dust grains: Comparison of measurement techniques

Identification of astronomical dust composition rests on comparison of Infrared (IR) spectra with standard laboratory spectra; frequently, however, a single mineralogical composition is assumed for spectral matching. Advances in laboratory instrumentation have enabled very precise IR spectra to be measured on single grains and zones within grains; with a more complete set of spectral data for planetary dust, better compositional matches will be achieved for astronomical dust. We have compared several FT-IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and powders and ATR spectroscopy of thin sections) to determine their utility for the direct measurement of the mid-IR spectra of small amounts of extraterrestrial grains. We have focussed our investigation on the spectra of the olivine series of silicates, (Mg,Fe)₂SiO₄, a species frequently identified as one of the major constituents of interstellar dust. The positions of three characteristic SiO₄ stretching bands at ∼10.4, 11.3 and 12 μm were measured for comparison of the techniques. All methods gave satisfactory results, although care must be taken to guard against artefacts from sample thickness and orientation effects. Single grains hand-picked from meteorites can be analysed, but results are inaccurate if the grain size is too large (>1-10 μm). Spectra for single grains also show variations that arise from sample orientation effects. Once the analytical artefacts are taken into account, we found that measurement of powder with a diamond compression cell is best suited for the analysis of small amounts of materials.     

Sintering of highly porous silica-particle samples: analogues of early Solar-System aggregates

I describe a new method to make particle layers which consist of SiO₂ spheres with 0.78 μm radius. The layers were produced by sedimentation of aggregates which had grown in ballistic particle collisions, and the layers had a porosity of 0.95. They were used for experiments on sintering, i.e., the samples were heated in an oven at varying temperatures and heating durations, and the samples were analyzed by scanning electron microscopy. Based on the change of particle diameter, surface diffusion sintering and viscous flow are identified as important transformation mechanisms. The first effect dominated at the start of restructuring and the latter at higher temperatures. The neck growth of adjacent particles was fitted to a surface diffusion sintering model and predicts neck radii as a heating temperature and duration function. Between the temperature range of neck formation and of melting, further restructuring occurred which lead to dissolution of particulate structure and to densification and which resulted in a porous object consisting of straight elongated substructures which connected kinks of higher material density. The thermal transformation is important for the change of strength, collisional behavior, light-scattering properties, and thermal conductivity with relevance to dust aggregates, planetesimals, comets, interplanetary dust particles, and regolith-covered celestial bodies.     

The Dust Coma of Comet C/1999 S4 (Linear)

Comet C/1999 S4 (LINEAR) showed a very special behaviour between 28 June and1 July 2000. Optical observations of the dust coma in two distinct continuum bandsrevealed that it changed morphologically as well as in colour. The two-dimensionalcoma morphology indicates a splitting of the nucleus which probably occurred shortlybefore the observations of 28 June 2000. The distribution of the dust particles in sunand tail direction reflected by the slopes of the radial profiles indicate the presence ofa considerable amount of disintegrating dust particles in the sunward hemisphere andan overabundance of dust, reflecting at 440 nm, within the first 18,000 km of the dusttail. The spatial profiles of the (BC–RC) colour index in sun direction are distinctly different on 28 June and 1 July, indicating the production of a large amount of particles observable in blue continuum after 28 June.     

An iras atlas of ysos in nearby dark clouds

We are using the newIRAS Sky Survey Atlas to construct a catalog of Galactic dark clouds and their associated Young Stellar Objects (YSOs). FromIRAS 100 and 60µm data we calculate images of the 100µm optical depth distribution in the clouds. Using theIRAS Faint Source Catalog we identify sources which have colors indicative of YSOs and have positions that lie inside cloud boundaries in the optical depth images. Here we present some preliminary results of applying this analysis to the Chamaeleon dark cloud complex.     

近10a中国沙尘气溶胶研究进展

沙尘气溶胶对全球气候变化的影响是当前国内外研究的热点问题。我国学者对沙尘气溶胶也开展了广泛的研究,尤其是在近10年,科研工作者对沙尘气溶胶的物理化学特性及其对气候的影响进行了重点研究。本文主要对沙尘气溶胶的物理特性（浓度、粒径谱分布、垂直分布特征及光学特性）、化学特性以及气候效应等方面的研究进行了综述,并对今后沙尘气溶胶研究中所面临的科学问题进行了展望。