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High-velocity impact among icy planetesimals is a physical phenomenon important to the planetary evolution process in the outer Solar System. In order to study this phenomenon, impact experiments on water ice were made by using a two-stage light gas gun installed in a cold room (-10 o C) to clarify the elementary processes of collisional disruption and to study the reaccumulation and the escape...
Cometary nuclei are composed mostly of water ice and mineral grains. When a comet passes near the Sun, volatiles sublimate. This process leads to the formation of a dust layer covering the nucleus. Various other metamorphic processes are also predicted to modify the subsurface layer of the nucleus. Consequently, it should become stratified and more cohesive than initially. At present, because of preparations...
During the impact of Comet Shoemaker-Levy 9 fragment K on Jupiter observers detected aurora-like emissions near the impact region as well as in the other hemisphere at approximately magnetic conjugate positions equatorward of auroral latitudes. A number of generation mechanisms were suggested, but investigations of their significance have been hampered by a lack of knowledge about the jovian internal...
Spacecraft imaging data allow an accounting of the occurrence and characteristics of the largest craters on small asteroids and satellites. Data covering Phobos, Deimos, Gaspra, Ida, Mathilde, Vesta, Amalthea, Thebe, Janus, Epimetheus, Hyperion, and Proteus show that ~50% of rocky objects support craters with diameters of 1 object mean radius (R m ), and the great majority have more than two...
We present observations of 40 asteroids taken mostly from Table Mountain Observatory, mostly from 1982, but with a few observations from late 1981 and early 1983. Several new or substantially revised periods are reported. Perhaps most interesting are observations of two asteroids, 288 Glauke and 3288 Seleucus, which are in ''tumbling,'' or nonprincipal axis rotation, states. Glauke has the longest...
This paper reports on a series of laboratory impact experiments designed to provide basic data on how simulated Edgeworth-Kuiper belt objects (EKOs) fragment in an impact event. In September-October 1997 we carried out 20 low-velocity airgun shots at the Ames Vertical Gun Range into porous and homogeneous ice spheres using aluminum, fractured ice, and solid ice projectiles. We found that the porous...
We perform three-dimensional N-body integrations of the final stages of terrestrial planet formation. We report the results of 10 simulations beginning with 22-50 initial planetary embryos spanning the range 0.5-1.5 AU, each with an initial mass of 0.04-0.13M . Collisions are treated as inelastic mergers. We follow the evolution of each system for 2x10 8 years at which time a few...
The protoplanetary nebular analog to the planetary Rossby wave is developed. Linear dispersion relations are derived. It is found that the nebular Rossby wave propagates freely in the nebular azimuthal direction and slowly grows in the radial direction. Possible ramifications for the nebula are raised for the reverse transition of turbulence into Rossby waves and zonal jets via the Rhines mechanism...
Measurements of the densities of interplanetary dust particles and unweathered stone meteorites indicate that both have significant porosity on the microscopic scale. In addition, the chondritic stone meteorites are generally inhomogeneous, typically consisting of strong, millimeter-size, olivine chondrules embedded in a weaker, fine-grained matrix. Since target porosity is known to influence energy...
The isotopic enrichment in argon, krypton, and xenon, when trapped in water ice, was studied experimentally. The isotopes were found to be enriched according to their (m 1 /m 2 ) 1/2 ratio. These enrichment factors could be useful for comparison among the uncertain cosmic or solar isotopic ratios, the hopeful in situ cometary ratio, and those in Earth's atmosphere,...
Paleolakes in impact craters on Mars are characterized at global scale using the Viking Orbiter data. We identified 179 paleolakes in impact structures formed by the influx of water and sediment derived from valley networks and channels that can be classified into three different fluviolacustrine systems: closed, open, and lake-chain systems. We show the hydrogeologic implications for each of the...
We have investigated the creation and destruction of asteroid families using a collisional code which follows the evolution of the size distributions of both the main-belt background population and individual families which are produced by collisions within the population. Starting with a small-mass initial belt, the number of families produced by collisional breakup of parent bodies larger than 100...
The causes of Io's variegated surface, especially the roles of sulfur, and the geochemical history of sulfur compounds on Io are not well understood. Suspecting that minor impurities in sulfur might be important, we have investigated the major and trace element chemistry and spectroscopic reflectance of natural sulfur from a variety of terrestrial volcanic-hydrothermal environments. Evidence suggests...
The analysis of recent observational data has clarified several general properties of the collisional processes originating the asteroid dynamical families. However, a few puzzling physical problems remain open. According to the observations, the fragments are usually ejected at high velocities; the available theoretical models of catastrophic fragmentation, based on hydrodynamical simulations, do...
The PROGRA 2 experiment allows measurements under microgravity conditions of polarization phase curves for clouds of dust particles. The sizes that can be used range from a few micrometers to hundreds of micrometers; regular or irregular particles can be tested. Measurements obtained on compact particles of boron carbide and silicon carbide are presented. Results for the silicon carbide, obtained...
Local radiative-convective thermal structure is calculated on Jupiter and Saturn under the assumption that a low opacity zone exists near the 2000-K depth, as suggested by T. Guillot et al. (1994a, Icarus112, 337-353). The opacity is uncertain, and a simple but qualitatively accurate model is used to explore a range of opacities. Under the assumption of local radiative-convective equilibrium, insolation...
During the 1998 Jupiter-Sun conjunction, the 60-year-old white ovals BC and DE at 33 o south latitude merged into a larger oval ''BE'' at the same latitude (J. Lecacheux, P. Drossart, F. Colas, G. S. Orton, B. Fisher, A. Sanchez-Lavega, R. Hueso, J. F. Rojas, and I. Miyazaki, IAU Circular No. 6942). Here we report our study of the motions, morphology, and reflectivity changes pre and postmerger...
We present images of Saturn at wavelengths of 0.35, 2.0, 3.6, and 6.1 cm taken in 1990-1995. These include the first radio images of the planet's entire southern hemisphere, which is shown to be ~5% brighter than the northern at 6.1 and 2.0 cm, and possibly at 0.35 cm. The latitudinal brightness distribution varies substantially over time. The bright band at latitude 30 o N seen throughout...
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