Formation of the saturnian system: A modern Laplacian theory

A theory for the formation of Saturn and its family of satellites, which is based on ideas of supersonic turbulent convection applied to the original Laplacian hypothesis, is presented. It is shown that if the primitive rotating cloud which gravitationally contracted to form Saturn possessed the same level of turbulent kinetic energy as the clouds which formed Jupiter and the Sun, given by % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSqaaSqaai% aaigdaaeaacaaIYaaaaOGaaiikaiabeg8aYnaaBaaajea4baGaamiD% aaWcbeaakiaadAhadaqhaaqcKfaGaeaadaWgaaqcKjaGaeaacaWG0b% aabeaaaSqaaiaaikdaaaGccaGGPaGaeyypa0ZaaSqaaSqaaiaaigda% aeaacaaIYaaaaOGaeqOSdiMaeqyWdiNaam4raiaad2eacaGGOaGaam% OCaiaacMcacaGGVaGaamOCaaaa!4D3D!\[\tfrac{1}{2}(\rho _t v_{_t }^2 ) = \tfrac{1}{2}\beta \rho GM(r)/r\] where =0.1065 ± 0.0015, then it would shed a concentric system of orbiting gas rings each of about the same mass: namely, 1.0 × 10^–3 M _S. The orbital radii R _n (n = 0, 1, 2, ...) of these gas rings form a geometric sequence similar to the observed distances of the regular satellites. It is proposed that the satellites condensed from the gas rings one at a time, commencing with Iapetus which originally occupied a circular orbit at radius 11.4 R _S. As the temperatures of the gas rings T _n increase with decreasing orbital size according as T _n 1/R _n , a uniform gradient should be evident amongst the satellite compositions: Mimas is expected to be the rockiest and Iapetus the least rocky satellite. The densities predicted by the model coincide with the Voyager-determined values. Iapetus contains some 8% by weight solid CH₄. Titan is believed to be a captured satellite. It was probably responsible for driving Iapetus to its present distant orbit. Accretional time-scales and the post-accretional evolution of the satellites are briefly discussed.     

The acceleration of minor ion species in the solar wind

This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be trapped between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.     

Infrared extinction of heavy ion irradiated and amorphous olivine,with applications to interstellar dust

A smooth surface layer of highly disordered olivine, (Mg, Fe)₂SiO₄, has been produced by exposure of polished, natural olivine to a dose of 5×10¹⁶ cm^–2 of 1.5 MeV neon ions from a Van de Graaff accelerator. The dielectric functions of the disordered silicate in the wavelength range from 8 to 30 m have been determined from analysis of specular reflectance data, and extinction for Rayleigh particles of such disordered olivine has been calculated. Extinction measurements for amorphous olivine smoke collected on a substrate are also presented. The small particle extinctions of both kinds of structurally disordered olivine are shown to agree well with the main features of the absorption and emission spectra from interstellar grains in the 10 and 20 m region.     

Temperature fluctuations in interstellar dust grains

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The equilibrium temperature is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

The rotations of 128 Nemesis and 393 Lampetia: The longest known periods to date

Extensive photoelectric lightcurves of the asteroids 128 Nemesis and 393 Lampetia show for both objects extremely long rotational periods, the longest known to date for minor planets. For 393 Lampetia the combined results suggest with high probability a period of 38.7 hr with a maximal amplitude of 0.14 mag; a double-wave characteristic of the lightcurve must be assumed. For 128 Nemesis the complete double-wave lightcurve was observed and a period of 39.0 hr with a total amplitude of only 0.10 mag was deduced. Observations of 128 Nemesis confirm without doubt the presence of small-scale features of amplitude 0.01 to 0.02 mag, corresponding to small topographic features of about 15 km in height and width on the surface.     

On matching the spectrum of Io: Variations in the photometric properties of sulfur-containing mixtures

The problem of comparing laboratory spectra of sulfur-containing binary mixtures with the spectrum of Io is discussed. For the satellite, the observable is the geometric albedo as a function of wavelength, whereas in the laboratory one often measures some other type of albedo. In a previous paper we demonstrated that for pure sulfur the multiplicative factor which converts the laboratory albedos to geometric albedos can be strongly wavelength dependent. The present paper demonstrates that this is also true for binary sulfur-containing mixtures. Furthermore, there is no universal conversion factor applicable to all binary mixtures, nor can the factor be interpolated for a particular mixture from the conversion factors of the two end members. The conversion factor is a function not only of the specific composition of a binary mixture, but of the relative particle size distributions of the two components, and must be measured specifically for each individual sample if a quantitative comparison between a laboratory sample and Io's surface is desired.     

Preirradiation history of Djermaia (H) chondritic breccia

Djermaia is a unique case among gas-rich chondrites. Twelve light xenoliths studied so far in the two available stones show unequivocal evidences of large preirradiation effects, which stand out most clearly when both cosmic-ray track and spallogenic rare-gas data are considered and compared to data from chondrites with a simple radiation history. Each of the two stones studied displays a specific xenolith population with distinct morphological and preirradiation characteristics. The first population consists of rounded and more- or less-shocked xenoliths which show evidences of short solar-flare irradiation (<1 my) and a longer residence under heavier shielding (～50 g/cm²). The second population shows neither clear indication of surface erosion nor evidence of solar-flare effects, and its preirradiation occured in deep-seated locations (～150 g/cm²).The size distribution of xenoliths is also different for the two populations, indicative of more active reworking processes in the top ～20 cm of the Djermaia regolith than at a depth of ～50 cm. The amount of ²¹Ne_sp accumulated during the preirradiation stage is substantial (25–60% of total ²¹Ne_sp), corresponding to a formal preexposure time of ～15 my for both stones, assuming static irradiation conditions. These results give an insight into the dynamics of the upper levels of the H-asteroid regolith, which appears to be in good agreement with the model of Housen, Wilkening, Chapman, and Greenberg (Icarus, 1979, 39, 317–351).     

Baroclinic instabilities in Jupiter's zonal flow

The baroclinic stability of Jupiter's zonal flow is investigated using a model consisting of two continuously stratified fluid layers. The upper layer, containing a zonal shear flow and representing the Jovian cloudy regions above p ～ 5 bars, is the same as Eady's (1949) model for the Earth. The lower layer has a relatively large but finite depth with a quiescent basic state, representing the deep Jovian fluid bulk below p ～ 5 bars. Due to the presence of the lower layer, the linearized non-dimensional growth rates are drastically reduced from the O(1) growth rates of the original Early model. Only very long wavelengths relative to the upper fluid's radius of deformation L₁ are unstable. Eddy transports of heat are also reduced relative to estimates based on scaling arguments alone. Since the hydrostatic approximation for the lower-layer perturbation breaks down at great depths, a second model is presented in which energy propagates downward in an infinitely deep lower fluid obeying the full linearized fluid equations. In this model, the growth rates are again very small, but now all wavelengths are unstable with maximum growth rates occurring for wavelengths O(1) relative to L₁. These results illustrate the importance for the upper-layer meteorology of the interface boundary condition with the lower fluid, which is radically different from the rigid lower boundary of the Earth's troposphere.     

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km² and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km.