Orbits of Saturn's F ring and its shepherding satellites

The shape and orientation of Saturn's F ring and the orbits of its two shepherding satellites have been determined from Voyager images. The data and processing are described, and orbital parameter estimates and associated uncertainties are presented. In addition, evidence that suggests that the F-ring braids are formed very near the conjunctions of the shepherding satellites is presented.     

Limb darkening of meteorites and asteroids

This paper presents the results of a laboratory study of the limb darkening, near opposition, of the carbonaceous chondrites Orgueil (C1), Murchison (C2), and Allende (C3), the ordinary chondrite Bruderheim (L6), and a stainless-steel powder. These materials represent possible analogs for the surface materials of C, S, and M asteroids respectively. At low phase angles, the limb-darkening behavior of all materials studied is well represented by Minnaert's law. For carbonaceous chondrites, the Minnaert limb-darkening parameter k is nearly independent of wavelength for wavelengths between 0.4 and 0.9 μm, with a typical value of k = 0.55. The reflectance parameter, B₀, varies from 0.045 to 0.065 over the same range of wavelengths. Both k and B₀ are larger for the stainless-steel powder and the ordinary chondrite, due to the increased importance of multiple scattering in the surface layer. If no limb darkening were present, k would equal $\text{1}\text{2}$ and the geometric albedo (p) of an asteroid would equal the normal reflectance $\text{(r}{}_{\mathrm{<mtext>n</mtext>}}\text{? B}{}_0\text{)}$ of its surface material. For bodies whose surface material is appreciably limb darkened, the geometric albedo measured at the telescope will be lower than the true normal reflectance of surface material; we estimate that for S and M objects r_n ? 1.05 p. In the case of nonspherical asteroids, because the distribution of incidence and emission angles varies as the asteroid rotates, the geometric albedo must change with aspect. If limb darkening is not considered when interpreting asteroid light curves, the values of b/a derived will be too extreme. This effect is probably too small to be observed for C asteroids, because of their intrinsically low reflectances, but could be appreciable for S and M objects.     

Production and condensation of organic gases in the atmosphere of Titan

The rates and altitudes for the dissociation of atmospheric constituents of Titan are calculated for solar UV, solar wind protons, interplanetary electrons, Saturn magnetospheric particles, and cosmic rays. The resulting integrated synthesis rates of organic products range from 10²–10³ g cm^?2 over 4.5 × 10⁹ years for high-energy particle sources to 1.3 × 10⁴ g cm^?2 for UV at $\text{λ < 1550}\text{A}\text{?}$, and to 5.0 × 10⁵ g cm^?2 if $\text{λ > 1550}\text{A}\text{?}$ (acting primarily on C₂H₂, C₂H₄, and C₄H₂) is included. The production rate curves show no localized maxima corresponding to observed altitudes of Titan's hazes and clouds. For simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at 2825 km. Such condensates comprise the principal cloud mass, with molecules of greater complexity condensing at higher altitudes. The scattering optical depths of the condensates of molecules produced in the Titanian mesosphere are as great as ～ 10²/(particulate radius, μm) if column densities of condensed and gas phases are comparable. Visible condensation hazes of more complex organic compounds may occur at altitudes up to ～ 3060 km provided only that the abundance of organic products declines with molecular mass no faster than laboratory experiments indicate. Typical organics condensing at 2900 km have molecular masses = 100–150 Da. At current rates of production the integrated depth of precipitated organic liquids, ices, and tholins produced over 4.5 × 10⁹ years ranges from a minimum ～ 100 m to kilometers if UV at $\text{λ > 1550}\text{A}\text{?}$ is important. The organic nitrogen content of this layer is expected to be ～ 10^?1?10^?3 by mass.     

Saturn's electrostatic discharges: Properties and theoretical considerations

The properties of Saturn's electrostatic discharges (SED) as observed by the Voyager Planetary Radio Astronomy experiment during the two Voyager encounters with Saturn are summarized. Several models for the formation of SED are discussed in light of these observations. The most likely source regions appear to be either the equatorial zone of the planet or the dense part of the B ring near 1.80 R_s. The strenghts and weaknesses of each of these possibilities are examined. Neither possibility accounts fully for the observed SED properties in a simple way. A search for an anomaly near 1.80 R_s in the data of other experiments aboard Voyager has been carried out, and at least one and possibly more such experiments do indeed obtain anomalous data at this point in the ring system. There thus appears to be unexplained phenomena at this point, independent of the PRA data, and it is a short step to postulate that a single object may be the cause of all such phenomena.     

On the ambiguity of rotational periods of asteroids: The peculiar case of 52 Europa

Recent results support a possible ambiguity for several adopted values of the spin rates. An analysis of the present data set on the rotational properties is made in order to select a number of asteroids, for which new observations should be performed to check the reliability of their adopted parameters. The most significant example is presented and it concerns the very large asteroid 52 Europa. Its lightcurve presented two maxima and two minima during 1976 opposition leading to a period of 11^h.26. In 1983 the same number of extrema appeared in 5^h.63 only. The amplitude was small in both oppositions (～0^m.08 and ～0^{m.10, respectively)} and suggests the possibility of surface irregularities which dominate the light variation. Other hypotheses can be made but generally the period determination of asteroids is more difficult than previously believed.     

Saturn's icy satellites: Thermal and structural models

Thermal histories of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus are constructed by assuming that they formed as homogeneous ice-silicate mixtures. The models include effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithospheric growth. Accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating. Mimas is so small that its thermal evolution is essentially purely conductive; at present it is a cold, nearly isothermal body. Any subsolidus convection or thermal activity in Mimas would have been confined to a brief period in its early history and would have been due to a warm formation. The four largest satellites are big enough and contain sufficient heat-producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions. Dione and Rhea have convective interiors for most of their thermal histories, while Tethys and Iapetus have mainly conductive thermal histories with early periods of convective 0activity. The thermal histories of the five satellites for the last 4 by are independent of initial conditions; at present they have cold, conductive interiors. The model thermal histories are qualitatively consistent with the appearances of these satellites: Mimas has an ancient heavily cratered surface, Tethys and probably Iapetus have both heavily cratered and more lightly cratered areas, and Dione and Rhea have extensively modified surfaces. Because of their similar sizes and densities, Mimas and Enceladus are expected to have similar surfaces and thermal histories, but instead Enceladus has the most modified surface of all the small icy Saturnian satellites. Our results suggest a heat source for Enceladus, in addition to radiogenic and accretional heating; tidal dissipation is a possibility. Because the water ice in these bodies does not melt, resurfacing must be accomplished by the melting of a low-melting-temperature minor component such as ammonia hydrate.     

Structure of the 3:1 Jovian resonance: A comparison of numerical methods

The motion of asteroids near the 3:1 Jovian resonance in the restricted planar case is studied using three numerical methods: (a) integrating the full equations of motion, (b) integrating the averaged equations of motion, and (c) using an algebraic mapping recently developed by Wisdom (1982, Astron. J.87, 577–593). The relative merits of each method are investigated. It is concluded that in the regular regions of the phase space, methods b and c give excellent agreement with each other and that provided the maximum eccentricity e_max < 0.4 differences with the exact solution (method a) are <6% in e_max and <27% in the period of the oscillations. The additions of higher order terms in the expansion of the averaged Hamiltonian provides marginally better agreement with the full integration. This is probably due to the slow convergence of the expansion of the disturbing function at large eccentricities (e > 0.3). In chaotic regions of the phase there is little agreement between the orbital elements at any given time calculated by each method. However, all methods reflect the qualitative behavior of the chaotic trajectories and give good agreement on the bounds of the motion. Since the map is at least 200 times faster than solving the full equations of motion it is an efficient method of rapidly exploring accessible regions of the chaotic phase space.     

Saturn meteorology: A diagnostic assessment of thin-layer configurations for the zonal flow

Voyager imaging, infrared, and radio observations for Saturn have been recently interpreted by Smith et al. (1982) as an indication that the jet streams observed at the cloud tops extend to depths greater than the 10⁴-bar level. This analysis assumes a maximum latitudinal temperature contrast of a few percent, a mean atmospheric rotation rate at depth given by Saturn's ratio period, and no variation with latitude of the bottom pressure level for the zonal flow system. These assumptions are not, however, firmly constrained by observation. The diagnostic analysis of plausible alternative configurations for Saturn's atmospheric structure demonstrates that a thin weather layer system (confined at mid to high latitudes to levels above 200 bar) cannot be excluded by any of the available observations. A quantitative estimate of the effects of moisture condensation (including the differentiation of mean molecular weight) suggests that these might provide the buoyancy contrasts necessary to support a thin-layer flow provided that Saturn's outer envelope is enriched approximately 10 times in water abundance relative to a solar composition atmosphere and strongly differentiated with latitude at the condensation level.     

Accumulation of planetesimals in the solar nebula

Characteristic time scales relevant to the accumulation of planetesimals in a gaseous nebula are examined and the accumulation toward the planets is simulated by numerically solving a growth equation for a mass distribution function. The eccentricity and inclination of planetesimals are assumed to be determined by a balance between excitation due to mutual gravitational scattering and dissipation due to gas drag. Two kinds of mass motion in the radial direction, i.e., diffusion due to mutual scattering and inward flow due to gas drag, are both taken into account. The diffusion is shown to be effective in later stages with a result of accelerating the accumulation. As to the coalescent collision cross section, the usual formula for a binary encounter in a free space is used but the effect of tidal disruption which increases substantially the cross section is taken into account. Numerical results show that the gravitational enhancement factor (i.e., the so-called “Safronov number”), contained in the cross section formula, always takes a value of the order of unity but the accumulation proceeds relatively rapidly owing to the effects of radial diffusion and tidal disruption. That is, a proto-Earth, a proto-Jupiter, and a proto-Saturn with masses of 1×10²⁷ g are formed in 5×10⁶, 1×10⁷, and 1.6×10⁸ years, respectively. Also, a tentative numerical computation for the Neptune formation shows that a proto-Neptune with the same mass requires a long accumulation time, 4.6×10⁹ years. Finally, the other effects which are expected to reduce the above growth times further are discussed.     

The retention of spallation products in interstellar grains

Stimulated by recent studies indicating the possible survival of presolar grains in certain meteorites, the importance of recoil loss for the retention of spallation products in submicrometer-sized interstellar dust during irradiation by high-energy cosmic ray protons has been calculated. The model presented incorporates range straggling effects, a realistic distribution of interstellar grain sizes, and utilizes an accurate theoretical formalism for the fragmentation recoil momenta. Apart from an only vague understanding of possible grain composition, the greatest uncertainty concerns the intrinsic material density of interstellar dust which determines the recoil range for any given momentum. It is found that even allowing for a fivefold variation of density values, the retention against recoil is substantial: for example, some 20 to 50% of all ³⁸Ar nuclei resulting from high-energy interaction remain trapped, depending on the target element considered. Retentivities for the various spallation reactions contributing to ³⁸Ar have been calculated and are used to deduce an interstellar spallogenic production rate. The results are then considered in the light of recent discoveries of ⁴⁰Ar³⁹Ar apparent ages in excess of 4.53×10⁹ years for some inclusions from the meteorite Allende. Limitations of both the theoretical and experimental efforts presently preclude conclusive statements regarding the question of interstellar grain survival. However, a procedure is outlined whereby this issue might be clarified in future investigations.     

Evidence for a low-altitude origin of lightning on Venus

Radio emissions attributed to lightning on Venus have been recorded by Venera 11 and 12 and by the Pioneer Venus Orbiter. The Venera descent records are compared to patterns of radio propagation within the Venusian atmosphere and an explanation is found for some timing trends that, if correct, indicates the lightning was below 33 km in altitude.     

The R asteroids reconsidered

JHK infrared photometry shows that R asteroids have two distinct infrared color domains. Most R asteroids have JHK and visual colors and albedos that fall amongst those observed for S asteroids, but a small subset is clearly different. These are designated as a new A class of asteroids.     

Angular momentum drain: A mechanism for despinning asteroids

It is proposed that a new mechanism—angular momentum drain—helps account for the relatively slow rotation rates of intermediate-sized asteroids. Impact ejecta on a spinning body preferentially escape in the direction of rotation. This material systematically drains away spin angular momentum, leading to the counterintuitive result that collisions can reduce the spin of midsized objects. For an asteroid of mass M spinning at frequency ω, a mass loss δM correspond to an average decrease in rotation rate $\text{δω ≈}\text{ωδM}\text{M}$. A. W. Harris' (1979), Icarus40, 145–153) theory for the collisional evolution of asteroidal spins is significantly altered by inlusion of this effect. While the modified theory is still somewhat artificial, comparison of its predictions with the data of S. F. Dermott, A. W. Harris, and C. D. Murray (1984, Icarus57, 14–34) suggests that angular momentum drain is essential for understanding the statistics of asteroidal rotations.     

A determination of Jovian ammonia abundance based on a 2 μm spectrum

A spectrum of Jupiter in the two micron region has been analyzed to determine the Jovian ammonia abundance. The result is a ?4 cm - amagat, assuming an airmass factor η = 2.5 and a single effective reflecting layer for this wavelength. This is compared with the abundances observed at other wavelengths.     

A note on the variational method of Stokes and DeMarcus for radiative transfer in planetary atmospheres

A generalized functional which yields the Milne integral equation on variation and whose extremum value is proportional to the reflectivity at arbitrary emergent angle is proposed. A similar functional exists for computing the transmissivity at arbitrary emergent angle. This work is a generalization of the variational method of Stokes and DeMarcus (1971, Icarus14, 307) based on the principle of reciprocity. In the special case of trial functions that are linear in the undetermined parameters, the calculation is greatly simplified. The computational value of our variational principle is demonstrated.     

A theoretical study of the time-dependent behaviour of O in the mid-latitude plasmasphere

The behaviour of O²⁺ at L = 3 in the plasmasphere is studied. Starting with a low O²⁺ flux-tube content to characterize post-magnetic-storm conditions the time-dependent equations of continuity and momentum for O²⁺ are solved to give densities and fluxes for a period of several days using both sunspotmaximum and sunspot-minimum parameters. Our results show large amounts of O²⁺ near the equator at sunspot maximum but relatively little at sunspot minimum, and emphasize the key role of the collisional process between O²⁺ and O⁺. It is the combined effects of O²⁺---O ⁺ collisions and thermal diffusion that lead to the large O²⁺ densities near the equator at sunspot maximum. Both of these mechanisms have less influence at sunspot minimum. At sunspot maximum the O⁺ layer acts as a collisional barrier below the O²⁺ production region preventing O²⁺ from sinking towards regions of high recombination rate. In this production region the effects of thermal diffusion are small and upward flow of O²⁺ results from the action of the O²⁺ pressure gradient and the polarization electric field. When the upward flowing O²⁺ reaches regions in which thermal diffusion has a strong influence it is accelerated to even higher altitudes. The O ⁺ barrier is so effective that the diurnal variation of the O⁺ layer is reflected in the diurnal variation of O²⁺ near the equator at sunspot maximum. Our sunspot maximum results also indicate that certain types of temperature profiles are more likely to enhance equatorial O²⁺ densities. The existence of large temperature gradients below 1000 km altitude does not help the flow of O²⁺ towards the equator. The associated changes in the O⁺ layer lead to more O²⁺-O ⁺collisions and a smaller O²⁺ thermal-diffusion coefficient, the latter being sensitive to the ratio n(H⁺)/n(O⁺).     

Mars: Subsurface properties from observed longitudinal variation of the 3.5-mm brightness temperature

Measurements at 3.5 mm of the disk-average brightness temperature of Mars during the 1978 opposition can be represented by

$\text{T}{}_{\mathrm{B}}\text{(Mars, 3 5 mm, Jan/Feb 1978)}\text{=}\textcolor{red}{}$

(The errors cited are from the internal scatter; the estimated absolute calibration uncertainty is 3%.) This longitudinal variation must be taken into account if Mars is to be used as a calibration source at millimeter wavelengths. The total range of the 3.5-mm variation is three to four times larger than both the 2.8-cm and 20-μm variations. This unexpected result can possibly be explained by subsurface scattering from rocks ?1.5-cm radius.     

A possible solar-wind cause of the segmented appearance and of the changes in orientation of the plasma-tail axis of Comet Austin 1982g

Several directional discontinuities in the plasma tail of Comet Austin 1982g are apparent on photographs obtained by different observers between 1982 Augut 17.84 and August 21.85. Furthermore, anomalous changes in the orientation of the inner tail axis with respect to the projection on the sky of the prolonged radius vector are noticed. An analysis based on the wind-sock theory of plasma comet tail orientations shows that changes in the azimuthal component of the solar-wind velocity would have produced the observed anomalies. No satellite data on interplanetary conditions were available to check the existence of such a solar-wind event. A type II–IV solar radio event observed on August 17.64, followed by a geomagnetic storm in August 20.67, might denote, however, the existence of a solar flare-generated interplanetary disturbance.     

Patterns of fracture and tidal stresses on Europa

the hypothesis that lineaments on Europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands, triple bands, and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity. If short, reticulate dark band nnear the anti-Jove point are tension cracks which formed in response to tidal distortion, they could only have been produced by orbital eccentricity. Long, arcuate dark band and triple bands peripheral to the anti-Jove point orientations which suggest that they are strike-slip faults which formed in response to orbital recession. If cuspate ridges are compressional features, their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume. Stresses due to orbital eccentricit could have produced tension cracks near the anti-Jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed. Surface fracturing, if a consequence of tidal deformation, places important constraints on the orbital evolution of Europa.     

Global and regional ridge patterns on Mars

Over 16,000 wrinkle ridges on Mars have been classified, mapped, and digitized to provide a large computer-accessible data base for analyzing regions subjected to possible compressive stresses. The survey has revealed major compressive structures that occur well beyond the Tharsis-dominated hemisphere. The large variety of affected geologic terrains indicate stresses not simply localized in the intercrater plains. One major area of inferred compression occurs in the southern cratered highlands near longitude 180°W where major ridges and scarps extend over 3000 km. The occurrence and orientation of many ridges are locally controlled by ancient impact basins. The Chryse basin in particular has an important effect on ridges in northern Lunae Planum. The removal of all basin-concentric ridges reveals, however, a complex global pattern. Although such patterns may yet be controlled by heavily degraded impact basins, major regional trends also emerge that appear to require broader scale global stresses. Most ridges in the Western Hemisphere are shown to be orthogonal to three centers corresponding to Tharsis (1°N, 122°W), Syria (12°S, 104°W), and Chryse (19°N, 47°W). Ridges not included in these three sets are generally more random and highly localized. Most, but by no means all, ridges in the Eastern Hemisphere are controlled by the Hellas and Isidis basins. A simple global grid is not yet identifiable and may not be recognizable owing to the large number of regional patterns.