Ephemeral Bodies Research Articles

Saturn's Rings: Optical Reflectance Polarimetry

The distribution of optical linear polarization over Saturn's rings was measured telescopically.Ring B, after correction for the polarization produced by illumination by Saturn's globe, was found to show two effects: (1) A polarization resulting from direct solar illumination, for which the phase angle and wavelength dependences are indicative of at least millimeter-sized high-albedo blocks with rough surfaces, and bright submicrometer-sized particles, probably icy grains, attached to their surfaces. (2) Another component of polarization, everywhere parallel to the direction of the globe center, or systematically slightly tilted, is probably caused by anisotropic multiple scattering of light between aligned grains, possibly due to the wakes of dynamical ephemeral bodies that may exist in the rings.Ring Awas found to show similar polarization due to direct solar illumination, suggesting similar aggregated blocks, although rather enriched with submicrometer coating particles. Azimuthal brightness variations suggest the presence of particles aligned as for ring B, but with no comparable component of polarization parallel to the direction of the globe center. Instead, a complex and patchy field of locally produced linear polarization was observed, variable in degree and in azimuth from place to place over ring A; this may reflect the vigorous and multistage process of accretion, wake formation, and disaggregation, as dynamically modeled by H. Salö (1992,Nature359, 619–621), and which is specific to ring A.

The Mimas 2:1 Eccentric Corotational Resonance in Saturn's Outer B Ring

We present evidence of an unusual and previously unidentified feature in Saturn's outer B ring at a radius of 117,130 km. The Voyager 2 optical occultation experiment shows a step in opacity, dropping from 1.4 to 0.5 across several kilometers and generally remaining low out to the edge of the ring. By contrast, the Voyager 1 radio occultation showed no distinct change at this location, a rare lack of correlation to a major optical feature. The ratio of opacities outside 117,130 km implies a nearly complete depletion of subcentimeter-sized ring particles unique in Saturn's main rings. We suggest this feature is the Mimas 2:1 eccentric corotational resonance (ECR). Unlike inner Lindblad resonances and inner vertical resonances, which produce waves on dynamic time scales (and which affect all sizes of particle equally), an ECR produces no observable effects on a dynamic time scale. However, we show that the torque exerted by the ECR can influence ring structure on the diffusion time scale. Moreover, a simple computation assuming two particle sizes and diffusion rates qualitatively reproduces the differences between the data sets for simulated times of a few million years. The ECR provides a new observational probe of some poorly known parameters of the rings: the physical mechanisms that determine the ring particle velocity dispersion and size distribution. Specifically, the difference between the optical and the radio opacities is the first observational evidence for the dependence of the diffusion coefficient on particle size, which is expected to arise from the size dependence of the velocity dispersion. Implicitly, it also places a lower limit of a few million years on the time scale for particle size redistribution processes. Hence, this is the first observational test of the suggestion of S. J. Weidenschilling et al. (1984, Planetary Rings, pp. 367-415, Univ. of Arizona Press, Tucson) that Saturn's ring particles are dynamical ephemeral bodies (DEBs) in a statistical equilibrium, accreting and fragmenting on a time scale of weeks. Detection of the ECR implies integrity of individual particles over much greater time scales, arguing against the DEBs model. The ECR time scale is consistent with ring formation due to breakup of a massive object with subsequent erosion of the remnant material. An interesting limit can be placed on the parameters of erosion by micrometeoroid bombardment.

Saturn's main ring particle size distribution: An analytic approach

An analytic theory for Saturn's ring particle size distribution is developed using the so-called “dynamic ephemeral bodies” (or DEBs) model of ring particles (S.J. Weidenschilling, C.R. Chapman, D.R. Davis, and R. Greenberg, 1984, in Planetary Rings, pp. 367–416, Univ. of Arizona Press, Tucson) . Accretion and erosion criteria are defined, and the fundamental integrodifferential equation describing the collisional evolution of the mass distribution function of ring particles is derived in the approximation that the ring velocity dispersion is independent of the particle size. A simple stationary solution of this equation is developed, which reproduces all the main features of the distributions determined from the Voyager I radio data (H.A. Zebker, E.A. Marouf, and G.L. Tyler, 1985, Icarus 64, 531–568) . The main results of this work are the following: (i) the upper cutoff of the distribution can be explained by a relative enhancement of the erosion of the large particles; (ii) the power-law index of the distribution can be related to parameters characterizing the particle collisional properties; (iii) the model yields estimates for the particle life times and for the dispersion velocity in various unperturbed regions of the rings; (iv) “hard-sphere” ring particle models are most probably ruled out, because they lead to stationary distribution functions in disagreement with the Voyager results.

Helminth Communities in Avocets: Importance of the Compound Community

This paper reports patterns of similarity and overlap in species presence and patterns of linear distribution of intestinal helminths in 22 avocets from 4 populations. Avocets collected from ephemeral bodies of water in Alberta and Manitoba had communities composed largely of species that are avocet specialists plus some that are host generalists. The composition of helminth communities in these hosts was similar to that reported in earlier surveys of avocet helminths. There was little evidence for competition between helminth species in these communities. In contrast, avocets collected from permanent bodies of water in Alberta had communities composed largely of species that are specialists in various duck species, particularly lesser scaup. These helminths were superimposed on the normal community, fitting into linear gaps along the intestine but also overlapping the distributions of avocet specialists. These lesser scaup specialists exhibit interactive patterns amongst themselves and, to some extent, with avocet specialists. Helminth communities in avocets from ephemeral bodies of water have vacant niches and are largely isolationist in nature. Those in avocets from permanent bodies of water are saturated and are more interactive in nature.

Acid Tolerance in Connecticut Wood Frogs (Rana sylvatica)

-Acid tolerance limits were measured in wood frog (Rana sylvatica) embryos and larvae from a population located in New London, Connecticut. Among embryos, relative hatching success was high and not significantly different in solutions with pH 7.2-7.6, pH 6.0, pH 5.0 and pH 4.0. However, at pH 3.75 relative hatching success dropped to 50% and less than 1% of embryos survived at pH 3.5 or below. At pH 4.0 the number of developmental abnormalities among surviving embryos increased significantly, and at pH 3.75 35% of surviving embryos appeared abnormal. Larvae were more acid tolerant than embryos, and this tolerance appeared to increase as the larvae developed. Measurements of acidity in the natural breeding habitat indicated that considerable temporal and spatial variation may occur within a single pond, but the levels of acidity measured did not exceed the species' tolerance limits. The effect of hydrogen ion concentration, or pH, on freshwater ecosystems has recently been the focus of intensive study (Drablos and Tollan, 1980; Hutchinson and Havas, 1980). In spite of a growing interest in environmental acidity and its influence on plants and animals, one large group of aquatic vertebrates, the amphibians, has received relatively little attention. Many temperate zone frogs and salamanders exhibit complex life history patterns in which courtship and spawning typically occur in small ephemeral bodies of water, embryos and larvae develop in the aquatic environment, and metamorphosis results in a transition to terrestrial habitats. (There are, however, many variations of this pattern; see Salthe and Mecham, 1964 and Dent, 1968). Aquatic life history stages may be particularly susceptible to acidity, but the importance of this environmental factor in limiting the natural distributions of amphibians is not yet clearly understood. Gosner and Black (1957) first provided evidence that frog species differ in their ability to survive acidic conditions. They measured the acid tolerance limits of 11 species from New Jersey, and found that those frogs breeding in the naturally acidic waters of the Pine Barrens were more acid tolerant than those breeding only in more neutral waters. Two other investigations have also examined embryo survival of several amphibian species in acid bog water, and both detected differences among species in bog water tolerance (Karns, 1983; Saber and Dunson, 1978). Pough and Wilson (1977) tested acid tolerance in two salamander species, and found that embryos of the spotted salamander (Ambystoma maculatum) displayed increased mortality in even slightly acidic pond water. Jefferson's salamander (Ambystoma jeffersonianum) appeared to be more acid tolerant. Furthermore, Pough (1976) correlated pond acidity with embryo survivial in field populations of Ambystoma maculatum and suggested that this animal is in danger of localized extinction as a result of increasing acidity by acid precipitation. However, Cook (1978) found that populations of spotted salamanders from the Connecticut Valley of Massachusetts were considerably more acid tolerant. Thus geographical variation in acid tolerances is implicated for this species. In other studies, Schlichter (1980) investigated acid tolerance in Rana pipiens, Porter and Hakanson (1976) evaluated the impact of acid mine drainage on the distribution of Bufo boThis content downloaded from 157.55.39.128 on Mon, 18 Jul 2016 05:41:09 UTC All use subject to http://about.jstor.org/terms

Saturn Ring Particles as Dynamic Ephemeral Bodies

Although Saturn's rings are within the Roche zone, the accretion of centimeter-sized particles into large aggregates many meters in diameter occurs readily, on a time scale of weeks. These aggregates are disrupted when tidal stresses exceed their very low strengths; thus most of the mass of the ring system is continually processed through a population of large "dynamic ephemeral bodies," which are continually forming and disintegrating. These large aggregates are not at all like the idealized ice spheres often used in modeling Saturn's ring dynamics. Their coefficient of restitution is low, hence they form a monolayer in the ring plane. The optically observable characteristics of the rings are dominated by the swarm of centimeter-sized particles.

Evaporative Water Loss from Three South-Eastern Australian Species of Water Turtle.

In the chelid turtles Chelodina expansa, C. longicollis and Emydura macquarii, over the temperature range 8-34�C and humidity range 25-60% RH, evaporative water loss (EWL; estimated as weight loss) corrected for differences in body weight was greatest in E. macquarii and least in C. longicollis. Interspecific differences were most pronounced at high temperature and low humidity, but may have been exaggerated by variations in experimental airflow. Under progressive dehydration, EWL declined by about 20% in C. expansa and E. macquarii and over 60% in C. longicollis. Cutaneous water loss greatly exceeded pulmonary loss in all three species, but made up the lowest percentage of total EWL in C. longicollis; dehydration further reduced the relative contribution of cutaneous loss in this species. C, longicollis also demonstrated an ability to take up free water via the cloaca. The water-conserving abilities of C. longicollis are ecologically advantageous, since it commonly migrates over land, occupies ephemeral bodies of water, and is capable of aestivation.

Behavioral Biology of the Annual Killifish, Nothobranchius guentheri

The behavior of Nothobranchius guentheri, an East African annual cyprinodontid is described from aquarium and outdoor pond observations. Some aspects of biology as they probably serve adaptive function in ephemeral bodies of water are also described. Nothobranchius guentheri is sexually dimorphic, reaches sexual maturity at less than 4 weeks of age and reproduces daily throughout life. Diel periodicity of both agonistic and reproductive behavior coupled with male linear hierarchies, absence of territories and mating promiscuity results in enhancement of opportunity for receptive females to come into contact with males. Initial contact between the sexes is largely dependent on positive orientation of the cryptically colored females towards the visually obvious males. Reproductive behavior has been reduced to those movements necessary for fertilization of the eggs which are deposited below the substrate surface. These features of the species' biology and behavior in combination with drought resistance of the eggs and embryological diapauses serve to maximize reproductive success in a temporally and spatially unreliable environment.