Cassini imaging of Saturn's rings: II. A wavelet technique for analysis of density waves and other radial structure in the rings

We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r∼118,800 km, 4.5 m at r∼120,700 km) and the inner A Ring (10-15 m for r<127,000 km).     

Core, Halo and Strahl Electrons in the Solar Wind

Electron velocity distribution functions (VDF) observed in the low speed solar wind flow are generally characterized by ‘core’ and ‘halo’ electrons. In the high speed solar wind, a third population of ‘strahl’ electrons is generally observed. New collisional models based on the solution of the Fokker-Planck equation can be used to determine the importance of the different electron populations as a function of the radial distance. Typical electron velocity distribution functions observed at 1 AU from the Sun are used as boundary conditions for the high speed solar wind and for the low speed solar wind. Taking into account the effects of external forces and Coulomb collisions with a background plasma, suprathermal tails are found to be present in the electron VDF at low altitudes in the corona when they exist at large radial distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.     

Complex refractive index of Martian dust: Wavelength dependence and composition

Mariner 9 ultraviolet spectra of the 1971 dust clouds were analyzed to obtain the phase function times single scattering albedo of the dust particles. The phase functions were matched with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. The absorption index of the dust particles was found to increase with decreasing wavelenght from 350 nm down to about 210 nm, and drop off shortward of this wavelength. A structural shoulder occurs in the absorption spectrum between 240 and 250 nm. Titanium dioxide (anatase) has the correct transitions at 210 and 250 nm to match the absorption curve of Martian dust, and is proposed as a candidate constituent in Martian surface material. The spectral neutrality of TiO₂ between 0.5 and 4 μm is consistent with visible and infrared observations of Mars. The high refractive index of TiO₂ can explain the large refringence of Martian dust. The titanium dioxide content of the dust particles is estimated to be a few percent or less. Uncertainties in the results due to limitations in the model and data are discussed.     

Towards a more habitable Mars — or — the coming martian spring

Two schemes are presented in which matter is moved closer to Mars so as to increase the equinoctal precession period of Mars. In such a way the Martian springs, during which Mars is habitable in the long winter model of Sagan (1971), could be extended and, in fact, prolonged indefinitely. The schemes involve using solar energy to move the satellite Phobos and/or material from the asteroid belt. Difficulties exist with the proposals but they are presented to stimulate discussion.     

A Mariner 9 atlas of the moons of Mars

This paper contains a complete set of the best enhancements of Mariner 9 high resolution television pictures of Phobos and Deimos, consisting of 27 different views of Phobos, and 9 of Deimos. Pertinent data about the pictures are arranged in convenient tabular and graphical form.     

Resonant behaviour of MHD waves on magnetic flux tubes

Resonant absorption of MHD waves on a nonuniform flux tube is investigated as a driven problem for a 1D cylindrical equilibrium. The variation of the fractional absorption is studied as a function of the frequency and its relation to the eigenvalue problem of the MHD radiating eigenmodes of the nonuniform flux tube is established. The optimal frequencies producing maximal fractional absorption are determined and the condition for total absorption is obtained. This condition defines an impedance matching and is fulfilled for an equilibrium that is fine tuned with respect to the incoming wave. The variation of the spatial wave solutions with respect to the frequency is explained as due to the variation of the real and imaginary parts of the dispersion relation of the MHD radiating eigenmodes with respect to the real driving frequency.     

Additional Analytical Data for Gabbro, GOG-1

Additional data for gabbro, GOG-1, were determined by instrumental-neutron-activation analysis, atomic-absorption spectrometry, and semi-quantitative spectrographic analysis. F ratios calculated in the analysis of variance for 26 sets of data for elements determined by the three methods were not significant, and hence the elements are distributed homogeneously among the bottles. The agreement between our data and the averages previously published ranges from very good to poor. More analytical data are necessary to establish reliable estimates of the concentrations of elements in GOG-1 and in two other gabbros so that three gabbros may be available to geochemists for use as standards.     

Climate change and crop yields in the Indian Cardamom Hills, 1978–2007 CE

In this study we analyzed climate and crop yields data from Indian cardamom hills for the period 1978–2007 to investigate whether there were significant changes in weather elements, and if such changes have had significant impact on the production of spices and plantation crops. Spatial and temporal variations in air temperatures (maximum and minimum), rainfall and relative humidity are evident across stations. The mean air temperature increased significantly during the last 30?years; the greatest increase and the largest significant upward trend was observed in the daily temperature. The highest increase in minimum temperature was registered for June (0.37°C/18?years) at the Myladumpara station. December and January showed greater warming across the stations. Rainfall during the main monsoon months (June–September) showed a downward trend. Relative humidity showed increasing and decreasing trends, respectively, at the cardamom and tea growing tracts. The warming trend coupled with frequent wet and dry spells during the summer is likely to have a favorable effect on insect pests and disease causing organisms thereby pesticide consumption can go up both during excess rainfall and drought years. The incidence of many minor pest insects and disease pathogens has increased in the recent years of our study along with warming. Significant and slight increases in the yield of small cardamom (Elettaria cardamomum M.) and coffee (Coffea arabica), respectively, were noticed in the recent years.; however the improvement of yield in tea (Thea sinensis) and black pepper (Piper nigrum L.) has not been seen in our analysis.     

Implementation of a non-hydrostatic, adaptive-grid dynamics core in CAM3. Part I: comparison of dynamics cores in aqua-planet simulations

We report on our implementation of EULAG as a dynamical core in the Community Atmospheric Model (CAM). EULAG is a non-hydrostatic, parallel computational model for all-scale geophysical flows. EULAG’s name derives from its two computational options: EULerian (flux form) or semi-LAGrangian (advective form). The model combines nonoscillatory forward-in-time (NFT) numerical algorithms with a robust elliptic Krylov solver. A signature feature of EULAG is that it is formulated in generalized coordinates. In particular, this enables grid adaptivity. In total, these features give EULAG novel advantages over the existing dynamical cores in CAM. This paper uses a series of aqua-planet simulations to demonstrate that CAM-EULAG results compare favorably with those from CAM simulations at standard CAM resolution that use current finite volume or Eulerian-spectral dynamical core options. We also show that the grid adaptivity implemented in CAM3-EULAG allows higher resolution in selected regions without causing anomalous behavior such as spurious wave reflection.