• NEANIAS Space Research Community

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part 1 presents determinations of the absolute reflectivities of several visually distinct regions of Jupiter between 3390 and 8400Å, at 10Å resolution, from observations made on the 60-inch telescope at Mt.Wilson between June 1973 and June 1974. These have been checked independently by observations using 150-200Å wide filters from 3400 to 6400Å. The absolute scale of Oke and Schild (1970) is used, and solar irradiance values are taken from Arvesen et al.(1969). The results are presented as a set of 9 figures showing the wavelength dependence of reflectivity. There is generally good internal consistency within the estimated errors. The effective reflectivities for several regions on the meridian in the 3390 to 8400Å range are: South Tropical Zone- 0.76±.05, North Tropical Zone- 0.68±.08, South Equatorial Belt- 0.63±.08, North Equatorial Belt- 0.62±.04, and Great Red Spot- 0.64±.09. Reflectivities nearer the limb are also observed. In support of the Pioneer imaging photopolarimeter experiment, the appropriate blue and red reflectivity values are also tabulated. For the regions on the meridian listed above, the equivalent widths of molecular bands vary as: CH[subscript 4] (6190Å): 14-16Å; CH[subscript 4](7250Å): 77-86Å; and NH[subscript 3](7900Å): 87-95Å. Significant differences from previous results of Pilcher et al. (1973) are noted. Part 2 presents latitude sectors of the 20 and 40 micron maps of Jupiter obtained by the Pioneer 10 infrared radiometer. These data are used to derive simple models for the average vertical thermal structure over the South Equatorial Belt and the South Tropical Zone, with additional examples of models for the North Equatorial Belt and the Great Red Spot. The models assume gaseous absorption by H[subscript 2] and NH[subscript 3] alone. The models are predominantly composed of H[subscript 2] with He dilution constrained to 0-35% by volume. For the South Equatorial Belt, the temperature is about 170°K at 1.0 atm pressure, assuming the deep atmosphere to be adiabatic. The temperature may be 113-121°K near 0.2 atm, depending on what is assumed for the overlying thermal structure. In a non-scattering model, as given above, the South Tropical Zone is some 8°K cooler than the SEB near 1.0 atm. However, the data may also be successfully modeled by a thermal structure at minimum variance from that of the SEB, but with an optically thick cloud close to the 150°K level. Such a model is consistent with the visible and 5 micron appearance of the planet, and the cloud is coincident with the location at which saturation of NH[subscript 3] is expected to begin. For this model, the STrZ temperature is 3°K cooler than the SEB near 0.2 atm. The Great Red Spot may be modeled by a thermal structure like the "cloudy" STrZ model, but 5°K cooler near 0.2 atm. The local effective temperatures for the SEB (129°K) and the STrZ (126°K) are both below the effective temperature of 134°K from earth-based measurements. The derived thermal structures are inconsistent with the neutral atmosphere inversion of the Pioneer 10 radio occultation (Kliore et al.,1974), but not with others in the literature, including the Gulkis et al. (1973) model for the microwave spectrum. Part 3 reports: (1) observations of the limb structure near the equator of Jupiter at 8.15 and 8.44 microns using the Palomar 200-inch telescope at a resolution of about 3 arc seconds and a cooled filter-wheelspectrometer ([...]λ/λ[...]0.015), and (2) a model of the thermal structure and cloud properties of the atmosphere which is most consistent with spatially and spectrally resolved observations of the planet in the 8 - 14 micron range, including those reported in (1). The thermal structure derived in Part 2 below the 0.2 atm level must be cooled by some 6°K in order to match the 12-14 micron spectrum, which is dominated by the opacity of H[subscript 2]. An NH[subscript 3] abundance defined by saturation equilibrium is consistent with the 9.5-12.0 micron spectrum, dominated by the opacity of that gas. The thermal structure above the 0.2 atm level is determined by fitting spectral and limb structure data in the 7.2-8.4 micron range, dominated by the opacity of CH[subscript 4]. The result is an inverted thermal structure with a base of about 110°K at 0.2 atm, rising through 150°K at about 0.03 atm. The mixing ratio of CH[subscript 4] most consistent with the spectral and limb structure data is 2.0 x 10[superscript -3] some three times that assumed in "solar abundance" models. The 8.2-9.5 micron spectral region is not eerily matched by simple gaseous opacity sources. However, a haze of solid NH[subscript 3] particles above a thick cloud (which exists only in zones, as implied in Part 2) is consistent with the observed spectrum. Difficulty is encountered, however, with limb structure data at 8.44 microns and with some observations outside the 8-14 micron range. Further observations of the separate spectral characteristics of belts and zones is recommended, as well as more accurate laboratory data for the opacity of atmospheric constituents in the relevant thermal regime and more sophisticated scattering approximations that used in this model.

The impact crater is the ubiquitous landform of the solar system. Theoretical mechanical analyses are applied to the modification stage of crater formation, both syngenetic (immediate or short term) and postgenetic (long term). The mechanical stability of an impact crater is analyzed via a quasi-static, axisymmetric slip line theory of plasticity. The yield model incorporated is Mohr-Coulomb and a simplified rectangular profile is used for the transient cavity. The degree of stability (or instability) is described as a function of internal friction angle, depth/diameter ratio, and a dimensionless parameter ρgH/c (ρ = density, g = acceleration of gravity, H = depth, and c = cohesion strength). To match the observed slumping of large lunar craters the cohesion strength of the lunar surface material must be low (less than 20 bars) and the angle of internal friction must be less than 2°. It is not implausible that these failure strength characteristics are realized by freshly shocked rock. A theoretical description of impact crater collapse is evolved which accounts for the development of wall scallops, slump terraces, and flat floors. A preliminary set of scale model experiments performed in a centrifuge corroborate the theory. The strength of terrestrial planet surfaces under impact is seen to vary by as much as a factor of two. Shortly after the excavation of a large impact crater the transient cavity collapses, driven by gravity. It is shown that at least one concentric fault scarp forms about the crater, if the strength of the target material decreases sufficiently rapidly with increasing depth. This is demonstrated by two classes of models: extrusion flow models which assume a weak layer underlying a strong layer, and plastic flow models which assume a continuous decrease of cohesion strength with depth. Both classes predict that the ratio of the radius of the scarp to the transient crater radius is between 1.2 and 2 for large craters. Large impact basins on Ganymede and Callisto are characterized by one or more concentric rings or scarps. The number, spacing, and morphology of the rings is a function of the thickness and strength of the lithosphere, and crater diameter. When the lithosphere is thin and weak, the collapse is regulated by flow induced in the asthenosphere. The lithosphere fragments in a multiply concentric pattern (e.g., Valhalla, Asgard, Galilee Regio, and a newly discovered ring system on Callisto). The thickness and viscosity of a planetary lithosphere increases with time as the mantle cools. A thicker lithosphere leads to the formation of one (or very few) irregular normal faults concentric to the crater (e.g., Gilgamesh). A gravity wave or tsunami induced by impact into a liquid mantle would result in both concentric and radial extension features. Since these are not observed , this process cannot be responsible for the generation of the rings around the basins on Ganymede and Callisto. The appearance of Galilee Regio and portions of Valhalla is best explained by ring graben, and though the Valhalla system is older, the lithosphere was 1.5-2.0 times as thick at the time of formation. The present lithosphere thickness is too great to permit development of any rings. It has been proposed that a mascon may be in the form of an annulus surrounding the Caloris basin on Mercury, associated with the smooth plains. The effects (stresses, deformation, surface tectonic style, gravity anomalies, etc.) of such a ring load on a floating elastic lithosphere of variable thickness are investigated. The main characteristics of the surface tectonic pattern are normal faulting within the basin and thrust faulting beneath the ring load~ both in agreement with observation Moreover, the dominant concentric trend of the basin normal faults is consistent with the ring load hypothesis provided the mercurian lithosphere was ≤125 km thick at the time of faulting. Simple updoming within the basin would produce normal faults of predominantly radial orientation.

This study describes the design and construction of a 5 µm imaging system used at the Hale 5 m (200 inch) telescope to acquire high spatial resolution infrared images of Jupiter. These images, recorded in a spectral region clear of terrestrial and Jovian gaseous absorption, offer a unique look into the deep atmosphere and provide direct observational evidence for the existence of multiple layers of clouds in the Jovian atmosphere. Evidence of layering is provided by the observed trimodal nature and persistence of the 5 µm flux-frequency distribution of equal areas on the Jovian disk. This indicates that three distinct brightness temperatures have a higher probability of being observed than a continuum of temperatures, and that, despite significant observed variations in the lateral 5 µm cloud distribution, this phenomenon is a long term stable vertical cloud feature. Furthermore, the visible color differences correlate with areas of different 5 µm intensity, implying that the colors are due to reflection from areas of different chemistry or state at different levels in the atmosphere. Also, short time scales are observed for large 5 µm flux variations over extensive areas of the Jovian disk, supporting the concept that the redistribution of obscuring clouds accounts for the contrasts at 5 µm. Finally, the 5 µm limb-darkening and opacity models, derived from imaging and spectroscopic measurements, are consistent with multiple layering of clouds in the Jovian atmosphere. Further information about the Jovian clouds results from the combination of 5 µm spectroscopic and imaging data sets. From the shape of the 5 µm spectrum true maximum brightness temperatures are derived, corrected for the clearest regions in the Jovian atmosphere. Furthermore, from data on spectral line saturation, limits are placed on the 5 µm cloud reflectivity over the field of view of the spectrometer. With this information, combined with the knowledge of the spatial flux distribution from imaging, constraints are derived for the optical properties of the upper Jovian clouds. A three layer cloud model is developed which is consistent with all of the observational data at 5 µm. The three model cloud layers have cloud top temperatures of T_1 ≤ 190°K (presumably T_1 ≃ 140°K), T_2 = 228 ± 2°K and T_3 = 292 ± 8°K. The highest layer, found only over the white zones and red spots, has optical depth near unity and transmits radiation from deeper levels. This upper level has a mean 5 µm cloud reflectivity less than 0.4, while the whole central 25% of the disk has a mean reflectivity less than 0.1. The middle cloud deck is present under the upper level clouds and over the brown colored Jovian belts. This level is optically thick everywhere except in regions where blue-gray areas are visible. Here the middle level thins to a mean optical depth of about 2 and allows radiation from the deepest and hottest level to be detected.

A range of theoretical models of the compositional structure of Io's dayside atmosphere and ionosphere are developed. The dominant neutral gas, SO2, is provided by sublimation of surface frost. Photochemical processes lead to the build up of O, S, SO, and O2 as minor gasses near Io's surface while O becomes the dominant gas near the exobase. The vertical column density of O2 in all models considered is less than 1014 cm-2. The dayside ionosphere is formed as a result of ionization of neutral species by solar UV radiation. Charge exchange and rearrangement reactions are important for determining the ionic composition of the ionosphere. The dominant ion in the models considered is SO+. A number of charge exchange reactions are identified whose rates need to be better determined in order to refine the present model of the ionosphere. The best matches of the model ionospheres to that observed by the Pioneer 10 radio occultation experiment require atmospheric surface concentrations of SO2 in the range of 2.5 x 109 to 1 x 1011 cm-3, and an exospheric temperature in the range of 960 K to 1230 K. The ratio of the escape fluxes of O to S from the exobase is ≥ 2 in the models considered, while the models which allow surface deposition of minor constituents always have a total sulfur depositional rate greater than 1/2 of the total oxygen depositional rate, thus a surface enrichment of S relative to that predicted by a pure SO2 surface. The depositional rate of this "excess" sulfur is in the range 100 m to 1 km thickness per billion years. Atmospheric Na is provided by surface sputtering of SO2 surface frost with Na impurities by MeV type magnetospheric ions. An upward flux of Na2O of S x 107 cm-2 s-1 leads to an escape flux of Na from the exobase of 1 x 107 cm-2 s-1. The chemistry (ion and neutral) of Na species in the atmosphere has only minor effects on the major characteristics of the atmosphere and ionosphere. It is generally accepted that Io is the source of S, O, Na, and K which, subsequent to ionization, form the constituents of the Io plasma torus. It is shown in chapter II that the escape of S and O from Io can be understood in terms of the photochemistry of a predominantly SO2 atmosphere created by the high vapor pressure of SO2. However, the vapor pressures of Na2S, K2S and other common compounds containing Na and K are negligible at the surface temperature of Io. In chapter III we propose that Na and K escape from Io in two stages. Atoms of Na and K (or molecules containing these atoms) are first sputtered into the atmosphere from the surface by high energy magnetospheric ions. Atmospheric sputtering by low energy corotating ions then removes these constituents (along with others present) out of Io's gravitational control. The estimated injection rates are sufficiently large to maintain the observed Na, K, and O clouds observed around Io.

Three investigations are conducted into the physical chemistry of volatiles in the outer solar system and the role of volatiles in icy satellite evolution. Part I: The thermodynamic stability of clathrate hydrate is calculated under a wide range of temperature and pressure conditions applicable to solar system problems, using a statistical mechanical theory developed by Van der Waals and Platteeuw (1959) and existing experimental data on properties of clathrate hydrates and their components. At low pressure, dissociation pressures and partition functions (Langmuir constants) for CO clathrate (hydrate) have been predicted using the properties of clathrate containing, as guests, molecules similar to CO. The comparable or higher propensity of CO to incorporate in clathrate relative to N2 is used to argue for high CO to N2 ratios in primordial Titan if N2 were accreted as clathrate. The relative incorporation of noble gases in clathrate from a solar composition gas at low temperatures is calculated, and applied to the case of giant planet atmospheres and icy satellites. It is argued that non-solar but well-constrained noble gas abundances would be measured by Galileo in the Jovian atmosphere if the observed carbon enhancement were due to bombardment of the atmosphere by clathrate-bearing planetesimals sometime after planetary formation. The noble gas abundances of Titan's atmosphere are also predicted under the hypothesis that much of the satellite's methane accreted as clathrate. Double occupancy of clathrate cages by H2 and CH4 in contact with a solar composition gas is examined, and it is concluded that potentially important amounts of H2 may have incorporated in satellites as clathrate. The kinetics of clathrate formation is also examined, and it is suggested that, under thermodynamically appropriate conditions, essentially complete clathration of water ice could have occurred in high pressure nebulae around giant planets but probably not in the outer solar nebula; comets probably did not aggregate as clathrate. At moderate pressures, the phase diagram for methane clathrate hydrate in the presence of 15% ammonia (relative to water) is constructed, and application to the early Titan atmospheric composition is described. The high pressure stability of CH4, N2, and mixed CH4-N2 clathrate hydrate is calculated; conversion back to water and CH4 and/or N2 fluids or solids is predicted for pressures ≳12 kilobars and/or temperatures ≳320 K. The effect of ammonia is to shrink the T-P stability field of clathrate with increasing ammonia concentration. A preliminary phase diagram for the high pressure ammonia-water system is constructed using new data of Johnson et al. (1984). These results imply that 1) clathrate is stable throughout the interior of Oberon- and Rhea-sized icy satellites, and 2) clathrate incorporated in the inner-most icy regions of Titan would have decomposed, perhaps allowing buoyant methane to rise. Brief speculation on the implications of this conclusion for the origin of surficial methane on Titan is given. A list of suggested experiments and observations to test the theory and its predictions is presented. Part II: We propose a global Titanic ocean, one to several kilometers deep, the modern composition of which is predominantly ethane. If the ocean is in thermodynamic equilibrium with an atmosphere of 3' (mole fraction) methane then its composition is roughly 70% C2H6, 25% CH4, and 5% N2. Photochemical models predict that C2H6 is the dominant end-product of CH4 photolysis so that the evolving ocean is both the source and sink for ongoing photolysis. The coexisting atmosphere is compatible with Voyager data. Two consequences are pursued: the interaction of such an ocean with the underlying "bedrock" of Titan (assumed to be water-ice or ammonia hydrate) and with the primarily nitrogen atmosphere. It is concluded that although modest exchange of oceanic hydrocarbons with enclathrated methane in the bedrock can in principle occur, it is unlikely for reasonable regolith depths and probably physically inhibited by the presence of a layer of solid acetylene and complex polymeric hydrocarbons a couple of hundred meters thick at the base of the ocean. However, the surprisingly high solubility of water ice in liquid methane (Rebiai et al., 1983) implies that topographic features on Titan of order 100 meter in height can be eroded away on a time scale ≾109 years; "Karst" topography could be formed. Finally, the large solubility difference of N2 in methane versus ethane implies that the ocean composition is a strong determinant of atmospheric pressure; a simple radiative model of the Titan atmosphere is employed to demonstrate that significant surface pressure and temperature changes can occur as the oceanic composition evolves with time. The model suggests that the early methane-rich ocean may have been frozen; scenarios for evolution to the present liquid state are discussed. Part III: A simple convective cooling model of a primordial, CH4-NH3-N2 Titan atmosphere is constructed, in an effort to understand the fate of volatiles accreted from a gaseous disk ("nebula") surrounding Saturn and released from accreting planetesimals during the satellite's formation. Near-surface temperatures are initially ≳400 K consistent with the large amount of energy supplied to the atmosphere during accretion. As a consequence of accretional heating, the upper mantle of the satellite consists of an ammonia-water liquid, extending to the surface. This "magma ocean" is the primary buffer of atmospheric cooling because it is ≳10 times as massive as the atmosphere. The radiative properties of the atmosphere are assumed independent of frequency and the resulting temperature profile is found to be adiabatic; if the atmosphere contains dark particulates surface temperatures could be lower than calculated here. Three major processes drive the cooling: (1) hydrodynamic escape of gas from the top of the atmosphere, which determines the cooling time scales, (2) atmospheric ablation by high velocity impacts (not modeled in detail here), and (3) formation of clathrate hydrate at the ocean-atmosphere interface, at T ≤ 250 K. Cooling time scales driven by escape are sufficiently long (108-109 years) to allow ~10 bars of N2 to be produced photochemically from NH3 in the gas phase (Atreya et al., 1978); however, the abundance of NH3 at temperatures ≾150 K (where the intermediate photochemical products condense out) is optically thick to the dissociative UV photons. Thus, N2 formation may proceed primarily by shock heating of the atmosphere during large body impacts, as well as by photochemistry (1) at T < 150 K if intermediate products supersaturate, or (2) in a warm stratosphere, with NH3 abundance fixed by its tropopause value. The clathrate formed during late stages of cooling sequesters primarily CH4, with some N2, and forces surface temperatures and pressures to drop rapidly. The clathrate is only marginally buoyant relative to the coexisting ammonia-water liquid. If it sinks, the atmosphere is driven to an N2-rich state with most of the methane sequestered in clathrate when the ocean surface freezes over at ~180 K. Implications of this scenario for the present surface state of Titan are contrasted with those obtained if the clathrate forms a buoyant crust at the surface.

Paper I: Goody's convolution theorem for obtaining the cumulative k-distribution of a gas mixture requires stronger assumptions than the multiplicative property of band transmission; thus new experimental investigations of its effectiveness were undertaken. The convolution was found to be a useful speed optimization of k-distribution calculations at high pressures. For low pressures a variety of mixing methods were compared, all taking advantage of the idea that stratospheric lines are too narrow to overlap. Appendix I discusses the context and application of k-distribution calculations. Paper II: We used a "quasi-random" radiative transfer model to estimate stratospheric radiative perturbations produced by SO_2 gas, silicate ash, and H_2SO_4 aerosols after the 1982 El Chichon eruptions. One week after the last eruption, net radiative heating perturbations exceeding 20 K/day were modeled at altitudes near 26 km. Silicate ash heating may have been balanced by global enhancement of stratospheric meridional circulation, with upward velocities of 1 cm/s near Chichon's latitude. Radiative forcing by silicate ash and SO_2 gas should be included in more comprehensive models of plume evolution. Particle size distributions inferred from ash fallout rates could be wrong if radiative heating is neglected. Paper III: Uncertainties in the solar spectrum can affect modeled net heating rates in the upper stratosphere by a factor of several. Variation among Antarctic surface albedo values in common use can affect modeled net heating rates in the lower stratosphere by tens of percent. Large uncertainties in polar cloud cover are less important to stratospheric heating models. I join Marcel Nicolet in urging support for a continuous solar observation program, and recommend that future intercomparisons of stratospheric radiation models prescribe a solar spectrum, to reveal other differences. Appendix 2 gives the details of some further validation and sensitivity tests for the quasi-random model. Paper IV: The Porcupine Plate was postulated in 1986 to explain difficulties in reconstructing anomalies 21 and 24 in the North Atlantic. Its main feature was thought to be a transpressive Eocene plate boundary along Charlie-Gibbs Fracture Zone. Eliminating data that could have been affected by subsequent movements of Greenland relative to North America leads to a picture that casts doubt on the Porcupine Plate hypothesis.

Craterform and related features on Ganymede and Callisto include bowl-shaped craters, craters with nearly fiat floors, craters with central peaks, craters with central pits, basins, crater palimpsests and penepalimpsests, and giant multiring systems of ridges and furrows. The large majority of all craters larger than 20 km diameter have a central pit. The pits are interpreted as formed by prompt collapse of transient central peaks. Most craters, in all size ranges, are highly flattened as a consequence of topographic relaxation by slow viscous or plastic flow. Analysis of the global distribution of craters and multiring structures on Callisto reveal that the large multiring structures are concentrated in the leading hemisphere, whereas craters are depleted here. Calculations of model crater retention ages based on a sample of 2000 craters ≥ 30 km in diameter show that the mean age of Callisto's surface is between 4.0 and 4.2 Gy. Variations in the surface ages, derived from different diameter craters, suggests that larger craters are not retained from as early a period in time as were the smaller craters; this is in agreement with the results predicted by viscous relaxation theory where large wavelength features relax at a faster rate than do small wavelength features. Most of the variations in the observed distribution of craters can be explained satisfactorily by the effects due to the formation of multiring structures, and on the viscous relaxation of craters beneath an insulating regolith. About 1000 topographic profiles of craters on Ganymede and Callisto were obtained by photoclinometry. Fresh craters on Ganymede and Callisto have depth-to-diameter ratios and rim height-to-diameter ratios similar to those of fresh lunar craters, but most craters are much shallower. Small craters have not flattened or relaxed as much as have large craters; comparison of the crater profiles with the results from theoretical of crater relaxation studies in a viscous medium, allows determination of the viscosity at the surfaces of Ganymede and Callisto, and, also, determination of the viscosity gradient with depth. The derived mean surface viscosity for the lithospheres of Ganymede and Callisto is 1.0 ± 0.5 x 1026 poise. For Ganymede, the estimated thermal gradient at ~3.9 Gya was ≥ 8 K/km; the thermal gradient can be modelled as decreasing approximately exponentially with time, with an e-folding time of about 108 years; the estimated present thermal gradient is ≤ 2.0 K/km. For Callisto, the thermal gradient was ≥ 3 K/km at ~4.1 GYA and the decrease in the thermal gradient can be modelled as an exponential dropoff with an e-folding time between about 5 x 107 and 2 x 108 years; the estimated present thermal gradient on Callisto ≤ 1.5 K/km. High resolution Voyager II images of Enceladus reveal that some regions on its surface are highly cratered; the most heavily cratered surfaces probably date back into a period of heavy bombardment. The forms of many of the craters, on Enceladus, are similar to those of fresh lunar craters, but many of the craters are much shallower in depth, and the floors of some craters are bowed up. Analysis of the forms of the flattened craters on Enceladus suggests that the viscosity at the top of the lithosphere, in the most heavily cratered regions, is between 1024 and 1025 poise. The exact time scale for the collapse of the craters is not known, but probably was between 100 My and 4 Gy. The flattened craters are located in regions in which the heat flow was (or is) higher than in the adjacent terrains. Because the temperature at the top of the lithosphere of Enceladus would be less than, or equal to that of Ganymede and Callisto, if it is covered by a thick regolith, and because the required viscosity, on Enceladus, is one to two orders of magnitude less than for Ganymede and Callisto, it can be concluded that the lithospheric material, on Enceladus, is different from that of Ganymede and Callisto. Enceladus possibly has a mixture of ammonia ice and water ice in the lithosphere, whereas the lithospheres of Ganymede and Callisto are composed primarily of water ice. New field measurements of elevation of Provo-level and Bonneville-level shoreline terraces, of Lake Bonneville, provide data for reanalysis of isostatic rebound in the Lake Bonneviile basin. Analysis of the differential rebound between the Provo shoreline (maximum rebound of 43 m) and the Bonneville shoreline (maximum rebound of 69 m) requires that the latter be an equilibrium shoreline. From the new data, the best estimate of the upper limit of effective viscosity of the uppermost mantle, assuming a half-space model and a 2000 year time interval between the Bonneville and Provo shorelines, is 2 x 1019 N sec m-2 (2 x 1020 poise). In addition, comparison of shoreline rebound profiles, for both shorelines, with theoretical plate flexure models indicates that the mean flexural rigidity of the Basin and Range lithosphere in this region is 1 x 1023 N m, or slightly less.

Observations of Mars at wavelengths of 2 and 6cm were made using the VLA in its A configuration. Two seasons were observed; late spring in the northern hemisphere (LS ~ 60°) and early summer in the southern summer (LS ~ 300°). The sub-earth latitudes were 25°N and 25°S, for each of these seasons respectively. So the geometry for viewing the polar region was optimal in each case. Whole-disk brightness temperatures were estimated to be 193.2 K ± 1.0 at 2 cm and 191.2 K ± 0.6 at 6 cm for the northern data set and 202.2 K ± l.0 at 2 cm and 195.4 K ± 0.6 at 6 cm for the southern data set (formal errors only). Since measurements of the polarized flux were taken at the same time, whole-disk effective dielectric constants could be estimated and from these, estimates of sub-surface densities could be made. The results of these calculations at 2cm yielded whole-disk effective dielectric constants of 2.34 ± 0.05 and 2.02 ± 0.03 which imply sub-surface densities of 1.24 g cm-3 ± 0.06 and 1.02 g cm-3 ± 0.05 for the north and south, respectively. The same calculations at 6 cm yielded effective densities of 1.45 g cm-3 ± 0.10 and 1.31 g cm-3 ± 0.07 from effective dielectric constants of 2.70 ± 0.09 and 2.48 ± 0.06 for the north and south data sets, respectively. From the mapped data these parameters were also estimated as a function of latitude between latitudes of 15°S and 60°N for the north data set; and between latitudes of 30°N and 60°S for the south data set. A region in which the brightness temperature behaves in an anomalous manner was discovered in both data sets. This region lies between about 10°S and 40°S. Here the brightness temperatures at both wavelengths in both data sets appears lower, by 4 K to 8 K, than a nominal model would predict. In addition to the effective dielectric constant and sub-surface density the radio absorption length of the sub-surface was estimated. The radio absorption length for most of these latitudes was about 15 wavelengths with formal errors on the order of 5 or 10 wavelengths. This is true for both data sets. The estimation of the effective dielectric constant at most latitudes was between 2 and 3.5 with only slight differences between the two different wavelengths. The two data sets show the same relative trends, but are off by a scaling factor. These estimates of the dielectric constant lead to estimation of the sub-surface densities as a function of latitude. Most calculations of the sub-surface density yielded results between 1 and 2 g cm-3 with errors on the order of 0.5 g cm-3. These results seem to imply that the sub-surface is not much different than the surface as observed by the Viking and Mariner missions. In line with this, an examination of the correlation of the dielectric constant at each wavelength with the thermal inertia, determined by the Viking infrared measurements, shows a relatively strong correlation, at both wavelengths, for the North data set. The South data set, however, shows little to nocorrelation between the radio parameters and the thermal inertia. Since the South data set is primarily composed of latitudes which contain the anomalous region, it is not suprising that the South data set shows no correlation. In addition, the thermal-radiative model used to estimate the above parameters was used to estimate the variability of the whole-disk brightness temperature of Mars. This was done in an effort to establish a background for those astronomers wishing to use Mars as a calibration source. The parameters investigated for their effect on the whole-disk brightness temperature of Mars were: the sub-earth longitude, the sub-earth latitude, the sub-earth time of day, the dielectric constant, and the radio absorption length. A nominal model was first created which established the variation of the brightness temperature as a function of season and radio absorption length. A nominal value of 2.2 was used for the dielectric constant, and the sub-earth latitude was set at 0°N and the sub-earth longitude was set at 75°W. The sub-earth time of day was held at noon for this nominal model. This is equivalent to a 0° phase angle. The most important geometric factor was the sub-earth latitude. The error in estimating the whole-disk brightness temperature of Mars by using the wrong sub-earth latitude can be as large as 5 to 10%. The charts presented will be useful to estimate the whole-disk brightness temperature which the thermal model would predict. It is believed that the error in this estimate is less than or equal to 5 K.

Part 1: Five major eccentric features in the rings of Saturn are studied. These are the outer A and B ring edges at 1.95 and 2.27 Rₛ and three narrow ringlets at 1.29, 1.45, and 1.95 Rₛ. Data acquired by four Voyager experiments - Imaging Science (ISS), Radio Science (RSS), Ultraviolet Spectrometer (UVS), and Photopolarimeter(PPS) - were used in this investigation. The shapes and kinematics of the A and B ring outer edges are determined by their proximity to strong low-order Lindblad resonances. The data for the A ring edge are consistent with a 7-lobed distortion rotating with the mass-weighted mean angular velocity of the co-orbital satellite system. The B ring edge has a double-lobed figure which rotates with the mean motion of Mimas. The Saturnian ringlets are narrow (mean widths vary from ~ 10-60 km) and have eccentricities of order 10-4. All have sharp edges, normal optical depths τ ~ 1-2, and are embedded in essentially empty gaps (τ < 0.05). The Titan ring at 1.29 Rₛ and the Huygens ring at 1.45 Rₛ exhibit positive linear width-radius relations; the Maxwell ring at 1.95 Rₛ does not. The kinematics of the Huygens ring are determined solely by Saturn's non-spherical gravity field. The kinematics of the Titan ring are apparently completely determined by its interaction with Titan. At present, the most plausible model for the Maxwell ring involves the superposition of two components: one which is freely precessing and the other which is forced by Mimas and the elliptical B ring. Masses, mean surface mass densities, and specific opacities have been calculated for the Titan and Huygens rings. Part 2: The discovery of a periodic variation in spoke activity in Saturn's rings from the analysis of Voyager images is reported. A Fourier power spectrum was computed using a data set generated by quantifying spoke activity observed on the morning (western) half of the rings in Voyager images spanning ~ 12 Saturn rotations and in Voyager 2 images spanning ~ 90 Saturn rotations. The period from Voyager 1 data is 631 ± 22 min; from Voyager 2, 640.6 ± 3.5 min. The latter result suggests that the fundamental modulation in spoke activity is due to the rotation of Saturn's magnetic field, the period of which is 639.4 min. Maximum spoke activity observed anywhere on the rings is most likely to be associated with the region of the magnetic field responsible for the most intense emission of the Saturn Kilometric Radiation (SKR). Passage of this region through Saturn's shadow may play a significant role in the creation and/or rejuvenation of spokes.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Land degradation is a serious and growing problem on a world-wide scale -- 11% of the Earth's vegetated surface having suffered serious damage in the last 45 years. Human activity, especially sprinkler irrigation agriculture, can cause dramatic changes in arid regions as the fragile natural plant cover is stripped off and its root system destroyed in the process of cultivation. Satellite and airborne remote sensing data covering the Manix Basin of Eastern California over the last two decades shows that abandoned fields there suffered progressive degradation, as the topsoil eroded due to the lack of protective plant cover. Blowing sand buried and disrupted the downwind plant cover, which caused the downwind area to lose its protection against wind erosion and expanded the region of damage. Because the amount and kind of plant cover is an important marker both of where wind erosion has occurred and where it is likely to occur in the future, especially designed satellite monitoring systems should be able to sense to signatures of undisturbed and disturbed vegetation cover in arid regions. However, this problem cannot be addressed by standard vegetation indices, because of the adaptation of arid region plants to the scarcity of water. Furthermore, weekly to monthly sampling will be necessary because blowing sand visible to satellite remote sensing is highly dependent on the local weather, and this can change within a few months. A new vegetative index suitable for arid regions is proposed for the wavelength region from 0.4-1.0 [...]. The detection and identification of arid region plant communities requires a highly calibrated remote sensing system with higher spectral resolution than that currently offered by Landsat Thematic Mapper. The way in which regions of blowing sand can appear and disappear with rapidity demonstrates the need for a remote monitoring system that can survey large areas on a regular basis. Such a system must be supported by focused ground observations and a continuing analysis of the satellite data.