Stephen D. Wall

The lakes of Titan

The surface of Saturn’s haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan’s surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface ‘liquid methane’ table.

Aeolian features on Venus - Preliminary Magellan results

Magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian, or wind processes. Wind streaks are the most common aeolian feature. They consist of radar backscatter patterns that are high, low, or mixed in relation to the surface on which they occur. A data base of more than 3400 wind streaks shows that low backscatter linear forms (long, narrow streaks) are the most common and that most streaks occur between 17°S to 30°S and 5°N to 53°N on smooth plains. Moreover, most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains. We infer that both of these geological settings provide fine particulate material that can be entrained by the low-velocity winds on Venus. Turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks. Thus, some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock; other high backscatter streaks may be lag deposits of dense materials from which low-density grains have been removed (dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns). Wind streaks generally occur on slopes < 2° and tend to be oriented toward the equator, consistent with the Hadley model of atmospheric circulation. In addition to wind streaks, other aeolian features on Venus include yardangs(?) and dune fields. The Aglaonice dune field, centered at 25°S, 340°E, covers ∼1290 km^2 and is located in an ejecta flow channel from the Aglaonice impact crater. The Meshkenet dune field, located at 67°N, 90°E, covers ∼17,120 km^2 in a valley between Ishtar Terra and Meshkenet Tessera. Wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds. Dunes on Venus signal the presence of sand-size (∼60 to 2,000 μm) grains. The possible yardangs are found at 9°N, 60.5°E, about 300 km southeast of the crater Mead. Although most aeolian features are concentrated in smooth plains near the equator, the occurrence of wind streaks is widespread, and some have been found at all latitudes and elevations. They demonstrate that aeolian processes operate widely on Venus. The intensity of wind erosion and deposits, however, varies with locality and is dependent on the wind regime and supply of particles.

Applications of spaceborne radar laboratory data to the study of aeolian processes

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ=24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ=5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems.

Multiple Incidence Angle SIR-B Experiment Over Argentina: Mapping of Forest Units

Multiple incidence angle SIR-B data of the Cordón la Grasa region of the Chubut Province of Argentina are used to discriminate nate various forest types by their relative brightness versus incidence angle signatures. The region consists of several species of Nothofagas which change in canopy structure with elevation, slope, and exposure. In general, the factors that appear to impact the radar response most are canopy structure, density, and ground cover (presence or absence of dead trunks and branches in particular). The results of this work indicate that 1) different forest species, and structures of a singlee-species, may be discriminated using multiple incidence angle radar-imagery and 2) it is essential to consider the variation in backscatter due to incidence angle when analyzing and comparing data collected at varying frequencies and polarizations.

The Lakes and Seas of Titan

Analogous to Earths water cycle, Titans methane-based hydrologic cycle supports standing bodies of liquid and drives processes that result in common morphologic features including dunes, channels, lakes, and seas. Like lakes on Earth and early Mars, Titans lakes and seas preserve a record of its climate and surface evolution. Unlike on Earth, the volume of liquid exposed on Titans surface is only a small fraction of the atmospheric reservoir. The volume and bulk composition of the seas can constrain the age and nature of atmospheric methane, as well as its interaction with surface reservoirs. Similarly, the morphology of lacustrine basins chronicles the history of the polar landscape over multiple temporal and spatial scales. The distribution of trace species, such as noble gases and higher-order hydrocarbons and nitriles, can address Titans origin and the potential for both prebiotic and biotic processes. Accordingly, Titans lakes and seas represent a compelling target for exploration.

Model-based engineering design for space missions

The basic elements of model-based design for space missions have existed for almost a decade, awaiting an opportunity to implement them in the same place at the same time. In early design phases, combinations of models, concurrent engineering methods, and scenario-driven design have been used for several years with results that have exceeded even optimistic expectations; but the goal of extending these methods to later phases of design has been more elusive. JPLs model-based engineering design (MBED) initiative provides opportunity to reach that goal. It enables advanced systems engineering practice through a series of integrated, increasingly detailed models that provide continuity from architectural concept through detailed design. It extends current capability for rapid conceptual design, allowing thorough exploration of design tradespaces and selection of an optimal design point with associated cost and rationale; and it provides seamless connection to subsystem models and detailed design tool suites. We review the goals and status of MBED and show the expected interconnectivity between conceptual and detailed design.

Titan, fractals, and filtering of Cassini altimeter data

Fractal behavior of one Titan profile acquired by the Cassini altimeter during its first flyby is demonstrated and quantitatively analyzed. The inadequacy of popular nonfractal models to represent scale-dependent behavior is also discussed. Our results lead to the conjecture that a proper geometrical model of the Titan surface is provided by the fractional Brownian motion stochastic process and that the particular acquired profile has a fractal dimension of about 2.4. In addition, use of the developed statistical model allows the design of model-based filters for a significant improvement of the measurement accuracy, at the expense of the horizontal resolution

A Lunar Surface Operations Simulator

The Lunar Surface Operations Simulator (LSOS) is being developed to support planning and design of space missions to return astronauts to the moon. Vehicles, habitats, dynamic and physical processes and related environment systems are modeled and simulated in LSOS to assist in the visualization and design optimization of systems for lunar surface operations. A parametric analysis tool and a data browser were also implemented to provide an intuitive interface to run multiple simulations and review their results. The simulator and parametric analysis capability are described in this paper.

Radiometric calibration analysis of SIR-B imagery

Abstract The second flight of the NASA Spaceborne Imaging Radar (SIR-B) collected nearly 8 hours of digital synthetic-aperture radar (SAR) data of the Earths surface. This paper analyses the performance of the SAR to determine the accuracy of the relative radiometric calibration of these data. Procedures are presented for deriving accuracy estimates as well as limitations under which these estimates are valid. The impact on calibration of an RF breakdown in the primary antenna feed system cable is evaluated. An analysis of the platform stability based on the SAR echo data is also presented in conjunction with its resultant effect on the calibration accuracy. Finally, numerical error bounds are derived with guidelines on their utilization.

Images and topographic relief at the North Pole of Venus

We report on the analysis of unique coverage of the north pole of Venus using a subset of 13 images from a total of about 775 synthetic aperture radar orbits during cycle 1 of the Magellan mission. We present images at 13 distinctly different azimuths that range over 360° in longitude and that show the topographic relief at vastly different imaging geometries at intervals of about 20°. A pair of intersecting or crossing orbits is used to assess the topographic relief with stereo radargrammetric techniques, and to refine this assessment using a multi-image clinometric approach. We show that the pole is located in accentuated topography with elevation differences in the range of no more than 1 km. We conclude that pinpointing the location of the pole from the overlapping images and standard mission data is accurate to within about 100 pixels or 8 km (RMS).