S. D. Wall

Hydrocarbon lakes on Titan: Distribution and interaction with a porous regolith

from <10 to more than 100,000 km 2 . The size and location of lakes provide constraints on parameters associated with subsurface transport. Using porous media properties inferred from Huygens probe observations, timescales for flow into and out of observed lakes are shown to be in the tens of years, similar to seasonal cycles. Derived timescales are compared to the time between collocated SAR observations in order to considertheroleofsubsurfacetransportinTitan’shydrologic cycle. Citation: Hayes, A., et al. (2008), Hydrocarbon lakes on Titan: Distribution and interaction with a porous regolith,Geophys. Res. Lett., 35, L09204, doi:10.1029/2008GL033409.

Overview of results of Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR)

The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was launched on the Space Shuttle Endeavour for two ten day missions in the spring and fall of 1994. Radar data from these missions are being used to better understand the dynamic global environment. During each mission, radar images of over 300 sites around the Earth were obtained, returning over a terabit of data. SIR-C/X-SAR science investigations were focused on quantifying radars ability to estimate surface properties of importance to understanding global change; and focused studies in geology, ecology, hydrology and oceanography, as well as radar calibration and electromagnetic theory studies. In addition, the second flight featured an interferometry experiment, where digital elevation maps were obtained by interfering data from the first and second shuttle flight, and from successive days on the second flight. SIR-C/X-SAR data have been used to validate algorithms which produce maps of vegetation type and biomass; snow, soil and vegetation moisture; and the distribution of wetlands, developed with earlier aircraft data. >

Titan Radar Mapper observations from Cassini's T3 fly-by

Cassinis Titan Radar Mapper imaged the surface of Saturns moon Titan on its February 2005 fly-by (denoted T3), collecting high-resolution synthetic-aperture radar and larger-scale radiometry and scatterometry data. These data provide the first definitive identification of impact craters on the surface of Titan, networks of fluvial channels and surficial dark streaks that may be longitudinal dunes. Here we describe this great diversity of landforms. We conclude that much of the surface thus far imaged by radar of the haze-shrouded Titan is very young, with persistent geologic activity.

Active shoreline of Ontario Lacus, Titan: A morphological study of the lake and its surroundings

Of more than 400 filled lakes now identified on Titan, the first and largest reported in the southern latitudes is Ontario Lacus, which is dark in both infrared and microwave. Here we describe recent observations including synthetic aperture radar (SAR) images by Cassinis radar instrument (λ = 2 cm) and show morphological evidence for active material transport and erosion. Ontario Lacus lies in a shallow depression, with greater relief on the southwestern shore and a gently sloping, possibly wave-generated beach to the northeast. The lake has a closed internal drainage system fed by Earth-like rivers, deltas and alluvial fans. Evidence for active shoreline processes, including the wave-modified lakefront and deltaic deposition, indicates that Ontario is a dynamic feature undergoing typical terrestrial forms of littoral modification.

Bathymetry and absorptivity of Titan's Ontario Lacus

Ontario Lacus is the largest and best characterized lake in Titans south polar region. In June and July 2009, the Cassini RADAR acquired its first Synthetic Aperture Radar (SAR) images of the area. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study the lakes nearshore bathymetry and complex refractive properties. Average radar backscatter is observed to decrease exponentially with distance from the local shoreline. This behavior is consistent with attenuation through a deepening layer of liquid and, if local topography is known, can be used to derive absorptive dielectric properties. Accordingly, we estimate nearshore topography from a radar altimetry profile that intersects the shoreline on the East and West sides of the lake. We then analyze SAR backscatter in these regions to determine the imaginary component of the liquids complex index of refraction (κ). The derived value, κ = (6.1_(−1.3)^(+1.7)) × 10^(−4), corresponds to a loss tangent of tan Δ = (9.2_(−2.0)^(+2.5)) × 10^(−4) and is consistent with a composition dominated by liquid hydrocarbons. This value can be used to test compositional models once the microwave optical properties of candidate materials have been measured. In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming the index of refraction is constant throughout the liquid. Accordingly, we construct a coarse bathymetry map for the nearshore region by measuring bathymetric slopes for eleven additional areas around the lake. These slopes vary by a factor of ∼5 and correlate well with observed shoreline morphologies.

Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolcanic activity

[1] Images obtained by the Cassini Titan Radar Mapper (RADAR) reveal lobate, flowlike features in the Hotei Arcus region that embay and cover surrounding terrains and channels. We conclude that they are cryovolcanic lava flows younger than surrounding terrain, although we cannot reject the sedimentary alternative. Their appearance is grossly similar to another region in western Xanadu and unlike most of the other volcanic regions on Titan. Both regions correspond to those identified by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) as having variable infrared brightness, strengthening the case that these are recent cryovolcanoes. Citation: Wall, S. D., et al. (2009), Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolcanic activity, Geophys. Res. Lett., 36, L04203, doi:10.1029/2008GL036415.

The shuttle imaging radar-C and X-SAR mission

The Shuttle Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR) (Figure 1) is a cooperative space shuttle experiment between NASA, the German Space Agency, and the Italian Space Agency. The experiment is the next evolutionary step in NASAs Spaceborne Imaging Radar (SIR) program that began with the Seasat Synthetic Aperture Radar (SAR) in 1978 and continued with SIR-A in 1981 and SIR-B in 1984. It also represents a continuation of Germanys imaging radar program, which started with the Microwave Remote Sensing Experiment flown aboard the Shuttle on the first SPACELAB mission in 1983. The SIR-C/X-SAR mission benefits from synergism with the Magellan mission to Venus, other international spaceborne radar programs, and prototype aircraft sensors such as the Jet Propulsion Laboratorys Airborne SAR (AIRSAR) and the German Aerospace Establishment (DLR) E-SAR.

Assessment of aerodynamic roughness via airborne radar observations

The objective of this research is to assess the relationship among measurements of roughness parameters derived from radar backscatter, the wind, and topography on various natural surfaces and to understand the underlying physical causes for the relationship. This relationship will form the basis for developing a predictive equation to derive aerodynamic roughness (z0) from radar backscatter characteristics. Preliminary studies support the existence of such a relationship at the L-band (24 cm wavelength) direct polarization (HH) radar band frequencies. To increase the confidence in the preliminary correlation and to extend the application of the technique to future studies involving regional aeolian dynamics, the preliminary study has been expanded by: 1) defining the empirical relationship between radar backscatter and aerodynamic roughness of bare rocks and soils, 2) investigating the sensitivity of the relationship to microwave parameters using calibrated multiple wavelength, polarization, and incidence angle aircraft radar data, and 3) applying the results to models to gain an understanding of the physical properties which produce the relationship. The approach combines the measurement, analysis, and interpretation of radar data with field investigations of aeolian processes and topographic roughness.

Cassini RADAR Sequence Planning and Instrument Performance

The Cassini RADAR is a multimode instrument used to map the surface of Titan, the atmosphere of Saturn, the Saturn ring system, and to explore the properties of the icy satellites. Four different active mode bandwidths and a passive radiometer mode provide a wide range of flexibility in taking measurements. The scatterometer mode is used for real aperture imaging of Titan, high-altitude (around 20 000 km) synthetic aperture imaging of Titan and Iapetus, and long range (up to 700 000 km) detection of disk integrated albedos for satellites in the Saturn system. Two SAR modes are used for high- and medium-resolution (300-1000 m) imaging of Titans surface during close flybys. A high-bandwidth altimeter mode is used for topographic profiling in selected areas with a range resolution of about 35 m. The passive radiometer mode is used to map emission from Titan, from Saturns atmosphere, from the rings, and from the icy satellites. Repeated scans with differing polarizations using both active and passive data provide data that can usefully constrain models of surface composition and structure. The radar and radiometer receivers show very good stability, and calibration observations have provided an absolute calibration good to about 1.3 dB. Relative uncertainties within a pass and between passes can be even smaller. Data are currently being processed and delivered to the planetary data system at quarterly intervals one year after being acquired.

Model-Based Engineering Design Pilots at JPL

This paper discusses two recent formulation phase model-based engineering design pilot projects at the Jet Propulsion Laboratory. It describes how model-based functional and state analyses were synthesized and integrated with system performance simulation and mission planning then piloted in the formulation phase of two deep space missions.