L. Wye

Stanford University, Palo Alto, California, United States

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Publications (75)205.44 Total impact

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    ABSTRACT: The existence of cryovolcanic features on Titan has been the subject of some controversy. Here we use observations from the Cassini RADAR, including Synthetic Aperture Radar (SAR) imaging, radiometry, and topographic data as well as compositional data from the Visible and Infrared Mapping Spectrometer (VIMS) to reexamine several putative cryovolcanic features on Titan in terms of likely processes of origin (fluvial, cryovolcanic, or other). We present evidence to support the cryovolcanic origin of features in the region formerly known as Sotra Facula, which includes the deepest pit so far found on Titan (now known as Sotra Patera), flow-like features (Mohini Fluctus), and some of the highest mountains on Titan (Doom and Erebor Montes). We interpret this region to be a cryovolcanic complex of multiple cones, craters, and flows. However, we find that some other previously supposed cryovolcanic features were likely formed by other processes. Cryovolcanism is still a possible formation mechanism for several features, including the flow-like units in Hotei Regio. We discuss implications for eruption style and composition of cryovolcanism on Titan. Our analysis shows the great value of combining data sets when interpreting Titan's geology and in particular stresses the value of RADAR stereogrammetry when combined with SAR imaging and VIMS.
    Journal of Geophysical Research 03/2013; 118(3):416-435. · 3.17 Impact Factor
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    ABSTRACT: On Sept. 10, 2007, the Cassini spacecraft encountered Saturn's third largest moon, Iapetus, during the closest encounter with the two-toned moon to date. During this experiment (called the IA49-3 experiment), the Cassini radar/radiometer observed the leading hemisphere of the moon, collecting a unique passive (radiometry) and active (radar) data set at the wavelength of 2.2-cm. At such a wavelength, the radiometer probes several tens of cm up to a few meters below the surface, depending on the absorbing properties of the regolith. In this paper, we show that the seasonal contrast between the high latitude terrains of Iapetus was captured during IA49-3 thus providing new constrains on the electrical and thermal properties of the moon's surface.
    09/2012;
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    ABSTRACT: Because of its large distance from Saturn and its high inclination, the Cassini spacecraft has made only one close (altitude<25 000 km) flyby (IA49) of Iapetus : on September 10, 2007. During this opportunity, the RADAR instrument scanned the antenna beam in a north-south raster pattern, mostly over the dark terrains (named Cassini Regio) of the leading hemisphere of the moon. During this scan, it collected a unique and concurrent set of passive (radiometry) and active (scatterometry) data at 2.2-cm wavelength and with a footprint size of ~120 km (~15% of Iapetus' diameter). The Cassini radiometer measures the surface microwave thermal emission, which varies with the emissivity (or reflectivity) and physical temperature profile of the near-surface. At such a wavelength, it probes several tens of cm up to a few meters below the surface, depending on the absorbing properties of Iapetus' regolith. Combined with the concurrent active data, the radiometry data acquired during IA49 can be used to constrain the electrical and thermal properties of Iapetus' dark region thus providing clues on the physical state (roughness, porosity) and composition of these terrains whose nature and origin are still under debate. In this paper, we will report on the Cassini microwave observations recorded during IA49 in the active and passive modes and describe the radiative transfer model we have developed in order to analyze the radiometry data. Comparison with this model indicates that the thermal inertia sensed by the Cassini radar radiometer at 2.2 cm over Cassini Regio significantly exceeds that measured in the thermal infrared by the Cassini's Composite Infrared Spectrometer (CIRS) instrument (~10 in Rivera-Valentin et al., 2011). This suggests a gradient in density with depth, which is typical for planetary regoliths. The radiometer also captured the temperature asymmetry around the Equator due to heat buried in ground on seasonal timescales while the different local solar times of the equatorial observations seem to be responsible for a variation of less than 10 K in the brightness temperature recorded over Cassini Regio.
    04/2012;
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    ABSTRACT: On November 6, 2011, Cassini RADAR obtained a unique data set during a flyby of Enceladus. We will discuss the observation design and processing and present the data in preliminary form.
    03/2012;
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    ABSTRACT: The scatterometry mode of the Cassini RADAR is the premier dataset with which to investigate the scattering properties of Titan's surface. The scatterometry mode observes a wider range of incidence angles, has acquired near-global coverage, includes more robust radiometric calibration, and can discern features at lower signal levels than is possible with the fine-resolution synthetic aperture radar (SAR) mode. The downside to scatterometry analysis is that the real aperture surface footprints are much coarser than the SAR resolution. Here we present high-resolution backscatter maps derived from Cassini scatterometer observations at ~5-20 km resolution, coarser than the SAR observations (0.3-1km resolution) but finer than the 100 km resolution offered by real aperture scatterometer data reduction. These new products are made possible by analyzing the range delay and phase of the scatterometry measurements, rather than using the total beam-integrated power computation approach in real-aperture reduction. Cassini scatterometer data are acquired using a low-bandwidth chirp modulation on the transmitted signal, and each observation consists of a burst of about 8 transmit pulses. Using a coherent back projection algorithm, we process the data to improved resolution by about a factor of 10 in each dimension over real aperture values, although not all pass geometries have range/Doppler surface contours to support this resolution. Nonetheless, the finer resolution offered on well-contoured passes implies that we can estimate the backscatter curve for features much smaller than has been possible to date. The existence of multiple observations of each of these finer features means that we can better constrain surface roughness and dielectric constant properties than is possible from the SAR data alone, where limited observations exist of any single feature. Here we present initial reductions of the scatterometry data set and show that we can predict resolution performance by examining the range and Doppler contour diagrams from each pass. These images display moderate resolution Titan backscatter maps of areas not before imaged at fine resolution. The contour analysis in addition provides a way to schedule future Cassini observations of un-investigated areas in order to make the best use of spacecraft resources.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Titan’s enigmatic Xanadu province has been seen in some detail with instruments from the Cassini spacecraft. The region contains some of the most rugged, mountainous terrain on Titan, with relief over 2000 m. Xanadu contains evolved and integrated river channels, impact craters, and dry basins filled with smooth, radar-dark material, perhaps sediments from past lake beds. Arcuate and aligned mountain chains give evidence of compressional tectonism, yet the overall elevation of Xanadu is puzzlingly low compared to surrounding sand seas. Lineations associated with mountain fronts and valley floors give evidence of extension that probably contributed to this regional lowering. Several locations on Xanadu’s western and southern margins contain flow-like features that may be cryovolcanic in origin, perhaps ascended from lithospheric faults related to regional downdropping late in its history. Radiometry and scatterometry observations are consistent with a water–ice or water–ammonia–ice composition to its exposed, eroded, fractured bedrock; both microwave and visible to near-infrared (v-nIR) data indicate a thin overcoating of organics, likely derived from the atmosphere. We suggest Xanadu is one of the oldest terrains on Titan and that its origin and evolution have been controlled and shaped by compressional and then extensional tectonism in the icy crust and ongoing erosion by methane rainfall.
    Icarus. 01/2011;
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    ABSTRACT: Since Cassini arrived at Saturn in 2004, its moon Titan has been thoroughly mapped by the RADAR instrument at 2-cm wavelength, in both active and passive modes. Some regions on Titan, including Xanadu and various bright hummocky bright terrains, contain surfaces that are among the most radar-bright encountered in the Solar System. This high brightness has been generally attributed to volume scattering processes in the inhomogeneous, low-loss medium expected for a cold, icy satellite surface. We can test this assumption now that the emissivity has been obtained from the concurrent radiometric measurements for nearly all the surface, with unprecedented accuracy (Janssen et al., and the Cassini RADAR Team [2009]. Icarus 200, 222–239). Kirchhoff’s law of thermal radiation relates the radar and radiometric properties in a way that has never been fully exploited. In this paper we examine here how this law may be applied in this case to better understand the nature of Titan’s radar-bright regions. We develop a quantitative model that, when compared to the observational data, allows us to conclude that either the reflective characteristics of the putative volume scattering subsurface must be highly constrained, or, more likely, organized structure on or in the surface is present that enhances the backscatter.
    Icarus 01/2011; 212(1):321-328. · 3.16 Impact Factor
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    ABSTRACT: Large expanses of linear dunes cover Titan’s equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini’s radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan’s geology and climate. We estimate that dune fields cover ∼12.5% of Titan’s surface, which corresponds to an area of ∼10millionkm2, roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ∼11°, dune fields tend to become less emissive and brighter as one moves northward. Above ∼11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ∼14°. In general, scattering from the subsurface (volume scattering and surface scattering from buried interfaces) makes interdunal regions brighter than the dunes. The observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north, thus allowing wave penetration in the underlying substrate. Altimetry measurements over dunes have highlighted a region located in the Fensal dune field (∼5° latitude) where the icy bedrock of Titan is likely exposed within smooth interdune areas. The hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and/or an increase in the ground humidity toward the north, which could be related to Titan’s asymmetric seasonal polar insolation. Alternatively, it may indicate that either the wind pattern or the topography is less favorable for dune formation in Titan’s northern tropics.
    Icarus 01/2011; 213(2):608-624. · 3.16 Impact Factor
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    ABSTRACT: Cassini RADAR images of Titan’s south polar region acquired during southern summer contain lake features which disappear between observations. These features show a tenfold increases in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries are transient, further supporting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately 1-m-per-year. On larger scales, we observe shoreline recession between ISS and RADAR images of Ontario Lacus, the largest lake in Titan’s south polar region. The recession, occurring between June 2005 and July 2009, is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3 m in lake depth.Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario’s shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are compared and shown to be consistent with available global circulation models. To date, no unambiguous changes in lake level have been observed between repeat images in the north polar region, although further investigation is warranted. These observations constrain volatile flux rates in Titan’s hydrologic system and demonstrate that the surface plays an active role in its evolution. Constraining these seasonal changes represents the first step toward our understanding of longer climate cycles that may determine liquid distribution on Titan over orbital time periods.
    Icarus. 01/2011;
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    ABSTRACT: Hayes et al. (JGR 2010) observed that the Cassini synthetic aperture radar (SAR) imaging magnitudes collected over Titan's Ontario Lacus vary exponentially with distance from the lake shore, as expected if there is a deepening liquid layer that is attenuating the reflection from a roughened bottom. They deduce near-shore slopes on the order of 10-3. Here, we extend this analysis across the entire width and length of the lake by applying the Hayes et al. approach to the real-aperture (beam-averaged) scatterometry and SAR mode data collected on Titan flyby T65 (12-January-2010). The real-aperture reduction provides longer integration times, thereby reducing the noise in the data. Consequently, we can detect bottom reflections from greater depths within the lake. . We create a depth profile along the diagonal of the lake using the T65 SAR mode data, assuming the dielectric properties inferred by Hayes et al. apply uniformly across the lake volume. The 8 km SAR beam footprint slightly smears out the actual depth profile. Nearly perpendicular to this track, the T65 scatterometry data, with 15 km footprints, yields a coarser depth profile across the dark waist of Ontario Lacus. The two profiles intersect at the darkest, and likely deepest, region of the lake. The shape of these profiles has implications for the lake geology. . Allowing for scatter from small-scale waves on the surface of the lake, we constrain the maximum depth of the dark region to be less than 9 meters over our km-scale resolution cell. Depths over the rest of the lake are less than 5 meters. These shallow depths may have implications for the lake's composition. We also model the rms wave heights to be less than 1 mm, consistent with the analysis of Wye et al. (GRL 2009). These are all conservative upper limits.
    10/2010;
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    09/2010;
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    ABSTRACT: Ontario Lacus' shoreline features include Earth-like rivers, deltas and flooded topography. Ontario is a dynamic lake, similar in many ways to terrestrial lakes, with active shoreline processes.
    03/2010;
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    ABSTRACT: We apply a multivariate statistical method to Titan data acquired by different instruments onboard the Cassini spacecraft. We have searched through Cassini/VIMS hyperspectral cubes, selecting those data with convenient viewing geometry and that overlap with Cassini/RADAR scatterometry footprints with a comparable spatial resolution. We look for correlations between the infrared and microwave ranges the two instruments cover. Where found, the normalized backscatter cross-section obtained from the scatterometer measurement, corrected for incidence angle, and the calibrated antenna temperature measured along with the scatterometry echoes, are combined with the infrared reflectances, with estimated errors, to produce an aggregate data set, that we process using a multivariate classification method to identify homogeneous taxonomic units in the multivariate space of the samples.In medium resolution data (from 20 to 100 km/pixel), sampling relatively large portions of the satellite’s surface, we find regional geophysical units matching both the major dark and bright features seen in the optical mosaic. Given the VIMS cubes and RADAR scatterometer passes considered in this work, the largest homogeneous type is associated with the dark equatorial basins, showing similar characteristics as each other on the basis of all the considered parameters.On the other hand, the major bright features seen in these data generally do not show the same characteristics as each other. Xanadu, the largest continental feature, is as bright as the other equatorial bright features, while showing the highest backscattering coefficient of the entire satellite. Tsegihi is very bright at 5 μm but it shows a low backscattering coefficient, so it could have a low roughness on a regional scale and/or a different composition. Another well-defined region, located southwest of Xanadu beyond the Tui Regio, seems to be detached from the surrounding terrains, being bright at 2.69, 2.78 and 5 μm but having a low radar brightness. In this way, other units can be found that show correlations or anti-correlations between the scatterometric response and the spectrophotometric behavior, not evident from the optical remote sensing data.
    Icarus 01/2010; · 3.16 Impact Factor
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    ABSTRACT: Radarclinometry is a powerful technique for estimating heights of landforms in synthetic aperture radar (SAR) images of planetary surfaces. In particular, it has been used to estimate heights of dunes in the sand seas of Saturn’s moon Titan (Lorenz, R.D., and 39 colleagues [2006]. Science 312, 724–727). In this work, we verify the technique by comparing dune heights derived from radarclinometry to known topography of dune fields in the Namib sand sea of western Africa. We compared results from three different image grid spacings, and found that 350 m/pixel (the same spacing at which the Cassini RADAR data was processed) is sufficient to determine dune height for dunes of similar morphometry to those of the Namib sand sea. At this grid spacing, height estimates derived from radarclinometry are largely representative of, though may underestimate by as much as 30%, or overestimate by as much as 40%, true dune height. Applying the technique to three regions on Titan, we estimate dune heights of 45–180 m, and dune spacings of 2.3–3.3 km. Obtaining accurate heights of Titan’s dunes will help to constrain the total organic inventory on Titan.
    Icarus 01/2010; · 3.16 Impact Factor
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    ABSTRACT: 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 Cassini's 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.
    01/2010;
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    ABSTRACT: The Cassini Titan Radar Mapper is providing an unprecedented view of Titan’s surface geology. Here we use Synthetic Aperture Radar (SAR) image swaths (Ta–T30) obtained from October 2004 to December 2007 to infer the geologic processes that have shaped Titan’s surface. These SAR swaths cover about 20% of the surface, at a spatial resolution ranging from ∼350 m to ∼2 km. The SAR data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth – volcanism, tectonism, impact cratering, and erosion and deposition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data and analyze their areal distribution and relative ages of modification in order to infer the geologic evolution of Titan’s surface. We find that dunes and hummocky and mountainous terrains are more widespread than lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures that may be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In terms of latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low latitudes (less than 30°), with no dunes being present above 60°. Channels formed by fluvial activity are present at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have been formed by impact appear to be uniformly distributed with latitude, but the well-preserved impact craters are all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes. Cryovolcanic features are not ubiquitous, and are mostly located between 30° and 60° north. We examine temporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountains and Xanadu are likely the most ancient.
    Icarus. 01/2010;
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    ABSTRACT: The Cassini RADAR instrument has observed ˜60% of Titan's surface poleward of 55° , re-vealing hydrocarbon lakes with morphologies and scales similar to terrestrial counterparts. Synthetic Aperture Radar (SAR) images of the South Polar Region, acquired during southern summer, show lake features which disappear between observations. These features show an or-der of magnitude increase in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries of these features are transient, further suggesting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately one meter-per-year. Ephemeral features have yet to be obserevd in the North Polar Region. On larger scales, SAR images of Ontario Lacus, the largest and best characterized lake in the South Polar Region, were first obtained in June and July 2009. Together with altimetry data acquired in December 2008, these observations provide a unique opportunity to study Ontario's near-shore bathymetry and complex refractive properties. The exponential decay of near-shore backscatter is used to estimate microwave optical absorption properties. Shoreline recession is observed between 2005 and 2009 images of Ontario Lacus. The recession is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3 m in lake depth. Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario's shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are consistent with available global circulation models. These observations constrain volatile flux rates in Titan's hydrologic system and demonstrate that the surface plays an active role in its evolution.
    01/2010;
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    ABSTRACT: Cassini radar tracks on Saturn’s icy satellites through the end of the Prime Mission in 2008 have increased the number of radar albedo estimates from 10 (Ostro et al., 2006) to 73. The measurements sample diverse subradar locations (and for Dione, Rhea, and Iapetus almost always use beamwidths less than half the target angular diameters), thereby constraining the satellites’ global radar albedo distributions. The echoes result predominantly from volume scattering, and their strength is thus strongly sensitive to ice purity and regolith maturity. The combination of the Cassini data set and Arecibo 13-cm observations of Enceladus, Tethys, Dione, Rhea (Black et al., 2007), and Iapetus (Black et al., 2004) discloses an unexpectedly complex pattern of 13-to-2-cm wavelength dependence. The 13-cm albedos are generally smaller than 2-cm albedos and lack the correlation seen between 2-cm and optical geometric albedos. Enceladus and Iapetus are the most interesting cases. We infer from hemispheric albedo variations that the E-ring has a prominent effect on the 13-cm radar “lightcurve”. The uppermost trailing-side regolith is too fresh for meteoroid bombardment to have developed larger-scale heterogeneities that would be necessary to elevate the 13-cm radar albedo, whereas all of Enceladus is clean and mature enough for the 2-cm albedo to be uniformly high. For, Iapetus, the 2-cm albedo is strongly correlated with optical albedo: low for the optically dark, leading-side material and high for the optically bright, trailing-side material. However, Iapetus’ 13-cm albedo values show no significant albedo dichotomy and are several times lower than 2-cm values, being indistinguishable from the weighted mean of 13-cm albedos for main-belt asteroids, 0.15 ± 0.10. The leading side’s optically dark contaminant must be present to depths of at least one to several decimeters, so 2-cm albedos can mimic the optical dichotomy; however, it does not have to extend any deeper than that. The fact that both hemispheres of Iapetus look Asteroid-like at 13 cm means that coherent backscattering itself is not nearly as effective as it is at 2 cm. Since Iapetus’ entire surface is mature regolith, the wavelength dependence must involve composition, not structure. Either the composition is a function of depth everywhere (with electrical loss much greater at depths greater than a decimeter or two), or the intrinsic electrical loss of some pervasive constituent is much higher at 13 cm than at 2 cm. Ammonia is a candidate for such a contaminant. If ammonia’s electrical properties do not depend on frequency, and if ammonia is globally much less abundant within the upper one or two decimeters than at greater depths, then coherent backscattering would effectively be shut down at 13 cm, explaining the Asteroid-like 13-cm albedo.
    Icarus. 01/2010;
  • L. C. Wye, H. A. Zebker
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    ABSTRACT: The Cassini RADAR instrument acquired backscatter measurements from Iapetus' dark leading hemisphere over a full range of incidence angles (1 to 60 degrees) during the 2007 targeted flyby. We present here the first radar backscatter curve of an icy moon of Saturn other than Titan, and it indicates the presence of a small quasi-specular term (~3% of the total radar albedo). Quasi-specular scattering has not been previously detected in distant radar spectra of this moon. We model the dark side's backscatter response as a composite of the linear superposition of two classical quasi-specular scattering laws plus a diffuse cosine power law. We find that Iapetus' dark hemisphere has a rather low bulk dielectric constant that is consistent with a surface largely composed of porous organic tholins. We compare the Iapetus backscatter to those that we have measured for various features on Titan and find that, at our at 2.2 cm wavelength, the dark side of Iapetus scatters similarly to Titan's Xanadu feature, albeit with a much lower albedo. This finding is consistent with the theory that Iapetus' dark side comprises an icy volume scattering substrate that is coated by an attenuating dark layer.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: Ephemeral hydrocarbon liquids have been observed in Titan's south polar region by the Cassini RADAR during southern summer. A collection of features, whose morphologies match that of previously identified partially-filled lakes, show more than an order of magnitude increase in backscatter. The morphologic boundaries of these features are transient between observations, suggesting surface change. Radiometrically, these changes are inconsistent with common scattering models. If not due to observational effects, the disappearance of these features represents volatile transport in Titan's hydrologic cycle. Lacustrine features identified on Titan have been grouped into three classes; empty lake basins, partially-filled lakes, and dark or liquid-filled lakes. Partially-filled lakes have radar returns consistent with incident radiation penetrating a liquid layer and interacting with the lakebed, while dark lakes completely reflect and absorb incident microwave energy. Empty lakes are brighter than their exteriors in both nadir and off-nadir observations, suggesting a strong volume scattering component. Backscatter models that include diffuse and quasi-specular scattering components are required to explain the incidence angle dependence of empty lakes. These scattering models are consistent with both individual empty lakes observed at multiple incidence angles and the collective set of empty lakes observed to date. Partially-filled lakes have lower backscatter, forbidding a significant diffusive component, and steeper slopes at lower angles. The increase in radar brightness between these feature classes suggests the exposure of diffusively scattering lakebeds that were previously covered by an attenuating liquid medium. A simple two-layer model is used to explain backscatter variations and estimate liquid depth changes in the ephemeral features. Changes in surface wave height may also be considered a possible explanation, but does not naturally explain the transient lake boundaries. In addition, the wave heights required to match radar returns are large compared with recent limits placed on the smoothness of Ontario Lacus and backscatter constraints from other lakes. Potential explanations for the observed surface changes include freezing, cryovolcanism, infiltration, and liquid evaporation. Freezing is thermodynamically discouraged during the summer season in Titan's south pole and there are no clearly observable cryovolcanic features in the study areas. Infiltration into a static hydrologic system is inconsistent with the observations. However, infiltration into a dynamic hydrologic system with a regionally varying phreatic surface is possible. Model results suggest evaporation rates are ~1 m/yr, similar to current GCM estimates of methane evaporation rates for the latitudes (60°S-65°S) and times (Ls between 309° and 360°) in question. An analysis of receding shorelines observed in Ontario Lacus also yield evaporation rates of ~1 m/yr and support the results of the two-layer model. These observations constrain volatile fluxes and hence, the seasonal evolution of Titan's hydrologic system.
    AGU Fall Meeting Abstracts. 11/2009; -1:02.

Publication Stats

512 Citations
159 Downloads
205.44 Total Impact Points

Institutions

  • 2005–2013
    • Stanford University
      • Department of Electrical Engineering
      Palo Alto, California, United States
  • 2005–2009
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, CA, United States
  • 2006
    • The University of Arizona
      • Department of Planetary Sciences
      Tucson, Arizona, United States