[Show abstract][Hide abstract] ABSTRACT: Since its arrival at Saturn, the Cassini spacecraft has had only a few opportunities to observe Iapetus, Saturn’s most distant regular satellite. These observations were all made from long ranges (>100,000 km) except on September 10, 2007, during Cassini orbit 49, when the spacecraft encountered the two-toned moon during its closest flyby so far. In this pass it collected spatially resolved data on the object’s leading side, mainly over the equatorial dark terrains of Cassini Regio (CR). In this paper, we examine the radiometry data acquired by the Cassini RADAR during both this close-targeted flyby (referred to as IA49-3) and the distant Iapetus observations. In the RADAR’s passive mode, the receiver functions as a radiometer to record the thermal emission from planetary surfaces at a wavelength of 2.2-cm. On the cold icy surfaces of Saturn’s moons, the measured brightness temperatures depend both on the microwave emissivity and the physical temperature profile below the surface down to a depth that is likely to be tens of centimeters or even a few meters. Combined with the concurrent active data, passive measurements can shed light on the composition, structure and thermal properties of planetary regoliths and thus on the processes from which they have formed and evolved. The model we propose for Iapetus’ microwave thermal emission is fitted to the IA49-3 observations and reveals that the thermal inertias sensed by the Cassini Radiometer over both CR and the bright mid-to-high latitude terrains, namely Ronceveaux Terra (RT) in the North and Saragossa Terra (ST) in the South, significantly exceed those measured by Cassini’s CIRS (Composite Infrared Spectrometer), which is sensitive to much smaller depths, generally the first few millimeters of the surface. This implies that the subsurface of Iapetus sensed at 2.2-cm wavelength is more consolidated than the uppermost layers of the surface. In the case of CR, a thermal inertia of at least 50 J m−2 K−1 s−1/2, and most probably >200 J m−2 K−1 s−1/2 is inferred. This suggests a gradient in density with depth or, more likely, that the Radiometer has probed the icy substrate underlying the dark layer. Furthermore, the measured thermal emission is found to arise from the upper few meters of the subsurface, which points to tholins, rather than iron oxide compounds, as the primary contaminants of the dark material. We also find that, although there is a latitudinal decrease probably related to the thinning of the dark layer away from the Equator, the CR region exhibits a high 2.2-cm emissivity, 0.87 in average, which is close to the emissivity of Phoebe, a putative source of the dark matter. In the case of RT + ST, model fitting points to a mean thermal inertia of ∼160 J m−2 K−1 s−1/2 along with the possible presence of an absorbing compound in the regolith of the bright terrains. Nevertheless, this layer is transparent enough for the Radiometer to capture the seasonal contrast between the northern and southern hemispheres. Lastly, a global decline of the microwave emissivity with latitude is revealed; it is probably indicative of a progressive increase of the water ice content in the near surface.
[Show abstract][Hide abstract] ABSTRACT: radar observations of the surface of Ligeia Mare collected during the 23 May 2013 (T91) Cassini flyby show that it is extremely smooth, likely to be mostly methane in composition, and exhibits no surface wave activity. The radar parameters were tuned for nadir-looking geometry of liquid surfaces, using experience from Cassini's only comparable observation, of Ontario Lacus on 21 December 2008 (T49), and also include coincident radiometric observations. Radar echoes from both passes show very strong specular radar reflections and limit surface height variations to 1 mm rms. The surface physical temperature at 80°N is 92 +/- 0.5 K if the sea is liquid hydrocarbon and the land is solid hydrocarbon, essentially the same as Cassini CIRS measurements. Furthermore, radiometry measurements over the surrounding terrain suggest dielectric constants from 2.2 to 2.4, arguing against significant surface water ice unless it is extremely porous.
[Show abstract][Hide abstract] 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 Atmospheres 03/2013; 118(3):416-435. · 3.44 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Since 2004, Cassini RADAR, operating at 13.8 GHz as a radiometer, scatterometer, altimeter and synthetic aperture radar (SAR), provides a vast amount of data, suggesting new scenarios for Titan’s morphology and evolution. An important result was the detection of lakes constituted by liquid hydrocarbons, thus supporting the hypothesis of a methane and ethane cycle similar to water cycle on Earth. In 2007 Ontario Lacus, a 200 km × 70 km lake, was detected near the South pole. To date Ontario is the only large liquid area sensed by Cassini RADAR in the southern hemisphere of Titan.In this work, we analyze the SAR data using two different electromagnetic modeling approaches to retrieve the optical thickness parameter of the liquid hydrocarbon layer. A physically-based model, IEM combined with a gravity capillary wave spectra and integrated into a Bayesian statistical inversion is compared with a semi-empirical model also based on a double-layer description. We consider the impact of the dielectric constant of the surface constituents, as well as wind speed and wave motion scenarios, on the retrieved optical thickness, and by extension, the lake depth and volume estimation. Wind speed can be constrained below 0.7 m/s, in good agreement with the forecasts of Global Circulation Models on Titan. Lake depths estimates depend on the hypotheses on wind speed and loss tangent of the liquid. The average depth lake estimates obtained with the physically based approach range from 2.7 and 8.3 m, with the 95% of the lake area not exceeding 30 m depth. The semiempirical model results confirm this interval, also considering the hypothesis of a low reflectivity lake bed: this would imply lower depth, with a significant part of the lake area not liquid-filled at the present.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 . 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 . 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.