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
AGU Fall Meeting Abstracts. 11/2009; -1:02.
ABSTRACT: 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 Titan's 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 Saturn's 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.
IEEE Transactions on Geoscience and Remote Sensing 07/2009; · 2.89 Impact Factor
ABSTRACT: The T39 Cassini radar pass over the south pole of Titan revealed few
lakes, in contrast to the north. A plausible hypothesis is that methane
lakes were present in southern spring but have evaporated. This leads to
lake depth estimates of tens's of meters.
ABSTRACT: 1] Synthetic Aperture Radar (SAR) images of Titan's north polar region reveal quasi-circular to complex features which are interpreted to be liquid hydrocarbon lakes. We investigate methane transport in Titan's hydrologic cycle using the global distribution of lake features. As of May 2007, the SAR data set covers $22% of the surface and indicates multiple lake morphologies which are correlated across the polar region. Lakes are limited to latitudes above 55°N and vary 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 consider the role of subsurface transport in Titan's hydrologic cycle.
Geophys. Res. Lett. 01/2008; 35.
ABSTRACT: Recent observations of Titan's Surface from Cassini's Synthetic Aperture
Radar (SAR) have revealed quasi-circular to complex features which are
interpreted as liquid hydrocarbon lakes (Stofan et al., 2007). We use
the global distribution of lake features to investigate methane
transport in Titan's hydrologic cycle, which includes atmospheric,
surface, and sub-surface interaction. Specifically, the latitudinal and
longitudinal division of hydrocarbon lakes combined with derived
topographic information is used to model subsurface transport and place
limitations on the properties of an isotropic porous regolith. Our
analysis of the dataset, which covers 22% of the surface, has led to
the identification of multiple lake morphologies which are correlated
across the polar region. Radar dark lake features are limited to
latitudes above 65°N and vary in size from the limits of observation
(a few km2) to more than 100,000 km2. Granular and
sub-granular lake features, which are distinguished by increased radar
backscatter relative to their surroundings as compared to dark lakes,
can be found as low as 55°N. Sub-granular lake features are inferred
to be empty basins while granular lake features are interpreted as
transitional between dark and sub-granular. The orientation, size, and
statistical correlations between dark, granular, and sub-granular lake
features provide constraints on precipitation conditions and the
importance of subsurface transport. Using preliminary porous media
properties inferred from Huygens probe results at 10°S, timescales
for flow between observed dark and empty lakes are calculated by solving
the groundwater flow equation. Derived lake equilibration timescales are
compared to the time between collocated SAR observations in order to
place limitations on the permeability of an isotropic porous regolith.
For permeabilities of 10-5cm2, equilibrium
timescales are found to be in the 10's of years and are similar to
Titan's seasonal cycles and lake evaporation estimates (Mitri et al.,
ABSTRACT: 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 degrees 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.
Nature 02/2007; 445(7123):61-4. · 36.28 Impact Factor
ABSTRACT: The first five Titan flybys with Cassini's Synthetic Aperture RADAR (SAR) and radiometer are examined with emphasis on the calibration and interpretation of the high-resolution radiometry data acquired during the SAR mode (SAR-radiometry). Maps of the 2-cm wavelength brightness temperature are obtained coincident with the SAR swath imaging, with spatial resolution approaching 6 km. A preliminary calibration shows that brightness temperature in these maps varies from 64 to 89 K. Surface features and physical properties derived from the SAR-radiometry maps and SAR imaging are strongly correlated; in general, we find that surface features with high radar reflectivity are associated with radiometrically cold regions, while surface features with low radar reflectivity correlate with radiometrically warm regions. We examined scatterplots of the normalized radar cross-section σ 0 versus brightness temperature, finding differing signatures that characterize various terrains and surface features. Implications for the physical and compositional properties of these features are discussed. The results indicate that volume scattering is important in many areas of Titan's surface, particularly Xanadu, while other areas exhibit complex brightness temperature variations consistent with variable slopes or surface material and compositional properties.
Icarus 01/2007; 191:211-222. · 3.38 Impact Factor
ABSTRACT: Cassini's Titan Radar Mapper imaged the surface of Saturn's 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.
Nature 07/2006; 441(7094):709-13. · 36.28 Impact Factor
ABSTRACT: The most recent Cassini RADAR images of Titan show widespread regions (up to 1500 kilometers by 200 kilometers) of near-parallel radar-dark linear features that appear to be seas of longitudinal dunes similar to those seen in the Namib desert on Earth. The Ku-band (2.17-centimeter wavelength) images show approximately 100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of approximately 0.5 meter per second resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create sand-sized (100- to 300-micrometer) particulates and a lack of persistent equatorial surface liquids to act as sand traps.
Science 06/2006; 312(5774):724-7. · 31.20 Impact Factor
ABSTRACT: The Cassini Titan RADAR Mapper has made two close passes of Titan's
southern hemisphere in 2005, discovering extensive drainage channels,
embayments, and broad areas of dark dunes. Together these emphasize the
relative youthfulness of the surface.
ABSTRACT: Results from four Titan fly-bys using SAR are summarized and show that
Titan has a complex and relatively young surface, with features formed
by cryovolcanism, fluvial and aeolian activity, cratering, and possibly
Nature 01/2006; 442(7100):322-594. · 36.28 Impact Factor
ABSTRACT: The Cassini Titan Radar Mapper obtained Synthetic Aperture Radar images
of the surface of Titan during the targeted fly-bys on October 26, 2004
(Ta) and February 15, 2005 (T3). These images revealed that Titan is
very complex geologically (see Elachi et al., 2005, Science 13, 970-4).
We will discuss the landforms interpreted to be cryovolcanic and
alluvial in origin. Cryovolcanic features include a circular structure
(provisionally named Ganesa Macula), which is the most prominent
geologic feature in the Ta SAR swath (see Wall et al. and Neish et al.,
this volume). This feature has morphological similarities to pancake
domes on Venus. Other landforms interpreted as cryovolcanic in origin
include numerous SAR-bright, lobate features that appear to be both
sheet-like and digitate cryovolcanic flows. These are seen in the Ta
swath, but none could be positively identified in the T3 swath. These
flow features extend from tens of kilometers to over 200 km in length
and appear to be topographically controlled. Several flows are seen over
Ganesa Macula, associated with linear features interpreted as possible
cryolava channels. Two craters elsewhere in Ta have flows emanating from
them. The irregular shape of these two craters and emerging
unidirectional flows supports a cryovolcanic origin. Preliminary
estimates of rheological properties (see Mitchell et al., this volume)
are consistent with those of ammonia-ice mixtures. Other flow features,
seen in both the Ta and T3 swaths, are associated with sinuous channels
and form fan-like deposits. Their morphologies are consistent with those
of alluvial fans, suggesting evidence of transported material on the
surface. The presence of both cryovolcanic and alluvial features on
Titan has long been suggested. The SAR data indicates that both types of
features are common on Titan.
ABSTRACT: The Cassini Titan Radar Mapper imaged about 1% of Titan's surface at a resolution of approximately 0.5 kilometer, and larger areas of the globe in lower resolution modes. The images reveal a complex surface, with areas of low relief and a variety of geologic features suggestive of dome-like volcanic constructs, flows, and sinuous channels. The surface appears to be young, with few impact craters. Scattering and dielectric properties are consistent with porous ice or organics. Dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons.
Science 06/2005; 308(5724):970-4. · 31.20 Impact Factor
ABSTRACT: The first Cassini Radar image of Titan has revealed no certain impact
craters, implying a very young surface. However, a 100-km scale possible
2 ring impact basin, if real, suggests differential preservation of
ABSTRACT: The first Cassini RADAR observations of Titan reveal a geologically
complex, rather smooth and radar-bright surface. Features include
multiple types that may be cryovolcanic, and radar-dark possible organic
deposits, but few candidate impact craters.
ABSTRACT: The Cassini Titan RADAR Mapper is a K(sub u)-band (13.78 GHz, lambda = 2.17 cm) linear polarized RADAR instrument capable of operating in synthetic aperture (SAR), scatterometer, altimeter and radiometer modes. During the first targeted flyby of Titan on 26 October, 2004 (referred to as Ta) observations were made in all modes. Evidence for topographic relief based on the Ta altimetry and SAR data are presented here. Additional SAR and altimetry observations are planned for the T3 encounter on 15 February, 2005, but have not been carried out at this writing. Results from the T3 encounter relevant to topography will be included in our presentation. Data obtained in the Ta encounter include a SAR image swath
ABSTRACT: The Cassini Titan Radar Mapper obtained Synthetic Aperture radar images of about 1.1% of Titan's surface during the spacecraft s first targeted fly-by on October 26, 2004 (referred to as the Ta fly-by). These images revealed that Titan is very complex geologically. Features identified include a possible volcanic dome or shield, craters that appear to be of volcanic origin, and extensive flows. We will discuss these features and others that will likely be revealed during Cassini s T3 Titan fly-by of February 15, 2005, during which a swath covering comparable amount of the surface will be obtained. Additional information is included in the original extended abstract.
ABSTRACT: Titan's atmospheric methane abundance suggests the likelihood of a surface reservoir of methane and a surface sink for its photochemical products, which might also be predominantly liquid. Although large expanses of obvious hydrocarbon seas have not been unambiguously observed, a number of rather radar-dark spots up to approximately 30 km across are observed in the Synthetic Aperture Radar (SAR) data acquired during the Cassini TA encounter on October 26th 2004. Here we review the properties and setting of these dark spots to explore whether these may be hydrocarbon lakes.