ABSTRACT: Introduction: This abstract is one in a series of reports on our efforts to map the topography of Saturn's satellite Titan at resolutions as high as a few kilometres [1–4], based on radargrammetric analysis of stereo image pairs from the Cassini RADAR . This instrument uses microwaves with 2.2-cm λ to form synthetic aperture (SAR) images with 350-1400 m resolution, as well as obtaining radiometric, scat-terometric, and altimetric data. To date, the instrument has obtained full resolution SAR swaths useful for stereomap-ping on 33 Titan flybys between Ta and T77. These images form 110 possible stereopairs, covering a combined area greater than 2% of Titan. In this abstract, we report on substantial recent progress in controlling the majority of the available SAR images and provide a first look at the most recent digital topographic models (DTMs), which are being produced rapidly now that the control is complete. Technical approach: To perform stereo analysis of Cas-sini SAR data, we use the software package SOCET SET (® BAE Systems)  for controlling sets of images so topo-graphic results are consistent with one another and with pri-ori data such as SAR topography elevations; creation of DTMs with powerful, highly adjustable image matching algorithms; and manual editing or even creation of DTMs. To apply these functions to Cassini data, we used the SOCET Developer's Toolkit to implement "sensor model" software that computes the transformations between pixel coordinates and ground coordinates by rigorous modeling of the physical process of SAR image formation . The USGS in-house cartographic software package ISIS  is used to prepare the images and metadata for ingestion into SOCET SET. Figure 1. Cassini RADAR SAR image coverage of Titan through flyby T77, showing the clusters of overlapping images forming the northern (blue) and southern (red) control networks. A few images (gray) have yet to be connected to either network. Simple Cylindrical projection with north at top, centered on 180°W, with 30° grid.
44th Lunar and Planetary Science Conference; 03/2013
ABSTRACT: Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation.
Science 03/2011; 331(6023):1414-7. · 31.20 Impact Factor
ABSTRACT: Determining whether cryovolcanism has occurred on Titan is of pressing
interest because eruptions have been suggested to resupply atmospheric
methane, replacing what is lost by photolysis. Cryovolcanism also has
intrinsic interest as a posited but largely undocumented geologic
process on Titan. Most candidate cryovolcanoes on Titan have been
proposed based on photointerpretation (i.e., analogy between their
morphology and that of volcanic features on the Earth and other silicate
bodies) based on RADAR SAR images with 300-1500 m resolution (e.g., Rohe
Fluctus, Ganesa Macula, Hotei Regio) or VIMS infrared images with
multi-km resolution (Tortola Facula, Tui Regio). RADAR provides a higher
resolution check of the VIMS candidates, weakening the case for Tortola
and supporting that for Tui, but definitive determination may require
even higher resolution imaging by a future mission. Fortunately,
topography provides important additional clues. Digital topographic
models (DTMs) produced by stereoanalysis of RADAR images are
particularly valuable because they permit features to be visualized in
3D; other methods provide only isolated topographic profiles. RADAR
stereo covers only a few percent of Titan but has provided compelling
evidence for or against several candidate cryovolcanoes. Ganesa Macula
was a leading early candidate because of its resemblance to steep-sided
volcanic domes on Venus, but stereo mapping showed that the feature is
irregular in relief, with a generally elevated eastern and low western
margin. It is clearly not a dome; it may have started as one but has
been drastically modified. Lobate radar-bright and -dark features in
Hotei Regio were suggested as volcanic flows, but have also been
interpreted as fluvial. Our DTM shows the flows to be 100-200 m thick,
with bright tops and dark margins. Towering over nearby fluvial
channels, they seem unlikely to be sedimentary deposits of fluvial
origin. Thus, by elimination, the cryovolcanic hypothesis is
strengthened. We recently mapped Sotra Facula (a 60 km subcircular
feature near 40°W 15°S from which lobate flows radiate northward
180 km) and found even clearer evidence for volcanism. Unlike most
dune-free areas in the equatorial sand seas (including Tortola), which
are nondescript rises with only a few hundred m of relief, Sotra
includes a 1000-m high peak and an adjacent 1500-m deep pit from which
the flows appear to originate. The apparent thickness of the flows
ranges from zero where they are locally crossed by dunes to 800 m,
suggesting that they have been extensively modified. No fluvial features
are found in the vicinity, but 450 km north of Sotra is a second 1000-m
peak surrounded by smaller flow lobes that resemble an area of western
Xanadu previously suggested to be volcanic. The Sotra area thus seems to
be a leading candidate for a cryovolcanic field on Titan.
AGU Fall Meeting Abstracts. 11/2010; -1:03.
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: Since Cassini's arrival at Saturn the season has progressed from
northern winter to just past the northern vernal equinox (the equivalent
of ~mid-January to late March on Earth), driving changes in the weather
patterns. Until shortly after Cassini arrived at Saturn, large
convective cloud systems were common over the South Pole. Since 2005,
such storms have been less common and elongated streaks of clouds have
been observed further and further to the north, becoming common at high
northern latitudes by 2007. Cassini's Imaging Science Subsystem (ISS)
has also observed changes in surface features at high southern
latitudes: a new large dark area appeared between July 2004 and June
2005 and may have subsequently faded; recent observations of Ontario
Lacus suggest that its boundary may have receded somewhat as well. Such
changes are interpreted to be the result of precipitation and ponding of
liquid methane and the subsequent evaporation thereof. Intriguingly,
Cassini RADAR observations of areas near Titan's south pole reveal far
fewer lakes than are observed by RADAR at high northern latitudes and
fewer than suggested by the number of dark features observed by ISS in
this area. This apparent discrepancy may simply be a result of the fact
that not all dark features identified by ISS are liquid-filled; however
another possible explanation is that evaporation has occurred between
the ISS observations in mid-2005 and RADAR observations of similar
territory starting in 2007. Further investigation of comparison of ISS
and RADAR observations is underway to better understand the implications
of the differences observed. We will present observations of Titan's
atmospheric behavior and surface features, documenting changes that have
resulted from weather and seasonal change.
AGU Fall Meeting Abstracts. 11/2009; -1:01.
ABSTRACT: Since 2004, the Cassini Titan RADAR Mapper instrument, a multimode
microwave multiple-beam sensor has observed the surface of Titan at
13.78 GHz. This instrument can operate as a high-resolution synthetic-
aperture radar (SAR) imager, profiling altimeter, scatterometer, and
radiometer, the latter able to observe simultaneously with, or
separately from, the active measurements. The comparison of the data
collected in these different modes of operation addresses a number of
compositional and geological questions. In particular, radiometry
observations near closest approach provide a powerful complement to SAR
reflectivity measurements, despite the difference in the resolution.
Among the 23 flybys of the Cassini prime mission for which SAR
measurements were performed, 14 provided observations of Titan's linear
dunes. They revealed that the fields of dunes cover a large portion of
Titan's surface, mainly in low-latitudes, within ± 30°. They
are radar-dark and exhibit a very high emissivity (with brightness
temperatures from 3 to 5 K above that of their surroundings), consistent
with a smooth surface and a low dielectric constant. Yet, many questions
remain relative to their composition and geometry. We will present the
results of our investigation of the correlation between the radar
backscatter and the brightness temperature of the dune fields that
suggests that interdunes are flat and with a higher dielectric constant
than the dunes. This interpretation is supported by data from
scatterometry and altimetry. It also accounts for the fact that the look
direction seems to have no significant importance in the identification
of the dunes. Also, both the emissivity and the reflectivity of the dune
fields depend on the incidence (or emission) angle and the look
direction. A few dunes were observed with a variety of geometries,
especially the ones at the overlap of several swaths. The backscatter
properties of these dunes as a function of the look geometry are
examined to provide an estimate of the dunes slopes.
AGU Fall Meeting Abstracts. 11/2008; -1:1310.
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.
ABSTRACT: Titan's equatorial regions are covered by vast fields of longitudinal dunes. Several observations point to solid hydrocarbons as the most likely candidate for the dune particle composition. Together with the polar lakes and seas, dune regions are thus the main reservoir of organic deposits on Titan. A refined estimate of the dune material volume and distribution is essential to constrain Titan total organic inventory and therefore to understand the carbon cycle on Titan. Using Cassini SAR observations we find that Titan's dune fields are generally hosted by basins and may cover ~12.5% of Titan's surface, which corresponds to an area of ~10 million km2 (roughly the area of the United States). Polarized radiometry observations indicate that dune particles are mainly composed of organic solids, consistent with spectroscopic measurements. This would imply that the dune particles were dominantly created by atmospheric photochemical production rather than fluvial erosion. However, it is not clear whether the aggregation occurred primarily during aerosol sedimentation from the stratosphere to the surface, or by subsequent sticking and growth during fluvial or eolian transport. Assuming that, everywhere, the dunes are 100m-high and that the interdunes spaces are clear of dune material and of equal area than the dunes, the volume of sand-sized sediments should approach ~250 000 km3, which is an order of magnitude higher than the current estimate of the volume of liquid hydrocarbons on Titan (Lorenz et al., 2008). However, the combined radar and radiometry measurements indicate regional variations among the dunes. In this paper we will show that differences in the microwave backscatter and emission of the dune regions can be well explained by various degrees of exposure of the icy bedrock of Titan in the interdunal corridors. In some regions, a thick sheet of sand-sized material covers the interdunes. In other places, the original substrate is peeking through. These variations need to be taken into account in order to estimate the volume of sand-sized sediments. Investigating them also bring new insights on the distribution of the available sand-sized sediments supply, which vary across Titan probably owing to differences in the ground humidity and wind patterns.
American Geophysical Union, Fall Meeting 2010, abstract #P22A-06.
ABSTRACT: Over the course of a year, Cassini RADAR obtained Synthetic Aperture Radar images covering 69 percent of Titan's polar region north of 65 degrees; the region being 1.4E6 km3 in extent, greater than double the land area of the USA. We observe several hundred lakes with a range of morphological expression, including areally massive and morphologically distinctive "seas", covering ~15% of the polar region. Lakes are extremely radar dark, consistent with a lossy liquid hydrocarbon. Preliminary laboratory estimates suggest that loss tangents in the range 10E4 to 2x10E3 are reasonable, which implies that one can see through at least a few to many tens of m of liquids before the noise floor is reached, consistent with observed brightening towards many lake shores. North polar lake volumes are most likely in the 8E3 - 1.4E6 km3 range. Uncertainties will be reduced as more data, both image-based and experimental, are obtained but we can conclude with a high degree of confidence that hydrocarbon lakes on Titan are more voluminous than known terrestrial oil reserves; current estimates range from 2248 - 3896 billion barrels of oil (J. Hakes, 2000, Long Term World Oil Supply, Meeting of the Am. Ass. Pet. Geol., 18th April 2000, New Orleans, LA, http://www.eia.doe.gov/pub/oil_gas/petroleum/presentations/2000/long_term_supply.), hence 357 - 619 km3 . Small lakes often occupy steep-sided depressions, and although thermal and cryovolcanic origins cannot be completely ruled out, we are seeing growing geomorphologic evidence for dissolution chemistry, indicative of karst-like geology. The dichotomy between small lakes over slightly more than one half of the region, and seas on the other half, may be best explained by a topographic anomaly causing sub-surface flow of materials from the lakes to the seas. This may also explain observations by the Cassini ISS team (E. Turtle et al., in prep.) of a putative massive sea extending considerably further south than other observed north polar lakes.
2007AGUFM.P23B1349M - American Geophysical Union, Fall Meeting 2007, abstract #P23B-1349.
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.
ABSTRACT: A set of lakes filled or partially filled with liquid hydrocarbon and empty lake basins have been discovered in the high latitudes of Saturn's moon Titan. These features were mapped by the radar instrument on the Cassini orbiter. Here we quantify the distribution of the lakes and basins, and show a pronounced hemispheric asymmetry in their occurrence. Whereas significant fractions of the northern high latitudes are covered by filled and empty lakes, the same latitudes in the southern hemisphere are largely devoid of such features. We propose that in addition to known seasonal changes, the observed difference in lake distribution may be caused by an asymmetry in the seasons on Titan that results from the eccentricity of Saturn's orbit around the Sun. We suggest that the consequent hemispheric difference in the balance between evaporation and precipitation could lead to an accumulation of lakes in one of Titan's hemispheres. This effect would be modulated by, and reverse with, dynamical variations in the orbit. We propose that much like in the Earth's glacial cycles, the resulting vigorous hydrologic cycle has a period of tens of thousands of years and leads to active geologic surface modification in the polar latitudes.