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R L Kirk,
E Howington-Kraus,
B Redding,
O Aharonson,
B G Bills,
A G Hayes,
L Iess,
R M C Lopes,
R D Lorenz,
A Lucas,
J I Lunine,
R Meriggiola,
K L Mitchell,
C D Neish,
J Radebaugh,
B W Stiles,
E R Stofan, S D Wall,
C A Wood
[show abstract]
[hide abstract]
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 [5]. 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) [6] 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 [7]. The USGS in-house cartographic software package ISIS [8] 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
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[show abstract]
[hide abstract]
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.
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[show abstract]
[hide abstract]
ABSTRACT: Dunes on Titan interact with topographic obstacles, leading to features
like those seen in the Namib and Saharan deserts. These results are
correlated with studies of wind directions from dune morphologies, not
with current GCM model wind directions.
02/2010; 41:2513.
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[show abstract]
[hide abstract]
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.
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[show abstract]
[hide abstract]
ABSTRACT: We have identified only 70 possible degraded impact craters on Titan;
dynamic processes have destroyed most of the early history and continue
to modify the surface. The existence of possible impact craters > 20
km is vexing.
02/2008; 39:1990.
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K. L. Mitchell,
R. M. C. Lopes,
J. Radebaugh,
R. D. Lorenz,
E. R. Stofan, S. D. Wall,
J. S. Kargel,
R. L. Kirk,
J. I. Lunine,
S. J. Ostro,
T. Farr,
Cassini RADAR Team
[show abstract]
[hide abstract]
ABSTRACT: Caldera-like depressions containing lakes near Titan's poles are
interpreted to be the result of karstic dissolution. We present a simple
model for dissolution of lake depressions on Titan, and find a wide
range of candidate solutes.
02/2008; 39:2170.
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J. I. Lunine,
G. Mitri,
C. Elachi,
E. R. Stofan,
R. D. Lorenz,
R. L. Kirk,
K. Michell,
R. M. C. Lopes,
C. A. Wood,
J. Radebaugh, S. D. Wall,
L. A. Soderblom,
Ph. Pallou,
T. Farr,
B. Stiles,
P. Callahan,
Cassini RADAR Team
[show abstract]
[hide abstract]
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.
02/2008; 39:1637.
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K. L. Mitchell, S. D. Wall,
E. R. Stofan,
R. M. C. Lopes,
M. Janssen,
B. Stiles,
P. Paillou,
G. Mitri,
J. Lunine,
S. Ostro,
R. D. Lorenz,
T. G. Farr,
R. L. Kirk,
J. Radebaugh,
Cassini RADAR Science Team
LPI Contributions. 07/2007; 1357:97-98.
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F Paganelli,
M A Janssen,
B Stiles,
R West,
R D Lorenz,
J I Lunine, S D Wall,
P Callahan,
R M Lopes,
E Stofan,
R L Kirk,
W T K Johnson,
L Roth,
C Elachi
[show abstract]
[hide abstract]
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
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Randolph L. Kirk, S. D. Wall,
R. D. Lorenz,
J. I. Lunine,
J. Radebaugh,
L. A. Soderblom,
B. W. Stiles,
M. A. Janssen,
F. Paganelli,
R. Lopes,
Cassini RADAR Team
[show abstract]
[hide abstract]
ABSTRACT: During the T13 Titan flyby on 2006 April 30, the Cassini RADAR obtained
a synthetic aperture (SAR) image from 62 to 170 W longitude at 10 S
latitude, crossing almost the entire 3600 km width of the IR-bright
region Xanadu. The region is revealed to be remarkably rugged and
different from the remainder of Titan seen so far. Earlier radiometric
observations showed Xanadu to have low emissivity and very low
polarization, suggesting a rough surface with low dielectric constant
and significant volume scattering (Janssen et al., this conference). The
T13 SAR data are consistent with these results, showing a radar-bright
surface with backscatter almost independent of incidence.
Morphologically, Xanadu is populated with ubiquitous, closely spaced
hills 5 km across, which locally form chains and appear to be dissected
by numerous channels and low areas filled by radar-dark sediments.
Radarclinometry indicates typical hills are at least 500 m high, but the
results are asymmetric, strongly suggesting that the foreshortened
bright slopes are unresolved. If so, the hills are 1000 m high with 30
degree slopes. In either case Xanadu contrasts strongly with the rest of
Titan, where topographic features are rare and mostly <300; m high.
Information about relief over distances >20; km is limited, but
Stiles et al. (this conference) offer evidence that Xanadu as a whole is
quite flat. Comparison with VIMS data (Soderblom et al., this
conference) suggests the surface is extensively coated with bright
tholin deposits, with some variation in water ice content correlated
with features in the SAR. We therefore hypothesize that Xanadu was
formed by an initial period of compressive tectonism and cryovolcanism
that triggered the erosion that sculpted the rugged surface, but that
current erosion rates are very low, allowing a thicker coating of IR-
and radar-bright material to accumulate than elsewhere on Titan.
08/2006; 38:579.
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R. M. Lopes,
E. R. Stofan,
F. Paganelli,
K. L. Mitchell,
R. Kirk,
R. Lorenz,
J. Lunine,
L. A. Soderblom, S. D. Wall,
C. Wood,
J. Radebaugh,
L. E. Robshaw,
C. Elachi,
Cassini RADAR Team
[show abstract]
[hide abstract]
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
tectonism.
02/2006; 37:1347.
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R. D. Lorenz, S. D. Wall,
E. Reffet,
G. Boubin,
J. Radebaugh,
C. Elachi,
M. D. Allison,
Y. Anderson,
R. Boehmer,
P. Callahan, [......],
R. Seu,
S. Schaffer,
L. A. Soderblom,
B. Stiles,
E. R. Stofan,
S. Vetrella,
R. West,
C. A. Wood,
L. Wye,
H. Zebker
02/2006; 37:1249.
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C. Elachi, S. D. Wall,
M. D. Allison,
Y. Anderson,
R. Boehmer,
P. Callahan,
P. Encrenaz,
E. Flamini,
G. Franceschetti,
Y. Gim, [......],
L. A. Soderblom,
B. Stiles,
E. Stofan,
S. Vetrella,
R. West,
C. A. Wood,
L. Wye,
H. Zebker,
B. Rizk,
L. McFarlane
[show abstract]
[hide abstract]
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.
02/2006; 37:1252.
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C. Elachi, S. D. Wall,
M. D. Allison,
Y. Anderson,
R. Boehmer,
P. Callahan,
P. Encrenaz,
E. Flamini,
G. Francescetti,
Y. Gim, [......],
R. Seu,
S. Shaffer,
L. A. Soderblom,
B. Stiles,
E. R. Stofan,
S. Vetrella,
R. A. West,
C. A. Wood,
L. Wye,
H. A. Zebker
[show abstract]
[hide abstract]
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.
02/2005; 36:2294.
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[show abstract]
[hide abstract]
ABSTRACT: Like Venus, the surface of Titan is hidden from view, with little known about its geology prior to Cassini. The first Synthetic Aperture Radar (SAR) swath across the surface of Titan has revealed a surprisingly complex surface, with few features that can be reliably identified as impact craters. More detailed reports on the results of the first radar encounter with Titan can be found in this volume.
02/2005;
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[show abstract]
[hide abstract]
ABSTRACT: The Cassini Titan RADAR Mapper [1] is a Ku-band (13.78 GHz,lambda = 2.17 cm) linear polarized RADAR instrument capable of operating in synthetic aperture (SAR), scatterometer, altimeter and radiometer modes. Radar observations on Titan passes Ta and T3 included rastered scatterometry, SAR, altimetry and rastered radiometry images of a full hemisphere in orthogonal linear polarizations. At this writing only the Ta data have been acquired, but data from both passes will be discussed in the presentation.
02/2005;
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C. Elachi,
M. D. Allison,
L. Borgarelli,
P. Encrenaz,
E. Im,
M. A. Janssen,
W. T. K. Johnson,
R. L. Kirk,
R. D. Lorenz,
J. I. Lunine, [......],
G. Picardi,
F. Posa,
C. G. Rapley,
L. E. Roth,
R. Seu,
L. A. Soderblom,
S. Vetrella, S. D. Wall,
C. A. Wood,
H. A. Zebker
[show abstract]
[hide abstract]
ABSTRACT: The Cassini RADAR instrument is a multimode 13.8 GHz multiple-beam sensor that can operate as a synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer. The principal objective of the RADAR is to map the surface of Titan. This will be done in the imaging, scatterometer, and radiometer modes. The RADAR altimeter data will provide information on relative elevations in selected areas. Surfaces of the Saturn’s icy satellites will be explored utilizing the RADAR radiometer and scatterometer modes. Saturn’s atmosphere and rings will be probed in the radiometer mode only. The instrument is a joint development by JPL/NASA and ASI. The RADAR design features significant autonomy and data compression capabilities. It is expected that the instrument will detect surfaces with backscatter coefficient as low as −40 dB.
Space Science Reviews 10/2004; 115(1):71-110. · 3.61 Impact Factor
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M. A. Janssen,
R. Lorenz,
C. Elachi,
Y. Z. Anderson,
R. A. Boehmer,
Y. Gim,
W. T. K. Johnson,
K. D. Kelleher,
R. M. Lopes,
L. E. Roth, S. D. Wall,
R. D. West
[show abstract]
[hide abstract]
ABSTRACT: The Cassini Radar instrument includes a passive microwave radiometer
operating at 13.78 GHz ( 2.2 cm wavelength). An objective for the
radiometer is the global mapping of the temperature and dielectric
constant of Titan's surface, to be achieved using raster scans to
measure the polarized thermal emission from Titan's disk on selected
inbound and outbound passes. Results will be presented from the first
such mapping of Titan, to be obtained during the initial Titan pass on
26 October, 2004. This work was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with
the National Aeronautics and Space Administration.
10/2004; 36:1075.
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[show abstract]
[hide abstract]
ABSTRACT: The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar
(SIR-C/X-SAR) was launched on the Space Shuttle Endeavour for two ten
day missions in the spring and fall of 1994. Radar data from these
missions are being used to better understand the dynamic global
environment. During each mission, radar images of over 300 sites around
the Earth were obtained, returning over a terabit of data. SIR-C/X-SAR
science investigations were focused on quantifying radar's ability to
estimate surface properties of importance to understanding global
change; and focused studies in geology, ecology, hydrology and
oceanography, as well as radar calibration and electromagnetic theory
studies. In addition, the second flight featured an interferometry
experiment, where digital elevation maps were obtained by interfering
data from the first and second shuttle flight, and from successive days
on the second flight. SIR-C/X-SAR data have been used to validate
algorithms which produce maps of vegetation type and biomass; snow, soil
and vegetation moisture; and the distribution of wetlands, developed
with earlier aircraft data
IEEE Transactions on Geoscience and Remote Sensing 08/1995; · 2.89 Impact Factor
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02/1992; 23:1369.
