P22B-01 INVITED
Topography of Titan from Cassini RADAR Stereo Data
The RADAR instrument onboard the NASA/ESA Cassini spacecraft uses 2.17 cm microwaves to see through the hazy atmosphere of Saturn's giant satellite Titan, forming a synthetic aperture image strip up to 5000 km long with 300-1400 m resolution during a single flyby. To date, 14 such image strips have been acquired, covering a total of 22% of the surface and revealing a strikingly diverse surface shaped by many of the same processes as Earth. Since late 2006, each new image has overlapped previous coverage, providing a stereoscopic view of 5% of Titan's surface. These image overlaps can be analyzed to provide information about Titan's topography at a resolution of a few km horizontally and one hundred to several hundred meters vertically, yielding more detail than any other available source except radarclinometry (shape from shading). Stereo analysis is made challenging by speckle noise and by the link between viewing geometry and illumination: image pairs with the greatest stereo convergence angle would give the most precise height measurements but differ most in illumination and are hardest to compare. We have therefore applied a combination of both manual feature measurements and automated image matching to produce both spot estimates of local relief and digital topographic models (DTMs) of areas where the image quality is suitable. Work to improve the efficiency and geometric rigor of these methods is ongoing at both USGS and JPL. Results to date reveal relief of 1 to1.5 km between the large hydrocarbon seas of the north polar region and surrounding mountains, and 300 to 600 m in caldera-like features containing smaller lakes. These stereo heights agree closely with collocated "SARTopo" elevation estimates obtained by comparison of the overlap between neighboring beams in a single image (Stiles, et al., this conference). The relief near the lakes and seas provides an indirect constraint on the probable depth of those bodies, and hence of Titan's reserves of liquid hydrocarbons (Mitchell et al., this conference). Topography of the extensive longitudinal dunes at low latitudes, and of the presumptive topographic obstacles that divert the dunes, is more subtle and has been difficult to quantify with stereo.
P22B-02
Analysis of selected Cassini VIMS and RADAR data over the surface of Titan through multivariate classification methods
We have searched through Cassini/VIMS hyperspectral cubes, selecting those data which have convenient viewing geometry and which overlap with Cassini/RADAR footprints having comparable ground resolution.\newline In RADAR data we have considered two geophysical quantities: the normalized backscatter cross-section obtained from the scatterometer measurement, corrected for the incidence angle, and the brightness temperature determined from the radiometer measurement, as found in publicly available data products.\newline In VIMS data, we have selected the infrared wavelengths in the methane windows, which provide the best optical depth to measure surface reflectance.\newline The two RADAR parameters are combined with the VIMS data, with estimated errors, to produce an aggregate data set, that we process using multivariate classification methods to identify homogeneous taxonomic units in the multivariate space of the samples.\newline A first analysis has been done with the G-mode method, which has been successfully used in the past for the classification of such diverse data sets as lunar rock samples, asteroids and planetary surfaces. This method can be used without any a priori knowledge of the taxonomic structure of the observations, which is in fact provided by the classification. Furthermore, independence of variables and samples is not required, although the relationship between variables and samples needs to be known.\newline Through this classification we search for information which would not be obtained from the individual data sets alone, to gain insight about the nature and physical structure of the surface.\newline This research is supported by the Italian Space Agency (ASI).
P22B-03
Saturn's Titan: Searching for Surface Change
The VIMS instrument on the Cassini spacecraft observes the surface of Titan through spectral ‘windows' in its atmosphere where methane, the principal absorbing gas is transmitting. We previously have used VIMS to document changes in spectral reflectance and that have occurred on Titan's surface during Cassini's orbital tour at (latitude 26S, longitude 78W), (AGU spring meeting 2007). Having removed the possibility that the observed changes are either an atmospheric phenomenon or are the result of viewing angle (phase) effects, we conclude that physical changes in the chemistry or structure of the surface must be occurring. The size of the region suggests it may exceed the size of the largest active volcanic areas in the solar system. We now have explored additional sections of Titan's surface and have developed new techniques for locating surface changes over time. While some additional candidate areas for surface activity are suggested, confirmation is possible with the support of additional instruments on the Cassini Orbiter, particularly the radar instrument. The principal difficulty in implementing a coordinated program of observations with both instruments is due to the radar instrument's higher spatial resolution but small footprint on the surface relative to VIMS. In addition, the two instruments can not be used simultaneously on the same pass. Overlapping coverage will only be available after repeated flybys during Cassini's extended mission. This work done at JPL/CALTECH under contract with NASA
P22B-04
Titan Surface Temperatures from Cassini RADAR Radiometry
The Cassini Radar instrument includes a passive microwave radiometer that operates at 13.78 GHz (2.2 cm wavelength). The radiometer is used to observe the thermal emission from Titan's surface at resolutions ranging from 5 – 500 km and at a variety of emission angles and polarizations. Nearly the entire surface has been observed through T30, enabling the construction of a mosaiced global map of the surface brightness temperature at normal incidence and the dataset now permits the separation of various contributing factors (dielectric constant and roughness/subsurface scattering as well as physical temperature). Voyager infrared measurements at 530 cm-1 show contributions of flux from the surface and near-surface atmosphere and suggested a symmetric equator-pole gradient of the of the order of 2-3K. Pure surface temperatures may be expected to show some asymmetry, with the south showing some effects of heat deposition from the long summer. Our data are much less influenced by the atmosphere and probe slightly into the subsurface. We present our preliminary results on the variation of surface temperature with latitude, and with terrain height. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.