P21A-1308
High resolution VIMS images of Titan's surface: implications for its composition, internal structure and dynamics
With a field of view of 0.5 mrad per pixel, the VIMS (Visual and Infrared Mapping Spectrometer) onboard the Cassini spacecraft can acquire images with a resolution of 500 m per pixel at closest approach during a typical Titan flyby. This resolution is comparable to the resolution of the radar instrument and allows comparisons between the radar images and optical images in the six infrared windows where the surface can be observed. Such opportunities were not set up for the nominal tour before Saturn insertion. The opportunity was offered during the TA flyby [Sotin et al., Nature, 2005] and the results lead the Cassini program to give VIMS the prime observations during closest approach at the T24 and T38 flybys. Two different implementations were experienced. During the T24 flyby (01/29/2007), we used a push-broom mode allowing VIMS to image a long path before pointing to a specific site at the limit between the light and dark terrains. This observation allowed us to see the dunes and to infer some information on their composition [Barnes et al., Icarus, 2008], to image channels and to infer information of erosion processes of the bright equatorial regions [Jaumann et al., Icarus, in press] and to observe the strong correlation between radar images and the VIMS images over a bright area interpreted as a flow feature [Lopes et al., Icarus, 2007]. During the T38 flyby over Ontario Lacus (12/05/2007), it was decided to point to the lake and get different images which provide us with a set of observations obtained with different emergence angles. This observation allowed us to infer the liquid nature of the lake and the composition of the lake [Brown et al., Nature, 2008]. In addition, this mode gives good information on the atmospheric component and will help us remove that component to get better spectra of Titan's surface. During the extended mission, two observations are forecasted at the beginning and at the end of the Cassini Equinox Mission. The first one will happen on November 19, 2008. The VIMS has been programmed to observe the Huygens landing site area at a resolution of 1 km/pixel. Before and after this observation, the push-broom mode will be used in order to cross-cut some of the radar paths. Because Titan's spin rate may be different from synchronous [Stiles et al., 2007; Lorenz et al., 2008], there is some uncertainty on the pointing. This study will report on the results of this flyby. This work has been carried out at the JPL, Caltech, under contract with NASA.
P21A-1309
The Dunes of Shangri-La : New Cassini RADAR results on patterns of aeolian features and the influence of topography
Recent flybys (T43, T44 - and just prior to this meeting, T48) provide SAR imagery of northern Shangri-La, the large dark region just to the WNW of Xanadu. Previous imaging of SE Shangri-La (T13) showed that dunes there take a pronounced southward dip compared with the E-W direction seen elsewhere. The new data show rather different directions for dunes in northern Shangri-La, and confirm a blocking or divergent influence of Xanadu. Application of monopulse radar methods to retrieve elevations from Cassini SAR images ('SARTopo') now allows us to explore the influence of topography on the local dune (and by implication, wind) patterns, and the relationship between elevation and sediment accumulation. The lack of large positive relief at Xanadu makes its influence on the dunes somewhat surprising. We consider the possible mechanisms of Xanadu's effect on the winds, using terrestrial analogs as a guide. We review the global pattern of dune orientations and their implications for atmospheric circulation: this orientation map presents a challenging constraint for modelers. We note preliminary indications that scatterometry of Titan's dunefields yields azimuth-dependent radar cross-sections (as is the case for terrestrial sand seas) and note future plans for dune studies on Titan with multi-angle observations that will provide constraints on dune-scale slopes and duneforms too small to resolve.
P21A-1310
Titan's dunes and interdunes: new insights from Cassini Radar observations
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.
P21A-1311
Different appearance of Titan's dunes
In this paper we analyze Cassini's Titan Radar Mapper recent flybys and yet more evidence of dark linear dunes, in the latitude between 30° S and 30° N, whose orientations are somewhat comparable to previous dune observations but at closer study show morphological differences. The appearance of Titan's dunes depends on the projected look direction of the Cassini Radar antenna, incidence angle and resolution. Dune fields are generally oriented East/West on Titan, and for many radar observations the flyby is in the equatorial plane. At closest approach the imaging direction is most nearly normal to the dune direction such as in the central portion of the T8 swath. Away from that configuration, and especially past the -/+10 minutes from close approach, the relative azimuth angle that the projected look direction of the Cassini Radar antenna has with respect to the surface changes rapidly along with incidence angle and resolution resulting in signal attenuation of the imaged features. Observational biases in the SAR images are key for dunes comparison across Titan's equatorial belt. The results show that in some regions the projected look direction could be on the order of 60° and parallel to the long axis of the radar dark features direction (i.e. T16, T25, T28), therefore suggesting that the variation in backscatter must be a combination of compositional dunes dark material and bright interdune material, varying roughness and topography when present. This suggests that we cannot assume that all the dune fields currently imaged can be characterized simply on the bases of their orientation and therefore we suggest that the characterization of the imaged surface features should be divided into at least two categories: -1) topography driven (in which Radar-clinometry can be applied); -2) compositional or due to varying roughness.
P21A-1312
Global Distribution of Dunes on Titan With VIMS
The carbon cycle on Titan involves several processes in the deep interior, in the sub-surface and in the atmosphere. The dunes, which were first seen by the Cassini/SAR (Synthetic Aperture Radar) [Lorenz et al., 2006], are thought to be composed of hydrocarbon grains deposited onto the surface after they formed in the atmosphere. Although their composition has not been determined during the nominal mission, pure water ice grains can be ruled out by the IR spectra obtained by the VIMS (Visual and Infrared Mapping Spectrometer). This study compares RADAR and VIMS images of dune fields to determine the spectral characteristics of these areas and uses the global VIMS spectral maps in order to provide a global map of the dune fields on Titan. Most of the Aeolian sand deposits are found in sand seas. In addition, isolated groups of "cat scratches", very sinuous short dunes [Radebaugh et al., 2008] and sand sheets [Lunine et al., 2008] are recognized. Their emplacement is most probably related to the available sand supply. In VIMS infrared dataset, the detailed study of dune fields by Barnes et al. [2008] shows that dune patterns are found mainly in brown units, which cover 18% of the whole Titan's surface and are found in equatorial regions. Dark blue units cover roughly 2% of Titan's surface. They are systematically associated with bright terrains and are never found isolated within brown units. The dune fields in SAR images generally end at the limit between infrared brown and bright units. Dunes can also be found on dark blue terrains as seen by Barnes et al. [2007] and Soderblom et al. [2007]. 82% of SAR dunes are located in brown units and 4.5% in dark blue units. The remnant dunes, corresponding to "cat scratches" or not well defined dune fields, appear in infrared bright units as isolated patches. These dunes may form with a low sand supply. They account for about 13.5% RADAR dunes. From the global mapping, we inferred that dunes in the RADAR data are highly correlated with brown infrared terrains, and can overlap dark blue areas. Observations of brown infrared terrains by VIMS will complete the dune field coverage found by SAR.
P21A-1313
The Changing Surface of Saturn's Titan: Cassini Observations Suggest Active Cryovolcanism
R. M. Nelson(1), L. Kamp(1), R. M. C. Lopes(1), D. L. Matson(1), S. D. Wall(1), R. L. Kirk(2), K. L Mitchell(1), G. Mitri(1), B. W. Hapke(3), M. D. Boryta(4), F. E. Leader(1) , W. D. Smythe(1), K. H. Baines(1), R. Jauman(5), C. Sotin(1), R. N. Clark(6), D. P. Cruikshank(7) , P. Drossart(9), B. J. Buratti(1) , J.Lunine(8), M. Combes(9), G. Bellucci(10), J.-P. Bibring(11), F. Capaccioni(10), P. Cerroni(10), A. Coradini(10), V. Formisano(10), G Filacchione(10), R. Y. Langevin(11), T. B. McCord(12), V. Mennella(13), P. D. Nicholson(14) , B. Sicardy(8) 1-JPL, 4800 Oak Grove Drive, Pasadena CA 91109, 2-USGS, Flagstaff, 3-U Pittsburgh, 4-Mt. Sac Col, 5- DLR, Berlin, 6-USGS Denver, 7-NASA AMES, 8-U Paris-Meudon, 9-Obs de Paris, 10-ISFI-CNR Rome, 11-U Paris –Sud. Orsay, 12-Bear Flt Cntr Winthrop WA, 13-Obs Capodimonte Naples, 14-Cornell U. Several Instruments on the Cassini Saturn Orbiter have been observing the surface of Saturn's moon Titan since mid 2004. The Visual and Infrared Mapping Spectrometer (VIMS) reports that regions near 26oS, 78oW (region 1) and 7oS, 138oW (region 2) exhibit photometric changes consistent with on-going surface activity. These regions are photometrically variable with time(1). Cassini Synthetic Aperture Rader (SAR) has investigated these regions and reports that both of these regions exhibit morphologies consistent with cryovolcanism (2). VIMS observed region 1 eight times and reported that on two occasions the region brightened two-fold and then decreased again on timescales of several weeks. Region 2 was observed on four occasions (Tb-Dec13/2004 ,T8-Oct27/2005, T10-Jan15/2006, T12-Mar18/2006) and exhibited a pronounced change in I/F betweenT8 and T10. Our photometric analysis finds that both regions do not exhibit photometric properties consistent with atmospheric phenomena such as tropospheric clouds. These changes must be at or very near the surface. Radar images of these regions reveal morphology that is consistent with cryovolcanoes. We conclude that the VIMS instrument has found two instances in which selected regions on Titan's surface became unusually reflective and remained reflective on time scales of days to months. In both cases the area of reflectance variability is large (~100000 sq km), larger than either Loki or the Big Island of Hawaii. This is a strong evidence for currently active surface processes on Titan. Pre-Cassini, Titan was thought of as a pre-biotic earth that was frozen in time. Cassini VIMS and SAR observations combined suggest that Titan is the present day is not frozen solid, and is instead an episodically changing or evolving world. References: [1] Nelson R. M. et al, LPSC 2007 , Europlanets 2007, AGU 2007, EGU 2008, Accepted in Icarus 2008. [2] Lopes et al (this meeting), Stofan et al. Icarus 185, 443-456, 2007. Lopes et al. Icarus 186, 395- 412, 2007. Kirk et al., DPS 2007. Acknowledgement: This work done at JPL under contract with NASA
P21A-1314
Laboratory Study of Titan's Surface Chemistry Induced by Meteoritic Impact Processing: Laser-Simulated Hypervelocity Impact on Ices
Titan's dense atmosphere, mostly composed of nitrogen and some methane, allows easy formation of long chains of organic molecules and high-molecular-weight organic solids, known as tholins. Over geologic time, both tholins and condensates of the organic gases accumulate in substantial amounts on the surface as liquid and solid. Titan's surface is then a repository of interesting organic molecules generated in the almost complete absence of water but sitting on top of ice. Until recently, researchers have been very careful in their speculations about what might be happening after these molecules get to the surface of Titan. What kind of organic chemistry occurs on the surface? Titan's thick atmosphere protects the surface and organics from harmful cosmic rays and ultraviolet radiation. It has been suggested that these organics could have been subjected to impact processing on Titan's and participate in the formation of products relevant to life such as amino acids, carboxylic acids, purines and pyrimidines. Subsequent impacts would probably have recycled some of the organic material back into the atmosphere. Furthermore the presence of condensable agents (C2N2, HCN, etc.) along with a natural concentrating mechanism makes polymerization of amino acids or others species likely. Laboratory simulations of meteoritic impact shocks onto Titan's icy surface have not yet been carried out, but preliminary experiments have been performed for planetary icy satellites. In these previous experiments, the possible chemical production induced by micrometeorite impact shocks on ices has been studied using a high-energy pulsed Nd-YAG laser to reproduce the shock phenomena during hypervelocity micrometeorite impacts into the icy material. The results show the production of various organics and inorganics. Here we have decided to extend those experiments to a simulated Titan's environment in order to study the effect of meteoritic impacts on the organic chemistry occurring on Titan's surface and to investigate the fate of tholins once condensated into the icy surface and bombarded by meteoritic impacts.
P21A-1315
Identification, Mapping, and Measurement of Titan Fluvial Features
Data from the Cassini-Huygens mission show various individual and networked curvilinear features on Titan's surface interpreted to have been formed by the flow of liquid methane. These inferred fluvial features are seen in the three Cassini surface imaging instrument datasets (from the Imaging Subsystem for Science, the Visual and Infrared Mapping Spectrometer, and the Cassini Titan RADAR Mapper). Such features are also seen in the Huygens Probe Descent Imager/Spectral Radiometer images, in which they have been classified as fluvial valleys. The features are visible at all latitudes, although the characteristics that suggest formation by fluvial flow change with latitude. To investigate the formation of Titan's fluvial features, we mapped out their locations in Synthetic Aperture Radar (SAR) images from the Cassini Titan RADAR Mapper and quantified their network parameters. First, released Cassini SAR images from flybys Ta, T3, T7, T13, and T23 were processed and reprojected using ISIS2 into the best map projections for obtaining accurate measurements, depending on the characteristics to be measured. Equidistant sinusoidal map projections were used to measure feature lengths and widths, whereas conformal mercator projections were used to measure junction angles at the confluence of fluvial features. Next, criteria were devised based on radar reflectance, illumination, and morphology with which to consistently identify the fluvial features. These criteria were then applied to the reprojected Cassini SAR images to create maps of the fluvial features. Finally, measurements were made of these mapped features to calculate their sizes, sinuosities, and junction angle. Using a published algorithm to classify terrestrial drainage network type from measured morphologic parameters, we found that the equatorial network of fluvial features over western Xanadu observed in the T13 radar swath would be classified as rectangular. On Earth, rectangular drainage networks are interpreted to be the result of surface flow over a jointed or fractured substrate. By analogy, the rectangular network over western Xanadu suggests faulting either previous to or simultaneous with surface fluvial flow in this location. Other fluvial networks on Titan showed dendritic and possibly parallel network drainage.
P21A-1316
Experimental investigation of fluvial incision on Titan by low-velocity sediment impacts
Images returned by the Cassini-Huygens mission reveal evidence for widespread fluvial incision in the polar regions of Titan. Dendritic channel networks draining to large lakes and the absence of cratering suggest active incision into Titan's water-ice bedrock surface. Previous work using the saltation-abrasion bedrock incision model suggests that a terrestrial channel transposed to Titan conditions would incise at remarkably similar rates, because the effects of Titan's lower gravity and less-dense sediments are offset by a much lower resistance to abrasion for ice than rock of similar strength. Here we report new laboratory measurements of ice erosion by low-velocity sediment impacts, part of a larger study investigating the temperature dependence of the material properties that control ice erodibility. We measure the energy required to erode a unit volume of ice using drop tests, in which a 110-150 g ice clast falls 5-10 cm onto a 20 cm diameter ice disk, and differences in mass and measurements of ice density are used to calculate the volume eroded. We construct the 10cm thick ice disks using 2-4 mm seed crystals and near-freezing distilled water. After freezing at 253 K a disk is placed in the bottom of a steel cylinder surrounded by dry ice and liquid nitrogen is pumped into the cylinder from below, chilling the ice to near-Titan temperatures for several hours but never submerging the samples (all drop test trials are completed in air). Our preliminary drop test results show that 4 J and 25 J are required to erode 1 cm3 of ice at temperatures of 205 K and110 K respectively, suggesting that ice may be no more than 2-3 times more erodible than previously-tested rocks of similar tensile strengths. A key limitation of this experimental method is the small size of our target disks, which fail catastrophically by through-cracking after several hundred drops. To avoid through-cracking and obtain direct measurements of ice surface erosion, we are preparing new experiments using a large ice block (~1.25x105 cm3) enclosed in an insulated test chamber, and a laser topographic scanning system. The drop-test results will then be used to design ice-flume experiments in a walk-in freezer to investigate controls on rates of ice incision by mobile sediments and the morphodynamics of incising ice channels.
P21A-1317
Analysis Of Selected VIMS And RADAR Data Over The Surface Of Titan Through Multivariate Statistical Methods
In this work 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, in order to properly look for correlations between the infrared and microwave ranges explored by
the two instruments.
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.
In VIMS data, combining spatial and spectral information, we have selected some atmospheric windows in the
spectral range between 2 and 5 μm, providing the best optical depth to measure surface reflectance.
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. The use of data sets from different instruments
onboard the Cassini spacecraft is a necessary step towards understanding the nature and history of Titan,
since correlations between different physical processes can be highlighted.
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. Due to the
large number of Titan data, the classification work is still ongoing; nevertheless the obtained results are
generally in agreement with previous works aimed both to the analysis of the scatterometry data through
physical models and to the correlation between SAR and radiometry data at a high resolution scale.
These evidences, evaluated for the first time through a multivariate statistical method, can provide
constraints on the geophysical models under development for the surface of Titan.
P21A-1318
Surface Parameters of Titan Feature Classes From Cassini RADAR Backscatter Measurements
Multimode microwave measurements collected by the Cassini RADAR instrument during the spacecraft's first four years of operation form a fairly comprehensive set of radar backscatter data over a variety of Titan surface features. We use the real-aperture scatterometry processor to analyze the entire collection of active data, creating a uniformly-calibrated dataset that covers 93% of Titan's surface at a variety of viewing angles. Here, we examine how the measured backscatter response (radar reflectivity as a function of incidence angle) varies with surface feature type, such as dunes, cryovolcanic areas, and anomalous albedo terrain. We identify the feature classes using a combination of maps produced by the RADAR, ISS, and VIMS instruments. We then derive surface descriptors including roughness, dielectric constant, and degree of volume scatter. Radar backscatter on Titan is well-modeled as a superposition of large-scale surface scattering (quasispecular scattering) together with a combination of small-scale surface scattering and subsurface volume scattering (diffuse scattering). The viewing geometry determines which scattering mechanism is strongest. At low incidence angles, quasispecular scatter dominates the radar backscatter return. At higher incidence angles (angles greater than ~30°), diffuse scatter dominates the return. We use a composite model to separate the two scattering regimes; we model the quasispecular term with a combination of two traditional backscatter laws (we consider the Hagfors, Gaussian, and exponential models), following a technique developed by Sultan-Salem and Tyler [1], and we model the diffuse term, which encompasses both diffuse mechanisms, with a simple cosine power law. Using this total composite model, we analyze the backscatter curves of all features classes on Titan for which we have adequate angular coverage. In most cases, we find that the superposition of the Hagfors law with the exponential law best models the quasispecular response. A generalized geometric optics approach permits us to combine the best-fit parameters from each component of the composite model to yield a single value for the surface dielectric constant and RMS slope [1]. In this way, we map the relative variation of composition and wavelength-scale structure across the surface. We also map the variation of radar albedo across the analyzed features, as well as the relative prevalence of the different scattering mechanisms through the measured ratio of diffuse power to quasispecular power. These map products help to constrain how different geological processes might be interacting on a global scale. [1] A. K. Sultan-Salem, G. L. Tyler, JGR 112, 2007.
P21A-1319
Dielectric Constant of Titan's South Polar Region from Cassini Radio Science Bistatic Scattering Observations
Four out of six Radio Science bistatic scattering (bistatic-radar) observations of Titan's surface completed during the Cassini nominal mission yielded detectable quasi-specular 3.6 cm-λ (X-band) surface echoes, making Titan the most distant solar system object for which bistatic echoes have been successfully detected. Right circularly polarized sinusoidal signal was transmitted by Cassini and both the right and left circularly polarized (RCP and LCP) surface reflected components were observed at the 70-m stations of NASA Deep Space Network. Cassini was maneuvered continuously to track the region of Titan's surface where mirror-like (quasi-specular) reflected signals may be observed. The experiments were designed for incidence angles θ close to the Brewster, or polarization, angle of likely surface compositions. Careful measurement of the system noise temperature allowed determination of the absolute power in each polarized echo component and hence their ratio. The polarization ratio, the known observation geometry, and Fresnel reflection theory were then used to determine the dielectric constant ε. Three near-equatorial (~ 5 to 15° S) observations on flyby T14 inbound and outbound and on flyby T34 inbound yielded weak but clearly detectable echoes. The echoes were intermittent along the ground track, indicating mostly rough terrain occasionally interrupted by patches of relatively flat areas. For the two observations on T14, polarization ratio measurements for two localized but widely separated surface regions (~ 15° S, ~ 14 and 140° W) conducted at angles θ ~ 56° and 64°, close to the Brewster angle for ices, imply ε ~ 1.6 for both regions, suggesting liquid hydrocarbons although alternative interpretations are possible (Marouf et al., 2006 Fall AGU, P11A- 07). In sharp contrast, a single high latitude (~81-86° S, ~ 45-155° W) observation on T27 inbound yielded much stronger surface echoes that lasted for almost the full duration of the experiment (~ 23 minutes). The relatively more grazing incidence geometry (θ ~ 70-79°) caused the RCP component to dominate the LCP component, as expected. Nonetheless, the later was mostly detectable, allowing estimation of the corresponding polarization ratio and hence profiling of the variability of the dielectric constant along the ground track. The inferred dielectric constant ε appears to vary over the large surface region probed but falls generally in the range 2 to 2.5, suggesting solid hydrocarbons or hydrocarbon "sludge" surface composition close to Titan's south pole. The small observed spectral Doppler broadening suggests that the echoes originate from gently undulating surface regions with RMS slopes of order few degrees.
P21A-1320
Constraints on Titan's Topography Through Fractal Analysis of Shorelines
The recent discovery of hydrocarbon lakes at Titan's North Pole by the Radio Detection and Ranging (RADAR) instrument onboard the Cassini spacecraft is one of the most exciting discoveries of the Cassini-Huygens mission. Previous analyses of terrestrial coastlines have revealed them to be closely approximated by self-similar fractals. Coastline length increases as the measuring scale decreases because smaller measuring scales are sensitive to smaller features of the coastline. The measured perimeter can be related to the measuring scale by a power law whose exponent is 1-D, where D is the fractal dimension. The value of D provides a means to quantify the complexity (ruggedness) of a coastline with higher values indicating higher complexity. As pooled liquids form equipotential surfaces, coastlines are equivalent to topographic contour lines. The complexity of a coastline can therefore be related to the complexity of the surface it is embedded in through fractal theory. Thus, a statistical characterization of Titan's topography can be extracted through analysis of these shorelines. We have carried out this analysis for coastlines on Titan and have related the coastline roughness parameters to topography parameters for Titan's landscape. In this study, we used projected Cassini Radar observations (resolution of about 350m/pixel near the centre of the swath). The shorelines of 290 of these North Polar Titanian lakes have been manually outlined at the full resolution of the dataset. Their fractal dimensions were calculated via two methods: the ruler method and the box-counting method. Our results show Titan's coastlines do exhibit fractal properties with fractal dimensions comparable to published estimates of the terrestrial coastlines of Britain and Germany. Such high values of this roughness parameter show that Titanian coastlines are intricate by terrestrial standards, which implies a rugged landscape. We will report on this statistical characterization of Titan's topography and spatial variations in landscape roughness.
P21A-1321
Observations of photoelectrons with CAPS ELS at Titan
Photoelectrons are generated in Titan's ionosphere as a result of the strong HeII (30.4nm) solar line ionising N2 [1]. The presence of these photoelectrons is discernible as a discrete peak in the electron energy spectrum observed by the ELS (ELectron Spectrometer) part of the Cassini Plasma Spectrometer (CAPS). They are generally observed in the dayside ionosphere. [2] discussed observations of photoelectrons in Titan's distant tail during the T9 encounter. This study describes other photoelectron observations near Titan. On some encounters photoelectrons are observed at large distances, some of which are near the nightside and in the tail. The origin of these is discussed. In some cases the photoelectrons may have travelled to the observation sites by means of a magnetic connection to lower altitudes on the dayside ionosphere where they could have been produced. To investigate this possibility, T15 observations are compared with different model results ([3], [4], [5]). The prospect of local production is also considered. References: [1] Galand et al (2006), Electron temperature of Titan's sunlit ionosphere, GRL, 33, L21101, doi:10.1029/2006GL027488. [2] Coates, A. J. et al (2007), Ionospheric electrons in Titan's tail: Plasma structure during the Cassini T9 encounter, GRL, 34, L24S05, doi:10.1029/2007GL030919. [3] Sillanpaa, I., Hybrid Modelling of Titan's Interaction with the Magnetosphere of Saturn, Ph.D. dissertation, Yliopistopaino, 2008. [4] Ma, Y. J.,et al (2008), Real-time Global MHD Simulations of Cassini T32 Flyby: from Magnetosphere to Magnetosheath, JGR, submitted. [5] Cravens, T. E. et al (2008), Model-Data Comparisons for Titan's Nightside Ionosphere, Icarus, submitted.
P21A-1322
Titan's Ionospheric Structure -- Dependence on Solar Illumination Conditions
The ionosphere of Titan is mainly created by photoionisation by solar EUV radiation and electron impact ionisation from Saturn's magnetospheric plasma. This study investigates the relative contribution of solar radiation versus magnetospheric input. We show that Titan's ionosphere exhibits a highly complex structure that largely varies with solar illumination conditions. Our investigations are based on in-situ data from Radio and Plasma Wave Science (RPWS) observations combined to Ion and Neutral Mass Spectrometer (INMS) observations from several deep (< 1000 km) ionospheric passes by the Cassini spacecraft. By using Langmuir Probe (part of the RPWS) data we determine electron number density profiles, the altitude and maximum number density of the ionisation peaks for each flyby. In conclusion, we show that solar photons are the main ionisation source of Titan's dayside atmosphere. However, magnetospheric electron precipitation is also of high importance and its contribution, especially on the nightside, cannot be neglected.
P21A-1323
Mapping Titan's Induced Magnetosphere
In this work we study the spatial distribution of the magnetic pressure in the vicinity of Titan from all the Cassini flybys to date in an attempt to characterize the average extent and shape of its induced magnetosphere at altitudes above 950 km. The role of parameters such as Saturn Local Time (SLT), total upstream pressure, and convective electric field is studied. Then we combine Cassini magnetometer (MAG), Plasma Spectrometer (CAPS), Plasma Wave/Langmuir probe (RPWS/LP), and Magnetospheric Imager (MIMI) observations in order to compare, when possible, the magnitudes of the dynamic, magnetic and plasma pressures at different altitudes. Finally, we discuss the constraints that these results set for the current theoretical models.
P21A-1324
Comparison of ion Escape From the Wake-side Ionospheres of Venus and Titan
Upward flow of ionospheric plasma into the induced magnetic tail of Venus was inferred some time ago from Pioneer Venus Orbiter (PVO) measurements, which were used to derive upward flow and acceleration of H+, D+ and O+ within the nightside ionosphere [1]. The measurements revealed that the polarization electric field in the nightside ionosphere produced the principal upward force on these light ions. Other electrodynamic forces were unimportant because the plasma beta in the nightside ionosphere is much greater than one. The resulting vertical flow of H+ and D+ was found to be the dominant escape mechanism of hydrogen and deuterium, corresponding to loss rates consistent with large oceans in early Venus [2]. Recently, plasma measurements made from Venus Express have clearly identified H+, D+ and O+ flowing away from Venus, down its magnetic tail [3]. The primary source of tail-flowing O+ is from the high altitude day-to-night flow system. Similarly, at unmagnetized Titan, ions have been observed to flow away from the moon along its induced magnetic tail by the Plasma Science Instrument (PLS) on Voyager 1 and the Casini Plasma Spectrometer (CAPS) on Cassini. In both cases, the ions have been inferred to be of ionospheric origin. Although the plasma beta is also greater than one in much of Titan's ionosphere, ion acceleration is expected to be more complex, especially because the subsolar point and the subflow points can be 180 degrees apart. Following what we learned at Venus, upward acceleration of light ions by the polarization electric field opposing gravity in the wake-side ionosphere of Titan is described. Additional electrodynamic forces resulting from the interaction of Saturn's magnetosphere with Titan's ionosphere will be examined using a recent hybrid model [4]. Comparisons between the wake-side flows on Venus and Titan will be made. [1] Hartle, R. E. and J. M. Grebowsky, Adv. Space Res., 15, (4)117, 1995. [2] Donahue, T. M. and R. E. Hartle, Geophys. Res. Lett., 19, 2449, 1992. [3] Barabash, S., et al., Nature, 450, 650, 2007. [4] Lipatov, A. S., E. C. Sittler, Jr. and R. E. Hartle, 2007 Fall AGU mtg., EOS, P23B-1366, 2007.
P21A-1325
How Surface-Atmospheric Separation Radiative Transfer Models Compare to Cassini VIMS Data for Titan
The Cassini orbiter's Visual and Infrared Mapping Spectrometer (VIMS) observes Titan's surface intensity at wavelengths where its methane-rich atmosphere is heavily absorbing and light is strongly scattering. We aim to quantify the amount of I/F (spectral signal) from each component of Titan's atmosphere and surface, to aid analyses of Titan's surface that require use of the VIMS dataset (e.g., photoclinometry, geologic interpretation, spectral identification of surface materials, photometry). In a previous work (Pitman et al. 2008, LPSC XXXIX meeting), we presented radiative transfer model images of I/F for the entire disk of Titan, generated for Titan aerosol optical properties derived from Cassini-Huygens data and varying surface bidirectional reflectance functions at 9 wavelengths, 5 of which are VIMS methane windows. In this work, we combine gas absorber, aerosol, and surface signatures and subset these model I/F images for direct mapping and comparison to VIMS Titan data at different locations on Titan acquired at different observation times. Degrees of perturbation required in the Cassini-Huygens assumptions to match VIMS Titan data, comparison/contrast between results of spherical shell and plane-parallel radiative transfer methods, and single versus multiple scattering contributions of Titan's atmosphere at VIMS-relevant wavelengths will be discussed. Work performed under contract to NASA and under appointment to the NASA Postdoctoral Program (ORAU).
P21A-1326
Saturn's Northern Hemisphere Ribbon; Characterizing the Wave Mode Composition
We analyze the wave-like feature in Saturn's northern mid-latitude known as the Ribbon. It was originally discovered by the Voyagers in 1980-81 (Sromovsky et al. 1983, JGR). In the visible wavelengths, the Ribbon appears as a dark line which meanders around 48 degree north planetographic latitude and completely engirdles the planet. Studies in the past suggest that the dark line traces the core of the zonal jet blowing at the latitude, and the Ribbon has been interpreted as a manifestation of instability modes (Godfrey and Moore 1986, Icarus). The wave's time dependent behavior suggests that nonlinear wave-wave interaction between different wave modes play an important role in shaping the Ribbon, which is remarkable because even though the oscillation amplitude is beyond the linear regime, the jet's meandering amplitude seems to reach an equilibrium and does not destroy the jet. The Ribbon was found again in ground-based observations in 1994-1995 (Sanchez-Lavega 2002, Icarus), and it can be also seen in recent Cassini images as well, which suggests that this is a long-lived feature on the planet. Our investigation first examines the conditions that allow the instability mode to have the zonal-wavelength characteristics and propagation phase speed observed by Voyager. Our simulations capture important nonlinear features of the observed Ribbon wave. Our modeling effort uses the EPIC atmosphere model by Dowling et al (1998 and 2006, Icarus). In addition to the dynamical modeling of the Ribbon, we examine the Ribbon captured in recent Cassini ISS observations. We measure the longitudinal wavelengths of the dominant oscillation modes and their phase speeds, and compare their current properties to those during the Voyager era.
P21A-1327
The Effects of Variable Upstream Conditions on Titan's Plasma Interaction
Cassini observations have found that the plasma and magnetic field conditions upstream of Titan are far more complex than they were thought to be after the Voyager encounter. Understanding the effects of the variable upstream conditions on Titan's plasma interaction and the evolution of Titan's ionosphere/atmosphere is one of the main objectives of the Cassini mission. To compliment the mission we perform hybrid simulations of Titan's plasma interaction to examine the effects of the incident plasma distribution function and the angle between the magnetic field and the flow velocity. We closely examine the results on Titan's induced magnetosphere and the resulting pickup ion properties.
P21A-1328
Deep Convective Cloud Constraints on the General Circulation of Titan
The influence and implications of deep convection on Titan's global heat and moisture budgets and on the mean meridional circulation (i.e., the Hadley cell) are explored. There is a strong coupling and interdependency between deep convective clouds and their environment. The environment, through the generation of convective available potential energy (CAPE) by radiative processes and through large scale advection, controls the nature of convective clouds. While the environment constrains the cloud dynamics, it is equally true that convective cloud processes feed back to the environment. Convective clouds consume CAPE and efficiently transport heat and moisture through vertical advection within the cloud cores, detrainment of cloudy air near cloud top, and through adiabatic descent of the cloud-free environment required to offset the upward mass flux within the convection. Based on numerical simulations with the Titan Regional Atmospheric Modeling System, deep convective clouds on Titan consume CAPE on a timescale ranging from 2-10 hours, depending on the initial relative humidity. Over these timescales, the convection cools/warms, by approximately several K/earth-day, the lower/upper levels of the atmosphere. Drying/moistening of the lower/upper atmosphere is also found to be several g/kg/earth-day. If the temperature and moisture profiles are to remain quasi-constant (i.e., CAPE remains quasi-constant), then there must be a net low level meridional flux of heat and moisture to balance the convective tendencies, since radiative processes operate on substantially longer timescales. For the case of southern summer convective clouds in the polar latitudes, this flux is shown to be ~1 m/s. However, unless heat is resupplied to the lower latitudes, this poleward heat flux would result in a change in the large-scale temperature gradient. The large-scale near-surface temperature gradient on Titan is on the order of ~5K from pole to pole. If this gradient is quasi-steady, then a meridional advective velocity of 100 m/s or more would be required to balance south polar convection. We provide a more complete energy and moisture budget analysis based on cloud model results to better constrain the magnitude of Hadley cell circulation of Titan.
P21A-1329
Clouds on Titan during the Cassini mission: a complete analysis of the VIMS data
The Cassini spacecraft reached Titan slightly after the summer solstice in the southern hemisphere. During its four year prime mission, Cassini conducted 45 flybys of Titan (ending on July 31, 2008), collecting data as the southern hemisphere transitioned from summer to fall. Using the Visual and Infrared Mapping Spectrometer's (VIMS) capability of observing different wavelengths, and therefore different layers of Titan's methane-rich atmosphere, we isolated and imaged methane clouds present in Titan's troposphere. We present maps of each flyby detailing the longitude and latitude locations and sizes of every cloud surveyed as well as their significance towards Titan's seasonal alterations.
P21A-1330
Cassini Imaging Observations of Lakes and Clouds at Titan's High Latitudes and the Implications of the Changes Therein
Cassini's Imaging Science Subsystem (ISS) imaged Titan's south polar region in July 2004 and June 2005, revealing convective cloud systems and dark surface features interpreted to be hydrocarbon lakes, e.g. 235- km-long Ontario Lacus (McEwen et al., B.A.A.S. 37, 2005). Recent evidence from Cassini's Visual and Infrared Mapping Spectrometer supports the interpretation that Ontario Lacus contains liquid ethane and methane (Brown et al., Nature 454, 2008). Although diffuse clouds or atmospheric scattering could play a role, differences between the two ISS observations taken a year apart may be due to changes in the lakes as a result of precipitation from a large cloud system observed in Fall 2004 (Schaller et al., Icarus 182, 2006). ISS observations of northern latitudes just emerging from northern winter have revealed much larger dark areas, including Mare Kraken (>1100 km long), as well as myriad smaller dark spots. Many of these features coincide with liquid-filled areas identified by Cassini RADAR (e.g., Lopes et al., EOS 88, 2007). Combined these features cover well over 600,000 km2, ~1% of Titan's surface area; however, as shown by Lorenz et al. (GRL 35, 2008), even if all were filled with liquid, they would not provide enough methane to keep Titan's atmosphere resupplied for a substantial amount of time, unless they are unexpectedly deep or other subsurface reservoirs exist. Intriguingly the surface coverage is unevenly distributed, with more total area and much larger seas occurring around the North (recently winter) Pole. The extents to which this variation depends on the season and/or local geology and its effects on atmospheric circulation may be revealed with the advent of northern spring and summer.
P21A-1331
Anomalous emission in the 3.28 μm in the Titan upper atmosphere
Earliest Cassini VIMS limb observations at Titan taken in October 26th, 2004 show a strong methane non- LTE limb emission at high atmospheric altitudes. During that pass at Titan, VIMS vertical resolution was about 110 km and the analyzed spectral interval corresponds to the methane emission band centered around 3.33 micron. A detailed analysis of the radiances versus altitudes shows an anomalous emission at altitudes higher than 900 km at wavelengths corresponding to the methane R branch around 3.28 microns. The nature of such emission is under investigation. Different spectral databases and codes both for calculating the methane expected non-LTE emissions and for the retrieval of limb observations have been used. The anomalous emission could not be reproduced using all the available data on methane in the simulations. According to the spectral position it can be guessed that the emission should belong to a molecule containing C-H or C-N bonds. However, different molecules and ions, whose spectral characteristics can be found in literature, have been tested and none could properly fit the data. The observation on Oct. 26th, 2008 was chosen for its very good signal to noise ratio and because of the favorable illumination of the atmosphere as seen from Cassini spacecraft (i.e. low phase angle). Successive observations confirm such a finding.
P21A-1332
Kinetic simulation of the distribution and escape of minor components of Titan's exosphere
Possessing the most significant atmosphere among icy satellites in Saturnian system, Titan attracts attention as a source of material in the Saturnian magnetosphere and makes the problem of understanding the distribution and escape of neutral components from its exosphere both interesting and important. Modeling of the neutral gas distribution throughout the exosphere is complicated because of the dramatic variation of its density. The collision regime of the gas flow in the exosphere starts as collision dominated at the boundary with a lower part of the atmosphere, where the gas is successfully described with its density, bulk velocity and temperature and becomes free molecular flow somewhat above the exobase, where momentum exchange between molecules has a negligible effect. In this case, adoptation of a kinetic approach is required for both the simulation of suprathermal components of the exosphere and possibly to correctly understand the structure of the exobase region itself. The presented work is aimed at the development of a kinetic numerical model that would be suitable for studying properties of suprathermal neutral components of upper planetary atmospheres, where the collision frequency is such that the validity of a fluid approach is limited. The model is applied to a case of the upper atmosphere of Titan in order to study distribution and escape of neutrals from its exosphere, the return flux to the lower atmosphere and influence of heating sources on neutrals distribution and ionization rate at the exosphere.
P21A-1333
Evolution and Origin of Titan's Atmosphere: Constraints from Carbon, Oxygen, and Hydrogen Isotope Ratios
Observations of the C-18O(3-2) and 13CO(3-2) rotational transitions were obtained 9 Nov 2007 using the Submillimeter Array (SMA). In addition, the first detection of the C-18O(6-5) rotational transition, at 658.553 GHz, was made using SMA data from 14 Dec 2005. Preliminary measurements of the isotopic ratios 13C/12C and 18O/16O will be presented. Moreover, the Cassini mission provides updated 13C/12C and D/H ratios in CH4 and C2H2. Recently 13C/12C and 18O/16O ratios in CO2 are obtained, including both singly and doubly isotope substituted compounds. Incorporating laboratory measured rate coefficients in isotope-variant chemical reactions (both two-body and photolytic reactions) into a photochemical model, the profiles of C- 16O, C-18O, 13CO, C-16O-16O, C-16O-18O, 13C-16O-16O, 13C-16O-18O, CH4, 13CH4, CH3D, 13CH3D, C2H2, C2HD, C2H6, C2H5D, and other C2 hydrocarbons are obtained. Implications of the model results to the evolution and origin of Titan's atmosphere are discussed.
P21A-1334
Titan's Prolific Propane
Propane (C3H8) was first detected on Titan by the Voyager 1 IRIS spectrometer in 1980, which identified four stratospheric emission bands (ν21 at 748 cm-1, ν16 at 922 cm-1, ν15 at 1054 cm-1 and ν7 at 1158 cm-1). Subsequent analyzes of this dataset have largely focused on the strongest of these bands (ν21) to infer the VMR (~ 7×10-7 in the stratosphere and relatively uniform with latitude), although it is significantly disadvantaged by being coincident with a strong R-branch line of acetylene. The Composite Infrared Spectrometer (CIRS) onboard Cassini is now providing the most complete infrared mapping of Titan yet obtained. Prior results have included the retrieval of the vertical and latitudinal profiles of minor gas species, including propane, again exclusively via the ν21 band, the only band for which a linelist is publicly available. We present new modeling of CIRS low-latitude limb spectra acquired from 2004 to 2008. The residuals after modeling all gases except propane clearly show all four bands detected by IRIS, now at much higher signal to noise. In addition, we show that four further bands are clearly evident once the emissions of methane (CH4) and ethane (C2H6) have been modeled and subtracted out: ν8 at 869 cm-1, ν14 at 1338 cm-1, ν13 at 1376 cm-1 and ν19 at 1472 cm-1). Using a new line list for the bands in the range 1300 to 1500 cm-1, we model the ν14, ν13 and ν19 emission bands and compare to abundances retrieved using the ν21 at 748 cm-1. This work has several purposes. Firstly, we demonstrate the urgent need for laboratory spectroscopic measurements of the propane bands at 869, 922, 1054 and 1158 cm-1 leading to line strength listings suitable for spectral calculations. Secondly, we show that the current line list for the 748 cm-1 band does not fit the data accurately, and requires a new spectroscopic study. Finally, we discuss the possibility of new gaseous molecular detections in the regions dominated by these ubiquitous propane bands, once they can be properly modeled.
P21A-1335
Modeling the Distribution of hydrocarbons in the atmosphere of Titan
The chemical and dynamical processes in the atmosphere of Titan are poorly quantified. In this presentation, we constrain the transport using the data obtained by the Cassini and Voyager spacecrafts, with emphasis on Cassini measurements. A two-dimensional photochemistry transport model is used to model the distribution of hydrocarbons at latitudes from pole to pole and altitudes from the tropopause (about 50 km) to about 1500 km. Currently no GCM is available to provide transport of species over such wide a range of altitudes. The transport is obtained from the model of Newman et al. (2008) by analogy with the Earth's transport, as the two objects have similar obliquity and major chemical composition, N2. The reference transport is too fast. Modifications of this transport are made to reproduce the observations. Cassini UVIS and CIRS measurements are used to constrain the model above and below about 500 km, respectively. Implications of the model are discussed.
P21A-1336
A 3-D Chemistry Transport Model for Titan's Thermosphere
MOZART-2 (Horowitz et al., 2003) has been adapted to investigate seasonal and diurnal differences in neutral composition in Titan's atmosphere between the surface and 1,200 km altitude. The chemical scheme with 64 solution species and 383 reactions is based on a simplified version of the Lavvas et al. (2008) scheme, without haze production. Wind and temperature fields were taken from the Cologne GCM (Tokano, 2007) or TitanWRF (Richardson et al., 2007) for the troposphere and stratosphere, and from the London TGCM (Mueller-Wodarg, 2000) for the thermosphere. Pronounced hemispheric concentration gradients develop in the thermosphere, and a strong diurnal cycle in composition is found, similar to the findings of Mueller-Wodarg (2003) for methane. Sensitivity experiments with different strengths of thermospheric circulation to account for uncertainty about the wind fields in that region are presented.
P21A-1337
Titan's plasma environment for T9 and T18 encounters: 3D hybrid simulation and comparison with observations
We discuss the results of the hybrid simulation of Titan's environment
in case of T9 encounter. The simulations are
based on recent analysis
of the Cassini Plasma Spectrometer (CAPS) ion
measurements during the T9 and T18 flyby through the induced magnetic
tail of Titan [Sittler].
This new result changes our previous model
of the interaction of Saturn's rotating magnetosphere with Titan
from one that was discussed in the recent publications.
The current simulation shows that mass loading by pickup ions H+,
H2+, CH4+ and
N2+ is stronger than in the previous simulations.
In our hybrid simulations
we use
Chamberlain profiles for the exosphere's
components.
We also include a simple ionosphere model.
Special attention will be paid to comparing our numerical
results with Cassini T-9 and T18 observations.
We shall estimate the mass loading rate and the
energy input to the upper atmosphere from ambient and
pickup ions for the
T9 and T18 encounters.
References
Sittler, E.C. et al. (2008) Spring AGU Meeting.
P21A-1338
Cassini's CAPS and MAG measurements during Titan flyby T15 compared to HYB model results
Titan's ionosphere and exosphere interact with the rotating plasma flow of the Saturn's magnetosphere. The plasma flow is magnetized by Saturn's magnetic field but also disturbed by the dynamics of the outer magnetosphere; e.g. the location of the plasma sheet and the motion of the magnetopause due to varying solar wind pressure. These cause changes in the density and velocity of the plasma flow as well as in the direction of the ambient magnetic field. Furthermore, near the magnetospheric current sheet (i.e. in the magnetodisk) oxygen and water-group ions have often higher density than hydrogen ions whereas outside the magnetodisk these heavy ion components are not detected and consequently H+ dominates. Cassini's T15 flyby of Titan's close wake on July 2, 2006 is studied using CAPS ion spectrometer (IMS) and magnetometer (MAG) data. The plasma flow was composed of water-group ions (0.09±0.05 cm-3) and H+ (0.07±0.03 cm-3) with the flow speed roughly 80 km/s. For T15 the magnetic field a one- lobe structure was observed similar to that seen in the wake further down Titan's tail during T9[1]. The data are compared with simulation results from a global hybrid plasma model HYB. [1] Sillanpää, I., Hybrid Modelling of Titan's Interaction with the Magnetosphere of Saturn, Ph.D. dissertation, Yliopistopaino, Helsinki, 2008. (Freely available at permalink http://urn.fi/URN:ISBN:978-951- 697-660-3)
P21A-1339
Unusually strong magnetic fields in Titan's ionosphere: T42 analysis
The observations of unusually large magnetic fields indicate periods of greatest stress on Titan's ionosphere and potentially of strongest loss rate of ionospheric plasma. During Titan flyby T42, the observed magnetic field maximized at 37 nT between an altitude of 1200 and 1600 km, over 20 nT stronger than previous measurements and close to five times greater in pressure. The strong fields occurred near the corotation- flow terminator rather than at the sub-flow point, suggesting that the flow that magnetized the ionosphere can be from a direction far from corotation. Extrapolation of solar wind plasma condition from Earth to Saturn using the UMich MHD predicts an enhanced solar wind dynamic pressure at Saturn close to this time. Earlier exits of Cassini from the magnetosphere support this prediction and the CAPS instrument saw a magnetopause crossing three hours before the strong-field observation. Thus it appears that Titan was magnetized when enhanced dynamic pressure compressed the magnetosphere and perhaps the magnetosheath magnetic field in onto Titan. The solar wind then relaxed leaving a strong fossil field in the ionosphere. When observed this strong magnetic flux tube had begun to twist further enhancing its strength. In this paper we examine the forces and stability of this structure, its possible source and its possible affects on the ionosphere.
P21A-1340
Recent Observations of Titan's Suprathermal Corona
Ion and Neutral Mass Spectrometer (INMS) measurements of nitrogen and methane densities as a function of radial distance during Cassini flybys are presented. These flybys cover a wide range of Titan latitudes, solar zenith angles and local times. Below the exobase the best fits are obtained using isothermal profiles. Most flybys exhibit breaks from isothermal profiles near the exobase. Large temperature increases would be required to fit the data above the exobase. Using an exospheric model based on the Liouville theorem, fits are produced for various exobase energy distributions. In general the best fits are found using energy distributions containing a suprathermal population. From the density of suprathermals at the exobase energy deposition rates are calculated. Implications for the escape of primary atmospheric constituents are also discussed.
P21A-1341
The role of methane in Titan's clouds, climate, winds, and spin
The atmosphere of Titan is laden with methane vapor, the equivalent of a 5 m global ocean. Methane is near its triple point at surface conditions on Titan, suggesting it is thermodynamically active. I will demonstrate how the coupling of methane thermodynamics to the large-scale circulation limits cloud coverage and duration, defines climate zones, limits low-altitude atmospheric superrotation, and damps the seasonal cycle. Cassini-Huygens and ground-based observations reveal these effects in the seasonality of methane clouds, the latitudinal distribution of surface morphologies, the vertical profile of winds, and a time lag in the observed seasonal spin rate change, respectively. These phenomena are consistent with the presence of an active but seasonal cycle of methane in the atmosphere.
P21A-1342
Planetary Waves on Titan
In April 2008 we observed tropospheric clouds on Titan located at near equatorial latitudes (30S-10S). These clouds were first detected on 13 April 2008 by our Titan IRTF spectral monitoring program (Schaller et al. 2008) and were subsequently observed in Gemini adaptive optics images until May 2008. In subsequent observations, cloud activity across the southern hemisphere of the moon resumed its previous low levels (Schaller et al. 2006b). During part of this ~4-week period of cloud activity near the equator and southern midlatidues, clouds were also observed near the south pole. These observations show that a large atmospheric perturbation at one latitude can trigger planetary waves capable of inducing cloud activity at latitudes otherwise stable against convection during the present season. South polar tropospheric cloud activity has been essentially non existent as seen from ground-based observations since 2005. The south pole is not predicted by general circulation models to be able to support convective cloud formation at the present season (e.g. Mitchell et al. 2006). Understanding Titan's methane-based meteorological cycle, including the causes and frequencies of these large cloud events and their subsequent evolution across the moon, holds the key for interpreting the fluvial surface features seen on Titan's surface by the Cassini Spacecraft.
P21A-1343
The Gravity Field of Titan From Four Cassini Flybys
Doppler tracking of the Cassini spacecraft across four flybys has been used for a preliminary determination of Titan's gravity field. The flybys occurred on February 27, 2006, December 28, 2006, June 29, 2007 and July 31, 2008, with closest approach altitudes between 1300 and 2100 km. X- and Ka-band Doppler data from each flyby have been combined in a multi-arc solution for the Stokes coefficients up to degree-3. The dynamical models employed in the data fit were limited to the static component of the gravity field and did not include eccentricity tides. Tidal variations of the quadrupole coefficients are expected at a level of a few percents if the surface hides an internal ocean, and are therefore accessible to Cassini measurements. As the flybys were evenly distributed about pericenter and apocenter of Titan's orbit, the current analysis provides a good representation of the static component of the quadrupole field. In one setup, Titan's ephemerides were also updated, leading to improved determination of the satellite's orbit and gravitational parameter (GM). The measured gravity field is dominated by a large, nearly hydrostatic, quadrupole component, consistent with an equilibrium response to the perturbations due to rotation and Saturn gravity gradient. The magnitude of the degree-3 coefficients accounts for about 1-3% of the overall field, with significant gravity disturbances (at a level of 2-5 mgal) over broad regions of the surface. The corresponding peak-to-peak geoid height variations amount to a few tens of meters. The ellipsoidal reference surface shows variations among the axes of a few hundred meters. The near hydrostaticity of Titan justifies the application of Radau-Darwin equilibrium theory, which provides the fluid Love number and the average moment of inertia. The latter is consistent with a partial, but not full, differentiation of the interior. This work was partly conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding in the USA and in Italy was provided by the Cassini Project and by the Italian Space Agency, respectively.
P21A-1344
The Influence of Internal and External Torques on Titan's Length-of-day Variations
Cassini radar observations show that Titan's spin is slightly faster than synchronous spin. Angular momentum exchange between Titan and its atmosphere is the most likely cause of the observed non-synchronous rotation. We study the effect of Saturn's gravitational torque and torques between Titan's internal layers on the length-of-day (LOD) variations driven by the atmosphere. Those torques depend on the equatorial flattening of Titan resulting from static tides raised by Saturn. We calculate Titan's flattening under the assumption of hydrostatic equilibrium and show that the gravitational forcing by Saturn, due to misalignment of the long axis of Titan with the line joining the mass centers of Titan and Saturn, reduces the LOD variations with respect to those for a spherical Titan by an order of magnitude. Internal gravitational and pressure coupling between the ice shell and the interior beneath a putative ocean tends to diminish any differential rotation between shell and interior and reduces further the LOD variations by a few times. For the current estimate of the atmospheric torque, we obtain LOD variations of a hydrostatic Titan that are more than 50 times smaller than the observations indicate when a subsurface ocean exists and more than 100 times smaller when Titan has no ocean. Moreover, Saturn's torque causes the rotation to be slower than synchronous in contrast to the Cassini observations. Those large differences with the observations suggest that non-hydrostatic effects in Titan are important. In particular, we show that the amplitude and phase of the calculated rotation variations would be similar to the observed values if non-hydrostatic effects strongly reduce the equatorial flattening of the ice shell above an internal ocean. Alternatively, the calculated LOD variations could be increased if the atmospheric torque is larger than predicted or if fast viscous relaxation of the ice shell could reduce the gravitational coupling, but it remains to be studied if a two order of magnitude increase is possible and if these effects can explain the phase difference of the predicted rotation variations.
P21A-1345
Titan after Cassini Huygens
In 2005, the Huygens Probe gave us a snapshot of a world tantalizingly like our own, yet frozen in its evolution on the threshold of life. The descent under parachute, like that of Huygens in 2005, is happening again, but this time in the Saturn-cast twilight of winter in Titan's northern reaches. With a pop, the parachute is released, and then a muffled splash signals the beginning of the first floating exploration of an extraterrestrial sea—this one not of water but of liquid hydrocarbons. Meanwhile, thousands of miles away, a hot air balloon, a "montgolfiere," cruises 6 miles above sunnier terrain, imaging vistas of dunes, river channels, mountains and valleys carved in water ice, and probing the subsurface for vast quantities of "missing" methane and ethane that might be hidden within a porous icy crust. Balloon and floater return their data to a Titan Orbiter equipped to strip away Titan's mysteries with imaging, radar profiling, and atmospheric sampling, much more powerful and more complete than Cassini was capable of. This spacecraft, preparing to enter a circular orbit around Saturn's cloud-shrouded giant moon, has just completed a series of flybys of Enceladus, a tiny but active world with plumes that blow water and organics from the interior into space. Specialized instruments on the orbiter were able to analyze these plumes directly during the flybys. Titan and Enceladus could hardly seem more different, and yet they are linked by their origin in the Saturn system, by a magnetosphere that sweeps up mass and delivers energy, and by the possibility that one or both worlds harbor life. It is the goal of the NASA/ESA Titan Saturn System Mission (TSSM) to explore and investigate these exotic and inviting worlds, to understand their natures and assess the possibilities of habitability in this system so distant from our home world. Orbiting, landing, and ballooning at Titan represent a new and exciting approach to planetary exploration. The TSSM mission architecture inherently provides the optimal balance between science, risk, and cost using three guiding principles: Achieve science well beyond the high bar set by Cassini Huygens. The TSSM orbiter, lander, and balloon have been configured with instruments and operational concept that go well beyond Cassini-Huygens capabilities, thus ensuring dramatic remote observation and in situ science discoveries. Build upon successful design and operational experience and lessons learned. ESA has successful experience in designing and landing probes on Titan (Huygens), as does NASA in implementing an orbiter at Saturn (Cassini). Long life design rules and extensive operational experience in the Saturn system have been applied to form the TSSM concept. Lessons learned from Galileo, Cassini, New Horizons, and MRO have been applied to reduce risk and lower cost. Development by ESA of the montgolfiere combines prior experience with Earth and planetary balloon systems to enable innovative science and unprecedented mobility for surface exploration. Strong international partnership. TSSM represents a collaborative effort between NASA and ESA that is structured to provide the best possible mission at a reasonable cost to NASA and to ESA. This NASA-ESA partnership leverages resources to maximize science return, distribute risk, and ensure technical readiness.
P21A-1346
The Titan Saturn System Mission
A mission to return to Titan after Cassini-Huygens is a high priority for exploration. Recent Cassini-Huygens
discoveries have revolutionized our understanding of the Titan system, rich in organics, containing a vast
subsurface ocean of liquid water, surface repositories of organic compounds, and having the energy sources
necessary to drive chemical evolution. With these recent discoveries, interest in Titan as the next scientific
target in the outer Solar System is strongly reinforced. Cassini's discovery of active geysers on Enceladus
adds an important second target in the Saturn system.
The mission concept consists of a NASA-provided orbiter and an ESA-provided probe/lander and a
Montgolfiere. The mission would launch on an Atlas 551 around 2020, travelling to Saturn on an SEP gravity
assist trajectory, and reaching Saturn about 9.5 years later. The flight system would go into orbit around
Saturn for about 2 years. During the first Titan flyby, the orbiter would release the lander to target a large
northern polar sea, Kraken Mare, and the balloon system to a mid latitude region.
During the tour phase, TSSM will perform Saturn system and Enceladus science, with at least 5 Enceladus
flybys. Instruments aboard the orbiter will map Titan's surface at 50 m resolution in the 5 micron window,
provide a global data set of topography and sound the immediate subsurface, sample complex organics,
provide detailed observations of the atmosphere, and quantify the interaction of Titan with the Saturn
magnetosphere. A subset of the instruments would provide spectra, imaging, plume sampling and particles
and fields data on Enceladus.
Instruments aboard the balloon will acquire high resolution vistas of the surface of Titan as the balloon
cruises at 10 km altitude, as well as make compositional measurements of the surface, detailed sounding of
crustal layering, and chemical measurements of aerosols. A magnetometer, will permit sensitive detection of
induced or intrinsic fields.
The probe/lander will splash into a large northern sea and spend several hours floating during which direct
chemical and physical sampling of the liquid would be undertaken. During its descent the probe would
provide the first in situ profiling of the winter northern hemispheric atmosphere, distinctly different from the
equatorial atmosphere where Huygens descended and the balloon will arrive. Radio science experiments
should be capable of providing detailed information on Titan's tidal response, and hence its crustal rigidity
and thickness.
http://www.lesia.obspm.fr/cosmicvision/tandem/
P21A-1347
Cassini Radar: Extended Mission Plans and Expected Results
Cassini completed its four year Prime Mission in July, 2008. This included a total of 45 close Titan flybys, with radar data obtained on 23 of these. An approved two year extended mission will provide an additional 25 close Titan flybys, with radar data collected on 12 of these. The prime mission radar data covered primarily the northern hemisphere of Titan, and the priority for the extended mission radar observations will be to fill in southern hemisphere coverage and provide limited repeat coverage for change detection in the north. During the prime mission, all but one flyby included some form of synthetic aperture imaging, leading to a wide range of viewing geometries. Imaging resolution varied from 300-500 m at the closest approach altitudes (1000 km) to about 2 km during high altitude (20,000 km) imaging segments. Altimetry, scatterometry, and radiometry mode data were also collected over multiple geometries to sample Titan's scattering and emission functions. The varying geometry and instrument parameters, which lead to varying resolution, SNR, polarization, incidence angle and noise characteristics, must be properly accounted for when interpreting these data. Here we review the coverage and other important characteristics of the radar data sets obtained at Titan in the prime mission, and compare these with plans for the extended mission. The first extended mission radar observation will occur on Dec 5, 2008, and we will present these timely preliminary results if the spacecraft operates as planned. Prime mission data along with corresponding surface coverage from ISS and VIMS have revealed a diverse Titan surface, which will be further augmented and analyzed by extended mission results. This work is supported by the NASA Cassini Program at JPL - Caltech.