P23B-1349
The north polar lakes of Titan as observed by Cassini Radar
Over the course of a year, Cassini RADAR obtained Synthetic Aperture Radar images covering 69 percent of Titan's polar region north of 65 degrees; the region being 1.4E6 km3 in extent, greater than double the land area of the USA. We observe several hundred lakes with a range of morphological expression, including areally massive and morphologically distinctive "seas", covering ~15% of the polar region. Lakes are extremely radar dark, consistent with a lossy liquid hydrocarbon. Preliminary laboratory estimates suggest that loss tangents in the range 10E4 to 2x10E3 are reasonable, which implies that one can see through at least a few to many tens of m of liquids before the noise floor is reached, consistent with observed brightening towards many lake shores. North polar lake volumes are most likely in the 8E3 - 1.4E6 km3 range. Uncertainties will be reduced as more data, both image-based and experimental, are obtained but we can conclude with a high degree of confidence that hydrocarbon lakes on Titan are more voluminous than known terrestrial oil reserves; current estimates range from 2248 - 3896 billion barrels of oil (J. Hakes, 2000, Long Term World Oil Supply, Meeting of the Am. Ass. Pet. Geol., 18th April 2000, New Orleans, LA, http://www.eia.doe.gov/pub/oil_gas/petroleum/presentations/2000/long_term_supply.), hence 357 - 619 km3 . Small lakes often occupy steep-sided depressions, and although thermal and cryovolcanic origins cannot be completely ruled out, we are seeing growing geomorphologic evidence for dissolution chemistry, indicative of karst-like geology. The dichotomy between small lakes over slightly more than one half of the region, and seas on the other half, may be best explained by a topographic anomaly causing sub-surface flow of materials from the lakes to the seas. This may also explain observations by the Cassini ISS team (E. Turtle et al., in prep.) of a putative massive sea extending considerably further south than other observed north polar lakes.
P23B-1350
Titan's Lake Distribution and Classification from the Cassini RADAR
A picture is emerging of surface hydrology in the north polar region of Titan. Quasi-circular, lobate to complex features, which take up 2.4% of the global coverage area (22.4% of the surface), are separated into 3 classes: dark lakes, granular lakes, and bright lakes. Dark lakes are interpreted as liquid filled while bright lakes are interpreted to be empty basins. Based on observed backscatter and geospatial position, granular lakes are inferred as transitional between dark and bright counterparts. In this work, the differences in distribution, morphology, and radiometric properties between the classes are explored using the Cassini Radar. The differences and similarities between the classes have implications for the interaction and evolution of hydrologic features on Titan. Dark lakes, which represent 84% of the mapped features, are found between 65°N and 90°N, and show a general trend of decreased off-nadir backscatter poleward. Granular lakes, which are distinguished from dark lakes by a higher backscatter cross-section relative to their surroundings, are found as low as 55°N and extend to 77°N. We have found no abrupt statistical change between dark and granular lakes, suggesting a smooth transition between the two classifications. Bright lakes, distinguished by their higher backscatter relative to their surroundings, represent ~10% of observed lakes. They are found in the same latitude range as granular lakes, often interspersed among them. Shoreline complexity, expressed as the fractal dimension, shows that bright and granular lakes are characteristically more circular than dark lakes. High resolution (~5 km) altimetry collected coincident with Synthetic Aperture Radar (SAR) images shows that bright lakes are empty basins 250-350 m in depth. Comparison between SAR images and integrated power in the altimetry waveforms shows that bright lakes have high return in both nadir and off-nadir backscatter relative to their surroundings. This allows distinguishing variations in roughness from intrinsic properties of the materials. Initial topographic measurements from stereo radar images constrain the relative elevations of nearby lakes, allowing comparison among lake classes. In cases where filled lakes lie above empty lakes, a state of disequilibrium with respect to subsurface transport is implied. In general, a single topographic contour cannot be used to separate empty and full lake positions. Combining morphology, radiometry, and topography provide complementary information on the nature of Titan's surface and sub-surface hydrocarbon hydrology.
P23B-1351
Titan Topography: A Comparison Between Cassini Altimeter and SAR Imaging from Two Titan Flybys
The Cassini RADAR has collected twelve altimeter data sets of Titan since the beginning of the Saturn Tour in 2004. Most of the altimeter measurements were made at high altitudes, from 4,000 km to 15,000 km, resulting in low spatial resolutions due to beam footprint sizes larger than 20 km, as well as short ground tracks less than 600 km. One flyby (T30) was dedicated to altimeter data collection from 15,000 km to the closest approach altitude of 950 km. This produced a beam footprint size of 6 km at the lowest altitude and an altimeter ground track of about 3,500 km covering Titan's surface from near the equator to high latitude areas near Titan's north pole. More importantly, the ground track is located inside the SAR swath viewed from an earlier Titan flyby (T28). This provides a rare opportunity to investigate Titan topography with a relatively high spatial resolution and compare nadir-looking altimeter data with side-looking SAR imaging. From altimeter data, we have measured the mean Titan radius of 2575.1 km +/- 0.1 km and observed rather complex topographical variations over a short distance. By comparing altimeter data and SAR images at altitudes below 2,000 km, we have found that there is a strong correlation between SAR brightness and altimeter waveform; SAR dark areas correspond to strong and sharp altimeter waveforms while SAR bright areas correspond to weak and diffused altimeter waveforms. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
P23B-1352
Estimating Titan Surface Topography from Cassini Synthetic Aperture RADAR Data
One of the more vexing dilemmas for RADAR remote sensing is the necessity to choose between altimetry and SAR imaging of a surface. Coincident surface height estimates are very useful in aiding the analysis of the unique surface features observed in the SAR imagery of Titan. Radar altimetry is optimally obtained from nadir observations, whereas SAR requires off-nadir observation in order to construct an image. Co-located nadir altimetry and SAR only occur when observations taken at different times happen to overlap. Stereo techniques can also be used to estimate topography in SAR images, but they also require multiple overlapping observations. Here we discuss a technique, SARTopo, for obtaining 10 km horizontal resolution and 75 m vertical resolution surface height estimates along each SAR swath. The height estimates comprise 1-3 cuts in each SAR pass that are 10 km wide by thousands of km long and extend along the entire long dimension of the SAR image strips. Because we obtain co-located topography along each SAR pass rather than only in regions with overlapping observations, the new technique extends the area over which we have colocated topography and SAR imagery by a couple orders of magnitude. The method is based upon Amplitude Monopulse Comparison, a technique for resolving RADAR targets developed prior to the advent of SAR. The technique requires: 1) accurate spacecraft pointing, 2) accurate spacecraft ephemeris, 3) precise knowledge of the antenna pattern of the RADAR, and 4) downlinked echo data covering the entire antenna footprint. The fourth requirement is met through synergy with Cassini SAR coverage requirements. Cassini SAR commanding and pointing is designed to utilize as much of the antenna footprint as possible in order to maximize cross-track coverage. We describe the technique and present the results for several SAR passes. We validate the technique through comparison with known features such as mountain ranges and dry lakes, and by comparison with colocated nadir altimetry and SAR stereo. In particular, we examine a strip of nadir altimetry obtained along a 1000 km strip observed by SAR a month earlier. The SARTopo height track is within 5-10 km of the nadir altimetry track for a 200 km long section. In this area, the two independent techniques agree closely. Furthermore the region contains prominent high spatial resolution topography, so it provides an excellent test of the resolution and accuracy of both techniques. SARTopo heights are also co-located and agree well with SAR stereo observations. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
P23B-1353
Titan's sand seas of longitudinal dunes as indicators of winds and sediment transport
The Cassini Titan Radar Mapper has revealed nearly ten thousand longitudinal dunes on the surface of Titan, mainly within +-30 deg latitude [1,2,3]. Based on dune orientations, divergence around topography, similarity to dune fields on Earth, and other features such as bright streaks, the prevailing wind hypothesis model for longitudinal dune formation is favored, in which winds blow generally parallel to the dune long axis [e.g.5,6,7]. On Titan, these winds are dominantly Eastward [2,3], but as observed in more recent Radar swaths (2,4,5/2007) there is Northeastward deviation from this direction at higher latitudes. Vast fields of longitudinal dunes at Titan's equatorial regions are described as sand seas [2,3]. On Earth, these regions are typified not so much by the transport as by the accumulation of sand. This may occur when there is a large nearby source of sand or when there is reduced sand removal due to decreased winds or topographic obstacles [7,8,9]. Sand transport could be inhibited both upwind and downwind of the topographically complex region Xanadu, for example. Seasonally reversing wind regimes could also lead to the stagnation of sand transportation and net accumulation within sand sea regions [7,8,9]. This scenario may help reconcile global wind model predictions with directionality opposed to that predicted from observation. Sand seas on Earth are formed in arid regions and in some places cover 30-40 percent of the surface [8], a percent coverage similar to that in the equatorial regions on Titan [2,3]. Sand seas on Earth are long-lived, having taken up to tens of thousands of years to accumulate; whether the Titan dunes are more or less stable is an outstanding question. [1] Elachi et al. 2006. [2] Lorenz et al. 2006. [3] Radebaugh et al. in rev. [4] Stofan et al. 2007. [5] Fryberger and Dean 1979. [6] Tsoar 1983. [7] Lancaster 1982. [8] Lancaster 1995. [9] Fryberger and Ahlbrandt 1979.
P23B-1354
Photoclinometry, Morphometry, and Spectroscopy of Titan's Sand Dunes from Cassini/VIMS
We present results from recent Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of the sand seas that cover Titan's equatorial region. High-resolution (~500 m/pixel) spectral mapping from the T20 Titan flyby on 2006 October 20 shows the dunes. The dunes themselves, and presumably therefore the sand of which they are comprised, are dark in all of Titan's spectral windows. The spectrum best matches organic material, but a small water-ice component cannot be ruled out. Thus the sand particles cannot be pure water ice but could still be mostly ice by volume but coated by an organic rind at least several microns thick. The dunes are separated by interdunes in some places, but are continuous in others. Where interdunes exist, we are able to map the extent of the substrate units. Where no interdunes exist, we use photoclinometry to ascertain crest-trough-crest heights of between 30 and 70 meters. Dune separations in the T20 observations are just over 2 kilometers from crest to crest; the dunes' orientations are predominantly east-west, but with variations of up to 10 degrees in either direction. A properly designed future VIMS observation would be capable of gathering a resolved profile of the dune slopes to ascertain wind direction and present activity status.
P23B-1355
Global Mapping of the Surface of Titan Using VIMS Infrared Images - Geodynamical Implications
The VIMS imaging spectrometer onboard Cassini spacecraft has been acquiring images of Titan in 352 spectral channels from 0.3 to 5.2 microns since 2004. The presence of a thick atmosphere makes the observation of surface spectral properties quite difficult to interpret since the aerosols induce a strong blurring of the images, and the atmospheric gas (in particular the few percent of methane) strongly absorb the flux, except in 7 narrow spectral windows which can be used to accurately map surface features. Merging data acquired at different flybys, sometimes with very different viewing geometries (incidence, emergence and phase angles), is challenging due to the strong effects of the atmosphere (additive and multiplicative components) and also to photometric effects. This makes uncorrected mosaics appear with seams, which can have about the same level than the surface spectral heterogeneities. Our goal is to retrieve homogeneous global maps of the different surface units, using a heuristic approach to correct the atmospheric and photometric effects. We show in particular that VIMS observations at high resolution (lower than 1 km/pixel) of dune fields can be used to evaluate the additive component of the signal, which is mostly due to the scattering by the aerosols. The multiplicative contribution of the atmosphere can then be removed, once the additive component has been empirically corrected. This process results in maps of surface heterogeneities, which emphasize surface variations that were not detected by previous analysis. Geological implications are being studied. We have already reported on possible flows that suggest that the outer icy layer of Titan has been active in a recent past.
P23B-1356
The Case for CO2 on Titan From the VIMS Reflectance Spectra
The Cassini Visual and IR Mapping Spectrometer (VIMS) is returning reflectance spectra of Titan's surface and atmosphere. The Titan atmosphere allows viewing of the surface only in a few narrow spectral windows in the near IR due to methane absorptions, and particulate scattering affects even these windows. We have analyzed many of the VIMS Titan data sets to determine what can be said about the surface composition of Titan from these data [McCord et al., Pl. and Sp. Sci., 2006; McCord et al., Icarus, in press]. One particularly interesting recent discovery using VIMS data is three-fold evidence of CO2 frost on the surface. First, it is possible to model the surface spectral units globally using Spectral Mixing Analysis and seven IR spectral windows with a combination of CO2, H2O, atmospheric scattering, an unknown bright material at 2 μm, and a dark spectrally-neutral material. Secondly, for at least the area called Tui Regio, the spectral contrast between the two 2.8-μm subwindows is matched by CO2 but not by other potential candidates, and thirdly a weak absorption near 4.92 μm for Tui Regio is very close to matching a CO2 absorption. No CO2 absorption features are missing where they should appear in the spectrum within the windows available. CO2 has been suggested before as a likely component of Titan's surface, but this is the first direct spectral absorption evidence.
P23B-1357
Surface-Atmospheric Separation Models for Titan: Plane Parallel vs. Spherical Shell Radiative Transfer Solutions for Cassini VIMS Data
The Cassini orbiter's Visual & Infrared Mapping Spectrometer (VIMS) observes Titan's surface intensity at wavelengths where its methane-rich atmosphere is heavily absorbing and light is strongly scattering. Therefore, most analyses of Titan's surface that require use of the VIMS dataset (e.g., photoclinometry, geologic interpretation, spectral identification of surface materials, photometry) are impeded until a method to separate the atmospheric from the surface spectral signature of Titan is fully developed. In a previous work (Pitman et al. 2007,AAS-DPS meeting #39), we presented a fully-functional plane-parallel radiative transfer (RT) correction method with core components extended from Mars surface-atmospheric separation models that can be used for modeling and removing Titan's atmosphere for VIMS observations which are far from the limb. This "Mars/Titan" hybrid plane-parallel RT correction model includes inputs from Cassini-Huygens c. 2007, allows for vertical variation of major atmospheric properties, and incorporates newly released methane absorption coefficients and haze scattering properties derived from in situ measurements by the Huygens DISR team. In this work, we attempt to resolve the issue of atmospheric variation as a function of Titan's geographic coordinates by utilizing a spherical-shell radiative transfer model, originally used by Cassini engineers to model radiation flow through Titan's atmosphere and used by other Cassini teams for atmospheric correction as well. Trade-offs on when and where to use which type of model will be discussed. Work performed under contract to NASA and under appointment to the NASA Postdoctoral Program (ORAU).
P23B-1358
A Mid-latitude Cloud Eruption on Titan Observed by the Cassini Visual Infrared Mapping Spectrometer (VIMS) in July 2007
Mid-latitude clouds on Titan have been monitored by the Cassini spacecraft since they were reported by ground- based observers (Roe et al. 2005, Ap. J. 618, L49). The Cassini Visual Infrared Mapping Spectrometer (VIMS) is especially suited to detecting and mapping these clouds because its wavelength range of 0.4-5.1 microns covers several key methane cloud filters. These clouds may be the result of atmospheric upwelling on Titan (Griffith et al. 2000 Science 290, p. 509; Rannou et al. 2006 Science 311, p. 201), or they may start as plumes coming from active geologic features on Titan (Roe et al. 2005, Science 310, p. 477). Mid-latitude clouds were observed in the early part of the nominal mission (Dec. 2004 and early 2005), but they had disappeared until a large cloud system was observed in summer 2006, in the 0-90 degrees W longitude mid-latitude regions of Titan. A new group of clouds was observed during the two flybys of July 2007, which dwarfs the previous mid-latitude system. These clouds originate in a region centered on ~200 W longitude and ~48 S latitude. Monitoring of mid-latitude clouds will show whether their timescales for formation are compatible with climate models for Titan's atmosphere. If mid-latitude clouds are the result of active geologic processes, there appears to be more than one source on Titan's surface. Work funded by NASA.
P23B-1359
PWA-HASI Measurements of Titan Surface Permittivity at Huygens Landing Point
After HUYGENS landing on Titan, ground permittivity measurements were performed by the mutual impedance (MI) technique during approximately 30mn. The MI probe designed mainly for atmospheric conductivity was set in a dedicated surface mode, working at 5 different frequencies between 45 and 5760 Hz. The measured impedance was processed in the Permittivity, Waves, and Altimetry (PWA) analyzer, a subunit of the Huygens Atmospheric Structure Instrument (HASI). The array configuration on the booms, together with the coupling between the soil and the atmosphere, required a careful evaluation of the attitude of the Huygens Probe on the surface for assessing the dielectric properties with good accuracy. As for atmospheric measurements a detailed numerical model of the HUYGENS conductive body on a flat surface was used for derivation of the ground permittivity. The ground was assumed homogeneous. The accuracy of the results as a function of the landing geometry is estimated and comparisons are made with laboratory samples measurements and CASSINI radar large scale measurements.
P23B-1360
Overview of the Huygens archive data set and demonstration of multi-instrument studies.
The Huygens Probe descended under parachute through Titan's atmosphere on 14 January 2005. After a 2.5 hour descent, it successfully landed and continued to function on the surface for more than 3 hours. Data were received through the Cassini Orbiter during the whole descent and for 72 min from the surface. A complementary data set was collected through a network of Earth-based radio telescopes. The integrated Huygens data set is now archived on ESA's Planetary Science Archive (PSA) and mirrored on NASA's Planetary data system (PDS). It is forseeen to maintain the data set when new high-level products will become available. A simple tool is being developped that allows to plot several data set on a common time scale. This poster will present the Huygens archive data set and illustrate studies that can be made by using a multi-instrument data subset.
P23B-1361
A Geochemical Model for the Origin of Methane on Titan
The existence of methane in Titan's atmosphere has been a mystery for years [1]. The short photochemical lifetime of methane in the atmosphere suggests that methane is replenished from the interior. Observations by Cassini-Huygens have offered new insights into the origin of methane on Titan. These data have confirmed that Titan's methane is endogenic [2], consistent with geophysical models [3]. Today, an issue is the origin of methane on Titan in general. Why does Titan have methane in the first place? Here, we show that methane formation would have been unavoidable on early Titan. It is likely that Titan accreted materials similar to carbonaceous chondrites and comets, except for extreme volatiles in comets, such as carbon monoxide. Thus, we assume that Titan started with Fe-Ni metals and sulfides, silicates and oxides of the rock-forming elements, organic matter, carbon dioxide, methanol, and ammonia. After accretion, radiogenic heat would have melted ice, facilitating water-rock separation and interaction. Mineral dissolution and precipitation, along with acid-base reactions, would have been facile throughout differentiation, despite the low temperature. In contrast, most redox reactions, notably organic matter decomposition, would have been slow in cold aqueous solution. Eventually, the interior would have segregated into a muddy core, covered by a high-pressure ice layer, overlain by a salty ocean, capped by an ice shell [3]. The primordial muddy core would have been composed of phyllosilicates, organic matter, carbonates, sulfides, and presumably, metals. The early salty ocean would have been rich in sodium chloride and bicarbonate, in addition to methanol and ammonium salts. Methane would not have formed in hydrothermal systems at the ocean floor because the high-pressure ice layer would have inhibited hydrothermal circulation. Instead, we propose that methane is a byproduct of the thermal evolution of the core. Specifically, our core devolatilization hypothesis states that high temperatures driven by radioactive decay [4] changed the chemistry of the core via metamorphism. Preliminary calculations indicate that hydrous minerals recrystallize into anhydrous minerals by releasing water, which oxidizes Fe metal, producing dihydrogen (i.e., reducing conditions). In response, organic matter in the core is broken down into carbon-bearing solids, liquids, and gases, including methane. In time, methane can migrate into the ocean, where it can be trapped in clathrate hydrates and subsequently released into the atmosphere [3]. References: [1] Owen T.C. (2000) P&SS 48, 747-752. [2] Niemann H.B. et al. (2005) Nature 438, 779-784. [3] Tobie G. et al. (2006) Nature 440, 61-64. [4] Grasset O. et al. (2000) P&SS 48, 617-636.
P23B-1362
Global Climate Models of Titan, Uranus, and Neptune
We present the formulation of and some results from global climate models for Titan, Uranus, and Neptune. The model for Titan is a fully three-dimensional, modified version of NCAR's terrestrial global climate model, CAM-3. It includes forcing by Saturn's gravitational tides, a treatment of the planetary boundary layer and surface interactions, scattering and absorption of short-wave radiation, and absorption and emission of long-wave radiation. The physical properties and distribution of aerosols are constrained by Cassini observations. The climate models for Uranus and Neptune are two-dimensional, radiative-diffusive models which calculate sensible heat fluxes and latent-heat fluxes due to ortho-para hydrogen conversion in terms of a mixing-length formulation. The vertical pressure range in these models extends from 100 bars up to 0.1 mbar. Our main goal for the 2-d models is to establish the relative roles of sensible and ortho-para latent-heat fluxes in transporting heat laterally and vertically. We are also currently developing three-dimensional models of Uranus and Neptune based on modification of CAM-3. We will discuss how the parameterization of heat fluxes in the 2-d models can be adapted to model small-scale convection in the presence of ortho-para conversion in the 3-d models. This research is supported by the NASA Outer Planet Research Program.
P23B-1363
Titan's Nitrogen Emissions: Spatial, Temporal, and Hemispherical Distribution and Variability as Measured by Cassini-UVIS.
As of September 1, 2007, the Cassini spacecraft has conducted 35 fly-by's of Titan in which the UltraViolet Imaging Spectrograph (UVIS) conducted observations. Used in this investigation is a sub-set of Titan observations, twenty-three in total, that viewed the entirety of Titan's dayside, nightside, or some combination of both bracketing the terminator, with the high resolution Far UltraViolet (FUV) channel (1120 - 1910 Å). Unlike observations during the Voyager missions, this extended monitoring period has provided greater insight into the distribution and temporal variability of emissions from Titan, in particular those of the nitrogen Lyman- Birge-Hopfield (LBH) bands. Not only does the UVIS instrument provide finer spatial and spectral resolution, but the observations spread over the first three years of the mission have provided the opportunity to examine the role of Saturn's magnetosphere on the distribution and intensity of Titan's nitrogen emissions as Titan has been observed at a variety of locations within the magnetosphere. Titan's nitrogen emissions have shown wide variability during Cassini's primary mission, in particular those emissions emanating from the nightside disk. Emissions from the nitrogen LBH bands are uniformly present on Titan's dayside disk with maximum intensities of approximately 0.5 Rayleighs/Å*pixel in the strongest of the LBH bands. Nitrogen LBH emissions, from observations that imaged both day and nightside disks, range from 0 to 50% the intensity of the dayside emissions. The variability of the nightside emissions appears uncorrelated with Titan's location within Saturn's magnetosphere. The high spatial resolution of the UVIS instrument has been used to probe the vertical and spatial variability of the nitrogen LBH emissions on both the day and nightside of Titan. Nitrogen emissions from Tita's dayside, both from the LBH and atomic N-I and N-II bands, peak at an altitude of 1100±50 km above the surface of Titan, while emissions from the nightside show a peak altitude of 1000±50 km.
P23B-1364
The Influence of Atmospheric and Surficial Processes on Titan Organics
The Cassini-Huygens mission has demonstrated that the atmosphere of Titan undergoes very complex organic chemistry. The synthesis of large molecules and ions take place in the upper atmosphere as revealed by the Cassini INMS[1] [2], which contributes to the hazes that are deposited onto the surface. Data from the GCMS instrument, moreover, has exposed the surface consisting of a much larger variety of species than what has been found in the stratosphere. These chemical species are affected by the heterogeneous nature of Titan's surface, which includes haze deposits, condensed organics, influx of high-energy particles, and methane replenishment from the interior [3]. We will present preliminary results from a study of the chemical processing of organics under these influences, constrained by GCMS data. References [1] J.H. Waite, et al., Science., 316, 870 (2007). [2] V. Vuitton, et al., Ap. J., 647, L175 (2006). [3] S.K. Atreya, et al., Planet. Space Sci., 54, 1177 (2006)
P23B-1365
Can a Warm (Aqueous Ammonium Sulfate) Ocean Survive Inside Titan?
The presence of an ocean inside Titan has yet to be confirmed, but is not unexpected. If gravity data do confirm a subsurface ocean, then what might its composition be? A recent model [1] predicts that it will be made of aqueous ammonium sulfate solution (AS), rather than an ammonia-water (AW) mixture [e.g. 2], formed by the leaching of sulfates from a hydrated core during differentiation. The freezing temperature of AS is significantly higher than that of AW, and therefore such an ocean would crystallize quicker upon cooling. The implications of the new internal structure model [1] are significant for surface chemistry and the nature of volcanism on Titan, and so the goal of this paper is to determine under what conditions such an ocean could survive to the present day. We model the thermal evolution of Titan using a parameterized convection scheme, using the new structural model as the starting reference. In this model, Titan consists initially of a rocky core (1900 km radius) made of the hydrated mineral antigorite, overlain by an icy mantle (700 km thick). At the base of the mantle there exists a high pressure ice VI phase (434 km thick), overlain by the AS ocean (153 km thick), and a crust made predominantly of methane clathrate (123 km thick) with small amounts of AS and ice I present. We allow the top and bottom boundaries of the ocean to change according to the rate of crystallization/melting over time. We use detailed material properties derived from experimental data to derive accurate models of each layer. Our nominal model predicts a present-day heterogeneous crust of 176 km above a warm (249 K) AS ocean 56 km thick. The exact thickness of the ocean is sensitive to parameter choice, but, for a wide range of input parameters and different scaling laws, an ocean made of AS can survive to the present day. Therefore this, and other, warm ocean compositions are plausible candidates for a sub-surface ocean inside Titan. References [1] Fortes et al., 2007. Icarus, 188, 139-153. [2] Tobie et al., 2006. Nature, 440, 61-64.
P23B-1366
Titan's plasma environment: 3D hybrid simulation and comparison with observations
A multiscale combined (fluid--kinetic) numerical method allows us to use more realistic plasma models at Titan. This method takes into account charge-exchange and photoionization processes. We study the Titan's plasma environment in case of high, intermediate and low exosphere density in Chamberlain models. The background ions H+, O+ and pickup ions H2+, CH4+ and N2+ are described in a kinetic approximation, where the electrons are approximated as a fluid. We also include an immobile ionosphere at the height 1300km. In this report we consider the multiscale spatial structure for plasma and electromagnetic field, and the velocity distribution of ions that results from the coupling between background ions and pickup ions. Special attention will be paid for the comparisons our numerical results with Voyager and Cassini observations (see e.g. [Sittler, Hartle, et al., 2005; Hartle, Sittler et al., 2006]). We shall estimate of mass loading rate for Ta, energy input to upper atmosphere from ambient +pickup ions, and the T9 encounter with two crossings. \newline E C Sittler Jr., R E Hartle, A F Vinas, R E Johnson, H T Smith and I Mueller-Wodarg, J. Geophys. Res., 110, A09302, 2005. R E Hartle, E C Sittler, F M Neubauer, R E Johnson, et al., Planet. Space Sci., 54, 1211, 2006.
P23B-1367
The Gravity Science Analysis of Cassini Flybys T11 and T22 and Future Work
The Cassini tour of the Saturnian system calls for DSN ground stations to track the spacecraft at X-band and Ka- band during five flybys of Titan for the purpose of determining the satellite's gravity field, including its Love number k2. This will constrain the models of interior structure. In particular, the determination of k2 will allow us to respond to the question of whether Titan contains an internal ocean. The first three flybys occurred on February 27, 2006, December 28, 2006, and June 29, 2007. Two additional flybys are planned in July 2008 and May 2010. The presentation will first focus on the estimation of the mass and quadrupole field of Titan from the first two flybys, carried out by the Cassini Radio Science Team using a short arc orbit determination technique (to avoid maneuvers), dual frequency downlink (to mitigate the effects of interplanetary plasma), and advanced water vapor radiometers (to calibrate the wet path delay). The first two flybys occurred intentionally while Titan was near apocenter on its orbit, so did not permit the determination of k2. The data from the two flybys were first independently fitted, and then combined in a multi-arc solution. Under the assumption that the higher degree harmonics are negligible, the gravity parameters from the combined, multi-arc solution are determined with relative accuracies of 2.8E-7 for the mass, 6.8E-3 for J2, and 3.6E-3 for C22. The excellent agreement (within 1.7 sigma) of the results from the two flybys analyzed independently further increases the confidence in the solution and provides an a posteriori validation of the dynamical model. The second part of the presentation will describe the method and expected results of the Love number measurement from the combination of the five flybys.
P23B-1368
Plasma Heating of Titan's Exobase and Corona
Cassini data have shown that the dominant heating process for Titan's atmospheric corona and exobase region is as yet uncertain (DeLaHaye et al. 2007). We have speculated that the incident plasma, both the slowed and deflected ambient ions and the pick-up ions, may be responsible for all or a significant fraction of the non-thermal component of Titan's corona (De La Haye et al. 2007). Our earlier models of the net incident plasma heating (Michael et al. 2004; 2005) fall short in describing the coronal structure seen by INMS on Ta, Tb and T5. Since heating of the corona and exobase affects atmospheric escape, it is critical for describing the evolution of Titan's atmosphere (Johnson 2004). Here we describe an empirical approach to this problem. INMS data and the preliminary CAPS flux data clearly indicate, not surprisingly, that the heating is spatially non-uniform and is variable, but there is as yet no correlation with the plasma flow models. Therefore, we haev analyzed INMS data for the atmospheric structure near the exobase for a large number of Cassini passes through the exobase region and we have analyzed certain CAPS data for the plasma flow near the exobase. The goal is to develop a model for the spatial variations in the plasma heating near the exobase with the goal of improving our knowledge of atmospheric escape. De La Haye, V.. et al., JGR 112, A07309, doi:10.1029/2006JA012222, 2007 Johnson, R.E. ApJ 609, L99, 2004 Michael, M., and R. E. Johnson. PSS 53, 1510, 2005. Michael, M., et al. Icarus, 175, 263, 2005.
P23B-1369
Pickup ion Phase Space Distributions at Titan in a Three Dimensional Exosphere
The composition and structure of neutral exospheres imbedded in moving plasmas can be determined by measurements of the velocity distributions of their pickup ion progeny. In turn, the velocity distributions are dependent on the spatial structure of the neutral source gases. Since Titan's neutral exosphere extends into the Saturn's magnetosphere (or solar wind) and well above its ionopause, it serves as a good place to analyze such characteristics. They are analyzed using pickup ion measurements made by the Cassini Plasma Spectrometer (CAPS) at Titan [e.g., Hartle et al., 2006] and an ion kinetic model. An early version of the model [Hartle and Sittler, 2007] is an expression describing the phase space density of pickup ions, which is derived from the Vlasov equation with an ion source that explicitly accounts for the velocity and spatial variation of the exosphere source gases. The current version used here includes exosphere source gases in three dimensions and is applicable where the uniform flow approximation is valid. A fundamental parameter of the phase space densities is the ratio of the gyroradius to the neutral scale height, α = rg/H. Titan's exosphere structure yields pickup ions whose phase space distributions are beam-like when α ≫ 1 and fluid-like when α ≪ 1. Downstream from the source peak, the light pickup ions, with α ≪ 1, are easily observed because their phase space densities are almost uniform over the orbit phases. In contrast, the phase space distributions of the heavier ions, with α ≫ 1, peak over narrow velocity and spatial ranges. This beam-like nature makes it considerably more difficult to observe heavy ions because their downstream positions and viewing directions are narrowly constrained. Examples of these extremes will be discussed. The results will also be compared with the distributions obtained from a new 3D hybrid simulation [Lipatov, Sittler and Hartle, 2007], which is applicable over a larger region, from the ionosphere to many Titan radii. Hartle et al., Planet. Space Sci., 54, 1211, 2006. Hartle and Sittler, J. Geophys. Res., 112, A07104, doi:10.1029/2006JA012157, 2007. Lipatov, Sittler and Hartle, Eos Trans. AGU, this meeting Abstract, Fall 2007.