P44A-01
Early History of Titan
We revisit models for the early history of Titan. Our models start a few My after the production of calcium- aluminum inclusions (CAIs), consistent with the dates required by our thermophysical-dynamical modeling of Saturn's medium-sized satellites. Depending on the time of formation with respect to CAIs, the accretion time scale, and the available accretional energy, models of Titan's interior after accretion are partially to fully differentiated. At one extreme of the models, Titan accretes incorporating a minimal amount of heat. This results in a relatively cold core that, over the long term, heats up and overturns, consistent with previous models of Titan. At the other extreme, accretional heat and heat fom the decay of short-lived radiogenic isotopes results in quick and complete differentiation. In this model there is no core overturn, and conditions soon develop for silicate serpentinization, and hydrothermal activity starts. We identify the periods during which conditions are suitable for hydrothermal geochemistry leading to the production of molecular nitrogen from ammonia decomposition and methane from the Fischer-Tropsch reaction. Key questions include the availability of suitable metal catalysts and/or clay minerals, storage of the reactants and products in the interior of Titan, and mechanisms by which they are released to the atmosphere. Acknowledgements: This work was carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA.
P44A-02
Titan's Geology as Viewed by the Cassini Titan Radar Mapper
Cassini's Titan Radar Mapper has imaged the surface of Titan on 8 flybys to date, collecting Synthetic Aperture Radar (SAR) data at spatial resolution ranging from about 300 m to about 2 km. These radar images reveal that Titan's surface has been modified by both endogenic (volcanism, tectonism) and exogenic (impact cratering, erosion) processes, with no process dominating in an obvious way. Although less than 15 % of the surface of Titan has been imaged to date using SAR, the acquired swaths are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. Cryovolcanic units have been identified in SAR images mostly at mid-latitudes (40-60 N), these include the construct Ganesa Macula, several calderas with associated flows, and large cryovolcanic flows. Flybys over high northern latitudes have shown lacustrine features, the distribution of these features is consistent with colder temperatures and more precipitation at high latitudes. Some of the depressions filled by the lakes may be volcanic calderas, but a thermokarstic origin is also possible (Mitchell et al., Lunar Planet Sci. Conf. XXXVIII, 2007). Ridges and mountains that are interpreted to be of tectonic origin have been seen mostly at low latitudes (Radebaugh et al., Lunar Planet Sci. Conf. XXXVIII, 2007), while drainage channels appear common at all latitudes (Lorenz et al., Plan. Space Sci., submitted). Fields of dunes (Titan's "sand seas") are mostly equatorial, but a few isolated patches of dunes extend as far north as ~60 degrees. The distribution and orientation of dunes is as expected from Titan's winds (Lorenz et al., 2006, Science 312; Radebaugh et al., Icarus, submitted). Erosion by fluvial processes is likely to have occurred on a global scale. The small number of definitive impact craters suggests that these geologic processes are erasing or burying the majority of impacts. Future data will allow us to further constrain the distribution of tectonism and volcanism, providing valuable input for models of Titan's interior.
P44A-03
The atmospheric structure of Titan from Voyager to Cassini
Titan's stratosphere has been studied in the past from the Earth and also from space with Voyager, ISO (Coustenis et al., 1998; 2003) and more recently Cassini observations. In particular, spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft became available during the Titan flybys spanning two years now since SOI (Flasar et al., 2005; Teanby et al., 2006, Vinatier et al., 2006; Nixon et al., 2006; Coustenis et al., 2007). The spectra characterize various regions on Titan from 85°S to 80°N with a variety of emission angles. We have studied the emission observed in the CIRS detector arrays (covering the 10-1500 cm-1 spectral range with apodized resolutions of 2.54 or 0.53 cm-1). We have used temperature profiles retrieved from the inversion of the emission observed in the methane _4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere (Coustenis et al., 2007). The composite spectra show several signatures of previously identified molecules: hydrocarbons, nitriles, H2O and CO2. Besides these well-known trace species, a firm detection of benzene (C6H6) is provided by CIRS at 674 cm-1 and allows for the study of its latitudinal variations. No longitudinal variations were found for any of the gases. Information is retrieved on the meridional variations of the trace constituents and tied to predictions by dynamical-photochemical models (Hourdin et al., 2004; Lavvas et al., 2007). Molecules showing a significant enhancement at northern latitudes are the nitriles (HC3N, HCN) and the complex hydrocarbons (C4H2, C3H4). The D/H ratio on Titan was also determined from the CH3D band at 8.6 micron and found to be about 1.2 ± 0.2 10-4. We have also identified the presence of C2HD at 678 cm-1 (Coustenis et al., 2006). Constraints are also set on the vertical distribution of C2H2. We will describe the most relevant results from ground or space regarding Titan's atmospheric structure. References : Coustenis et al., 1998, A&A 336, L85; Coustenis et al., 2003, Icarus 161, 383; Coustenis et al., 2006, BAAS 38; Coustenis et al., 2007, Icarus, in press; Flasar et al., 2005, Science 308, 975 ; Hourdin et al., 2004, J. Geophys. Res. 109, E1205; Nixon et al., 2006, BAAS 38; Lavvas et al., 2007, Plan. Space Sci., submitted; Teanby et al., 2006, Icarus 181, 243; Vinatier et al., 2006, Icarus, in press.
P44A-04
Plasma-ion Induced Sputtering and Heating of Titan's Atmosphere
Titan is unique among the outer solar system icy satellites in having an atmosphere with a column density about ten times that of the Earth's atmosphere and an atmospheric mass to solid mass ratio comparable to that of Venus. Atmospheres equivalent in size to that at Titan would have been removed from the icy Galilean satellites by the plasma trapped in the Jovian magnetosphere (Johnson 2004). Therefore, the use of Cassini data to determine the present erosion rate of Titan's atmosphere provides an important end point for studying the erosion and heating of planetary and satellite atmospheres by an ambient plasma. In this paper we describe the deposition of energy, the erosion and the expansion of the upper atmosphere of Titan using Direct Simulation Monte Carlo models (Shematovich et al. 2003; Michael et al. 2005; Michael and Johnson 2005). These calculations are used to calibrate semi-empirical models of atmospheric sputtering (Johnson 1994) that are used to interpret Cassini data at Titan. Using a number of plasma conditions, the temperature and density vs. altitude above the exobase and the rate of escape are calculated. References: Johnson, R.E. "Plasma-induced Sputtering of an Atmosphere" in Space Science Reviews 69 215-253 (1994). Johnson. R.E., " The magnetospheric plasmadriven evolution of satellite atmospheres" Astrophys. J. 609, L99-L102 (2004). Michael, M. and R.E. Johnson, "Energy deposition of pickup ions and heating of Titan's atmosphere", Planetary & Space Sci.53, 1510-1514 (2005). Michael M., R.E. Johnson, F. Leblanc, M. Liu, J.G. Luhmann, and V.I. Shematovich, "Ejection of nitrogen from Titan's atmosphere by magnetospheric ions and pick-up ions", Icarus 175, 263-267 (2005). Shematovich, V.I., R.E. Johnson, M. Michael, and J.G. Luhmann, "Nitrogen loss from Titan", JGR 108, No. E8, 5087, doi:10.1029/2003JE002094 (2003).
P44A-05
Titan's surface near the Huygens landing site
The Cassini/Huygens probe landed on a dark flat area near a bright hilly terrain. Channels and islands are indicative for a large scale flow along the coastline-like boundary separating the dark from the bright area. The cobbles seen in the images after landing seem to be distributed by a flood with a flow speed of about 1 ms- 1. The hilly terrain is furrowed by valleys forming dendritic systems. The dark appearance of the valley bottoms in the images could be caused by varying illumination and reflectivity of the topographic shapes. No dark material is needed. During time of precipitation the hills are eroded and bright material is transported into the lake. The analysis of the down looking spectrometer data show a weak gradient of the surface reflectivity perpendicular to the coastline. The hilly terrain exhibits a redder spectrum than the lake area. A terrain with similar properties can be identified south of the landing spot. The surface reflectivity shows variations with phase angle and a back scattering enhancement is found when the surface lamp dominates the illumination.
P44A-06
Saturn's Titan: The Case for Surface Activity
The Saturn orbital tour of the Cassini spacecraft has produced VIMS instrument observations of Titan's surface through spectral ‘windows' in its atmosphere where methane, the principal absorbing gas, is transmitting. We have found that the reflectance of a region on Titan's surface (latitude 26S, longitude 78W) increased twofold between July 2004 and March-April 2005. It returned to the July 2004 level by November 2005. In late December 2005 the reflectance surged upward again to a new maximum. It then declined for the next three months. Detailed analyses indicate that the brightening episodes are a surface phenomenon, making these the first changes seen on Titan's surface. The spectral differences between the region and its surroundings rule out the deposition of many ices including H2O, CO2, and CH4 as possible causes. Remarkably, the change is spectrally consistent with the deposition and removal of ammoniated materials. NH3 has been proposed as a constituent of Titan's interior but not its surface or atmosphere. This transitory NH3 spectral signature is consistent with occasional effusion events in which juvenile ammonia is brought to the surface. Its decomposition may feed nitrogen to the atmosphere. The size of the region suggests it may exceed the size of the largest active volcanic areas in the solar system. This work done at JPL/CALTECH under contract with NASA
P44A-07
Cassini radio occultations of Titan's ionosphere
We report results on Titan's ionosphere from the Cassini radio occultation of March 26, 2007 (T27), as well as those of March 19, 2006 (T12), and May 20,2006 (T14) . The 2006 occultations occurred at low Southern latitudes of 14.7S, 36.2S, 19.8S, and 21.9S. The 2007 occultation was nearly polar, at latitudes of 81S and 59N. The solar zenith angles for all occultations were near the terminator, ranging from 85 to 95 deg. The ionosphere peak was observed to lie close to an altitude of 1200 km, and the observed peak densities ranged from about 1.2 to 2.0x103 cm-3, which is in fairly good agreement with other Cassini observations and the previous Voyager radio occultation results. Radio occultation observations of the Titan ionosphere are difficult because of its low density and small size, and it was facilitated by the unprecedented Cassini radio science system, which has three frequencies that can operate simultaneously: S-band (2.3 GHz), X-band (8.4 GHz), and Ka-band (32 GHz). In particular, Ka-band hads never been used before to probe Titan's ionosphere, and the signal-to-noise ratios at all frequencies of 42, 54, and 48 dB-Hz., respectively, have never before been achieved.
P44A-08
Pickup Ion Phase Space Distributions at Titan: Effects of Atmospheric Spatial Gradients
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. 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. A fundamental parameter in the phase space density expression is the ratio of the gyroradius to the neutral scale height., α = rg/H. Titan's exosphere includes H, H2, CH4 and N2, with scale heights near the exobase of ~ 2400, 1200, 149 and 85 km, and α of ~ 0.1, 0.4, 27, and 82, respectively. This structured exosphere 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 the phase space density is almost uniform over the orbit phases. On the other hand, the phase space distribution of the heavier ions, with α >> 1, peaks over a narrow velocity and spatial range. This beam-like nature makes it more difficult to observe the heavy ions because the downstream positions and viewing directions are constrained. Examples of these extremes will be discussed. Hartle et al., Planet. Space Sci. vol. 54, 1211, 2006 Hartle and Sittler, J. Geophys. Res., in review, 2007