P53A-1440 1340h
Titan's Surface Seen by VIMS/Cassini
On October 26th 2004, the Cassini spacecraft will fly over Titan at a distance of 1000 km at closest approach. The Visual and Infrared Mapping Spectrometer (VIMS) will observe Titan during more than 20 hours. More than 100 hyperspectral images should be recorded with spatial resolution ranging from 100 km to 1 km at closest approach. The images will include views of the Huygens landing site (10S, 190W) with a spatial resolution of 20 km and it should provide a context map for the probe that will land there in January 2005. During Saturn Orbit Insertion, the first observations of Titan by VIMS prove how well this instrument can see Titan's surface in several infrared windows at 1.3, 1.6, 2, and 5 µm. These observations should allow us to map tectonic features and morphological features such as impact craters. Titan's large eccentricity (0.03) may be responsible for the presence of large cracks at its surface as is the case for Europa. It will be possible to compare Titan's surface with that of other large satellites Ganymede, Callisto, and Europa. A major issue concerns the source of methane in Titan's atmosphere : oceans, subsurface reservoirs or cryo-volcanism. The data should allow us to determine whether an ocean is present at Titan's surface. Moreover, analyses of these data should help us determine the surface composition and its variations with latitude and longitude.
P53A-1441 1340h
Characteristics Of Titan's Clouds from VIMS T0 Observations
Over the past 4 years, ground-based images have shown that Titan sports high cloud systems on a daily basis, which have been observed exclusively in Titan's south polar region. These clouds are composed of methane ice and may be a component of a liquid cycle similar to Earth's hydrologic cycle, with clouds, rain and seas. This past July, Cassini gave us the first direct view of Titan's high clouds as the spacecraft passed below Titan's south pole. Observations by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) illuminate the altitudes, thicknesses and extents of Titan's clouds, which were dramatically imaged in detail by the ISS instrument. We find, consistent with ground-based observations, that the clouds reside in the high troposphere. In addition, at times the clouds are optically thick over a region of at least 100 km. Here I will discuss the characteristics of Titan's clouds as measured by VIMS, and implications of these results in terms of the formation of Titan's clouds.
P53A-1442 1340h
Radiative Equilibrium - Photochemical Model of Titan's Atmosphere
Titan's thermal structure has been characterized and studied by radiative/convective models [1,2,3,4] that are based on an assumed height distribution of species. Likewise, photochemical models [5,6,7,8,9] also employ a temperature profile as basis for the calculations. In this presentation, we discuss preliminary results of a coupled self-consistent radiative equilibrium/photochemical model for the middle and lower atmosphere of Titan. The radiative equilibrium model includes CH$_4$ as the principal absorbent of thermal radiation, minor constituents, and the effect of pressure - induced absorption. The vertical distribution of species is calculated by a comprehensive photochemical model [9]. Our coupled radiative equilibrium/photochemical model is self-consistent, so that the temperature calculated in the radiative equilibrium model is used in the photochemical model to calculate the species vertical distribution, and this produced vertical distribution - in the radiation code, reiterating until both reach equilibrium. We are in the process of adding haze contribution to the radiative model for it serves as a main source of heating in the stratosphere. 1. Samuelson et al., Icarus, 53, 1983; 2. McKay et al., Icarus, 80, 1989; 3. Samuelson and Mayo, Icarus, 91, 1991; 4. McKay et al., Science, 253, 1991; 5.Yung et al., Astrophys. J. Suppl.,55,1984; 6. Toublanc, Icarus, 113, 1995; 7. Lara et al., JGR, 101, E10, 1996; 8. Lebonnois et al., Icarus,152, 2001; 9. Wilson and Atreya, JGR, E06002, 2004.
P53A-1443 1340h
Numerical Modeling of Cloud Convection With High Condensation Threshold: Implication to Methane Convetive Clouds in Titan's Atmosphere
Recent ground-based observations and the first Cassini flyby reveal prominent cloud activities near the south pole of Titan. Their characteristics imply their convective origin. On the other hand, it has been proposed that a large degree of super-saturation is required for condensation of methane to occur. Here, we examine how such high condensation threshold affects the nature of cloud convection and over-all structure of the atmosphere through explicit numerical modeling of cloud convection. As a first step, we perform sensitivity experiments designed to isolate the effects of the large super saturation in the setup of the earth's tropical atmosphere because the condition for Titan's atmosphere is not well constrained. We conduct long-term integrations of a two-dimensional non-hydrostatic cloud convection model that extends 4,096km in the horizontal direction including three-category (vapor-cloud-rain) parameterized microphysics. We compare the simulated cloud convection in the case with "ordinary" condensation scheme with that in the case with "Titan's" condensation scheme, where water vapor is allowed to condense into cloud water only at a highly super saturated condition; after the nucleation, water vapor rapidly condense onto the cloud water toward exactly saturated state, and cloud water also evaporates towards exactly saturated state in appropriate conditions (e.g., in the downward flow of the air). The results show that, in "Titan's" case, individual convective clouds are much stronger, larger and longer-lived. The convective towers occur only at one or two limited locations in the 4,096km domain instead of occurring in rather scattered manner in the "ordinary" case. The average atmosphere in the "Titan's" case is super saturated around the condensation level and the tropopause, but the degree of super saturation is much smaller than that specified as the condensation criterion. The temperature structure is maintained to be conditionally unstable. Although direct comparison between the present results and the real situations on Titan requires cautions, the simulated cloud convection shares some common properties with Titan's convective clouds, which are rather long-lived and concentrated around a localized area.
P53A-1444 1340h
New results From HIPWAC Measurements of Ethane in the Stratosphere of Titan
The Goddard Space Flight Center Heterodyne Instrument for Planetary Wind And Composition, HIPWAC, was used at the National Astronomical Observatory of Japan Subaru Telescope on Mauna Kea, Hawaii to measure Ethane emission features near 12 $\mu$m. HIPWAC spectral resolving power of $ > $10$^6$ and the 0.36 arcsec diffraction limited FOV on Titan with the 8.2 m Subaru telescope enabled recording of true emission line profiles on the East and West hemispheres of Titan. The direction and magnitude of global stratospheric wind on Titan was determined by analyzing differences in the line frequencies (Doppler shifts). The line profiles were fit to retrieve ethane abundances and constraints on the thermal structure near 0.5 mbar pressures. The near equatorial FOV positions overlapped the Huygens probe entry latitude. Analyses retrieved a prograde wind of 170$\pm$90 m/s. Ethane abundance was 0.7$\pm$0.3 x 10$^{-5}$. The indication of temperature profile variations between the two hemispheres of Titan will be presented and comparisons to 1993-96 InfraRed Heterodyne Spectrometer, IRHS at NASA's InfraRed Telescope Facility will be made. Possible explanations for the temperature differences will be discussed. An outlook for expected results from measurements on Subaru during the descent of Huygens into Titan's atmosphere on January 14, 2005 UT will be given. Work was supported by the NASA Planetary Astronomy Program.
P53A-1445 1340h
Radiometric Characteristics of Cassini RADAR Imagery
The Cassini RADAR instrument on-board the Cassini Orbiter is currently being employed to obtain SAR imagery of the surface of Saturn's largest moon, Titan. The viewing geometry of Cassini RADAR is different from most imaging radars because the Cassini Orbiter flies by Titan rather than entering into orbit about it. This unusual viewing geometry leads to variable noise characteristics throughout the SAR swath. Due to large changes in range to target and number of looks, noise characteristics and effective resolution vary widely throughout the swath. A good understanding of these parameters is important in order to draw scientific conclusions from the SAR images. Changes in noise bias could be misinterpreted as changes in reflectivity from the surface. Changes in resolution or noise variance could be misinterpreted as changes in the heterogeneity of the surface. The purpose of this paper is to quantify noise variance, bias, and effective radiometric resolution throughout the SAR swath in order to aid scientists in interpreting the data. Of the three parameters, the easiest to model is noise bias which increases with the range to the target. Noise variance is more complicated. The thermal noise (SNR) contribution to the overall variance increases with range, but the fading (speckle) noise contribution varies inversely with number of looks and thus with range. Effective resolution becomes coarser as range increases, but cross track and along track resolution vary differently. Along track resolution varies continuously, but cross track resolution has a discontinuity at 1600 km altitude, due to a change in commanded bandwidth. This paper presents the equations governing the noise characteristics and effective resolution as well as providing pseudo-color images of each quantity in SAR image coordinates for the October 2004 Cassini RADAR observation of Titan. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.
P53A-1446 1340h
Techniques for Calibrating the Cassini RADAR
From July 2004 through July 2008, the Cassini spacecraft will be conducting an extensive survey of the Saturn system with many instruments. The Cassini RADAR instrument will collect data in both active and passive modes providing co-polarized backscattering cross-sections and brightness temperatures. These data will be obtained for Titan at close range and relatively high resolution (500 m), and for several of the icy satellites at longer range with low (whole disk) resolution. Passive data will also be collected on the Saturn disk and the ring system. Calibrating the various operating modes of the Cassini RADAR will be an important and challenging task during the Saturn Tour. This paper will discuss the procedures and observations being planned to address absolute and relative calibration of the Cassini RADAR active and passive data sets. The first few observations will provide some data on the performance of these calibration procedures. We will discuss these results and the implied limits on the ultimate accuracy of brightness temperatures and backscattering cross-sections.
P53A-1447 1340h
Titan: Predicted Bulk Chemical Composition and Interior Structure for a Capture Origin
I report a predicted bulk chemical composition and internal structure for Titan based on the idea that this body is a captured satellite of Saturn which originally condensed within the gas ring shed by the proto-Solar cloud (PSC) at Saturn's initial helio-centric distance $\sim 8.1$ AU . The case for capture rests on the large disparity (by a factor of $\sim $58) between the masses of Titan and Rhea. Rhea's mass ($2.3 \times 10^{24}$ g) is consistent with the mass $m_{cond} = 9.3 \times 10^{24}$ of rock, H$_{2}$O, and NH$_{3}$ ices expected for a native moon of Saturn, had Rhea condensed from a gas ring shed by the proto-Saturnian cloud (Prentice, \textit{JPL Pub.} 80-80 1980; \textit{Proc. Astron. Soc. Australia} 4 164 1981; \textit{Earth, Moon Planets} 30 209 1984). Here I assume an efficiency of 25% in the process of satellite accretion and adopt the proto-solar elemental abundances of Lodders (\textit{Astrophys.J} 591 1220 2003). Titan's mass exceeds $m_{cond}$ by a factor of $\sim 14$, so speaking against a native origin (http://www.aas.org/publications/baas/v36n2/aas204/887.htm). Previously it has been supposed that the process of shedding discrete gas rings by the parent gas cloud comes about solely through the action of large turbulent stresses arising from powerful convective motions (Prentice, \textit{Moon & Planets} 19 341 1978, \textit{Earth, Moon & Planets} 87 11 2001, http://www.lpi.usra.edu/meetings/mercury01/pdf/8061.pdf). This has necessitated convective speeds $v_{t}$ up to $\sim 5$ times the local adiabatic sound speed $v_{s}$, which is unacceptable. An exact numerical simulation of supersonic turbulent convection in a model atmosphere which represents the outer layers of the PSC shows, however, that the upper layers are strongly super-adiabatic (Prentice & Dyt, \textit{MNRAS} 341 644 2003). This results in a natural density inversion at the top boundary . Gas ring shedding can now be achieved for speeds $v_{t} \leq 3v_{s}$, which is OK. A new model for PSC has thus been constructed to include the influence of very strong super-adiabaticity. The controlling paramters are chosen so that the mean density of the condensate at the orbit of Mercury matches the inferred uncompressed value $\rho_{unc} = 5.3$ g/cm$^{3}$ and that the fraction of water vapour in the gas ring at Jupiter's orbit which condenses is $\phi_{\mathrm{H_{2} O}}$ = 0.665. This later accounts for the densities of Ganymede and Callisto, following condensation from the gas rings shed by proto-Jovian cloud (Prentice 2001). At Saturn's initial orbit, where the gas ring temperature is $T_{n}= 94$ K and the mean orbit pressure $p_{n}= 4.7 \times 10^{-7}$ bar, the bulk chemical constituents of the condensate are anhydrous rock (mass fraction 0.494), water ice (0.474) and graphite (0.032). The mean density is 1.52 g/cm$^{3}$. Structural models for a present-day Titan based on this composition yield mean densities of 2.10 g/cm$^{3}$ (homogeneous case) and 1.93 g/cm$^{3}$ (differentiated 2-zone case). For the latter, $C/MR^{2}$ = 0.32. Titan is thus most likely fully differentiated between its rock, graphite and water ice constituents. It is predicted that Titan has no internal ocean or induced magnetic field but it may possess a small magnetic dipole moment of magnitude $\sim 2\times 10^{11}$ T m$^{3}$. This was acquired through thermoremanence at $\sim 1.5 \times 10^{9}$ yr after satellite formation. Capture of Titan was achieved by gas drag within the proto-Saturnian envelope whose initial size was $\sim 60 R_{Sat}$. Titan's surface should thus look much like that of Triton. I thank John D. Anderson [NASA/JPL] for much support, and Nicole Rappaport and Bob Jacobson for helpful discussions.
P53A-1448 1340h
The fate of methane clathrate hydrate within Titan
Titan has a thick atmosphere composed primarily of nitrogen and methane. Methane is known to be irreversibly consumed photochemically on a timescale of a few tens of million years, which implies that any replenishment process must occur to maintain the methane abundance to its current value. Methane is believed to have been trapped within clathrate hydrate, a particular structure of ice, in Saturn's subnebula environnement, and then to have been incorporated within Titan's interior. Although the major part of clathrates is likely to have been devolatilized during the late stage of Titan's accretion, a significant portion could have "survived" within the deeper interior and could have been released later in Titan's history. Through coupled thermal and orbital calculations including a full description of the tidal dissipation, the heat transfer,the H$_2$O-NH$_3$ phase diagram and the methane clathrate stability, we study the possible evolution of clathrate distribution within Titan's interior, its effect of the thermal evolution, and the process of degassing from the interior. We show that only models with a few percent of ammonia and a significant fraction of methane clathrate within the interior can explain both the conservation of Titan's high eccentricity over the age of the solar system and the methane replenishment of the atmosphere. In our preferred scenario, two episodes of methane degassing are predicted: one during the first billion years and a second one after 3.5-4 Ga, explaining the present-day atmospheric methane abundance. Forthcoming data from the NASA/ESA Cassini-Huygens mission will allow us to test the present predictions.
P53A-1449 1340h
Tidal Loading On Titan
The tidal response of Titan as a function of interior structure modeling is revised. The Love numbers of the solid body are estimated assuming thermal and mechanical equilibrium. Atmospheric tides are also taken into account. The influence of Saturn's gravitational tide on the atmosphere cause significant surface pressure oscillations as predicted from analytical as well as numerical models. The results are discussed with respect to expected Cassini-Huygens measurements.
P53A-1450 1340h
Wave Activity Above the Ionosphere of Titan - Predictions for the Cassini Mission
Beam driven wave generation mechanisms are investigated in linear approximation, which are viable in the flowside plasma mantle of Titan. The flowside plasma mantle is defined - by analogy with the dayside plasma mantle of the planet Venus and Mars - as being the interaction region between the "cold" ionospheric plasma and the "hot" streaming plasma of magnetospheric or solar wind origin, with both types of plasma being present in comparable densities. In order to match the various plasma and field parameters expected to be measured by Cassini during its numerous encounters with Titan, we have performed our model calculations in a broad parameter space encompassing the plasma characteristics determined by Voyager 1 in Titan's wake, and also making use of the results of available MHD model calculations concerning the plasma environment of Titan. Two types of beam-instability modes were found to be dominant: a fluid-like (non-resonant) modified two stream instability (MTSI) and the kinetic (beam resonant) ion-ion acoustic instability (IIAI). The two instability modes are characterized by distinct frequency ranges (an order or below the lower hybrid frequency for the MTSI, and a few times the lower hybrid frequency for the IIAI), and are found to be dominant in well separated spatial regions determined by the presence/absence of cold ionospheric electrons. Giving a global rather than a specific description of the instability types expected to be the most important growing modes within Titan's flowside mantle we make predictions concerning the wave characteristics and the spatial location of the dominant wave modes measurable by the plasma wave instrument onboard Cassini. In particular, the extension of the waves towards the flanks is investigated, with comparisons to the observations by Cassini during its first Titan encounter.
P53A-1451 1340h
Cassini's First Distant and Close Encounters With Titan: Initial Analysis From CAPS
Cassini performed a distant Titan flyby (T0) at ~330,000 km on 2 July 2004, and a much closer flyby at ~1200 km (TA) is anticipated on 26 October 2004. We present an initial analysis of CAPS data during the two flybys. For the T0 flyby, some of the data are consistent with the passage of the spacecraft near Titan's magnetic flux tube. We examine the evidence and conditions which may have caused this to occur. For the TA encounter, assuming that Titan is in Saturn's magnetosphere at the time, the spacecraft should provide close wake and ionospheric sampling in addition to more distant signs of the interaction. If Titan is in the magnetosheath, a more cometary interaction will occur with a quite different geometry. We present aspects of the first results, in particular concentrating on the structure of the interaction region and on the input of energy into Titan's atmosphere via electron heating.
P53A-1452 1340h
Predictions of the Electrical Conductivity and Charging of the Aerosols in Titan's Atmosphere
The electrical conductivity and electrical charge on the aerosols in the atmosphere of Titan will be presented for altitudes between 0 and 400 km as well as for both daytime and nighttime conditions. Ionization due to both galactic cosmic rays and electron precipitation from Saturn's magnetosphere is included. This ionization results in the production of free electrons and the primary ions N2+ and N+ which are then rapidly converted into secondary ions such as H2CN+ and NH4+ which in turn form ion clusters such as H2CN+ (HCN)n. and NH4+(NH3)m. Because polycyclic aromatic hydrocarbons (PAHs) have been observed in the laboratory and found to be electrophillic, we also include the formation of negative ions. The ubiquitous aerosols observed in the Titan atmosphere play an important role in determining the charge distribution in the atmosphere acting as a store for negative charge on the night side by attaching electrons. This is particularly the case if large concentrations of negative ion-forming particles are present. On the day side, on the other hand, solar UV causes the aerosols to copiously emit electrons because of the low ionization potential of graphitic material even when the effect of the finite size of the particles (the Coulomb term) is included. The prediction of large electron densities on the day side as a result of photo emission of electrons by the aerosol particles is amenable to checking by the radio occultation method. Charge distributions on the aerosol particles have been computed and will be presented; they predict the accumulation of large electric charge on the 0.2 micron particles, peaking at about 400 charges at some altitudes. We will also present the computed electrical conductivities, which can be measured by the Huygens probe.
P53A-1453 1340h
Preliminary Interpretation of Titan Plasma Interaction as Observed by the Cassini Plasma Spectrometer: Comparisons With Voyager 1
The Cassini Plasma Spectrometer (CAPS) instrument is scheduled to observe the plasma environment at Titan October 26, 2004 from the Cassini Orbiter. Preliminary CAPS ion measurements from this encounter (1, 2) will be compared with measurements made by the Voyager I Plasma Science Instrument (PLS). The comparison will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements (3, 4). The comparisons will focus on the composition and nature of the ambient plasma and pickup ions. Using the CAPS ion measurements, some of the questions to be addressed, as stimulated by the previous interpretations and predictions made evaluating PLS data, are the following: A) Are H+ and N+ the major ion components of Saturn's rotating magnetosphere in the vicinity of Titan? B) Are finite gyroradius effects apparent in ambient N+ as the result of its interaction with Titan's atmosphere? C) Are the principal pickup ions composed of H+, H2+, N+, N2+ and CH4+?, D) Is there evidence of slowing down of the ambient plasma due to pickup ion mass loading and as the ionopause is approached are heavier pickup ions becoming dominant such as N2+? During the Voyager I flyby, Titan was in Saturn's magnetosphere. If Titan is in Saturn's magnetosheath or the solar wind at the encounter, questions similar to the above will be addressed as appropriate. 1. Young et al., this issue, Fall AGU session, 2004. 2. Coates, et al., this issue, Fall AGU session, 2004 3. Hartle et al., J. Geophys. Res., 87, 1383, 1982. 4. Sittler et al., Titan Symposium Proceedings, ESTEC, Editor, Jean-Pierre Lebreton, 2004.
P53A-1454 1340h
Global Hybrid Simulations of Titan's Plasma Interaction During the Cassini TA Flyby
The interaction of Titan's ionosphere with Saturn's magnetosphere plasma is complicated by the significant size of the ion gyroradii relative to the size of Titan. Voyager 1 found a two species plasma consisting of H$^{+}$ and N$^{+}$ with number densities of 0.1 and 0.2 cm$^{-3}$ respectively, near Titan. The gyroradii of the incident N$^{+}$ was found to be 2.25 R$_{T}$ (1 R$_{T}$ = 2575 km), while the gyroradii of the H$^{+}$ was smaller at 0.16 R$_{T}$. The plasma conditions near Titan at TA could be significantly different from the plasma conditions during the Voyager encounter, since Titan is located closer to the sub-solar point in the magnetosphere. Depending on the solar wind conditions present at TA Titan may find itself in the outer magnetosphere, the magnetosheath or even the solar wind. We investigate Titan's interaction with its surrounding plasma environment during TA using a global hybrid simulation with predicted upstream conditions from a global MHD simulation of Saturn's magnetosphere.
P53A-1455 1340h
Titan solar reflection and emission properties in the EUV/FUV
\noindent Cassini UVIS imaging spectrograph exposures of Titan in the first fly-by show the resolved absorption structure of acetylene (C$_{2}$H$_{2}$ ) and ethylene (C$_{2}$H$_{4}$) in the 130 -- 190 nm region of the solar reflection spectrum. A simple model of the reflection and extinction process indicates the underlying scattering layer is consistent with a constant albedo in the 130 - 180 nm region and a relatively sharp rise between 180 - 190 nm. In reality the reflection is a more complex interplay between gaseous absorption and aerosol scattering. Molecular absorption by CH$_{4}$, C$_{2}$H$_{2}$, and C$_{2}$H$_{4}$ dominate in the 130 - 170 nm region. As the gaseous absorptions becomes weaker beyond 170 nm, the albedo increases. At 190 nm it is more than a factor of three higher than that at 130 nm. The brightening is tentatively attributed to aerosols formed from the condensation of heavy hydrocarbons. The spatially averaged spectrum contains abundances of 4. $\times$ 10$^{16}$ cm$^{-2}$ and 2.2 $\times$ 10$^{16}$ cm$^{-2}$ in C$_{2}$H$_{2}$, and C$_{2}$H$_{4}$ respectively. Atmospheric emission is also present, showing a distinctive spectrum containing N$_{2}$ bands, atomic nitrogen, atomic carbon, and possibly CH emission features. The atomic nitrogen emission lines indicate that most of the population is in the $^{2}$D state, inferring that the emission is at a high enough altitude that quenching by electrons and other species are not effective against the 17 hour emission lifetime of the state. A preliminary analysis will be presented.
http://lasp.colorado.edu/cassini
P53A-1456 1340h
Cassini Observations of Co-rotating Heavy Ions Near Saturn's Rings
During Saturn orbit insertion on July 1, 2004, Cassini passed over the B, A, and F rings and the Cassini division before descending, just inside the G ring, through the ring plane. The ion mass spectrometer (IMS), a component of the Cassini plasma spectrometer (CAPS), observed ion flux during this ring plane crossing whenever the instrument viewing was into the co-rotation direction. This occurred in two time intervals, the first from about 03:36 to 03:58 UT (interval 1) and the second from about 04:14 to 04:48 UT (interval 2). Interval 1 is about 0.2 Rs above the A and B rings and the Cassini division. Interval 2 includes the ring plane crossing, inside the G ring. A predictive model that incorporates the IMS view direction and detector response, the spacecraft velocity, and the assumption of co-rotation reproduces many of the salient features in these data. In interval 1, the IMS flux peaks at about 20 and 40 eV in the IMS frame, consistent with co-rotating ions with mass per charge of 16 and 32. It is plausible that these peaks are produced by the heavy ions O+ and O2+, a by product of erosion of the main icy rings. The interval 2 data are similar, with the most intense flux at ~80 eV, extending down to about 40 eV, again in the IMS frame. These energies are also consistent with the co-rotating model predictions for mass per charge values of 16 and 32. Quantitative analysis will be presented at the meeting, including ion identification via the IMS time of flight spectra.
P53A-1457 1340h
Cassini Radio Occultation of Saturn's Rings: a Bayesian Approach to Particle Size Distribution Recovery
The radio occultation technique was first used to study Saturn's rings through their effects on quasi-monochromatic radio signals transmitted from Voyager 1 during its flyby of Saturn in 1980. Almost a quarter of a century later, Cassini is planned to conduct a more extensive set of radio occultation experiments during its tour of the Saturn system. Cassini enjoys the advantage of a wide range of ring viewing geometry as well as the unique new capability of simultaneously transmitting 0.94, 3.6 and 13 cm-wavelength coherent radio signals (Ka-, X-, and S-band, respectively). Observed extinction of the direct signal and time-sequence spectra (spectrogram) of the near-forward scattered signal can be used to infer the size distribution of particles of resolved ring features (among other objectives). The inference requires solving three distinct inversion problems to recover from the measurements: i) the multiply-scattered collective diffraction lobe of a resolved ring feature, ii) the first-order scattering contribution to the collective lobe, and iii) the corresponding particle size distribution. Although various classical regularization techniques may be used for this purpose, a subjective valuation of solution smoothness usually needs to be introduced. We investigate an alternative approach based on Bayesian function learning schemes which provides a rigorous probabilistic framework to address the tradeoff between data fit residuals and prior knowledge about the character of the solution. In contrast with the regularization approach, the Bayesian approach provides estimates of confidence intervals for the most-likely solution achieved, an important advantage. The approach is particularly adaptable to some Cassini occultations of relatively unfavorable alignment between contours of constant Doppler shift in the ring plane and circular boundaries of ring features, as the approach naturally "fuses" time-sequence of spectra each containing contributions from adjacent ring features. We also use the Bayesian approach to combine in a single step inversion of (simulated) extinction and diffraction lobe observations to recover the particle size distribution over the centimeter to several meters size range without assuming an explicit model. Only the first-order scattering approximation has been considered in our investigation so far, an idealization to be removed in future work.
P53A-1458 1340h
Dust Impacts Observed by Cassini at Saturn's Ring Plane Crossings
During the Cassini Saturn encounter on July 1, 2004, the radio and plasma wave instrument on Cassini detected a region of intense impulsive noise at the inbound and outbound ring plane crossings. The noise has been attributed to small micron-sized particles hitting the spacecraft. When a small particle strikes the spacecraft at a high velocity, the particle is instantaneously vaporized and heated to a high temperature, producing a cloud of ionized gas that expands away from the impact site. As the ionized gas cloud sweeps over the antenna, it causes a voltage pulse, thereby producing the impulsive noise. The maximum impact rate detected was about 680 per second and the north-south thickness of the impact region was about 2000 km centered on the ring plane. The dust particles are thought to have radii of about 1 to 10 micrometer. The impact rate distribution as a function of height above Saturn's equator will be discussed and compared with previous Voyager measurements.
P53A-1459 1340h
Stochastic Acceleration of Dust Grains in the Saturn Rings
In a previous work we developed a model in which it is possible to estimate the gain and loose of energy of a population of ions in the Saturnian's magnetospheric region. Ions like H+, C+, O+, N+, and N2+ are being accelerated to energies of hundreds of MeV. In this work we applied the same model to charged and neutral dust grains in the Saturn rings using stochastic forces of electromagnetic and gravitational origin. The charged dust grains were more accelerated than the neutral ones. This could explain the levitation of the material of the spokes.
P53A-1460 1340h
Tenuous Ring of Captured Dust at Saturn
Small dust particles, less than 300 nm in radius, on orbits with low eccentricity and small inclinations may be captured from the interplanetary medium through interactions with Saturn's magnetosphere. Previous work at Jupiter has shown that particles will be preferentially captured into retrograde orbits. We use simple two-dimensional numerical simulations to attribute this asymmetry to the conservation of the generalized Jacobi constant. Additionally, we use a more sophisticated fully three-dimensional code to predict the existence of a faint ring of captured dust particles in orbit about Saturn. In our two-dimensional code, dust grains with various radii and impact parameters are followed in the simulations until they crash into the planet, are ejected from orbit, or get captured into a circular orbit. We demonstrate that the range of values of the Jacobi constant for circular orbits is larger than retrograde orbits than for prograde ones. Since incoming particles will have a random distribution of Jacobi constants, the class of orbits with the largest available range should be the most populated. The three-dimensional code uses the Z3 model for the magnetic field and plasma parameters provided by Richardson [1995]. It includes solar radiation pressure, interplanetary magnetic field, planetary oblateness, and sputtering of the dust grains in the magnetosphere. The particles' initial conditions are calculated assuming a random distribution of eccentricities, semimajor axes, and inclinations about the sun. These particles are then input into the simulations until an equilibrium number of particles is reached. The predicted ring has a peak density of about $50 x 10^{-15}$ cm$^{-3}$ and extends from just outside the main ring to about 10 planetary radii $(R_S)$ and is about three $R_S$ thick at its widest point. The Cassini spacecraft's dust detector will verify these predictions.
P53A-1461 1340h
Faint Rings and Things According to Cassini
During Saturn Orbit Insertion and the months surrounding that event, Cassini imaged various ring structures having low optical depth. These include the broad diffuse E ring, the narrower G ring, the contorted F ring, diffuse ringlets within the Encke gap and a very faint sheet lying between the A and F rings. We have also discovered a very faint, narrow ringlet surrounding Atlas's orbit. If other small satellites are found in this region, they may also be surrounded by faint ringlets. We will present preliminary results on the morphology and photometry of these features, along with first-cut explanations for some observed features. So far, scattered light has hampered our characterization of some faint features that lie close to the bright globe or rings of Saturn.
P53A-1462 1340h
Collision, Rotation, and Accretion of Particles in Planetary Rings
Collisions between particles play an essential role in dynamical evolution of planetary rings: they result in either rebound, accretion, or fragmentation of particles, and cause angular momentum transport in rings. Furthermore, oblique impacts between particles with rough surfaces lead to rotation. Recent works have shown that small moonlets embedded in planetary rings would spin slowly in the prograde direction when impacting particles are much smaller than the moonlets. In this case, the rotation of a moonlet is determined by the mean angular momentum brought by a number of small impacts. However, when the mass of impacting particles is comparable to the moonlet's mass, dispersions in the rotation rates become significant. This random component of rotation needs to be taken into account in the case of rotation of ring particles. We have derived an equation which describes the evolution of the dispersion of rotation rates of ring particles, and obtained the rate of evolution using three-body orbital integration. Using these results and N-body simulation, we will discuss rotation of ring particles and moonlets. We also obtained gravitational capture probability of colliding particles using three-body orbital integration, which takes into account velocity distribution and surface friction of particles. On the basis of our numerical results, we will also discuss accretional evolution of ring particles.
P53A-1463 1340h
Saturn's A Ring as Seen by the Voyager IRIS and Cassini CIRS Experiments
During Cassini's successful orbital insertion manuever at Saturn its Composite Infrared Spectrometer (CIRS) obtained thermal spectra of Saturn's rings at a range of geometries not seen since the Voyager flybys. CIRS is a Fourier-transform spectrometer that measures infrared radiation from 7 microns out to 1 millimeter (1400 to 10 $ \text{cm}^{-1} $). Of the main rings, the A Ring was observed at the greatest range of phase and emission angles during Saturn orbit insertion. Scans of the lit and unlit sides of the A Ring were obtained at a spectral resolution of 15.5 $ \text{cm}^{-1} $ and at low ($ \sim 60^{\circ} $) and intermediate ($ \sim 130^{\circ} \text{-} 140^{\circ} $) phase angles. The infrared interferometer spectrometer (IRIS) experiments aboard Voyagers 1 and 2 also obtained thermal spectra of the rings. IRIS, the predecessor of the CIRS instrument, is a Michelson interferometer that records spectra between 4 and 55.5 $ \mu \text{m} $ (2500 to 180 $ \text{cm}^{-1} $). Spectra of the lit and unlit A Ring with a resolution of 4.3 $ \text{cm}^{-1} $ were obtained at phase angles somewhat lower than CIRS SOI scans ($ \sim 30^{\circ} $) and at intermediate phase angles comparable to those observed by CIRS. We will interpret Cassini observations of the A Ring in the context of the earlier Voyager observations. A Ring brightness temperatures retrieved from the SOI scans clearly show a dependence on viewing geometry, varying by $ \sim 10 \, \text{K} $. The wide range of viewing geometries provided by the two sets of observations will allow us to determine the effect of viewing geometry on the ring's brightness temperature. By separating out such viewing geometry effects, we can constrain the physical properties of the ring as a whole as well as those of its constituent particles. Additionally, the time separation between the CIRS and IRIS observations will allow us to identify any changes in ring temperatures between the Voyager and Cassini epochs as might be expected from, for example, differences in solar elevation angle.