Cassini/Huygens at Saturn and Titan I
Presiding: D Matson, Jet Propulsion Laboratory, California Institute of Technology; K K Khurana, Institute of Geophysics and Planetary Physics, University of California, Los Angeles
P13A-01 13:30h
Cassini VIMS at Saturn
The Cassini Visual and Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer on the Cassini spacecraft that covers the spectral range of 0.35-5.2 Îœm in 352 spectral channels, a nominal instantaneous field of view of 0.5 mrad and an image format of 64x64 pixels. It has completed it's first 7 months in orbit around Saturn. During that time it has made extensive observations of Saturn's rings, it's icy satellites, in particular Phoebe and Iapetus, and had 1 distant and 2 close flybys of Titan. The surface of Phoebe is dominated by water ice and bound water, but it has significant amounts of ferrous-iron-bearing silicates, CO2 as liquid or gaseous inclusions in minerals, organics, CN compounds and several as yet unidentified compounds. Phoebe's surface composition is consistent with other outer solar system objects Iapetus shows the presence of water ice, bound water, CO2 complexed in a similar fashion as on Phoebe, organics and several as yet unidentified surface components. The rings are seen by VIMS to be composed of almost pure water ice with small amounts of contamination/coloration that seem to be more abundant in the inner portion of Saturn's rings and in the ring gaps. VIMS has detected radial structure in the composition of Saturn's rings all the way down to the roughly 2-km resolution limit of the data obtained during Saturn Orbit Insertion (SOI). Titan is shown to have strong atmopheric fluorescence at 3.3 Îœm due to methane, a variegated surface of roughly a factor of 2 in albedo contrast that seems mostly due to textural differences rather than compositional differences, and definite evidence of topography and geologic structures.
P13A-02 13:45h
Cassini UVIS Results from Saturn, Titan, Icy Satellites and Rings
Dynamic interactions between neutrals, ions, rings, moons and meteoroids produce a highly structured and time variable Saturn system. UVIS has detected neutral oxygen, which dominates the Saturn inner magnetosphere, in contrast to Jupiter. The O is probably the product of water physical chemistry, and derived ultimately from water ice. Observed fluctuations indicate close interactions with plasma sources. Stochastic events in the E ring may be the ultimate source. Saturn's aurora varies by at least a factor of 4, associated with solar wind shocks and kilometric radio emission. Prior to Cassini's orbit insertion at Saturn, UVIS detected the spectral signature of water ice on Phoebe and in Saturn's rings, mixed non-uniformly with darker constituents. Recent observations of Enceladus, Tethys, Dione, Rhea and Iapetus allow us to begin to compare the surfaces of Saturn's icy satellites. Titan emissions show atomic, molecular and ionized nitrogen. Haze layers have been detected in reflected light from Titan's atmosphere, and show a difference in structure between the equator and the poles. The spacecraft made a close flyby of Titan on 13 December 2004 and stellar occultations of alpha Vir (Spica) and lambda Sco were observed. The occultation probed the night side of Titan in the southern hemisphere. Six hydrocarbon species were identified, CH4, C2H2, C2H4, C2H6, HCN, and C4H2. The UVIS derived temperature at the top of the atmosphere agrees with the results of the Ion Neutral Mass Spectrometer. The transition from convective to diffusive separation occurs between 800 km and 1100 km.Additionally, we find a previously unobserved 80 km layer at 1350 km, showing abundances of CH4 and C2H4 a factor of ~5 above the ambient underlying distribution.
P13A-03 14:00h
Titan's Surface Observed With the Cassini RADAR Scatterometer
The Cassini Titan RADAR, a multibeam Ku-band (13.78 Ghz, λ = 2.17 cm) linearly polarized radar instrument [1], includes an active, real-aperture scatterometer mode in which the central antenna beam is used to measure regional-scale backscatter across large areas of a target's surface. Raster scans permit the study of backscatter variation over diverse incidence angles, helping to constrain surface structure and composition. During the first targeted Titan fly-by (Ta) on October 26, 2004, the scatterometer observed Titan with resolutions around 100 km and incidence angles ranging from 0° to 60°, covering a total of about 107 km2. Backscattered power includes specular and diffuse components, and varies strongly over the surface. Initial modeling of the collected Ta inbound backscatter measurements and, separately, the collected Ta outbound measurements show an average angular dependence that can be described by the sum of a specular Hagfors Law and a diffuse Cosine Law. The best-fit model for the specular term implies rms slopes of a few degrees and bulk dielectric constants between 1 and 2, consistent with those inferred from radiometer measurements. The model for the diffuse component gives estimates of 0.7 (inbound) and 0.4 (outbound) for Titan's disc-integrated albedo in the same-linear polarization (SL). The Ta inbound pass, in particular, shows at least two populations of scatterers with varying reflectivity, both of which are highly correlated with near-IR reflectivity [3]. The albedo estimates are much larger than expected given the Earth-based 13 cm-λ Arecibo [2] and 3.5 cm-λ Goldstone results [4]; however, calibration of the instrument is still somewhat uncertain. Data from the upcoming T3 and T4 Titan fly-bys should be collected by the time of this meeting. With a specular component in Cassini's 2.2-cm echoes and also in Arecibo's 13-cm echoes [2], Titan's radar signature is unique among icy solar system bodies. It requires the existence of extended surface regions that are nearly flat at centimeter scales, suggesting that some smooth terrain is present. The high radar backscatter at larger incidence angles and the observed negative correlation of radar cross-sections and microwave brightness temperatures imply that volume scattering, rather than surface slope or roughness, dominates the echo power. Thus, the surface likely contains a layer of material that is highly transparent at Ku-band. The near-surface structure and composition must be heterogeneous to produce the large backscatter variations observed. References: [1] Elachi C. et al. (2005). Science (submitted); [2] Campbell D. et al. (2003). Science, 302, 241; [3] Smith P. H. et al. (1996). Icarus, 119, 336-349. [4] Muhleman D. O. et al. (1995). Annu. Rev. Earth Planet. Sci. 23, 337-374.
P13A-04 14:15h
Cassini First Diametric Radio Occultation of Saturn's Rings
We present preliminary results expected from the first planned Cassini radio occultation observation of Saturn's rings, to be conducted on May 3rd, 2005. The path of Cassini as seen from Earth (the occultation track) has been designed to cross the rings from the west to the east ansa almost diametrically, allowing for occultation of all major ring features at two widely separated longitudes (about 180 deg apart). The duration of the geometric occultation is about 1.5 hours on each side. During the occultation, Cassini transmits through the rings three coherent monochromatic radio signals of wavelength 0.94, 3.6, and 13 cm (Ka-, X-, and S-band respectively), a capability unique to Cassini. The perturbed signals received at the Earth are recorded at the NASA DSN complexes at Goldstone and Canberra. Both direct and forward-scattered components of the signal may be identified in spectrograms of the received signals. The time history of the extinction of the direct signal is expected to yield high-spatial-resolution optical depth and phase shift profiles of ring structure. The timing of the occultation was optimized to allow probing the rings when the ring-opening-angle B (the angle between the line-of-sight and the ring plane) is relatively large (B = 23 deg), hence maximizing chances of measuring for the first time the structure of the relatively optically thick Ring B. In a similar experiment by Voyager in 1980, excessive signal attenuation along the long path within the nearly closed rings (B = 5.9 deg) limited the utility of the observations in relatively thick ring regions, in particular the main Ring B. For the Cassini optimized occultation geometry, a large B, slow radial velocity along the occultation track, and much improved phase stability of the reference ultrastable oscillator (USO) on board Cassini combine to promise achievable radial resolution approaching 100 m over a good fraction of the rings. Measurement of the amplitude and phase of the diffracted signal enables reconstruction of the observations to remove diffraction effects. Reliable high resolution profiling of ring structure at multiple ring longitudes is at the heart of investigating ring kinematics and dynamics, a major scientific objective of this experiment. In addition, observations of the scattered signal combined with measurements of the differential extinction of the three radio signals are expected to yield complementary information about ring physical properties, including particle size distribution and thickness, another major scientific objective.
P13A-05 14:30h
Saturn Ring Temperature Roll-off at Submillimeter Wavelengths From Cassini CIRS Observations
The Cassini Composite Infrared Spectrometer (CIRS) spatially resolved Saturn's main rings, measuring the spectrum over a broad range of wavelengths, in particular covering the submillimeter band between 100 microns and 0.5 mm. For each ring, fitting a blackbody curve to the CIRS thermal spectrum between 25 and 100 microns (400 and 100 cm-1), and accounting for the ring opacities, we derived ring brightness temperatures that also accounted for the ring filling factor. Using the best fits to the CIRS spectra, we find that the roll-off in ring temperature occurs at ~ 200 microns (50 cm-1) for each ring. We used these data to investigation the spectral location of the roll-off that occurs somewhere between mid-thermal and centimeter wavelengths. The CIRS measurements unambiguously reveal a trend in brightness temperature that is much less steep between 50 and 100 microns than has been reported from earth-based observations. This more gradual trend implies that the main rings are not dominated by particles smaller than ~1 millimeter because larger particles do not show any significant variation in emission over the 50 to 100 micron wavelength range. This conclusion is consistent with results from particle eclipse cooling and high phase angle Voyager imaging observations. The roll-off in temperature measured by CIRS is principally due to material properties of the ring particles that change dramatically in this region. This work was performed at JPL under contract with NASA.
P13A-06 14:45h
Cassini magnetometer observations at Saturn
An overview of the magnetic field observations at Saturn from the dual technique magnetometer onboard the Cassini orbiter will be described. This analysis includes a description of the observed internal magnetic field, clear field periodicities observed at the rotational period of the planet, observations of diamagnetic cavities observed at the edge of the E-ring, strong wave activity both upstream of Saturn as well as deep within the magnetosphere as well as observations from the first close Titan flybys and the first Enceladus flyby.