P51C-01 INVITED 08:00h
Saturn's Rings on the Arrival of Cassini
In this talk we will review the basic properties of Saturn's rings, and the main processes which act upon them, to place Cassini's first ring science results in context. Issues touching on the origin and evolution of Saturn's rings will also be discussed.Voyager, groundbased, and HST contributions will be described, recent theoretical developments will be discussed, and key outstanding problems will be noted. The properties of the Cassini tour relevant to ring observations will be reviewed. Specifically, the geometry of the Saturn Orbit Insertion maneuver, the geometry of the first ring-intensive periapse pass (Rev A, October 2004), and notable elements of the geometry of the rest of the tour will be highlighted as "coming attractions".
P51C-02 INVITED 08:05h
Cassini UVIS Observations Show Active Saturn's Rings
The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of the NASA/ESA Cassini spacecraft. This spectrograph includes channels for extreme UV and far UV spectroscopic imaging, high speed photometry of stellar occultations, solar EUV occultation, and a hydrogen/deuterium absorption cell. We report our initial results from UVIS observations of Saturn's rings. Dynamic interactions between neutrals, ions, rings, moons and meteoroids produce a highly structured and time variable Saturn system Oxygen in the Saturn system dominates the magnetosphere. Observed fluctuations indicate close interactions with plasma sources. Stochastic events in the E ring may be the ultimate source. The spectral signature of water ice is seen on Phoebe and in Saturn's rings. Water ice is mixed non-uniformly with darker constituents. The high structure of the UV ring reflectance argues that collisional transport dominates ballistic transport in darkening the rings. Our preliminary results support the idea that rings are recycled fragments of moons: the current processes are more important than history and initial conditions. The spectra along the UVIS SOI radial scan indicate varying amounts of water ice. In the A ring, the ice fraction increases outward to a maximum at the outer edge. This large-scale variation is consistent with initially pure ice that has suffered meteoritic bombardment over the age of the Solar system (Cuzzi and Estrada 1998). We also see variations over scales of 1000 - 3000 km, which cannot be explained by this mechanism. Ballistic transport of spectrally neutral extrinsic pollutants from meteoroids striking the rings has a typical throw distance of 6000 km (Durisen et al 1989), too long to explain this finer structure. We propose a class of smaller renewal events, in which a small moon residing within the rings is shattered by an external impactor (Colwell and Esposito 1993, Barbara and Esposito 2002, Esposito and Colwell 2003). The interior of such a body has been shielded from external meteoritic bombardment, and thus contains purer ice. Since the amount of meteoroid pollution provides a rough clock to estimate the age of the rings (Cuzzi and Estrada), these random events reset that clock locally, making the material at that radial location younger and purer. As these purer ring particles collide with others, they exchange regolith, and the range of purer water ice spectrum spreads radially. The radial variation we interpret as due to differential pollution in our data set is consistent with the disruption of several small bodies in the A ring in the last 107 to 108 years. When the small moon Pan (Showalter 1991, R ~ 10 km, now residing in the nearby Encke Gap) is eventually shattered by an external impact (Colwell et al 2000), the gap will close up, and for some 10 to 100 million years thereafter a brighter radial swath of purer water ice at its former location will gradually spread and darken.
P51C-03 INVITED 08:20h
Splendor in the Rings...as Seen by the Cassini Imaging Science Experiment
Since Cassini's approach to Saturn began in early February 2004, the Imaging Science experiment has collected multi-spectral observations of Saturn's rings at intermediate phase angles, images designed to search for previously unseen satellites near and within the ring system, images of known ring-region satellites for the purposes of orbit refinement, and a striking set of images acquired immediately after Cassini was placed in orbit, as the spacecraft flew across the unilluminated side of the rings, and then again, after it crossed the ring plane onto the illuminated side. New ring structures and objects have been found in the vicinity of the F ring, orbits for these objects as well as the F ring have been determined, the phase/color behavior of the rings has been extended into new geometries, and the highest resolution images have revealed a variety of features -- some brand new, and some previously known but seen now in greater two-dimensional detail -- all created by the perturbations of moons, both external and internal to the rings. Some of these features indicate the presence of unseen moons in ring gaps; analysis of others will allow reliable measures of ring viscosity and masses for the perturbing bodies. These new results and others, as well as their implications, will be discussed.
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P51C-04 INVITED 08:35h
Cassini-VIMS Observations of Saturn's Rings at SOI.
Following the Cassini spacecraft's Saturn Orbit Insertion (SOI) burn on 1 July 2004, the Visual and Infrared Mapping Spectrometer (VIMS) obtained near-infrared spectra from 0.9 to 5.1~$\mu$m in two continuous radial scans across the unlit side of the rings, at ranges of $\sim30,000$~km. The first scan covers the outer C and inner B rings at a phase angle, $\alpha = 82^\circ$ and an emission angle, $e = 47^\circ$, while the second covers the Cassini Division and entire A ring at $\alpha = 59^\circ$ and $e = 63^\circ$. The solar incidence angle was $114^\circ$ and the radial resolution of both scans is $15-20$~km, with sampling intervals of $2-3$~km. Structurally, the rings appear to have changed little, if at all, since the Voyager observations in 1980/81 and the 28~Sgr occultations in 1989. This similarity extends even to the quasi-irregular structure which characterizes the inner B ring on scales of $\sim100$~km. Spectrally, all regions of the rings scanned are dominated by water ice, with prominent absorption bands at 1.55, 2.0 and 3.0~$\mu$m, as well as weaker bands at 1.04 and 1.25~$\mu$m seen primarily in the A and B rings. The ice bands are strongest in the middle A ring, somewhat weaker in the B ring, and much weaker in the C ring and Cassini Division. However, the transitions between the C and B rings and between the Cassini Division and A ring are marked by gradual changes in band depth over radial distances of a few thousand km, perhaps indicative of ballistic redistribution of material. Besides water ice, the most noteworthy spectral feature is a broad, shallow absorption in the $0.9-1.8~\mu$m region which we tentatively attribute to Fe-bearing minerals, most likely silicates. This feature is seen primarily in the outer C ring and the Cassini Division, but like the ice band depths it pays scant attention to structural boundaries. This work was supported by NASA and ESA under contracts with the Cassini-Huygens Project.
P51C-05 INVITED 08:50h
Cassini CIRS: Preliminary Results on Saturn's Rings
The Cassini Composite Infrared Spectrometer (CIRS) obtained thermal spectra of the rings during Saturn approach, orbit insertion (SOI), and post-SOI. CIRS is a Fourier-transform spectrometer that measures radiation in the thermal infrared from 7 microns to 1 millimeter (1400 to 10 cm-1). For 65 minutes immediately following the SOI burn, the spacecraft flew over the unilluminated side of the main rings and was nearly an order of magnitude closer to the rings than it will be at any other time in the mission. CIRS obtained a scan of segments of the main rings at an average spatial resolution of 200 km with a spectral resolution of 15.5 cm-1. Following the outbound ring plane crossing, CIRS obtained a single scan of a portion of the lit A ring. After SOI, CIRS acquired azimuthal scans across the shadowed regions of the A, B and C rings CIRS retrieved temperatures for the unlit side of Saturn's rings from the SOI scan. Temperatures varied from approximately 70 K to 110 K. The data show that opaque sections of the rings, like the A and B rings, are cooler than the more transparent sections, like the Cassini Division and the inner C ring. Distinct cool and warm regions are also present in the C ring and Cassini Division, with the cool regions corresponding to the more optically thick ringlets. CIRS observed the lit A ring at a phase angle of ~130 degrees and the unlit A ring at ~60 degrees phase. In this case, for very different phase angles, the retrieved lit and unlit A ring temperatures appear comparable, providing new evidence for possible slowly rotating particles. During the azimuthal scans across the shadow boundaries, ring particles entering the shadow cooled relatively quickly, indicative of a low thermal conductivity. C ring particles cooled over 10 K after entering the shadow. The total amount of cooling is not as pronounced in the A ring, which is only partially shadowed at this epoch. Preliminary information on retrieved temperatures and ring characteristics will be reported. This work was performed at JPL under contract with NASA and at CEA Saclay supported by the "Programme National de Planetologie".
P51C-06 INVITED 09:05h
Evidence of Meteoroid Impacts on the Rings from Cassini Plasma Wave Measurements
During the passage of the Cassini spacecraft through the inner region of the Saturnian system on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many impulsive narrowband emissions that appear to be closely associated with the passage over the rings. In particular, the signals began and ended at the outer edge of the A ring, and were observed almost continuously (except for data gaps) over the A and B rings, with a well-defined gap at the Cassini division. Typically the signals occur in the frequency range from about 1 to 10 kHz, with bandwidths of a few hundred Hz and durations of about 1 to 3 seconds. The peak occurrence rate was about 1 event every few seconds. A good case can be made that this radiation is propagating near the whistler-mode resonance cone, which means that the source lies on the surface of a cone extending downward centered on the magnetic field with a half-angle ranging from about 10 to 60 degrees. The impulsive character of the emission suggests that the radiation is generated by some transient energetic process in the rings. The leading candidate appears to be meteoroid impacts on the rings. Rough estimates suggest that the flux of meteoroids with radii of about 1 cm would be adequate to explain the observed occurrence rates and emission intensities.
P51C-07 INVITED 09:20h
Plasma Near Saturn's Rings: CAPS observations
During the Saturn Orbit Insertion on 1 July 2004, the Cassini-Huygens spacecraft flew over Saturn's rings, performing the closest ever encounter with these spectacular features. The CAPS instrument made plasma measurements immediately over the B and A rings, as well as over the Cassini, Encke and Keeler gaps. The electron spectrometer measured electrons between 0.5 and 28,000 eV with 2s time resolution, within a 160 degree fan-shaped field of view directed partly towards the rings in this interval. The Ion Mass Spectrometer measured ions between 1 and 50,000 eV with a similar field of view. During much of this time the spacecraft was in solar occultation, and spacecraft-produced photoelectrons were therefore absent. When the spacecraft left the protection of the main rings and features, penetrating radiation associated with Saturn's radiation belts appeared, confirming that the rings absorb radiation belt particles. The new low energy plasma results over the rings showed: (1) significant enhancements in the fluxes of electrons above the Cassini, Encke and Keeler gaps, (2) spectral signatures of electrons above the A ring which may contain information about ring material, (3) the presence of electrons within the ring ionosphere with changing properties throughout the period and (4) ion signatures consistent with corotation of oxygen. Here we will present the CAPS observations and preliminary interpretations.
P51C-08 INVITED 09:35h
Saturn's A-ring Ionosphere as Observed by the Cassini Ion and Neutral Mass Spectrometer
The Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini Orbiter jettisoned its cover and started making measurements shortly after the spacecraft's main engine shut off. The mass spectra obtained with the INMS open source ion mode when the spacecraft was over the A-ring showed clear peaks at mass numbers associated with O2+, O+, and H+ ions. The O2+ peak was by far the largest and suggests the existence of a significant ring-plane ionosphere. The existence of O2+ ions also points to the existence of neutral O2 in the vicinity of the A-ring. This neutral oxygen must then be ionized, perhaps by photoionization by solar ultraviolet photons. Determining absolute ion densities from the INMS data alone is difficult due to the limited INMS field of view but density estimates have been made and can be compared with data from other Cassini plasma instruments. Ring-atmosphere measurements made by INMS in its closed and open source neutral modes appears to be dominated by background signal but possible upper limits to the ring plane neutral density will be briefly discussed.
P51C-09 09:50h
Stream Particles as Messengers from Saturn's Rings
Beginning in 2004 when the Cassini spacecraft was closer to Saturn than 1200 $\mathrm{R_S}$ (Saturn radius $\mathrm{R_S} = 60\,330$ km), the on-board dust sensor, CDA discovered a few streams of tiny ($ < 20$ nm), high-velocity dust particles. A detailed analysis of the impact signals revealed that the registered particles were expelled from within the Saturnian system at speeds exceeding 100 km\,s$^{-1}$. Such impact velocities can only be explained if the particles were released from the outskirts of Saturn's A ring. An interesting consequence of this result is that material from the dense inner rings can be analysed by in-situ dust detectors like the CDA. Until the end of August 2004, the dust instrument obtained about 400 time-of-flight mass spectra of stream particles. There is clear indication that the bulk material of the stream particles has to be a silicate. This implies that Saturnian stream particles are rather the inclusions of ring material than the ice particles itself.