P13C-1426
Compositional Mapping of Saturn's Satellite Iapetus with Cassini VIMS and Implications of Dark Material in the Saturn System
The Cassini Visual and Infrared Mapping Spectrometer (VIMS) has obtained spatially resolved imaging spectroscopy data on Iapetus and numerous other satellites of Saturn. Multiple spectral features seen in spectra of dark material on Iapetus match those seen on Phoebe, Hyperion, Dione, Epimetheus, and the F-ring, implying the dark material has a common composition throughout the Saturn system. Water, carbon dioxide and, tentatively, ammonia have been detected in the dark material on Iapetus and other satellites. A blue peak due to Rayleigh scattering, and a strong UV-Visible absorption, is observed in spectra on all satellites which contain dark material, again pointing to a common origin. The Rayleigh scattering effect has been confirmed with laboratory experiments using ice and 0.2-micron- diameter carbon grains when the carbon abundance is less than about 3% by weight. Cassini will fly by Saturn's moon Iapetus on September 10, 2007, at a closest approach altitude of about 1644 km. The low fly-by velocity will allow imaging spectroscopy data to be obtained with VIMS over dark terrain, the transition region into bright terrain, and bright (icy) regions of Iapetus, including good views of the equatorial bulge. Using VIMS data, which covers the spectral range from 0.35 to 5.1 microns in 352 wavelengths at spatial resolutions as high as 0.5 x 1 km, we will map materials on Iapetus' surface and study their geologic placement. Materials under study include water ice, carbon dioxide, ammonia ice, and materials contributing to absorptions seen at 1.94, 2.05, 2.16, and 2.42 microns. Other weaker absorptions observed in spectra of dark material on Iapetus in previously obtained VIMS data will be examined.
P13C-1427
Iapetus: Ultraviolet Measurements from Cassini
The Cassini spacecraft will make an unprecedented flyby of Iapetus, the enigmatic moon of Saturn, on September 10, 2007. The flyby, at an altitude of ~1645 km, will provide spectacular opportunities for observations of the bright-dark boundary on the anti-Saturnian hemisphere, as well as the first high-resolution views of the higher-albedo trailing hemisphere. The Ultraviolet Imaging Spectrograph (UVIS) will participate in the remote sensing campaign to study the surface composition of Iapetus, and will also perform a stellar occultation to search for any atmospheric gases. The previous Iapetus flyby (December 2004), at ~123,000 km, focused on the low-albedo leading hemisphere and yielded intriguing results about the dark terrain; UVIS spectra showed that the water ice absorption band is present even in the darkest, lowest latitudes of the central leading hemisphere. This result suggests the possibility of an ongoing coating process by an exogenic source of the dark material. Water ice has a distinctive absorption feature at ~165 nm and is readily observed by UVIS, allowing for compositional and grain size mapping.
P13C-1428
Cassini RADAR Icy Satellite Observation Designs and Results
Although Titan is the primary target of interest for the Cassini RADAR, whenever possible the radar has observed other icy satellites in the Saturn system. Nearing the end of the prime mission, some have been observed several times, and some have been observed with multi-point scans with real-aperture resolution providing some regional data. Most of the observations are tone transmissions that are processed for their Doppler signature. In a few cases, a full chirp is transmitted which offers the possibility of range processing. In a recent Iapetus observation, the range was low enough to permit synthetic aperture imaging of portions of the facing hemisphere. This presentation will discuss the observation designs used, and the status and prospects for processing of the overall data set. The latest results from the Iapetus imaging observation will be shown. In addition to the active echo data obtained, passive radiometer data has also been obtained and the two data sets have the potential to complement the optical and infrared imaging of the icy satellites and shed more light on the structure and composition of their surfaces. This work is supported by the NASA Cassini Program at the Jet Propulsion Laboratory, California Institute of Technology.
P13C-1429
Painting Iapetus White
One of the intriguing features of Iapetus is about the remarkable dichotomy of its surface albedo distribution with the leading hemisphere being very dark (albedo about 0.05) and the trailing hemisphere being relatively bright (about 0.5). This factor of 10 difference in the surface albedos has been generally interpreted to be the result of collection of collisional ejecta from Phoebe which is also very dark (about 0.06). Being moving outside Iapetus"orbit in retrograde direction, such mechanism certainly has its distinct advantage. The first close encounter of the Cassini spacecraft in September, 2007, will bring many new measurements and insights. We would like to take this opportunity to explore a different scenario. That is, the surface material of Iapetus was initially as dark as Phoebe and the albedo dichotomy actually came from the surface impact and accretion of large amount of icy materials emitted from the inner Saturnian region during the formation of the Saturn system. For this to happen, an intrinsic magnetic field of Saturn much stronger than the present value would be required so that the water-group ions can be trapped inside the partially rotating magnetosphere containing the orbit of Iapetus. In case this possibility of reverse-coating can not be ruled out by theoretical models and scaling arguments, it would have interesting implication on the origin of Iapetus itself.
P13C-1430
The Black Axiom: an A Priori Explanation for Iapetus's Dark Side
Most theories to explain Iapetus's hemispheric dichotomy begin with the observation of the dichotomy and guess its cause. Only one theory stands on solid footing independent of Iapetus, yet requires that Iapetus be marked just as we see it. That one theory is the modern updating of the 200-year-old exploded planet hypothesis (EPH), the scientific justification for which is extensive. A comprehensive review paper on the status of the theory just appeared in "The challenge of the exploded planet hypothesis", Int'l J.AstroBio. 6:185-197, 2007. The so-called "black axiom" first appeared in "Dark Matter, Missing Planets and New Comets", T. Van Flandern, North Atlantic Books, Berkeley, 1993; 2nd ed. 1999. It notes that the EPH event sends a carbonaceous blast wave spreading throughout the solar system and coating surfaces facing the wave as it passes by over a period of a few weeks. Most moons rotate fast enough to get coated on all sides. But a few can be only partially coated. The most outstanding examples are Iapetus (half black and half white because of very slow rotation) and Triton (tilted spin axis, protecting part of one hemisphere). But all old-surface solar system moons are coated to the extent the EPH would expect. For Iapetus, the Cassini photos have already revealed some confirming evidence. The dark material is streaked perpendicular to the border, consistent with grazing arrival of the blast wave in those border regions. A prediction for future imagery is that the near-equator borders of the dark hemisphere will taper off gradually as the moon rotated during the weeks of the blast wave passage. The polar regions should show no such tapering. Scientists trying to understand Iapetus will be interested in considering this model along with the others already on the table so that the correct model ultimately prevails. http://metaresearch.org/publications/bulletin/2006issues/1215/Mrb06dp3.asp
P13C-1431
The Relative Surface Roughness of the two Sides of Iapetus
We apply Cassini ISS (Imaging Science Subsystem) data from the January 1st, 2005 flyby of Iapetus to a surface roughness model originally developed by Buratti and Veverka (1985). Since macroscopic features of topography alter the scattering properties of a planetary surface (Schoenberg, 1925; Hameen-Antilla et al., 1965; Hapke, 1966, 1984; Veverka and Wasserman, 1972; Lumme and Bowell, 1981; Buratti et al., 1985), this model uses the observed scattering behavior to provide a depth to radius factor q quantifying the size of craters on the surface. Relative surface roughness of the low albedo (leading) hemisphere and high albedo (trailing) hemisphere can then be determined by comparing the value for the two hemispheres, and any differences observed will provide an estimate of the depth of the dark material. Our preliminary findings show marked differences in macroscopic roughness between the high and low albedo hemispheres, indicating that the surface on the dark side is much smoother than the bright. Our results further suggest that the dark material is substantial enough to cause significant infilling of the craters on the dark side. Funded by the NASA Space Grant.
P13C-1432
Iapetus: a Prediction for Bulk Chemical Composition, Internal Physical Structure and Origin
I report calculations for the predicted chemical composition and physical structure of Iapetus. The results are based on the hypothesis that Iapetus is a native moon of Saturn which initially formed closer to the planet at orbital distance of ~ 12RSat , where RSat = 60268 km (Prentice 1984 Earth Moon Planets 30 209- 228; 2006 Publ. Astron. Soc. Australia (PASA) 23, 1-11). Iapetus was scattered tidally to its current orbit by Titan, which is assumed to be a captured moon of Saturn (2005 LPSC XXXVIII, # 2402). That is, Titan condensed as a secondary body in the same solar orbit as Saturn. Initially Titan's orbit was very eccentric, having a peri-Saturn distance of ~ 10RSat. All of Saturn's native moons, including 2 former moons that existed at radii ~ 17RSat and ~ 24RSat, condensed from a concentric family of gas rings. The rings were shed by the proto-Saturnian cloud to rid spin angular momentum during gravitational contraction. If the contraction of the cloud is homologous, then the sequence of gas rings (n = 0,1,2,…) are geometrically spaced and their temperatures Tn vary with mean orbital distance Rn according as Tn ~ 1/Rn, setting aside the heat due to the early Sun. If the Tn are scaled so that Enceladus condenses just inside the stability field of liquid water, then for Iapetus Tn=95 K and the condensate consists of hydrous rock (mass fraction 0.336), water ice (0.344), ammonia ice (0.267) and clathrated methane (0.053). The density of the rock at 76 K and 0.1 Mpa is 3.154 g/cc. The condensate mean density is ρ = 1.185 g/cc. A chemically uniform Iapetus model for this mix has ρ = 1.210 g/cc. The Iapetus density is 1.083 g/cc. Next, a 4-zone differentiated model was constructed. This consists of a rocky core surrounded by separate layers of water ice, ammonia ice and methane ice – all in the same proportions as above. This model has ρ = 1.123 g/cc. Lastly, a 5-zone model was made by adding a crust composed of the initial homogeneous mix and having a porosity of 58%. A crustal thickness of ~ 16 km yields a model whose ρ=1.083 g/cc. The axial moment-of-inertia coefficient of this model is 0.311. It is proposed that tidal heating due to the satellite's orbital displacement by Titan caused all of the ices (except for those of the crust) to melt and for the moon to differentiate. Most likely, the equatorial ridge of Iapetus was formed by the subsequent solidification and contraction of the liquid interior.