P22C-01 INVITED
Targeted Iapetus Flyby of Cassini: Imaging Results
On September 10, 2007, the first and sole targeted Iapetus flyby of the Cassini spacecraft within six years took place (5 days after submission of this abstract). The inbound imaging plan covered the crescent over the leading side down to 480 m/pxl (at 145 deg phase angle). Within the time frame between closest approach (C/A) minus 5:25 hrs and C/A+5:44 hrs, observations of the dark/bright transition zone, the ridge, and a large (~500 km) basin on the trailing side were in the plan. From C/A-55 min to +180 min, the top-priority observation contained 11 different targets on the surface, including high-phase observations of the dark ridge (~30 m/pxl, 140W longitude), 10 m/pxl samples of the dark terrain, two large transition zone mosaics over the anti-Saturn hemisphere, and high-res. imaging (~50 m/pxl) of the bright "Voyager mountains". Outbound observations included a global, 15- panel mosaic of the complete trailing hemisphere (~450 m/pxl), as well as multiple regional color panels and a global color observation at 1.6 km/pxl and 34 deg phase. Imaging data will hopefully help to give answers to questions like: What is the cratering record at Iapetus? How old is the surface? Might Iapetus indeed act as a reference point for surface age determinations in the whole Saturn system? What about the huge basins? How did the ridge form? Are there more geologic features than the craters and the ridge, especially at smaller scales? How is the dark and bright material distributed? Are there more than two kinds of dark material? How thick is the dark blanket, and might we even see bright holes deep within the dark terrain? Why is Iapetus' shape not spherical? What are the implications of the global color dichotomy? Etcetc... And, most important: What causes the extreme brightness dichotomy already recognized by J.-D. Cassini 335 years ago? http://www.geoinf.fu-berlin.de/projekte/cassini/cassini_fu_iapetus_flyby.php
P22C-02 INVITED
Iapetus as Seen Through the Multispectral Eyes of Cassini VIMS
The Cassini targeted flyby of Saturn's moon Iapetus on Sept. 10, 2007 will enable the first detailed study of the composition of this satellite within the context of geologic features and evolution. Detailed maps of the volatiles and components constructed by data from the Visual Infrared Mapping Spectrometer (VIMS) will follow on earlier detections. These materials include carbon dioxide (Buratti et al. Ap. J., 2005), polycyclic aromatic hydrocarbons (PAHS; Cruikshank, Icarus, 2007) and trace ammonia (Clark et al. Icarus, 2007). The flyby offers good coverage of the bright-dark interface, and it should enable observations of compositional markers in low-albedo features (e.g., dark floor craters) on the moon's high-albedo hemisphere. Funded by NASA.
P22C-03 INVITED
New Cassini RADAR Results for Iapetus and Saturn's Other Icy Satellites
During the past two years, radar tracks on the icy satellites have more than doubled the number of observations reported by Ostro et al. (2006, Icarus 183, 479-490). Our scatterometry has now yielded estimates of the 2.2-cm- wavelength radar albedo in the same linear (SL) polarization as transmitted for Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe. Our tracks have sampled widely separated subradar locations on most of the satellites, in some cases using beams comparable to or larger than the target's disc and in others using beams significantly smaller than the disc. Hence we can begin to assess the dispersion in these objects' distributions of radar reflectivity. The 2.2-cm SL radar albedo varies by at least several tens of percent on at least Dione, Rhea, Enceladus, and Iapetus. For Enceladus, a scatterometric observation centered on (29 S, 243 W) with a beamwidth of 1.2 Enceladus diameters gives the largest disc-integrated SL radar albedo obtained at any wavelength for any solar system object (including Europa), probably because of the extreme purity of at least the uppermost few decimeters of the water-ice regolith on much of the hemisphere facing the radar. For Iapetus, multi-beam, disc-resolved measurements disclose that the disparity in the 2.2-cm SL radar albedos between the optically bright and dark terrains is greater than that inferred by Ostro et al. (2006). During the Iapetus 49 flyby shortly after this meeting's abstract deadline, radar observations will include SAR imaging with 2-to-12-km surface resolution covering much of the visible disc, using beam sizes of about 120 km (less than one tenth of an Iapetus diameter), plus a short altimetric measurement with 35-m range resolution. The radar imaging will be mostly of the optically dark terrain and will include tracks across the equatorial ridge and the largest impact structures in Cassini Regio. The Iapetus SAR images will be the first of an icy satellite for which high-resolution optical images are available, and therefore will provide useful lessons for interpreting Titan SAR images.
P22C-04
Cassini Composite Infrared Spectrometer (CIRS) Observations of Iapetus' Thermal Emission, and Implications for the Hemispheric Asymmetry
During the flyby of Iapetus on September 10th 2007, The Composite Infrared Spectrometer (CIRS) on the Cassini Saturn orbiter spacecraft will provide high-resolution maps of thermal emission from the dark leading hemisphere at night, and from the bright terrain and the bright/dark transitional region on the trailing hemisphere during the day. These data, combined with daytime observations of the dark hemisphere from January 2005, will provide a much improved picture of diurnal temperature variations on Enceladus, allowing comparison of the thermal inertias of the dark and bright terrains. The temperature data will also yield improved constraints on the stability of H2O ice and other ices on the different terrains on Iapetus, perhaps shedding light on the peculiar distribution of bright and dark terrain on this satellite. The bright terrain on Iapetus is concentrated on the trailing hemisphere but extends over the poles onto the leading hemisphere, while the dark terrain, centered on the leading hemisphere, extends around the equator onto the trailing side. Our 2005 model (Spencer et al. 2005, Division for Planetary Sciences Meeting, Cambridge, abstract #39.08) explains the shape of this boundary in terms of modification of a simple exogenic leading/trailing asymmetry by ice migration. Ice is proposed to sublime from the warm, dark leading hemisphere and collect in the polar regions, which become brighter, while bright low-latitude regions on the trailing hemisphere may experience sufficient ice sublimation to darken by formation of a surface lag deposit. CIRS observations of the daytime temperatures of the bright terrain at low latitudes will determine whether this terrain is indeed warm enough for significant ice sublimation, thus providing a test of the sublimation model.