Cassini/Huygens at Saturn and Titan II
Presiding: M K Dougherty, Space and Atmospheric Physics, Imperial College London; L Spilker, Jet Propulsion Laboratory, California Institute of Technology
P14A-01 15:30h
Cassini VIMS Photometric Investigation of Selected Features on the Surface of Titan: Relevance to Widespread Precipitation.
Photometric analysis of Cassini VIMS images of Titan suggests a hemispheric albedo dichotomy and that palimpsests are present. This implies long term surface albedo stability, inconsistent with widespread precipitation on Titan. We calculate linear phase coefficients for Titan's surface and find that it behaves like most planetary regoliths. Photometric anlaysis of Titan's approximately opposite hemispheres finds the most reflective regions (r=20%) are between longitude 50-130°. The lowest albedo regions (A~6%) are found near zero deg, consistent with previous observations (1,2,4,5,6). The most reflective units on Titan's leading side have higher reflectance than the most reflective units on the approximately opposite hemisphere. This is also true for the most absorbing units. VIMS finds features on Titan which are similar to circular features exhibiting topographic relief caused by impact events on a wide range of solar system objects. For two circular regions we searched for vertical relief by comparing photometric profiles with the profiles expected from model craters using the bi-directional reflectance equations developed by Hapke (1993). Despite our best effort to adjust the depression parameters to fit our data, no model is consistent with a craterlike depression. We suggest that these are albedo features and are not caused by topographic relief. They are consistent with palimpsests - impact events where the vertical relief has been lost to lithospheric plastic flow over time and remain only as expressions of darker reflectance on the surface. If so, their persistence on the surface suggests that widespread weathering processes, such as a planet-wide precipitation of aerosols, on Titan are severely limited. 1Campbell et al, Science, 302, 431-434, 2003 2Combes et al., Icarus, 129, 482-497, 1997 3Gibbard et al., Icarus, 139, 189-201, 1999. 4Hapke 1993, Theory of Reflectance and Emittance Spectroscopy, Cambridge. 5Meier et al., Icarus, 145, 463-473, 2000. 6Smith et al., 119, 336-349, 1996
P14A-02 15:45h
The Huygens Doppler Wind Experiment: Results from Titan
The ESA Huygens Probe entered and descended for nearly 2.5 hours through the atmosphere of Titan on 14 January 2005. Huygens survived impact on the surface and continued its telemetry broadcast to the NASA Cassini spacecraft on two separate radio links, denoted Channels A and B, respectively, for an additional 1.2 hours. The instrumentation for the Huygens Doppler Wind Experiment (DWE) consisting of two Ultra-Stable Oscillators in the transmitter (TUSO) and receiver (RUSO), were implemented only in Channel A. Whereas Channel B functioned flawlessly during the entire mission, the receiver for Channel A was never able to lock onto the Huygens signal because the DWE-RUSO had not been properly programmed into the critical probe radio relay sequence. All data on Channel A, including the DWE measurements and probe telemetry, were thus lost. In spite of this setback, the Channel A signal was successfully received at many radio telescopes on Earth. The precision of these Doppler measurements, considered as an aggregate, is roughly equivalent to that which had been foreseen from the measurements on board Cassini. We present an overview of the DWE ground-based observations and the Titan wind profile derived from them.
P14A-03 16:00h
Reflections on the Origin of Iapetus' Odd shape
Iapetus is a puzzling body in many respects: orbital characteristics, surface composition variations, shape and geology. This satellite presents unusual topography variations at different scales, as presented by Denk et al. (LPSC 2005 &35;2268). It can be described as a triaxial ellipsoid with radii 732x726x722 km (Denk et al. LPSC 2005 &35;2262) and large-scale topography variations of up to 10 km. Moreover, a narrow ridge up to 20-km high and 1300 km long makes Iapetus unique among the icy satellites family. We explore scenarios combining thermal, geodynamic and dynamic conditions suitable for Iapetus to develop these topographic features. We study the hypothesis that in its early history, this body went through a stage of high spin rate during which it acquired a hydrostatic ellipsoidal shape. We explore the scenario that while it was in high rotation speed, the body went through a phase of partial melting due to radiogenic decay and tidal heating. We model the differential buoyancy of diapirs originating in different places. The equator appears to be a preferential place for diapirs to rise to the surface. We investigate the potential connection between the latter result and the formation of Iapetus' ridge. We discuss the evolution of these topographic features with time, resulting in a present non-hydrostatic state of the satellite. This allows us to infer some information on its interior and composition. The above scenarios are also considered in the perspective of models of formation and orbital evolution available from the literature, scenarios proposed for the origin of the dark leading side, and comparison with other Saturnian medium-sized satellites.
P14A-04 16:15h
Saturn's Dynamic Magnetosphere: Energetic Particles and Neutrals from the Magnetospheric Imaging Instrument (MIMI)
The MIMI investigation comprises three sensors: the Ion and Neutral Camera (INCA) provides images using energetic neutral atoms (ENA) and ions; the Charge-Energy-Mass-Spectrometer (CHEMS) determines the mass and charge state of ions; and the Low Energy Magnetospheric Measurement System (LEMMS) measures ion and electron distributions using a dual field-of-view telescope (Krimigis et al., 2004). Measurements by MIMI following Saturn orbit insertion on 1 July 2004 revealed: (1) a dynamical magnetosphere with a day-night asymmetry and an 11-hour periodicity; (2) several water-product ions (O+, OH+, H2O+), but little N+; (3) inferred quantities of neutral gas sufficient to cause major losses in the trapped ions and electrons in the middle and inner magnetosphere; (4) a Titan exosphere that is a copious source of energetic neutral atoms (ENA); (5) a previously unknown radiation belt residing inward of the D-ring that is most likely the result of double charge-exchange between the main radiation belt and the upper layers of Saturn's exosphere. Finally, MIMI data show evidence of injections of plasma on the night-side of the planet, some substorm-like in the magnetotail and others in the 7-10 RS region that subsequently corotate with the planet for a number of days before dissipating. The observations will be presented and discussed in the context of current theoretical models of Saturn's magnetosphere. Krimigis, S. M., et al., Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci. Rev., volume 114/1-4, pp 233-329, 2004.
P14A-05 16:30h
Saturn's Planetary Magnetic Field: Cassini In-orbit Observations
Insertion of Cassini into orbit approximately one year ago produced the equivalent of another close flyby of Saturn after a lapse of 23 years. Field measurements within 1.3 radii reconfirmed the remarkable axial symmetry of the field and showed that little, if any, secular change had occurred since the earlier Pioneer 11 and Voyager 1 and 2 flybys. Observations of the planetary field have continued during the in-orbit phase of the mission. Over half a dozen additional periapses have occurred including one with a closest approach of 2.6 Saturn radii. Although operational problems prevented the acquisition of scalar magnetic field data at SOI, subsequent periapses provide additional opportunities to obtain highly accurate measurements of the field magnitude. These accumulating vector- and scalar measurements enable further investigation of various aspects of the planetary field. Investigation continues of intermittent field variations with quasi-periods near the planetary rotation period that would not normally be expected for an axially symmetric field. A possible cause is a "magnetic anomaly" in Saturn's field that is revealed during "special" circumstances. Alternatively, shielding of the non- axially symmetric components hypothesized by Stevenson could be less efficient at high latitudes in which case a tilt angle and periodicity of the core could become evident with increasing latitude or possibly in the outer magnetosphere.
P14A-06 16:45h
First Cassini Three-frequency Radio Occultation Observations of the Atmosphere and Ionosphere of Saturn
The first Cassini radio occultation observations of the atmosphere and ionosphere of Saturn were made on May 3rd, 2005. The path of Cassini as seen from Earth (the occultation track), which had been designed to cross Saturn's rings almost diametrically, has also provided two near-equatorial occultations of the atmosphere and ionosphere at longitudes separated by about 180°. During each occultation, three coherent monochromatic radio signals at wavelengths of 0.94, 3.6, and 13 cm (Ka-, X-, and S-band respectively) were transmitted through the Saturn atmosphere and ionosphere, a capability unique to Cassini. The refracted signals were received at the Earth and recorded at the NASA/JPL DSN complexes at Goldstone and Canberra. Analysis of these signals has provided the electron density profiles in the near-terminator ionosphere and the pressure - temperature profiles in the neutral atmosphere, as well as the first three-frequency measurement of microwave absorbers in Saturn's lower atmosphere.