37th DPS Meeting, 4-9 September 2005
Session 39 Icy Satellites II
Oral, Wednesday, September 7, 2005, 2:15-4:00pm, Law LG19

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[39.02] Gravity Science In The Saturnian System: The Masses of Phoebe, Iapetus, Dione, and Enceladus

N. J. Rappaport (JPL), L. Iess (U. Roma, Italy), P. Tortora (U. Bologna, Italy), S. W. Asmar (JPL), L. Somenzi (U. Roma, Italy), A. Anabtawi, E. Barbinis, D. U. Fleischman, G. L. Goltz (JPL)

We will present the determination of the masses of Phoebe, Iapetus, Dione, and Enceladus. The mass and the volume yield the density, a crucial quantity in the modeling of the evolution of the Saturn system.

Cassini gravity experiments were designed to benefit from two new technologies. First, Ka-band and multi-frequency radio links allow for a reduction of plasma noise. This reduction is limited to a factor of 1.5 to 2 due a malfunction of an onboard hardware component. Second, the Advanced Media Calibration system precisely measures the noise due to the Earth troposphere. This calibration is especially useful when plasma noise is small.

The data analysis uses the JPL-developed Orbit Determination Program. Short arcs of data are analyzed. A small number of parameters are estimated, including the satellite's mass and the initial state of the spacecraft. For close flybys, it is also necessary to improve the satellite's ephemeris.

Phoebe's flyby occurred on June 11, 2004. The distance at closest approach was 2000 km. We obtained GM = 0.5517 ± 0.0007 km3/s2, which implies a density of ~q 1.60 ±0.02 g/cm3.

Cassini flew twice by Iapetus, first on October 17, 2004 at a distance of 1.11 million km, and then on December 31, 2004 at a distance of 123,000 km. We obtained GM = 120.20 ± 0.06 km3/s2, which implies a density of ~q 1.105 ±0.027 g/cm3.

We also have tracking data for the Dione flyby, which occurred on December 15, 2004, with a closest approach distance of 72,600 km. The Enceladus flyby occurred on February 16, 2005, with a closest approach distance of 1420 km. We are in the process of analyzing the Dione and Enceladus data.

Rappaport acknowledges funding from the Cassini Project. Iess, Tortora and Somenzi acknowledge funding from the Italian Space Agency.

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