The Determination Of Titan'S Rotational State From Cassini Sar Images

Details The Determination Of Titan'S Rotational State From Cassini Sar Images The Determination Of Titan'S Rotational State From Cassini Sar Images

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.6306p&link_type=abstract

American Astronomical Society, DPS meeting #39, #63.06; Bulletin of the American Astronomical Society, Vol. 39, p.543

Astronomy and Astrophysics

Astronomy

Scientific paper

SAR images acquired by the spacecraft Cassini in overlapping strips have been used to determine the vectorial angular velocity of Titan. The method entails the tracking of surface landmarks at different times (and mean anomalies), spanning a period from 2004 to 2007. Each image is referenced both in an inertial frame and in the IAU, Titan-centric, body-fixed reference frame. This referencing is quite precise (accuracy of Cassini relative to Titan position smaller than 100 m).
The IAU body-fixed frame assumes a spin axis different from the actual one. By correlating the two images of the same surface region, one gets a two-dimensional vector, which retains information about the true spin axis. This vector provides the magnitude and direction of the displacement to be applied to a reference point of each image in order to produce maximum correlation. The correlation results therefore in a new Titan-centric, inertial referencing of the images, R(t1) and R(t2). The spin axis s is then obtained by requiring that [R(t2) - R(t1)] • s = 0 for each overlapping image pairs. Left hand sides cannot be simultaneously zeroed because Titan doesn't move respect to the known polar axis and the real spin axis must be determined by means of a least square procedure. The magnitude of the angular velocity is then derived from the angle and time between two observations.
The Titan pole position estimated is consistent with the occupancy of a Cassini state. If Titan were a rigid body in a Cassini state (with an icy crust anchored to the mantle), one could use theoretical arguments to derive the moment of inertia from the obliquity and the second degree gravity field. However, our results suggest that those theoretical arguments cannot be straightforwardly applied to Titan, whose rotational state is more complex than expected.

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