DDA 2015 – Constraints on Titan’s rotation from Cassini mission radar data

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Bruce Bills (JPL)

Abstract

We present results of a new analysis of the rotational kinematics of Titan, as constrained by Cassini radar data, extending over the entire currently available set of flyby encounters. Our analysis provides a good constraint on the current orientation of the spin pole, but does not have sufficient accuracy and duration to clearly see the expected spin pole precession. In contrast, we do clearly see temporal variations in the spin rate, which are driven by gravitational torques which attempt to keep the prime meridian oriented toward Saturn.

Titan is a synchronous rotator. At lowest order, that means that the rotational and orbital motions are synchronized. At the level of accuracy required to fit the Cassini radar data, we can see that synchronous rotation and uniform rotation are not quite the same thing. Our best fibng model has a fixed pole, and a rotation rate which varies with time, so as to keep Titan’s prime meridian oriented towards Saturn, as the orbit varies.

A gravitational torque on the tri-axial figure of Titan attempts to keep the axis of least inertia oriented toward Saturn. The main effect is to synchronize the orbit and rotation periods, as seen in inertial space. The response of the rotation angle, to periodic changes in orbital mean longitude, is modeled as a damped, forced harmonic oscillator. This acts as a low-pass filter. The rotation angle accurately tracks orbital variations at periods longer than the free libration period, but is unable to follow higher frequency variations.

The mean longitude of Titan’s orbit varies on a wide range of time scales. The largest variations are at Saturn’s orbital period (29.46 years), and are due to solar torques. There are also variations at periods of 640 and 5800 days, due to resonant interaction with Hyperion.

For a rigid body, with moments of inertia estimated from observed gravity, the free libration period for Titan would be 850 days. The best fit to the radar data is obtained with a libration period of 645 days, and a damping time of 1000 years.

The principal deviation of Titan’s rotation from uniform angular rate, as seen in the Cassini radar data, is a periodic signal resonantly forced by Hyperion.

Notes

  • Titan:
    • hard to see surface
    • Cassini’s radar intended for mapping surface
      • didn’t get much by way of repeat observations (“tie points”), which are needed to constrain rotation
      • most data near poles — not terribly helpful
  • Rotation model from tie-point observations
    • Stiles et al. 2008: 50 tie points over 2.8 yr
    • Now: 2602 tie points over 10 yr
    • solve for 3 params (RA & DEC of spin pole, angular rate)
    • $P = 15.94547727 \pm 6.03 \times 10^{-7}$ d
    • spin pole precession
      • gravity model: ~250 yr
      • not clearly seen in data
    • spin rate variations
      • seen in data
      • dynamical model
        • assume Titan in synch. rotation
        • gravity torque
        • dissipation
        • $\rightarrow$ libration period ~850 d
        • Hyperion has nontrivial influence
    • fit: libration period = 645.4 d, damping time = 430 yr, rotation period slightly changed

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