NASA Science Highlight: Planetary Program Support
Research by N. Rambaux (1,2), J. Castillo-Rogez (3), V. Dehant (4), and P. Kuchynka (3) (1) Universit? Pierre et Marie Curie, UPMC, (2) IMCCE, Observatoire de Paris, CNRS UMR 8028, (3) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, (4) Royal Observatory of Belgium, Brussels, Belgium.(Nicolas.Rambaux@imcce.fr)
Ceres, the first asteroid to be discovered, is by far the largest and most massive body in the main asteroid belt. In fact, it comprises about a third of the total mass of the entire main belt. While Ceres is much smaller than any of the planets, it is large enough for its shape to be spherical. Therefore, Ceres is considered to be a "dwarf planet," and appears be the only dwarf planet in the asteroid belt.
NASA's Dawn Mission will be the first to reach Ceres and one of its main goals is to understand its thermal evolution and current state. Ceres contains about 25 wt.% (weight percent) of water and is probably stratified in a rocky core and icy shell. Detailed knowledge of the rotational motion can help constrain the mass distribution inside Ceres itself, which provides clues into its geophysical history. Scientists from the Universitie Pierre et Marie Curie, the Observatoire de Paris, NASA's Jet Propulsion Laboratory, as well as from the Royal Observatory of Belgium computed the polar motion, precession-nutation, and length-of-day variations of Ceres using a stratified model of Ceres' interior constrained by recent shape data and surface properties.
The figure of Ceres appears to be an oblate body. Hence the Sun exerts a non-zero torque and raises tides that perturb Ceres rotational velocity. The precession time of the axis of Ceres is very long, about 220,000 years.
The obliquity, defined as the angle between the normal to the orbital plane and the figure axis, brings information on the moment of inertia only if it has reached an equilibrium position. Pole positions derived from adaptive optics, Hubble Space Telescope images and JPL's Horizons ephemerides indicate that Ceres' obliquity is likely greater than 3 deg. The scientists showed that the equilibrium value should in theory be 0.01 deg. This tells us that Ceres' rotation has been excited or never relaxed from an originally large inclination, which in itself is important information on its origin.
Summing all the contributions (nutation, precession) yielded an amplitude no greater than 1 millimeter. On top of this, Ceres could also present a Chandler Wobble. In the inertial frame, its period would be equal to 9h40 minutes, i.e., an increase of 36 minutes with respect to the proper rotation of the body.
As a general result, the amplitudes of oscillations in the rotation appear to be small, and their characterization from spaceborne techniques will be challenging. Hence, the Chandler wobble's signature should stand out in a precise determination of the rotational motion. This offers the prospect to better constrain Ceres' thermal state, provided that the asteroid is excited by endogenic and/or exogenic processes. The potential role of a liquid reservoir in exciting that wobble remains to be modeled in preparation for the arrival of Dawn at Ceres in 2015.
The protoplanet Ceres is one of the targets of the ongoing NASA's Dawn mission, which is expected to provide crucial information on the history of the Solar System. An accurate determination of Ceres' shape is essential for the determination of its internal structure and, therefore, for understanding its evolution. Available observations on the shape of Ceres show it as a rotationally symmetric, almost spherical, oblate spheroid. However, deviations from axisymmetry could happen to Ceres at the level of observational accuracy, and it is known that small deviations from axisymmetry may show non-negligible effects on the rotational dynamics of rigid bodies.
Significance to Solar System Exploration:
These recent observations suggest that Ceres is a very interesting body, and the upcoming arrival of DAWN is an exciting prospect. For example, like the planets and large moons, Ceres has "differentiated," meaning that its interior is separated into crust, mantle, and core layers, with the densest materials being at the center.? The outermost layer is likely made of icy material.? The fact that Ceres has not lost these volatile components in destructive impact events means that its composition provides ultimate constraints on the Solar system formation and early evolution.
Last Updated: 21 January 2014