Estimate of Tidal Q of Mars Using MOC Observations of the Shadow of Phobos

Emma Rainey, California Institute of Technology

Phobos, the larger of the two small moons of Mars, is undergoing rapid orbital evolution due to tidal interaction with Mars. Phobos is very close to Mars, with a mean distance of only 2.8 RMars, within the synchronous orbital distance. Thus, Phobos’s orbit is moving inward. The rate of this secular acceleration can give us information about tidal dissipation in the interior of Mars. Although Phobos has been extensively studied since its discovery in 1877, few new observations have occurred since 1990. We have used images from the Mars Orbiter Camera (MOC) to indirectly observe Phobos using its shadow on the surface of Mars. The observed position of Phobos’s shadow agrees well with predictions based on the current orbital model, except that Phobos is getting slightly ahead of its predicted position, a result also found by Bills et al. (2005) using Mars Orbiter Laser Altimeter (MOLA) detections of the Phobos shadow. Using the MOC and MOLA datasets, we calculate a revised value for the secular acceleration of Phobos, and use this to estimate the tidal Q of Mars.

Abstract Author(s): Emma S. G. Rainey<br />Oded Aharonson