Models and Measurements of the Rotation of Mars

The rotation of Mars has been determined more accurately than for any other planet except Earth. This has been done using radio tracking data from spacecraft orbiting Mars or landed on Mars, starting with Mariner 9 in 1972 continuing through the present with several orbiters currently in operation. The Viking landers in 1976 provided the first clear measurements of variation in length of day. Mars Pathfinder combined with Viking lander provided the first estimate of the martian precession rate. The model for rigid Mars rotation developed by Reasenberg and King for Viking data analysis is accurate enough to fit the currently available measurements. With the InSight mission to be launched in 2018 and the ExoMars lander mission to be launched in 2020, nutation of Mars due to non-rigid effects are expected to be detectable, requiring improved models for the effects of the martian fluid core. We will present an overview of the current measurements sets, including comparisons of length-of-day variations from independent subsets, plans for the InSight and ExoMars missions, and summarize potential modeling improvements.