Venus length-of-day variations are due primarily to the exchange of angular momentum between the atmosphere and the solid planet. A time history of spin rate measurements can therefore provide crucial constraints on the atmospheric dynamics and climate of Venus. We have been trying to obtain measurements since 2002. Our initial data indicate a change in the spin rate since the Magellan era. Additional observations are needed to measure daily, seasonal, and secular variations in the rotation rate.
We are also attempting to characterize the evolution of the spin axis orientation, which would provide a direct measurement of the moment of inertia of Venus, an unmeasured yet fundamental constraint on models of the interior. Understanding the obliquity of Venus is also important as it may be excited by mantle convection, volcanic or seismic activity, resurfacing, or atmospheric changes.
This project requires small amounts of telescope time (1 hour per measurement) on the Goldstone 70-m antenna (DSS-14) and the Green Bank Telescope (GBT). However, progress has been slow due to extraordinary difficulties in securing DSS-14 antenna time. A table documents the history of attempted measurements and their outcomes. Many of our requests for observations have been denied, often to accommodate spacecraft telecommunications that do not seem particularly time-sensitive.
We are continuing our efforts to convince the Interplanetary Network Directorate at JPL, the organization in charge of managing DSS-14, to schedule time for these important measurements.