We used radar speckle tracking observations to measure Venus's moment of inertia with 7% uncertainties (Margot et al., Spin state and moment of inertia of Venus. Nature Astronomy 5, 2021). With additional observations and improved precision, we will ultimately reduce the uncertainties on the size of the core of Venus and rule out certain classes of interior models. Understanding the obliquity of Venus is also important as it may be excited by mantle convection, volcanic or seismic activity, resurfacing, or atmospheric changes.
We also obtained measurements of Venus's length of day with 5 ppm precision. Our data demonstrate that Venus exhibits substantial length-of-day variations. These variations are due primarily to the exchange of angular momentum between the atmosphere and the solid planet. Our observations provide key data towards elucidating Venus's fascinating but poorly understood atmospheric dynamics.
This investigation requires small amounts of telescope time (less than 1 hour per measurement) on the Goldstone 70-m antenna (DSS-14) and the Green Bank Telescope (GBT). A table documents the history of attempted measurements and their outcomes.
We are producing an improved geodetic control network for Venus on the basis of Magellan synthetic aperture radar (SAR) images at ~100 m resolution. This work will produce an improved estimate of the spin axis orientation at the Magellan epoch, which will yield an improved measurement of the moment of inertia. It may also yield measurements of length-of-day variations at high temporal resolution over a ~500 day period. Finally, it may yield an improved measurement of the k2 tidal Love number.
Results of our observations to date are published and also available on arXiv.