Impact of Earth-based radar observations

The moment of inertia of our sister planet has never been measured, until now. We have been using the Goldstone Solar System Radar and the Green Bank Telescope to address fundamental questions related to the interior and atmosphere of Venus. We have obtained a measurement of Venus's moment of inertia with 7% uncertainties and a rough estimate of the size of the core (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. We have also obtained a record of length-of-day variations, which is providing key data towards elucidating Venus's fascinating but poorly understood atmospheric dynamics.

The length of day on Venus changes, but by how much?

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 of Venus, enabling key tests of hypotheses related to the superrotation or planet-scale atmospheric structures. We have been obtaining measurements of the length of day for over a decade. Our initial data indicate that Venus clearly exhibits length-of-day variations. Additional observations are needed to measure daily, seasonal, and secular variations in the rotation rate.

What is the size of the core? Is the core solid or liquid? Nobody knows.

We have been characterizing the evolution of the spin axis orientation, which provides a direct measurement of the moment of inertia of Venus, a fundamental constraint on models of the interior. While we have detected the precession with 7% precision, additional observations are needed to secure a measurement with smaller uncertainties. 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 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.

Results of our observations to date are published and also available on arXiv. They were also presented at the 2012 DPS meeting, 2013 AAS meeting, 2016 DPS meeting, and 2019 DPS meeting.