Spin state and moment of inertia of Venus

TL;DR

This study uses radar observations from 2006-2020 to determine Venus's internal properties, including its tilt, precession, moment of inertia, and length of day variations, revealing insights into its core and atmospheric dynamics.

Contribution

First measurement of Venus's spin axis, precession rate, and moment of inertia using Earth-based radar observations over 14 years.

Findings

Venus's tilt is 2.6392 degrees with respect to its orbital plane.

The spin axis precesses at 44.58 arcseconds per year.

The average sidereal day is approximately 243.0226 Earth days.

Abstract

Fundamental properties of the planet Venus, such as its internal mass distribution and variations in length of day, have remained unknown. We used Earth-based observations of radar speckles tied to the rotation of Venus obtained in 2006-2020 to measure its spin axis orientation, spin precession rate, moment of inertia, and length-of-day variations. Venus is tilted by 2.6392 $\pm$ 0.0008 degrees ($1\sigma$) with respect to its orbital plane. The spin axis precesses at a rate of 44.58 $\pm$ 3.3 arcseconds per year ($1\sigma$), which gives a normalized moment of inertia of 0.337 $\pm$ 0.024 and yields a rough estimate of the size of the core. The average sidereal day on Venus in the 2006-2020 interval is 243.0226 $\pm$ 0.0013 Earth days ($1\sigma$). The spin period of the solid planet exhibits variations of 61 ppm ($\sim$20 minutes) with a possible diurnal or semidiurnal forcing. The length-of-day variations imply that changes in atmospheric angular momentum of at least $\sim$4% are transferred to the solid planet.