Spin state and moment of inertia of Venus
Jean‐Luc Margot, D. B. Campbell, Jon D. Giorgini, Joseph S. Jao, L. G. Snedeker, F. D. Ghigo, Amber Bonsall
Abstract
Fundamental properties of the planet Venus, such as its internal mass\ndistribution and variations in length of day, have remained unknown. We used\nEarth-based observations of radar speckles tied to the rotation of Venus\nobtained in 2006-2020 to measure its spin axis orientation, spin precession\nrate, moment of inertia, and length-of-day variations. Venus is tilted by\n2.6392 $\\pm$ 0.0008 degrees ($1\\sigma$) with respect to its orbital plane. The\nspin axis precesses at a rate of 44.58 $\\pm$ 3.3 arcseconds per year\n($1\\sigma$), which gives a normalized moment of inertia of 0.337 $\\pm$ 0.024\nand yields a rough estimate of the size of the core. The average sidereal day\non Venus in the 2006-2020 interval is 243.0226 $\\pm$ 0.0013 Earth days\n($1\\sigma$). The spin period of the solid planet exhibits variations of 61 ppm\n($\\sim$20 minutes) with a possible diurnal or semidiurnal forcing. The\nlength-of-day variations imply that changes in atmospheric angular momentum of\nat least $\\sim$4% are transferred to the solid planet.