Astronomers using terrestrial radar observations have measured some of the fundamental properties of the planet Venus: the precise length of a day, the tilt of its axis, and the size of its core.
“Venus is the closest planetary neighbor to Earth and its closest analogue in the solar system in terms of mass, radius and density,” said Professor Jean-Luc Margot of the University of California at Los Angeles and her colleagues.
“However, Venus remains enigmatic on various fundamental levels: the size of its nucleus is unknown, whether the nucleus is solid or liquid is uncertain, and estimates of its average rotation period are discordant.”
“Venus is also distinguished by its 243-day retrograde rotation and 4-day atmospheric superrotation, neither of which is fully understood.”
On 21 occasions between 2006 and 2020, astronomers directed radio waves at Venus from the 70m-wide Goldstone antenna in California’s Mojave Desert.
A few minutes later, these radio waves bounced off Venus and returned to Earth. The radio echo was picked up at Goldstone and at the Green Bank Observatory in West Virginia.
Radar measurements show that an average day on Venus lasts 243.0226 Earth days, or about two-thirds of an Earth year.
In addition, the rate of rotation of the planet is constantly changing: a value measured at any given time will be a little larger or smaller than a previous value.
The researchers estimated the length of a day from each of the individual measurements and observed differences of at least 20 minutes.
“This probably explains why the previous estimates did not match,” Professor Margot said.
“The heavy atmosphere of Venus is probably to blame for the variation. As it laps around the planet, it exchanges a lot of momentum with solid ground, speeding up and slowing down its rotation.
“This also happens on Earth, but the exchange adds or subtracts a millisecond each day.”
“The effect is much more dramatic on Venus because the atmosphere is about 93 times as massive as Earth’s, and therefore has a lot more momentum to exchange.”
Scientists also found that Venus tilts to one side precisely 2.6392 degrees (Earth is tilted about 23 degrees), an improvement over the accuracy of previous estimates by a factor of 10.
Repeated radar measurements further revealed the ice speed at which the orientation of Venus’ axis of rotation changes, much like a spinning child’s top.
On Earth, this precession takes about 26,000 years to cycle. Venus needs a little more time: around 29,000 years.
With these precise measurements of Venus’ rotation, the authors calculated that the planet’s core is about 3,500 km in diameter – quite similar to Earth – although they cannot yet deduce whether it is liquid or solid.
The results were published in the journal Nature astronomy.
J.-L. Margot et al. State of rotation and moment of inertia of Venus. Nat Astron, published online April 29, 2021; doi: 10.1038 / s41550-021-01339-7