Astronomers using Earth-based 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.
This composite image, taken by JAXA’s Akatsuki spacecraft, shows Venus. Image credit: JAXA / ISAS / DARTS / Damia Bouic.
“Venus is Earth’s nearest planetary neighbor and closest analogue in the Solar System in terms of mass, radius and density,” said Professor Jean-Luc Margot from the University of California, Los Angeles and his colleagues.
“However, Venus remains enigmatic on a variety of fundamental levels: the size of its core is unknown, whether the core is solid or liquid is uncertain and estimates of its average spin period are discordant.”
“Venus is also distinctive because of its 243-day retrograde rotation and 4-day atmospheric superrotation, neither of which is fully understood.”
On 21 separate occasions from 2006 to 2020, the astronomers aimed radio waves at Venus from the 70-m-wide Goldstone antenna in California’s Mojave Desert.
Several minutes later, those radio waves bounced off Venus and came back to Earth. The radio echo was picked up at Goldstone and at the Green Bank Observatory in West Virginia.
The radar measurements show that an average day on Venus lasts 243.0226 Earth days — roughly two-thirds of an Earth year.
What’s more, the rotation rate of the planet is always changing: a value measured at one time will be a bit larger or smaller than a previous value.
The researchers estimated the length of a day from each of the individual measurements, and they observed differences of at least 20 min.
“That probably explains why previous estimates didn’t agree with one another,” Professor Margot said.
“Venus’ heavy atmosphere is likely to blame for the variation. As it sloshes around the planet, it exchanges a lot of momentum with the solid ground, speeding up and slowing down its rotation.”
“This happens on Earth too, but the exchange adds or subtracts just one millisecond from each day.”
“The effect is much more dramatic on Venus because the atmosphere is roughly 93 times as massive as Earth’s, and so it has a lot more momentum to trade.”
The scientists also found that Venus tips to one side by precisely 2.6392 degrees (Earth is tilted by about 23 degrees), an improvement on the precision of previous estimates by a factor of 10.
The repeated radar measurements further revealed the glacial rate at which the orientation of Venus’ spin axis changes, much like a spinning child’s top.
On Earth, this precession takes about 26,000 years to cycle around once. Venus needs a little longer: about 29,000 years.
With these exacting measurements of how Venus spins, the authors calculated that the planet’s core is about 3,500 km across — quite similar to Earth — though they cannot yet deduce whether it’s liquid or solid.
The results were published in the journal Nature Astronomy.
J.-L. Margot et al. Spin state and moment of inertia of Venus. Nat Astron, published online April 29, 2021; doi: 10.1038/s41550-021-01339-7
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