Planetary scientists agree that a Venusian day is a little longer than 243 Earth days, but for decades their measurements have disagreed about the additional number of minutes and seconds. What’s more, those measurements haven’t been precise enough to infer other fundamental properties of Venus, like how the length of a day changes, the tilt of the planet’s spin relative to its orbital plane, and the precession of that tilt. Without these precise values, a craft attempting to land on Venus might miss its target zone by as much as 30 kilometers.
In a recent paper in Nature Astronomy, astronomers used 15 years of radar measurements to reveal a few of these fundamental properties of our closest planetary neighbor that have long remained elusive. The team also explained why these properties have been so hard to pin down before. To do this study, the team repeatedly pinged Venus’s surface with bursts of transmitted radio waves and listened for the reflected waves’ return.
“We use Venus as a giant disco ball,” lead author Jean-Luc Margot, an astronomer at the University of California, Los Angeles, said in a statement. The 70-meter radio antenna at the Goldstone Deep Space Communications Complex in California served as the flashlight, and Venus’s surface worked like millions of tiny reflectors. “We illuminate it with an extremely powerful flashlight—about 100,000 times brighter than your typical flashlight. And if we track the reflections from the disco ball, we can infer properties about [its] spin.”
The Answer Is Blowing in the Wind
The researchers carefully measured the timing of the returned waves with two radio telescopes: Goldstone in California and the Green Bank Telescope in West Virginia. Each reflection yielded a measurement of Venus’s rotation speed and spin axis tilt at the moment of reflection. With these measurements they found that Venus’s spin axis is tilted 2.6392° from its orbital plane and that tilt precesses once every 29,000 Earth years, 3,000 years longer than Earth’s precession. These measurements are 5–15 times more precise than what was achieved by NASA’s Magellan mission to Venus, which ended in 1994 and had provided the most precise radar measurements prior to this study.
The tilt of a planet’s spin axis (black arrow) relative to its orbital plane (gray grid) changes direction over thousands of years like a slowly wobbling top. On Venus this precession (blue circle) takes 29,000 years, and its tilt is more upright than Earth’s. Credit: NASA/JPL-Caltech
They found that the 15-year average length of day on Venus is 243.0226 Earth days (243 days, 32 minutes, and 30 seconds). What’s more, the team discovered that their individual measurements of the length of a Venusian day varied by about 3 minutes when measured on consecutive Earth days and up to 21 minutes over the 15-year study period. (Earth’s length of day varies by about 4 milliseconds averaged over 20 years.) “That probably explains why previous estimates didn’t agree with one another,” Margot said.
The momentum needed to change Venus’s rotation speed by such a large amount can only be provided by the planet’s thick atmosphere. Momentum transferred from Earth’s atmosphere can lurch the length of a day by a few milliseconds at most, but Venus’s atmosphere has 100 times more mass than Earth’s does and 180 times the momentum. The viscous atmosphere rotates faster than the rest of the planet does, and as it sloshes around, it exchanges momentum with the surface below and changes the surface’s rotation speed.
By continuing to study how the exchange of momentum between the atmosphere and the surface changes the planet’s rotation speed, planetary scientists will gain a better understanding of the interior structure of Venus, such as whether it has a liquid or solid core and how the planet formed and evolved.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
1 June 2021: The effect of Earth’s atmospheric momentum has been corrected.
Cartier, K. M. S. (2021), Fifteen years of radar reveal Venus’s most basic facts, Eos, 102, https://doi.org/10.1029/2021EO159150. Published on 01 June 2021.
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