Fifteen Years of Radar Reveal Venus’s Most Basic Facts

Source: By Kimberly M. S. Cartier, Eos/AGU

Excerpt: Venus’s heavy atmosphere tugs the planet’s surface enough to change the length of its day by up to 21 minutes [per day]. ...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 70-meter radio antenna at the Goldstone Deep Space Communications Complex in California served as the flashlight, ...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. ...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 missionto Venus, which ended in 1994.... ...They found that the 15-year average length of day on Venus is 243.0226 Earth days (243 days, 32 minutes, and 30 seconds). ...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.) ...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....

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