Venus is a mystery. This is the neighboring planet, and yet it reveals little about itself. An opaque blanket of clouds suffocates the harsh landscape shed acid rain and baked at temperatures that can liquefy lead.
New observations of Earth’s safety are now raising the curtain on some of Venus’ most basic properties. By repeatedly bouncing the radar off the planet’s surface over the past 15 years, a UCLA-led team determined the exact length of Venus’ day, the tilt of the axis and the size of the core. The findings are published in the journal Natural astronomy.
“Venus is our sister planet, and yet these basic properties remain unknown,” said Jean-Luc Margot, a professor of Earth, Planetary and Space Science at the University of California who led the study.
Earth and Venus have a lot in common: Both rocky planets have almost the same size, mass and density. Yet they have evolved wildly in different ways. Basics such as how many hours are on a Venus day provide critical data for understanding the different stories of these neighboring worlds.
Changes in the rotation and orientation of Venus reveal how the mass is distributed inside. Knowledge of its internal structure in turn nurtures insight into the formation of the planet, its volcanic history and how time has changed the surface. In addition, without accurate data on how the planet is moving, all future landing attempts could be ruled out by up to 30 kilometers.
“Without those measurements,” Margo said, “we’re essentially flying blind.”
New radar measurements show that the average day on Venus lasts 243.0226 Earth days – approximately two-thirds of the Earth year. Moreover, the speed of rotation of Venus always changes: The value measured at the same time will be slightly higher or lower than the previous value. The team calculated the length of the day from each of the individual measurements and observed differences of at least 20 minutes.
“This probably explains why previous assessments disagreed,” Margo said.
The heavy atmosphere of Venus is probably to blame for the variation. As it unravels around the planet, it exchanges a lot of inertia with the solid earth, accelerating and decelerating its rotation. This also happens on Earth, but the exchange adds or subtracts only one millisecond of each day. The effect is much more dramatic for Venus, as the atmosphere is approximately 93 times more massive than that of Earth and therefore has much more momentum to trade.
The UCLA-led team also reported that Venus tilted to one side by exactly 2.6392 degrees (the Earth tilted by about 23 degrees), improving the accuracy of previous estimates by a factor of 10. Repeated radar measurements further revealed the glacial velocity at which the orientation the axis of rotation of Venus changes, like the rotating child’s tip. On Earth, this “precession” takes about 26,000 years to revolve about once. Venus needs a little more time: about 29,000 years.
With these rigorous measurements of how Venus rotates, the team calculated that the planet’s core is about 3,500 kilometers long – much like Earth – although they still can’t determine if it’s liquid or solid.
Venus like a giant disco ball
In 21 separate cases from 2006 to 2020, Margo and colleagues transmitted radio waves to Venus from the 70-meter-wide Goldstone antenna in the Mojave Desert in California. A few minutes later, these radio waves bounced off Venus and returned to Earth. The echo of the radio was spotted at Goldstone and at the Green Bank Observatory in West Virginia.
“We’re using Venus as a giant disco ball,” Margo said, with the radio acting like a flashlight and the planet’s landscape acting like millions of small reflectors. “We illuminate it with an extremely powerful flashlight – about 100,000 times brighter than your typical flashlight. And if we trace the reflections from the disco ball, we can conclude about the properties of the rotation [state]. “
Complex reflections chaotically illuminate and obscure the return signal that envelops the Earth. The Goldstone antenna sees the echo first, and then Green Bank sees it approximately 20 seconds later. The exact delay between receipts in the two facilities gives a snapshot of how fast Venus rotates, while the specific time window in which the echo is most similar reveals the planet’s tilt.
Observations require time to ensure that Venus and Earth are correctly positioned. Both observatories had to work perfectly – which was not always the case. “We’ve found that it’s actually a challenge to make everything work in exactly 30 seconds,” Margo said. “In most cases, we get some data. But it is unusual that we get all the data we hope to get. “
Despite the challenges, the team moved forward and focused Jupiterthe moons Europe and Ganymede. Many researchers strongly suspect that Europe in particular is hiding an ocean of liquid water under a thick ice sheet. Ground-based radar measurements could strengthen the ocean case and reveal the thickness of the ice sheet.
And the team will continue to repel the radar from Venus. With each radio echo, the curtain over Venus raises a little more, bringing our sister planet to a sharper view.
Reference: “Spin state and moment of inertia of Venus” by Jean-Luc Margot, Donald B. Campbell, John D. Georgini, Joseph S. Zhao, Lawrence G. Snedecker, Frank D. Gigo and Amber Bonsal, April 29, 2021 ., Natural astronomy.
DOI: 10.1038 / s41550-021-01339-7
This study was supported by NASA, The Jet Propulsion Laboratory and the National Science Foundation.
Other researchers who have contributed to the study include Donald Campbell of Cornell University; John Georgini, Joseph Zhao and Lawrence Snedecker of the Jet Propulsion Laboratory; and Frank Gigo and Amber Bonsal of the National Radio Astronomical Observatory in West Virginia.