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Why Venus Rotates, Slowly, Despite Sun’s Powerful Gravitational Pull

A sequence of images from Solar Dynamic Observatory in 171 angstroms of the Venus transit, merged together to show the path of Venus across the sun. Credit: NASA/SDO

The planet’s atmosphere explains the gravity of the situation.

VenusVenus, the second planet from the sun, is named after the Roman goddess of love and beauty. After the moon, it is the second-brightest natural object in the night sky. Its rotation (243 Earth days) takes longer than its orbit of the Sun (224.7 Earth days). It is sometimes called Earth's "sister planet" because of their similar composition, size, mass, and proximity to the Sun. It has no natural satellites.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Venus, Earth’s sister planet would likely not rotate, if not for its soupy, fast-moving atmosphere. Instead, Venus would be fixed in place, always facing the sun the way the same side of the moon always faces Earth.

The gravity of a large object in space can keep a smaller object from spinning, a phenomenon called tidal locking (also known as gravitational locking and captured rotation). Because it prevents this locking, a University of California, Riverside (UCR) astrophysicist argues the atmosphere needs to be a more prominent factor in studies of Venus as well as other planets.


These arguments, as well as descriptions of Venus as a partially tidally locked planet, were published on April 22, 2022, in the journal Nature Astronomy.

Bright Venus is seen near the crescent moon. Credit: NASA/Bill Dunford

“We think of the atmosphere as a thin, almost separate layer on top of a planet that has minimal interaction with the solid planet,” said Stephen Kane, UCR astrophysicist and lead paper author. “Venus’ powerful atmosphere teaches us that it’s a much more integrated part of the planet that affects absolutely everything, even how fast the planet rotates.”

Venus takes 243 Earth days to rotate one time, but its atmosphere circulates the planet every four days. Extremely fast winds cause the atmosphere to drag along the surface of the planet as it circulates, slowing its rotation while also loosening the grip of the sun’s gravity.

Slow rotation in turn has dramatic consequences for the sweltering Venusian climate, with average temperatures of up to 900 degrees FahrenheitThe Fahrenheit scale is a temperature scale, named after the German physicist Daniel Gabriel Fahrenheit and based on one he proposed in 1724. In the Fahrenheit temperature scale, the freezing point of water freezes is 32 °F and water boils at 212 °F, a 180 °F separation, as defined at sea level and standard atmospheric pressure. ” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Fahrenheit — hot enough to melt lead.

“It’s incredibly alien, a wildly different experience than being on Earth,” Kane said. “Standing on the surface of Venus would be like standing at the bottom of a very hot ocean. You couldn’t breathe on it.”

Image of Venus acquired by the Akatsuki mission, the first Japanese probe to enter orbit around a planet other than the Earth. Credit: ISAS/JAXA

One reason for the heat is that nearly all of the sun’s energy absorbed by the planet is soaked up by Venus’ atmosphere, never reaching the surface. This means that a rover with solar panels like the one NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It's vision is "To discover and expand knowledge for the benefit of humanity."” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>NASA sent to MarsMars is the second smallest planet in our solar system and the fourth planet from the sun. Iron oxide is prevalent in Mars' surface resulting in its reddish color and its nickname "The Red Planet." Mars' name comes from the Roman god of war.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Mars wouldn’t work.

The Venusian atmosphere also blocks the sun’s energy from leaving the planet, preventing cooling or liquid water on its surface, a state known as a runaway greenhouse effect.

It is unclear whether being partially tidally locked contributes to this runaway greenhouse state, a condition that ultimately renders a planet uninhabitable by life as we know it.

Not only is it important to gain clarity on this question to understand Venus, but it is also important for studying the exoplanets likely to be targeted for future NASA missions.

Most of the planets likely to be observed with the recently launched James Webb Space TelescopeThe James Webb Space Telescope (JWST or Webb) is an orbiting infrared observatory that will complement and extend the discoveries of the Hubble Space Telescope. It covers longer wavelengths of light, with greatly improved sensitivity, allowing it to see inside dust clouds where stars and planetary systems are forming today as well as looking further back in time to observe the first galaxies that formed in the early universe.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>James Webb Space Telescope are very close to their stars, even closer than Venus is to the sun. Therefore, they’re also likely to be tidally locked.

Since humans may never be able to visit exoplanets in person, making sure computer models account for the effects of tidal locking is critical. “Venus is our opportunity to get these models correct, so we can properly understand the surface environments of planets around other stars,” Kane said.

“We aren’t doing a good job of considering this right now. We’re mostly using Earth-type models to interpret the properties of exoplanets. Venus is waving both arms around saying, ‘look over here!’”

Gaining clarity about the factors that contributed to a runaway greenhouse state on Venus, Earth’s closest planetary neighbor, can also help improve models of what could one day happen to Earth’s climate.

“Ultimately, my motivation in studying Venus is to better understand the Earth,” Kane said.

Reference: “Atmospheric dynamics of a near tidally locked Earth-sized planet” by Stephen R. Kane, 22 April 2022, Nature Astronomy.
DOI: 10.1038/s41550-022-01626-x

Source: SciTechDaily