Venus is an odd planet. It may once have supported liquid water and even a habitable environment, but today it rains acid and is hot enough to melt lead.
One of the most bizarre things about Venus though is its rotation. It takes the planet about 243 Earth days to complete a rotation (and 225 days to orbit the Sun), but its atmosphere is much faster, completing a rotation in just four Earth days. This is known as super-rotation.
Attempts to measure the exact rotation period of the planet, however, have been difficult. Different spacecraft have all come up with slightly different answers, on the order of a few minutes. NASA’s Magellan mission (1989 to 1994) and ESA’s Venus Express mission (2005 to 2015), for example, measured a difference of seven minutes.
Now we might have an answer. Reporting in the journal Nature Geoscience, researchers led by Thomas Navarro from the University of California, Los Angeles suggest that the thick Venusian atmosphere might quite literally be tugging the planet, causing it to speed up and slow down by about two minutes every day on Venus.
“This interplay between the solid planet and atmosphere may explain some of the difference in rotation rates measured by spacecraft over the past 40 years,” the team wrote in their paper.
To come to this conclusion, they used data from the Japanese Akatsuki spacecraft, which is currently in orbit around Venus. Last year, that spacecraft discovered a vast stationary wave on the planet, spanning about 10,000 kilometers (6,200 miles).
That wave is thought to be the result of air flowing over a mountain, moving from high to low temperature. At the top of the mountain, air on both sides balances out and produces this huge stationary structure, which is known as a gravity wave.
Navarro and his team simulated the circulation of the Venusian atmosphere, and found that this structure likely formed in the afternoons and vanished by dusk – which, remember, is an extremely long time on Venus. And the waves were found to cause fluctuations in the atmosphere of the planet, which change how fast the planet is spinning.
“There is a torque from the atmosphere onto the solid body that speeds up the rotation rate of the solid body itself,” Navarro told Cosmos Magazine. “We suspect that the rotation rate of Venus constantly varies.”
This, therefore, could explain why it’s been so hard to nail down the rotation of Venus – because it’s constantly changing very slightly by a matter of minutes. It is only a simulation though, so we'll need actual data to find out for sure.
Up next will be working out why the atmosphere of Venus rotates so much faster than the planet. And if the winds are pulling the planet, how are they able to maintain their speed? It's a good reminder, if one were needed, that we need to focus more on Venus.