The length of a day on Venus is always changing

Nearly full view of Venus covered in white and salmon clouds.
The thick cloud cover of Venus – which is impenetrable in visible light – has made it tough for astronomers to measure the length of the planet’s day. A new study at radio wavelengths may have some answers. Image via NASA/ JPL-Caltech.

Scientists said that they’ve finally answered the question of the length of a “day” on Venus. And the answer is … it’s always changing, by as much as 20 minutes! We knew that Venus had an exceedingly long day. A day on Venus – a single spin of the planet on its axis – is equal to approximately 243 Earth-days. What’s new is that the length of a Venus-day doesn’t stay fixed. That is, the spin of this neighboring planet regularly speeds up and slows down, by an amount measured in only minutes on earthly clocks, as the planet’s thick atmosphere interacts with its topography, or surface features.

This same study also unveiled the precise tilt of Venus’ axis and the size of its core.

The team of scientists led by Jean-Luc Margot of UCLA announced their findings in a new study published April 29, in the peer-reviewed journal Nature Astronomy. Margot explained how basic information about Venus is hidden behind its thick atmosphere:

Venus is our sister planet, and yet these fundamental properties have remained unknown.

Man with dark hair in suit and red tie.
Jean-Luc Margot lead the team that made precise measurements of Venus’ spin rate and axial tilt. Image via Jean-Luc Margot/ UCLA.

We said the day on Venus is approximately 243 Earth-days. A more precise number is 243.0226 Earth-days. That’s the same as 5832.5424 Earth-hours. If you’re someone who feels like there are never enough hours in a day, maybe Venus is for you? Just be aware that it’s hot enough on the surface of this world to melt led. Still, Venus’ day is a curiosity. In fact, the Venus-day is so long that its year is shorter, at 225 Earth-days. Its day is longer than its year! Let that rattle around in your mind a little …

As mentioned above, the inconstant timing of a day on Venus is the fault of its incredibly thick atmosphere. The atmosphere of Venus – which is largely carbon dioxide – is about 93 times as massive Earth’s, and therefore it presses down onto the planet’s surface with greater force. The surface pressure on Venus may be up to 100 times greater than on Earth. When Venus’ thick atmosphere – with its winds that can reach 224 miles (360 km) an hour – interact with solid ground, momentum is exchanged, speeding up and slowing down the planet’s rotation. Earth has the same interplay between atmosphere and ground, but its exchange only adds or subtracts a millisecond from each day.

Being able to measure a precise length of a day on Venus is important for any future exploration on the planet, because without accurate measurements on the planet’s movements, landings could be off by as much as 18.6 miles (30 km)!

In this new study, the scientists also acquired precise measurements of Venus’ axis and core. Venus is almost completely upright, with a tilt in its axis of just 2.6392 degrees. Contrast that slight tilt to Earth’s larger tilt of 23 degrees, and you’ll understand why Venus has no seasons. It takes Venus an extremely long time to complete one circuit in the tiny wobble on its axis: 29,000 years. Just as on Earth, this wobble in Venus’ axis is known as precession.

Sphere with arrow emerging at poles and red, blue, and green mottlings.
This topographic map of Venus shows the spin axis of the planet (yellow arrow), which is less than 3 degrees, meaning Venus spins nearly upright with only the smallest wobble. Image via Jean-Luc Margot/ UCLA/ NASA.

Once the scientists had the radio data on Venus’ spin, they were able to calculate the size of the planet’s core. They measure the core at 2,175 miles (3,500 km) in diameter, basically the same size as Earth’s core.

But how did the scientists come up with all these new measurements for Venus? They used radio waves, that they bounced off Venus over the course of 15 years (radar). Using the 70-meter–wide Goldstone antenna in California’s Mojave Desert, the scientists sent photons in the form of radio waves to Venus, where they penetrated the thick atmosphere, bounced off the ground, and then returned to Earth. Both the Goldstone telescope and the Green Bank Observatory in West Virginia picked up their return signals. Margot explained, likening the planet to a disco ball:

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 the spin.

The delay between the return signals at Goldstone and Green Bank helped the scientists estimate Venus’ spin rate, and the window of time in which the echoes from Venus are most similar revealed the planet’s tilt.

This process of bouncing photons off Venus and interpreting the return signal can be (and has been) used on other worlds in the solar system. The team want to target Jupiter’s moons Europa and Ganymede to learn some of their distant secrets as well, including more about the ocean that may hiding under a shell of ice on Europa.

Bottom line: Scientists used radio waves to precisely measure Venus’ spin rate and the tilt of its axis. Their findings revealed a core similar in size to Earth’s and a day that regularly changes in length.

Source: Spin state and moment of inertia of Venus


May 5, 2021

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