Day rate: winds accelerate the rotation of Venus
The planet’s dense atmosphere and steep mountains combine to affect the planet’s speed. Richard A Lovett reports.
Strong winds blowing across the tops of mountains on Venus appear to be causing that planet’s rotation rate to speed up by as much as couple of minutes per Venusian “day,” scientists say.
“There is a torque from the atmosphere onto the solid body that speeds up the rotation rate of the solid body itself,” says Thomas Navarro, a planetary scientist at the University of California, Los Angeles, in the US.
The rotation rate of Venus is difficult to measure because of the planet’s constant cloud cover. Radar soundings from two orbital missions, NASA’s Magellan spacecraft (1990 to 2004) and the Europe’s Venus Express spacecraft (2006 to 2014) have produced intriguingly different results.
“We suspect that the rotation rate of Venus constantly varies,” Navarro says. The difference between the two measurements, he adds, was about 7 minutes each Venusian day.
One of the oddities of Venus is that that while its solid surface takes 243 Earth days to make a 360-degree revolution, its atmosphere is dominated by winds that blow much faster. The effect is mild at the surface but mounts with altitude, with the cloud tops seeing speeds in excess of 360 kilometres per hour — so fast they circle the entire planet every four Earth days.
When these winds hit a mountain range, Navarro says, they rise up and over it, just as winds do across Earth’s mountains. In the process, they push against the surface and change the solid planet’s spin.
On Earth, the effect on the length of the day is tiny and transient, changing with the weather. “[It’s] only a few milliseconds,” Navarro says. “It’s not something we can feel, but something we can measure.”
Furthermore, as winds rise up and over mountains on Earth, the resulting “mountain waves” break like ocean waves on a beach, creating turbulence that mitigates their impact. On Venus, however, the slow progression of the day allows them to build to enormous proportions, particularly in the planet’s long, slow afternoon.
Then, they can rise to altitudes of 70 kilometres and extend into huge, bow-shaped patterns over distances as long as 10,000 kilometres. “That’s almost as big as the planet,” Navarro notes.
These waves, he adds, have been seen from space by Japan’s Akatsuki spacecraft, which has been orbiting the planet since late 2015. But it is only now that Navarro and his colleagues attempted to model how airflow in the thick Venusian atmosphere interacts with mountain ranges that can reach heights of 5,000 to 6,000 metres, in the process affecting the planet’s spin.
Navarro adds that his work is only the beginning. Scientists don’t yet know why the Venusian atmosphere rotates faster than the planet itself — a process called super-rotation, which creates the conditions conducive to the giant mountain waves affecting its spin.
“We have many possible explanations,” he says, “but at the moment we do not understand exactly what causes the super-rotation.”
He notes, however, that this seems to be common among slow-rotating bodies. Saturn’s giant moon Titan, with days 16 times as long as Earth’s, also has a super-rotating atmosphere, he says.
Nor, he says, do scientists understand what keeps the Venusian atmosphere spinning so rapidly. In theory, wind speeds should slow down as they impart torque to the solid surface, until eventually atmosphere and planet are rotating at the same speed.
But since that’s obviously not happened, Navarro says, there must be countervailing processes at work, keeping the atmosphere circulating faster than the planet itself. Another question is whether the rotation rate of Venus is steadily accelerating or changing back and forth, in a tug-of-war between competing forces.
Lori Glaze, acting director of NASA’s Planetary Science Division, is intrigued.
“We know that Venus’s atmosphere is extremely dense, but it is amazing to think that the atmosphere could actually be contributing to the observed changing rotation rate of the solid planet,” she says.
What’s needed now, she adds, are new, long-term observations of the mountain-wave features observed by Akatsuki, as well as detailed observations of the planetary rotation rate and how it changes over time.
Navarro agrees that his finding supports the case for a new mission to Venus, as soon as possible. “Oh yes!” he laughs, when asked about the idea.
Such an expedition, he adds, would also help scientists figure out the nature of the Venusian core.
“We don’t know anything about the interior of Venus,” he says. “This is frustrating because Venus is the closet planet to Earth in terms of size, and yet we don’t know what the interior looks like.”
Navarro’s work is published in the journal Nature Geoscience.