A mysterious bow-shaped "gravity" cloud that hovers over mountains on Venus could speed up the planet's day by a couple of minutes, according to a study.

Key points: We don't know exactly how long it takes Venus to complete a single rotation.

We don't know exactly how long it takes Venus to complete a single rotation. New modelling shows fleeting formation of massive mountain waves in atmosphere may speed up rotation at surface

New modelling shows fleeting formation of massive mountain waves in atmosphere may speed up rotation at surface Understanding rotation rate can help us understand other mysteries such as what lies at Venus's core

The study, published today in Nature Geoscience, may help explain variations in the planet's rotation seen by different spacecraft over the years.

Venus spins at a leisurely pace, taking an average of 243 Earth days to complete one full rotation.

Measurements taken 16 Earth years apart by the Magellan and Venus Express missions differ by up to seven minutes, said Thomas Navarro, a planetary scientist at the University of California, Los Angeles.

"It is very difficult to measure the rotation rate because Venus rotates very slowly," Dr Navarro said.

"We have to average the displacement over a long period of time to get an estimate.

"We kind of know what the average duration of the day is, what we don't know very well is how the fluctuations of the rotation rate could make the day vary."

Slow planet vs super-fast atmosphere

Venus is also hard to study because it is shrouded in thick clouds of sulfuric acid.

Unlike the leisurely pace taken by the planet itself, these clouds rip right around Venus in just four to five Earth days.

"We do not understand much about why the atmosphere is super rotating," Dr Navarro said.

But not all the atmosphere appears to super rotate.

Recently, the Japanese Akatsuki mission detected a massive 10,000km-long bow-shaped wave that extended 65 kilometres into the atmosphere over the Aphrodite Terra region — an elevated plateau about the size of Africa at the planet's equator.

"It was very surprising to find this wave that was not moving with the super rotating atmosphere but instead it was moving with the surface," Dr Navarro said.

Modelling a mountain wave

Dr Navarro and his colleagues wanted to find out whether the wave, which is produced by the upward flow of winds hitting the mountains, could affect the planet's spin.

"You have winds blowing up the mountain in the same direction as the planet so it's going to transfer what we call angular momentum," he said.

"We need to take this into account if we want to understand what is causing this super rotation."

A solar day is the time it takes for a planet to return to the same point at noon. A solar day on Venus takes 116.75 Earth days.

Their models of atmospheric conditions and topography on Venus showed the wave appeared during the afternoon — the equivalent of about 30 Earth days — and disappeared by dusk.

They estimated the mountain waves could speed up the rate of rotation at the planet's surface by two minutes per solar day.

"Some of the angular momentum transferred to the solid body is going to speed up the planet, whereas the atmosphere is going to slow down," Dr Navarro said.

Winds pushing across the surface of mountains create gravity waves. ( Supplied: ESA )

Mountains are quite evenly spaced around the equator of Venus, so this is likely to be a common event, Dr Navarro said.

But, he added, more research is needed to pin down these figures even further.

What can the rotation rate tell us about Venus?

NASA atmospheric scientist Michael Way said mystery surrounded the differences in the rotation measurements by the Magellan and Venus Express missions.

Two types of day There are two types of day: sidereal and solar

There are two types of day: sidereal and solar A sidereal day is the time it takes for a planet to complete a full rotation in relation to background stars.

A is the time it takes for a planet to complete a full rotation in relation to background stars. A solar day is the time it takes for the Sun to appear in the same place in the sky.

A is the time it takes for the Sun to appear in the same place in the sky. On Earth a sidereal day is about 23 hours, 56 minutes and 4 seconds, and a solar day is about 24 hours.

On Earth a sidereal day is about 23 hours, 56 minutes and 4 seconds, and a solar day is about 24 hours. On Venus a sidereal day takes an average of 243 Earth days. A solar day takes an average of 116.75 Earth days.

On Venus a sidereal day takes an average of 243 Earth days. A solar day takes an average of 116.75 Earth days. Why the huge difference on Venus? It rotates in the opposite direction to its orbit around the Sun.

"Many thought that Magellan was right, and the other measurements were probably in error," said Dr Way from the Goddard Institute for Space Studies.

"Although some of us thought it might also be the dense atmosphere of Venus acting on the surface."

Dr Way, who was not involved in the current research, said the modelling by Dr Navarro and his colleagues was an "impressive achievement".

Not only did Dr Navarro's team replicate the waves seen by the Akatsuki mission, and smaller waves detected by the Venus Express mission, they were able to calculate the effect of the atmosphere on the topography of the planet, he said.

"So they have managed to kill two birds with one stone," Dr Way said.

"But at the same time they are right that we need more data … to better constrain the orbit history of Venus.

"This is important in understanding how Venus's rotation rate may have evolved through time."

Fluctuations in the length of the day are not unique to Venus. They also happen on Earth.

The motions of the atmosphere, oceans and ice constantly cause millisecond shifts in our 24-hour solar day, Dr Navarro said.

"It's not something you feel but is something that we are able to measure very accurately because we are on Earth," he said.

Being able to pin down the scale of the much larger fluctuations on Venus could help unlock more of our sister planet's secrets.

"Venus is the planet that is the closest to Earth in terms of size, but yet we don't know what [its core] is made of," Dr Navarro said.