Russia-Led Mission Might Shed Light On Venus' Odd Clockwise Rotation

Tweet This
Credit: NASA

As bizarre as Venus appears today --- a hellish cloud-cloaked world of extraordinary surface pressures and surface temperatures not fit for man or beast --- it is also one of only two full-sized planets in our solar system that rotates in a clockwise (or retrograde) manner. And it does so at a very slow rate; taking 243 Earth days to make one complete rotation.

With the exception of Venus and Uranus, why do planets in our solar system rotate counterclockwise?

Planet formation models show that most planets will end up spinning in a prograde (or counter-clockwise) direction. That’s largely due to how the planets gravitationally interacted during the earliest days of a forming solar system. The idea is that a protoplanetary disk’s direction of rotation around its protostar likely dictated that planets forming inside such disks also rotated in the same direction.

As to why would Venus rotate clockwise? Here are a couple of ideas.

It may have been hit early on by a large impactor.

It could be that a large impactor hit the planet in the late stages of its accretion and that either flipped the axis of the planet (as was probably the case for Uranus) or caused it to change its direction of rotation, Michael Way, a planetary scientist at The NASA Goddard Institute for Space Studies in New York City, told me.

Many believe that if Venus was struck by a moon-like impactor then its moon should still be around, but clearly it’s not, says Way. So, one idea is that a second moon-like object came along and knocked the original Venusian moon out of its orbit. The two moons would have then gone sailing off into the Sun, he says.

“It’s rather unlikely, but not impossible,” said Way.

Nobody has really done the heavy lifting to find out, he says because it is computationally expensive and there’s a dearth of Venus data .

Venus may have started out with prograde rotation, but then its spin axis may have flipped at some point in its history due to what is termed ‘core-mantle friction’.

Core mantle friction, literally the friction between a planet’s interior core and mantle, can slow its spin rate and even affect its obliquity (or axial tilt). On Venus, such friction may have actually flipped the planet’s tilt by 180 degrees; keeping it there for perpetuity.

But whether Venus has ever experienced such core-mantle friction remains unknown.

As we find more and more exoplanets we will eventually start to collect statistics on their obliquities and rotation rates, says Way. Within the next decade or two, hopefully, that will enable researchers to produce some statistically meaningful hypotheses, he says.

Way says the fact that Venus is spinning slowly is perhaps more important than whether it spins retrograde or prograde. That’s because the rate of rotation may play a key role in the climates of planets that receive higher amounts of their parent stars’ radiation.

However, Way says the only mission in the foreseeable future that might help resolve this rotation mystery is the proposed Russian-led Venera-D mission which has been four years in planning and already has a joint (NASA/Roscosmos) Science Definition Team (JSDT). Way will be in Moscow in October to present his ideas on potential Venera-D science goals and landing sites. The mission would likely touch down on one of Venus’ large volcanic plains.

“Thus far, NASA has only contributed funds for studying the mission and NASA's possible contribution to it,” said Way. But he says NASA may contribute an instrument to the mission ; perhaps a balloon-borne experiment or a surface station.

Credit: Wikipedia

If Russia funds Venera-D and it sees launch within the next decade, the mission would try to assess Venus’ past and present habitability using an orbiter, a lander and one or more surface stations that may include seismic instruments. The idea is that the surface stations could survive surface pressures 93 times that of Earth and 864 degrees F. temperatures for perhaps as long as a couple of weeks.

If Venera-D were to find strange isotopic anomalies that can only be explained by an impactor, that might help solve Venus’ rotation mystery, says Way.

How much does rotation direction really matter?

Computer modeling of Earth’s own atmosphere shows that imposing a retrograde rotation on our own present-day planet would likely lead to large shifts in continental climates and in patterns of precipitation, the authors of a 2018 paper in the Journal of Earth System Dynamics note.

Would such large shifts in continental climate have made life more or less difficult to arise on ancient Earth?

We don’t know, says Way; we don’t even know when and where life began on Earth.

As for when Venus’ own rotation mystery might actually be solved?

Probably not until we have rovers on Venus with the same capabilities as the best ones currently running around Mars, says Way.

“We are probably decades [away] from having that sort of capability,” he said.

Follow me on Twitter or LinkedIn. Check out my website.
