Last week the BepiColombo spacecraft swung by Earth for a boost on its journey to Mercury via Venus, bringing Solar System exploration firmly back in the spotlight. Sometimes astronomy news can seem dominated by distant and exotic galaxies, exoplanets, black holes, and gravitational waves, making it easy to overlook the many unknowns in our own back yard, the Solar System. We may have put landers on the surface of Mars, asteroids and even one of the moons of Saturn, Titan; nevertheless, our neighbours in space have all sorts of weirdnesses that we don’t yet understand.
For example, we all know that all the (known) planets orbit in the same direction and roughly the same plane around the sun, and they also rotate on their axis, generally more or less perpendicular to the plane of their orbit (within 30 degrees). But oddly, Uranus rotates almost on its side. And Venus not only rotates in the opposite directly to all the other planets, it also rotates so slowly that its year (the time it takes to go around the sun, 225 days) is shorter than its day (the time it takes to rotate once on its axis, around 243 days). How weird is that?
One of the more convincing theories attempting to explain Uranus’ extreme tilt is a massive impact with another space object, but hard observational evidence is hard to come by as only one spacecraft has visited Uranus to date, Voyager 2, and that was back in 1986. However, astronomers have many tools, including theoretical modelling which helps to envisage the likely physical results of an event such as a collision. Last month a team of Japanese astronomers reported on how they used modelling to show that the current configuration of Uranus and its moons and rings (also equally tilted) can be explained by a massive impact with another icy object, almost all of which has now vaporised, leaving Uranus’ faint rings and many small icy moons as the only remnants of the huge impactor.
So how about Venus? Why does it rotate backwards and so slowly? And why has its exact rotation rate proved so difficult to pin down – measurements show variation of around 7 minutes over the last few years. Research based on both observations and modelling may offer part of the answer. Venus has an extremely dense atmosphere with winds that blow constantly in one direction, so that the atmosphere moves around the planet much faster than the planet rotates, completing a circuit once every 4 days.
Simulations have shown that this difference in rotation rate between the atmosphere and solid surface, combined with the mountainous nature of the surface in places, could lead to the build-up of waves in the atmosphere above mountains, and a long-lived 10,000 km cloud-top wave has actually been observed on Venus. Because of the high density of the atmosphere, such waves could have a braking effect on the solid planet, which the simulation researchers estimate could account for over a quarter of the observed 7 minute variation. Interactions between the atmosphere and solid surface could possibly contribute to the slow rotation rate in other ways too, but the obscuring of surface features by the thick atmosphere makes research work more difficult.
As for Venus’ backwards spin direction, various theories have been proposed, including a massive impact that flipped it upside-down, gravitational effects of other bodies, and even a slowing down of rotation, perhaps due to the drag of the atmosphere, so much that it eventually started rotating in the opposite direction. It’s still an open question.
Going back to BepiColumbo, this joint ESA/JAXA mission will be flying past Venus twice (in October 2020 and August 2021) on its way to Mercury. These flybys will be unique opportunities to learn more about the atmosphere of Venus by working in conjunction with the Japanese spacecraft Akatsuki, which has been orbiting the planet since 2015, maybe bringing us one step further towards understanding some of the unsolved mysteries of the Solar System.
1. NASA/Erich Karkoschka (Univ. Arizona) https://www.jpl.nasa.gov/images/voyager/20161021/UranusHST20161021.jpg
2. NASA/JPL https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA00254
3. ISAS/JAXA http://akatsuki.isas.jaxa.jp/en/gallery/spacecraft/