For over 50 years, humans have tried to pull back the thick clouds of Venus to study its surface. Most observations have tapped the long wavelengths of radar to pierce the cloud layers. Now, scientists have combined long-term radar observations of the planet’s surface to try to clarify a longstanding mystery behind the length of Venus' day. On Earth, it's easy to measure how long a day lasts. You simply pick a feature and track how long it takes for the Sun to return to the same position. Venus isn’t so simple. On Venus, a day lasts about 243 Earth-days. That's longer than it takes the planet to complete an orbit around the Sun. So, a Venusian year actually spans just 225 Earth-days. The clouds make things even more challenging. They cover the surface and make it difficult to locate features. Still, thanks to radar and spacecraft like NASA's Magellan mission, which spent four years visiting Venus in the early 1990s, astronomers thought they had a handle on the length of a Venus day.But when the European Space Agency's Venus Express mission returned to the world, a team of scientists found something odd. In research published in 2012, they showed that the average rotation period between 1991 and 2007 was shorter than the average rotation period measured by Magellan between 1991 and 1993.
The mystery makes it challenging to understand what's happening on the planet's surface, information that can help explain why a world that started out so much like Earth turned into an extreme hotbox. The lack of knowledge could also make it difficult to send a lander to Venus even though that goal has recently gained significant scientific support.
Now, researchers have combined almost 30 years of Earth-based observations of Venus to calculate an average rotation rate for the planet. The new average is slower than the average calculated by Magellan over a significantly shorter observation period of 487 days.
