ESA’s Venus Express spacecraft has discovered that Venus spins a little slower than previously measured.
According to the European Space Agency, a team of researchers used the VIRTIS instrument on-board the orbiter to penetrate the cloud cover over our neighbor at infrared wavelengths.
They studied surface features and discovered that some were displaced by up to 20 km from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s. The discovery appears in the February issue of the journal Icarus.
In the 1980s and 1990s, the Venera and Magellan spacecrafts made radar maps of the surface of Venus, which gave us the first detailed global view of this unique and hostile world.
Magellan’s data allowed researchers to determine the length of the day on Venus as being equal to 243.0185 Earth days.
However, surface features seen by Venus Express some 16 years later could only be lined up with those observed by Magellan if the length of the Venus day is 243.0230 Earth days, that is about 6.5 min longer than Magellan measured.
This also agrees with the most recent long-duration radar measurements from Earth.
“When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of,” said Dr. Nils Müller, a lead author on the paper and a planetary scientist at the DLR German Aerospace Center.
Researchers looked at the possibility of short-term random variations in the length of a Venus day, but concluded these should average themselves out over longer timescales.
On the other hand, recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period. Other effects could also be at work, including exchanges of angular momentum between Venus and the Earth when the two planets are relatively close to each other.
“An accurate value for Venus’ rotation rate will help in planning future missions, because precise information will be needed to select potential landing sites,” concluded Dr. Håkan Svedhem, ESA’s Venus Express project scientist.