Venus Spinning Slower Than Thought—Scientists Stumped

• In the early 1990s scientists with NASA's Magellan mission calculated that a single rotation of Venus takes 243.015 Earth days, based on the speed of surface features passing beneath the orbiting spacecraft.
• But scientists now mapping Venus's surface with the European Space Agency's Venus Express orbiter were surprised to find the same features up to 12.4 miles (20 kilometers) from where they were expected to be, based on the previous measurements.
• According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.
• Mysterious decrease could affect future exploration missions.
• One possible cause for the slowed spin is friction caused by Venus' thick atmosphere and high-speed winds. The motion of the atmosphere on Earth, for example, has been observed to affect the planet's rotation rate, albeit to a much smaller degree.
• Due to a heavy blanket of carbon dioxide-laden air, the surface pressure on Venus is 90 times what we experience on Earth at sea level, and opaque clouds of caustic sulphuric acid constantly whip around the planet at hurricane speeds.
• "The origin of this could lay in the solar cycle or in long-term weather patterns that modify the atmospheric dynamics. But this puzzle is not yet solved."