Scientists mapping Venus's surface with the European Space Agency's Venus Express orbiter recently received a shock when features on the planet's surface appeared to have moved up to 12.4 miles from where they were expected to be, reports National Geographic.

The measurements, if correct, would seem to indicate that Venus' rotation has slowed by 6.5 minutes — a dramatic decrease on a planetary level — compared to when it was last measured just 16 years ago.

That last measurement was taken during NASA's Magellan mission in the 1990s, when a single rotation of Venus was calculated to take 243.015 Earth days. Magellan used the passing speed of surface features on the planet to make its calculation, and scientists have long held that measurement as the standard.

"When the two maps did not align, I first thought there was a mistake in my calculations, as Magellan measured the value [of Venus's spin] very accurately," said planetary scientist Nils Müller. "But we have checked every possible error we could think of."

This leaves a rather large question: What could possibly be causing a planet's spin to decelerate so rapidly? Since Venus is also Earth's closest neighbor, should we be worried?

Interestingly, the Earth's rotation is also slowing down, but scientists attribute this to tidal acceleration, frictional "drag" caused by the Moon's gravitational pull. This explanation cannot apply to Venus' slowing spin, however, because Venus has no moon of its own.

Some scientists have speculated that Venus' thick atmosphere and high-speed winds could be to blame. The planet's murky carbon dioxide-rich atmosphere gives it a surface pressure 90 times that of Earth's. This fact, combined with the hurricane-like speeds of the winds around the planet, could possibly generate enough friction to slow down Venus' rotation.

Other scientists are skeptical. While a planet's atmosphere has been proven to effect its rotation before, these effects are minimal compared to the degree of slowing that has been witnessed for Venus.

"It is difficult to find a mechanism that will cause the average rotation rate to change this much in only 16 years," said Venus Express project scientist Håkan Svedhem. "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."

Venus' slowing spin isn't the only peculiar thing about its rotation. Venus is unique in our solar system for being the only planet that spins clockwise; all the other planets spin counter-clockwise. This effect, called "retrograde" rotation, is another mystery about Venus that has yet to be adequately solved. Venus' rotation is also by far the slowest in the solar system, which makes the rapid deceleration of its spin especially curious. So far, though, no theory exists that links these other peculiar facts to the planet's decelerating spin.

Whatever is causing Venus' pirouette to dawdle, scientists will need to adjust their measurements before any new space missions are planned for the rocky planet. Without precise measurements, future probes could end up landing somewhere completely different than anticipated.