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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 to a high 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 an entire planet's spin to decelerate so rapidly? Since Venus is also Earth's closest neighbor, should we be worried?   ... >>>