The Earth may once have been surrounded by rings, like Saturn, which would have cast a shadow on parts of the planet all day long. The rings could have been formed after a glancing blow from an asteroid, when a space rock carves debris from a planet, then bounces off into the atmosphere. The resulting debris would also have shot off into space, and some of it would have ended up orbiting the Earth, forming a ring.
Peter Fawcett of the University of New Mexico and Mark Boslough of Sandia Labs say the debris ring would have cooled the planet by blocking or reducing the amount of sunlight it received in the tropics and subtropics. The rest of the planet would cool down too, because less heat would be transported from tropical regions to higher latitudes.