Puzzle
Provident
NASA
Mercury continue to bewilder NASA despite Messenger mission updates about the planet
Mercury may not be stealing the spotlight like Mars so often does, but the planet is as intriguing as the Red Planet. The surface of Mercury is marked by mysterious hollows, which have puzzled scientists for decades. These depressions vary in size from 60 feet to over a mile across and can be up to 120 feet deep, with no known origin. The planet's lack of atmosphere means that wind and water couldn't have created these hollows, leaving scientists searching for alternative explanations. And this is despite the fact that NASA had sent its Mercury Messenger spacecraft to study the planet thoroughly.