'Pie crust' craters observed on Mercury

Several strange crater "pies" with wrinkled "crusts" and cracked "fillings" have been spotted on the planet Mercury by a NASA probe, the space agency says. NASA said the pies found in Mercury's northern highlands are unlike anything seen on other rocky worlds and add to the diversity of geologic processes discovered in the solar system, NewScientist.com reported Thursday. The odd craters spotted by NASA's Messenger spacecraft are located in a region of Mercury flooded repeatedly with volcanic eruptions early in the planet's history, astronomers said. These floods buried underlying impact craters, but as the lava cooled, the crater rims became visible as the material above them wrinkled and split, forming the edges of the pie crust. The interiors of the craters are criss-crossed with cracks called graben, which form when rock is stretched horizontally, and scientists said the cracks bring to mind the fissures that sometimes form in a pumpkin pie when it cools. "I haven't really done the tectonics of the pie," Tom Watters of the Smithsonian Institution said. "But there are a number of similarities." "We've never seen anything like that on Mercury [before], and we've never really seen anything like that on any of the other terrestrial planets," Watters said. "What is happening here? Something definitely different is going on on Mercury that hasn't happened on the moon or Mars."