WASHINGTON :An exhaustive examination of lunar gravity using data obtained by two NASA robotic spacecraft is offering new clues about why the two sides of the moon - the one perpetually facing Earth and the other always facing away - look so different. The data from the U.S. space agency's GRAIL, or Gravity Recovery and Interior Laboratory, mission indicates that the moon's deep interior has an asymmetrical structure, apparently caused by intense volcanism on its nearside billions of years ago