VOLCANIC TERRAIN During a 2009 flyby, Messenger spotted Rachmaninoff, a
170-mile-wide, double-ringed impact basin. Now the probe will take a closer view, hopefully finding clues to the
violent volcanic eruptions that apparently flooded the basin with lava, creating the smooth inner plains less than 2 billion years ago.
IRON CORE Mercury’s iron-rich core extends three-quarters of the way to the surface and makes up 60 percent of the planet’s mass. (By comparison, Earth’s core
accounts for 30 percent.) Messenger will attempt to confirm recent measurements suggesting that part of Mercury’s core is liquid, like Earth’s. The finding would overturn the assumption that small planets cool too rapidly to maintain molten, dynamic cores.
RAYS OF LIGHT Messenger’s first snapshot from orbit captured the Debussy crater, the site of a billion-year-old major collision, one of the planet’s most recent. Bright markings, including rays stretching halfway around the planet, reveal minerals kicked up from the impact. Over time the constant bombardment of charged particles from the sun, along with space dust, will darken the debris.
POLAR ICE? Although Mercury’s daytime temperatures exceed 800 degrees Fahrenheit, radar studies indicate that the planet has vast
deposits of ice within the perpetually dark, frigid craters at its poles. Messenger’s instruments will help determine if ice was slowly deposited by comets over billions of years of collisions.