THE OTHER HALF OF MERCURY

When Mariner 10 observed Mercury in 1974 and 1975, the spacecraft viewed the same half of the planet during each of its three flybys. Globes of Mercury were blank on one side, and the limited resolution of images and lack of compositional information meant that Mercury continued to hold onto many of its secrets for decades. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission was selected for flight in 1999 and launched in 2004. MESSENGER’s trajectory included three flybys of Mercury in 2008 and 2009 before entering orbit in 2011. Fortuitously, each of the flybys revealed terrain that had not been previously seen, which made for a host of discoveries about Mercury’s geologic history, including the first view of the full Caloris basin and the confirmation of volcanic activity on the planet. Most of MESSENGER’s compositional information did not come until the spacecraft was in orbit, and there Mercury defied most expectations in that it was not volatile poor, and that its crust contains graphite. Although some findings came as immediate revelations and others slowly revealed themselves throughout the mission, each of MESSENGER’s discoveries provided such excitement and awe that it is easy to remember where you were, who you were with, and the details of the moment it all came together. We will review some of these discoveries and their lasting scientific impact.