UNDERSTANDING THE PLANET MERCURY Thirty years have elapsed since the one and only mission to Mercury, Mariner 10, performed three ?ybys of the planet, capturing moderate-reso- tion (100 m at best) images of one hemisphere (45% of the surface) and d- covering that Mercury could be the only other terrestrial planet to have a global magnetic ?eld and core dynamo analogous to the Earth's. At the time of this writing, the MESSENGER mission to Mercury has...