Mercury's Surface Dominated by Volcanism and Iron Deficiency, NASA/JHUAP/ASU
"Volcanism has played a more extensive role in shaping the surface of Mercury than scientists had thought. This result comes from multispectral imaging data gathered in January 2008 by MESSENGER, the latest spacecraft to visit the Sun's innermost planet.
MESSENGER data has also identified and mapped surface rock units that correspond to lava flows, volcanos, and other geological features. At the same time, the spacecraft's suite of instruments has confirmed an apparent planet-wide iron deficiency in Mercury's surface rocks."