On March 18, 2011, the MESSENGER spacecraft entered orbit around Mercury to become that planet’s first orbiter. The spacecraft’s instruments are making a complete reconnaissance of the planet’s geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment. MESSENGER is providing a wealth of new information and some surprises. For instance, Mercury’s surface composition differs from that expected for the innermost of the terrestrial planets, and Mercury’s magnetic field has a north-south asymmetry that affects interaction of the planet’s surface with charged particles from the solar wind.
Tens of thousands of images reveal major features on the planet in high resolution for the first time. Measurements of the chemical composition of the planet’s surface are providing important clues to the origin of the planet and its geological history. Maps of the planet’s topography and magnetic field are offering new evidence on Mercury’s interior dynamical processes. And scientists now know that bursts of energetic particles in Mercury’s magnetosphere are a continuing product of the interaction of Mercury’s magnetic field with the solar wind.
“MESSENGER has passed a number of milestones just this week,” offers MESSENGER principal investigator Sean Solomon of the Carnegie Institution. “We completed our first perihelion passage from orbit on Sunday, our first Mercury year in orbit on Monday, our first superior solar conjunction from orbit on Tuesday, and our first orbit-correction maneuver on Wednesday. Those milestones provide important context to the continuing feast of new observations that MESSENGER has been sending home on nearly a daily basis.”
Among the fascinating features seen in flyby images of Mercury were bright, patchy deposits on some crater floors, but they remained a curiosity. New targeted MDIS observations reveal these patchy deposits to be clusters of rimless, irregular pits with horizontal dimension from hundreds of meters to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters.