Mercury is the smallest of the planets, and despite being known from antiquity, due to its proximity to the Sun, it has proven hard to study. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is designed to fill in many of the gaps in our knowledge of our small planetary cousin. Two new papers from the MESSENGER team respectively highlight the surface and internal structure of Mercury, from detailed observations of the northern hemisphere.
By mapping the fluctuations in Mercury's gravitational field, the researchers measured the variations in mass, indicating how the planet's interior structure differs from place to place. In addition, the spacecraft probed the forces shaping the planet's surface using laser altitude measurements as it flew over the surface. These results show Mercury to be a recently active world with an interior markedly different from the other terrestrial planets—Venus, Earth, and Mars—as well as from Earth's Moon. Specifically, the team found evidence for a solid shell just outside the planet's core, a feature found on no other world.