Mercury, Venus, Earth, and Mars are terrestrial (rocky) planets. Among these, Mercury is an extreme: the smallest, the densest (after correcting for self-compression), the one with the oldest surface, the one with the largest daily variations in surface temperature, and the least explored. Understanding this "end member" among the terrestrial planets is crucial to developing a better understanding of how the planets in our Solar System formed and evolved. To develop this understanding, the MESSENGER mission, spacecraft, and science instruments are focused on answering six key outstanding questions that will allow us to understand Mercury as a planet. For additional, detailed information about the driving science questions of the MESSENGER mission, check out some of the articles given on the MESSENGER publication list.
Mercury's density implies that a metal-rich core occupies at least 60% of the planet's mass, a figure twice as great as for Earth! MESSENGER will acquire compositional and mineralogical information to distinguish among the current theories for why Mercury is so dense.
Before the MESSENGER mission, only 45% of the surface of Mercury had been photographed by a spacecraft! Using its full suite of instruments, MESSENGER will investigate the geologic history of Mercury in great detail, including the portions of the planet never seen by Mariner 10.
Mercury has a global internal magnetic field, as does Earth, but Mars and Venus do not. By characterizing Mercury's magnetic field, MESSENGER will help answer the question of why the inner planets differ in their magnetic histories.
Through a combination of measurements of Mercury's gravity field and observations by the laser altimeter, MESSENGER will determine the size of Mercury's core and verify that Mercury's outer core is molten.
At Mercury's poles, some crater interiors have permanently shadowed areas that contain highly reflective material at radar wavelengths. Could this material be ice, even though Mercury is the closest planet to the Sun? MESSENGER will find out.
MESSENGER will measure the composition of Mercury's thin exosphere, providing insights into the processes that are responsible for its existence.