“Each of the terrestrial planets consists of a dense iron-rich core surrounded by a rocky mantle, composed largely of magnesium and iron silicates. The topmost layer of rock, the crust, formed from minerals with lower melting points than those in the underlying mantle, either during differentiation early in the planet’s history or by later volcanic or magmatic activity. “
“Mercury’s core occupies a large fraction of the planet, about 85% of the planetary radius, even larger than previous estimates. Because of the planet’s small size, at one time many scientists thought the interior should have cooled to the point that the core would be solid. However, subtle dynamical motions measured from Earth-based radar, combined with MESSENGER’s newly measured parameters of the gravity field and the characteristics of Mercury’s internal magnetic field that signify an active core dynamo, indicate that the planet’s core is at least partially liquid.”
“A planet’s topography can reveal fundamental information about its internal structure and its geological and thermal evolution. Ranging observations from MESSENGER’s Mercury Laser Altimeter (MLA) have provided the first-ever precise topographic model of the planet’s northern hemisphere and characterized slopes and surface roughness over a range of spatial scales. From MESSENGER’s eccentric, near-polar orbit, the MLA illuminates surface areas as wide as 15 to 100 meters (50 -325 feet), spaced about 400 meters apart (1,300 feet).
The spread in elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive area of lowlands at high northern latitudes that hosts the volcanic northern plains. Within this lowland region is a broad topographic rise that formed after the volcanic plains were emplaced.”
“Theories of Mercury’s formation have been developed to explain its unusually large metal-to-silicate ratio compared to Venus, Earth, and Mars. These theories generally fall into one of two categories; physical removal of silicates, or differences in the composition material from which Mercury formed compared with other solar system bodies. Two of the physical models invoke one or more giant impacts (left) or the vaporization of surface by a hot solar nebula to remove the planet’s original crust and outer mantle. Chemical models describe the material from which Mercury formed, for example, refractory condensates or primitive precursory material (right). The abundances of potassium, thorium, and uranium on the surface of Mercury measured by the MESSENGER Gamma-Ray Spectrometer rule out the giant impact, vaporization, and refractory condensation models.”
“When MESSENGER flew close to the planet, its magnetometer collected data on the magnetism of rocks in Mercury’s surface. Those tiny signals revealed that Mercury’s magnetic field is very ancient, between 3.7 and 3.9 billion years old.”