MESSENGER results have shown that Mercury contracted by as much as 7 km in radius, a number substantially larger than previously believed.
Mercury’s tenuous atmosphere scatters sunlight, and the brightness of the emission is proportional to its content.
The first frame shows an Earth-based Arecibo radar image in red overlaid on a mosaic of MESSENGER’s Mercury Dual Imaging System images, enabling for the first time the identification of the host craters for all of the radar-bright deposits.
In these global views, the Caloris impact basin is initially in the center, and the colors on the spinning globes represent the ratios by weight of magnesium to silicon and aluminum to silicon.
Silicon is known to be relatively homogenous across the surface, so these maps demonstrate variations in the abundances of magnesium and aluminum, both of which are sensitive to the details of the interior melting that produced the lavas that formed the surface volcanic deposits.
Relatively high abundances of other volatile elements, including sulfur (right side of the animation), sodium, and chlorine, provide further evidence that Mercury is volatile-rich.
The high sulfur contents combined with low amounts of iron on the planet's surface additionally indicate that Mercury formed from materials with less oxygen than those that formed the other terrestrial planets, providing an important constraint on theories for the formation of all of the planets in the inner Solar System.
MESSENGER has provided multiple lines of evidence that Mercury’s polar regions host water ice.
Shown here is a view looking down on Mercury’s north polar region, with 0° longitude on the bottom of the view and extending to 65°N latitude.
The formation of hollows remains an active area of research, but the etched nature of the features, such as in this image, suggests that material is being lost from the surface to create the hollows.
Volcanism has played a critical role in shaping Mercury’s surface.
Variations in topography are shown in color (red: high-standing terrain; blue: low-lying terrain).
Tectonic landforms such as Carnegie Rupes formed by horizontal shortening in response to cooling and contraction of the planetary interior.
MESSENGER measurements have revealed that Mercury is surprisingly abundant in volatile elements that evaporate at moderately high temperatures, ruling out many of the models for its formation and early history that had been proposed before the mission.