The density of Mercury does not conform with that of the other terrestrial planets, nor with that of the Moon. When corrected for compression due to size, Mercury has the highest density of all. The following theories may account for this anomaly:
- the concentration of iron, a heavy element, may have been higher in the region of the primordial nebula from which Mercury formed
- high solar-radiation levels may have reduced lighter oxides in Mercury to their heavier, metallic form
- the heat of the Sun may have vaporised a large amount of Mercury's outer crust
- one or more gigantic impacts may have removed a substantial part of Mercury's rocky mantle, leaving a relatively large metallic core
By mapping the elements and minerals in Mercury's surface, BepiColombo will help establish which of these possibilities is likely. For example, the ratio of different elements on the planet's surface will provide an indication of the temperature (and hence composition) of the zone in the gaseous nebula from which Mercury grew.
Density diagram for the inner planets and the Moon.
Mercury's high density also suggests that it has a large core, accounting for 70 - 80% of the planet's mass. Scientists will be able to check this using BepiColombo's accurate measurements of the planet's shape, rotation, gravity field and tilt of its axis of rotation.
Last Update: 25 Jan 2008