The density of Mercury does not conform with that of the other terrestrial
planets, nor with the Moon. When corrected for compression due to size, Mercury
has the highest density of all. The following theories may account for this
- the concentration of iron, a heavy element, may
have been higher in the region of the primordial nebula from which Mercury
- 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
or more gigantic impacts may have removed a substantial part of Mercury's
rocky mantle, leaving a relatively large metallic core
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.