Mercury 121
FIGURE 2 (a) A high-resolution radar image of
the north polar deposits down to 80◦N is shown.
The deposits are within the permanently shaded
areas of fresh craters. Degraded craters with low
rims do not have the deposits. (b) A map of the
deposits (shown in blue) down to 70◦N. (Courtesy
of John Harmon, Arecibo Observatory, Puerto
Rico.)of 5300 kg/m^3. This means that Mercury contains a much
larger fraction of iron than any other planet or satellite in
the solar system (Figs. 3 and 4). If this iron is concentrated
in a core, then the core must be about 75% of the planet
diameter, or some 42% of its volume. Thus, its silicate man-
tle and crust is only about 600 km thick. For comparison,
Earth’s iron core is only 54% of its diameter, or just 16% of
its volume.
Aside from Earth, Mercury is the only other terres-
trial planet with a significant magnetic field.Mariner 10
first encountered Mercury’s magnetosphere at a distance
of 1.9 radii from its surface. It took measurements of the
field for only 30 minutes;∼17 minutes during Mercury’s
first equatorial pass, and∼13 minutes during the third
high latitude pass. These short observations are all we know
about Mercury’s magnetic field, magnetosphere, and parti-
cle environment. However, investigators constructed a pic-
ture of the magnetic field environment at Mercury based
on analogy with that of Earth’s magnetic field and particle
environment. Because Mercury probably lacks the iono-
sphere and trapped radiation zones of Earth’s magneto-
sphere, many comparisons are inappropriate.