122 Encyclopedia of the Solar System
FIGURE 3 Comparison of terrestrial planet sizes and core radii.
The percent of the total planetary volume of the cores is also
shown in yellow. The size of the Moon’s core is not known, but
the maximum possible size is shown.
FIGURE 4 AMariner 10photomosaic together with an accurate
artist’s rendition of the size of Mercury’s core compared to the
silicate portion. The outer part of the core is still in a liquid state.
(From Strom, 1987.)
The measured magnetic field is strong enough to hold
off the solar wind and form a bow shock. As the spacecraft
approached the planet, it measured a sudden increase in
the field strength that represented the bow shock. Also the
instruments measured signals indicating the entrance to
and exit from amagnetopausesurrounding a magne-
tospheric cavity about 20 times smaller than the Earth’s
(Fig. 5). Also because of the small size of Mercury’s magne-
tosphere, magnetic events happen more rapidly and repeat
more often than in Earth’s magnetosphere. The nominal
magnetopause subsolar distance is estimated to be about
1.35±0.2 Mercury radii, and the bow shock distance is
about 1.9±0.2 Mercury radii. The polarity of the field is
the same as Earth’s. The magnetic strength increased as the
spacecraft approached the planet. The interplanetary field
is about 25 nT (nano-Tesla) in the vicinity of Mercury, but
it increased to 100 nT at closest approach to Mercury. If
that rate of increase continued to the surface, the surface
strength would be about 200–500 nT. This is about 1% of
the Earth’s strength.
Although other models may be possible, the mainte-
nance of terrestrial planet magnetic fields is thought to
require an electrically conducting fluid outer core surround-
ing a solid inner core. Therefore, Mercury’s dipole magnetic
field is taken as evidence that Mercury currently has a fluid
outer core of unknown thickness. Recent high-resolution
radar measurements of the magnitude of Mercury’slibra-
tionsindicate that the mantle is detached from the core
confirming the outer core is fluid. Although the thickness
of the outer fluid core is unknown at present, theoretical
studies indicate that a dipole magnetic field can be gener-
ated and maintained even in a thin outer fluid core. Thermal
history models strongly suggest that Mercury’s core would
have solidified long ago unless there was some way of main-
taining high core temperatures throughout geologic history.
Most theoretical studies consider the addition of a light, al-
loying element to be the most likely cause of a currently
molten outer core. Although oxygen is such an element,
it is not sufficiently soluble in iron at Mercury’s low inter-
nal pressures. Metallic silicon has also been suggested, but
sulfur is considered to be the most likely candidate. Some
models require only a small amount of sulfur, whereas oth-
ers support greater amounts. Currently we do not know
how much sulfur is in the core, but it is possible that the
MESSENGERmission will provide the answer. If sulfur is
the cause of Mercury’s outer fluid core, then estimates of
its abundance can be used to estimate the thickness of the
outer fluid core. For a sulfur abundance in the core of less
than 0.2%, the entire core should be solidified at the present
time, and for an abundance of 7% the core should be en-
tirely fluid at present. Therefore, if sulfur is the alloying
element, then Mercury could contain between 0.2 and 7%
sulfur in its core. As discussed later, possible sulfur abun-
dances can be estimated from the planetary radius decrease
derived from thetectonic framework.