CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 459
The Interior of Mercury
One of the most striking diff erences between Mercury and the
moon is the composition of their interiors. You have seen that
the moon is a low-density world that contains no more than a
small core of metals. In contrast, evidence suggests that Mercury
has a large metallic core.
Mercury is over 60 percent denser than the moon. Yet
Mercury’s surface appears to be normal rock, so you can con-
clude that its interior contains a large core of dense metals,
mostly iron. In proportion to its size, Mercury must have a larger
metallic core than Earth (see the diagram in Celestial Profi le 4).
If Mercury had a large metallic core that remained molten,
then the dynamo eff ect would generate a magnetic fi eld (see
Chapter 20). Th e Mariner 10 spacecraft found a magnetic fi eld
only about 0.5 percent as strong as Earth’s, and this weak fi eld
made it diffi cult to understand the interior. Because Mercury is
a small world, it should have lost most of its internal heat long
ago and should not have a molten core. Nevertheless, radar
observations of Mercury’s rotation show that the surface of the
planet is shifting back and forth slightly in response to the sun’s
gravity. Th at must mean that at least the outer core is molten.
If Mercury’s iron core contains a higher-than-Earthly concen-
tration of sulfur, the melting point would be lowered, and the
outer core, where the pressure is lower, could be molten. It is
not clear, however, why a planet that formed so close to the sun
could contain so much sulfur, which is a volatile material. Th eMuch of Mercury’s surface is old, cratered terrain (■ Figure
21-16), but other areas called intercrater plains are less heavily
cratered. Th ose plains are marked by meteorite craters less than
15 km in diameter and secondary craters produced by chunks of
ejecta from larger impacts. Unlike the heavily cratered regions,
the intercrater plains are not totally saturated with craters. As an
expert in comparative planetology, you can recognize that this
means that the intercrater plains were produced by later lava
fl ows that buried older terrain.
Smaller regions called smooth plains appear to be even
younger than the intercrater plains. Th ey have fewer craters and
appear to be lava fl ows that occurred after most cratering had
ended. Much of the region around the Caloris Basin is composed
of these smooth plains (Figure 21-16), and they appear to have
formed soon after the Caloris impact.
Given the available evidence, planetary astronomers con-
clude that the plains of Mercury are solidifi ed lava fl ows much
like the maria on the moon. Unlike the maria, Mercury’s lava
plains are not signifi cantly darker than the rest of the crust. Th is
may be due to a compositional diff erence between Mercury’s
lava fl ows and the moon’s. Except for a few bright crater rays,
Mercury’s surface is a uniform gray with an albedo of only
about 0.1. Th at means Mercury’s lava plains are not as dramati-
cally obvious on photographs as the much darker maria on
our own moon, which show up in contrast to the lighter
highlands.
■ Figure 21-14
A lobate scarp (arrow) crosses craters,
indicating that Mercury cooled and shrank,
wrinkling its crust, after many of its craters
had formed. (NASA)
Visual-wavelength imageVisual-wavelength image