The Solar System

CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 451

meteorites. Th e smallest meteorites, the micrometeorites, do the

most damage by constantly sandblasting the lunar surface, grind-

ing the rock down to fi ne dust. Th e Apollo astronauts found that

the dust coated their spacesuits and equipment, and then the

interior of the Lunar Module after they climbed back inside.

Impact cratering, a theme of this chapter, dominates the lunar

surface and is responsible for the lunar regolith.

Th e moon rocks are old, dry, igneous, and badly shattered by

impacts. You can use these facts, combined with what you know

about lunar features, to tell the story of the moon.

The History of the Moon

Evidence preserved in the Apollo moon rocks shows that the

moon must have formed in a molten state. Planetary geologists

now refer to the newborn moon as a magma ocean. Evidently,

denser materials sank to form a small core, and, as the magma

ocean cooled, low-density minerals crystallized and fl oated to

the top to form a low-density crust. In this way the moon dif-

ferentiated into core, mantle, and crust. Th is corresponds to the

fi rst of the four stages of Terrestrial planet development displayed

Some of this rock is anorthosite, a light-colored rock that contrib-
utes to the highlands’ bright contrast with the dark, iron-rich
basalts of the lowlands. Th e rocks of the highlands, although badly
shattered by impacts, represent the moon’s original low-density
crust, whereas the mare basalts rose as molten rock from the deep
crust and upper mantle. Th e highland crustal rocks range in age
from 4.0 to 4.5 billion years old, signifi cantly older than the mare
basalts.
Moon rocks are igneous, but many are classifi ed as breccias,
rocks that are made up of fragments of earlier rocks cemented
together by heat and pressure. Evidently, after the molten rock
solidifi ed, meteorite impacts broke up the rocks and fused them
together time after time.
If you went to the moon, you would get your spacesuit dirty.
Both the highlands and the lowlands of the moon are covered by
a layer of powdered rock and crushed fragments called the rego-
lith. It is about 10 m deep on the maria but over 100 m deep in
certain places in the highlands. About 1 percent of the regolith is
meteoric fragments; the rest is the smashed remains of moon
rocks that have been pulverized by the constant rain of

■ Figure 21-5

Rocks returned from the moon show that the moon formed in a molten state, that it was heavily fractured by cratering

when it was young, and that it is now affected mainly by micrometeorites grinding away at surface rock. (NASA)

The Apollo astronauts found that all moon rocks are

igneous, meaning they solidified from molten rock.

Rocks exposed on the

lunar surface become

pitted by micrometeorites.

Vesicular basalt contains

bubbles frozen into the

rock when it was molten.

A breccia is formed by

rock fragments bonded

together by heat and

pressure.