The Solar System

454 PART 4^ |^ THE SOLAR SYSTEM

components while it was in space, so the moon would have formed

lacking volatiles. Such an impact would have melted the proto-

Earth, and the material falling together to form the moon would

also have been heated hot enough to melt completely. Th is fi ts the

evidence that the highland anorthosite in the moon’s oldest rocks

formed by diff erentiation of large quantities of molten material.

Th e moon is small, and small worlds cool rapidly because
they have a large ratio of surface area to volume. Th e rate of heat
loss is proportional to the surface area, and the amount of heat in
a world is proportional to the volume. Th e smaller a world is, the
easier it is for the heat to escape. Th at is why a small cupcake fresh
from the oven cools more rapidly than a large cake. Th e moon lost
much of its internal heat when it was young, and it is the outward
fl ow of heat that drives geological activity, so the moon is mostly
inactive today. Th e crust of the moon rapidly grew thick and
never divided into moving plates. Th ere are no rift valleys or
folded mountain chains on the moon. Th e last lava fl ows on the
moon ended about 2 billion years ago when the moon’s internal
temperature fell too low to maintain subsurface lava.
Th e overall terrain on the moon is almost fi xed. On Earth a
billion years from now, plate tectonics will have totally changed
the shapes of the continents, and erosion will have long ago worn
away the mountain ranges you see today. On the moon, with no
atmosphere and no water, there is no Earth-like erosion. Over
the next billion years, impacts will have formed only a few more
large craters, and nearly all of the lunar scenery will be unchanged.
Micrometeorites are the biggest infl uence; they will have blasted
the soil, erasing the footprints left by the Apollo astronauts and
reducing the equipment they left behind to peculiar chemical
contamination in the regolith at the six Apollo landing sites.
You have studied the story of the moon’s evolution in detail
for later comparison with other planets and moons in our solar
system, but the story has skipped one important question: Where
did Earth get such a large satellite?

The Origin of Earth’s Moon

Over the last two centuries, astronomers developed three diff er-
ent hypotheses for the origin of Earth’s moon. Th e fi ssion hypoth-
esis proposed that the moon broke from a rapidly spinning young
Earth. Th e condensation hypothesis suggested that Earth and its
moon condensed together from the same cloud of matter in the
solar nebula. Th e capture hypothesis suggested that the moon
formed elsewhere in the solar nebula and was later captured by
Earth. Each of these older ideas had problems and failed to sur-
vive comparison with all the evidence.
In the 1970s, after moon rocks were returned to Earth and
studied in detail, a new hypothesis originated that combined some
aspects of the three older hypotheses. Th e large-impact hypoth-
esis proposes that the moon formed when a very large planetesi-
mal, estimated to have been at least as massive as Mars, smashed
into the proto-Earth. Model calculations indicate that this colli-
sion would have ejected a disk of debris into orbit around Earth
that would have quickly formed the moon (■ Figure 21-9).
Th is hypothesis explains several phenomena. If the proto-
Earth and impactor had each already diff erentiated, the ejected
material would have been mostly iron-poor mantle and crust,
which would explain the moon’s low overall density and iron-poor
composition. Furthermore, the material would have lost its volatile

■ Figure 21-9

Sometime before the solar system was 50 million years old, a collision

produced Earth and the moon in its orbit inclined to Earth’s equator.

The Large-Impact Hypothesis

Volatiles are lost to

space as the particles

in the ring begin to

accrete into larger

bodies.

Eventually the moon

forms from the iron-

poor and volatile-poor

matter in the disk.

Iron-poor rock from

the mantles of the two

bodies forms a ring

of debris.

Another body that has

also formed an iron

core strikes the larger

body and merges,

trapping most of the

iron inside.

A protoplanet nearly

the size of Earth

differentiates to form

an iron core.