The Solar System

452 PART 4^ |^ THE SOLAR SYSTEM

about 2 billion years ago. Th is formed the maria (■ Figure 21-6).

It is a Common Misconception that the lava that

fl oods out on the surfaces of Earth and other planets comes from

the molten core. Th e lava comes from the lower crust and upper

mantle. Th e pressure is lower there, and that lowers the melting

point of the rock enough that radioactive decay can melt portions

of the rock. If there are faults and cracks, the magma can reach

the surface and form volcanoes and lava fl ows. Whenever you see

lava fl ows on a planet, you can be sure heat is fl owing out of the

interior, but the lava did not come all the way from the core.

Some maria on the moon, such as Mare Imbrium, Mare

Serenitatis, Mare Humorum, and Mare Crisium, retain their

round shapes, but others are irregular because the lava overfl owed

the edges of the basin or because the shape of the basin was modi-

fi ed by further cratering. Th e fl oods of lava left other characteristic

features frozen into the maria. In places, the lava formed channels

that are seen from Earth as sinuous rilles. Also, the weight of the

maria pressed the crater basins downward, and the solidifi ed lava

was compressed and formed wrinkle ridges visible even in small

telescopes. Th e tension at the edges of the maria broke the hard

lava to produce straight fractures and faults. All of these features

are visible in Figure 21-2. As time passed, further cratering and

overlapping lava fl oods modifi ed the maria. Consequently, you

should think of the maria as accumulations of features refl ecting

multiple events during the moon’s complex history.

Mare Imbrium is a dramatic example of how the great basins

became the maria. Its story can be told in detail partly because of

evidence gathered by Apollo 14 astronauts, who landed on ejecta

in Figure 20-2. Th e radioactive ages of the moon rocks show that
the surface solidifi ed between 4.6 and 4.1 billion years ago. Th e
moon has a low average density and no magnetic fi eld, so its
dense core must be small. Th e core may still retain enough heat
to be partially molten, but it can’t contain much molten iron, or
the dynamo eff ect would produce a magnetic fi eld.
Th e second stage, cratering, began as soon as the crust solidi-
fi ed, and the older highlands show that the cratering was intense
during the fi rst 0.5 billion years or so—during the heavy bombard-
ment at the end of planet building. Th e cratering rate should have
fallen rapidly as the solar system was cleared of debris. However,
there is evidence from lunar crater counts and rock sample ages that
there was a temporary surge in impact rate, called the late heavy
bombardment, about 4 billion years ago, near the end of the heavy
bombardment era. Models of the solar system’s evolution men-
tioned in Chapter 19 and 23 indicate that the late heavy bombard-
ment could have been caused by an episode of Jovian planet
migration that would have scattered remnant planetesimals into
collisions with all the solar system’s planets and moons.
Th e moon’s crust was shattered to a depth of 10 kilometers or
so, and the largest impacts during the heavy bombardment and
late heavy bombardment formed giant multiringed crater basins
hundreds of kilometers in diameter, such as Mare Imbrium and
Mare Orientale. Th is led to the third stage—fl ooding. Although
Earth’s moon cooled rapidly after its formation, radioactive decay
heated material deep in the crust, and part of it melted. Molten
rock followed the cracks up to the surface and fl ooded the giant
basins with successive lava fl ows of dark basalts from about 4 to

Aitken Basin

Mare Imbrium Mare Serenitatis

Mare Crisium

Near Side Far Side

200 km

Major impacts broke the crust,

and lava welled up to flood the

largest basins, forming maria.

■ Figure 21-6

Much of the near side of the moon is marked by great, generally circular lava

plains called maria. The crust on the far side is thicker, and there is much

less fl ooding. Even the huge Aitken Basin contains little lava fl ooding. In

these maps, color marks elevation, with red the highest regions and purple

the lowest. (Adapted from a diagram by William Hartmann; NASA/Clementine)