The Solar System

450 PART 4^ |^ THE SOLAR SYSTEM

were extremely dry. Almost all Earth rocks contain 1 to 2 percent

water, either as free water trapped in the rock or as water mole-

cules chemically bonded with certain minerals. But moon rocks

contain no water at all.

Rocks from the lunar maria are dark-colored, dense basalts

much like the solidifi ed lava produced by the Hawaiian volcanoes

(■ Figure 21-5). Th ese rocks are rich in heavy elements such as

iron, manganese, and titanium, which give them their dark color.

Some of the basalts are vesicular, meaning that they contain holes

caused by bubbles of gas in the molten rock. Like bubbles in a

carbonated beverage, these bubbles do not form while the magma

is under pressure. Only when the molten rock fl ows out onto the

surface, where the pressure is low, do bubbles appear. Th e vesicular

nature of some of the basalts shows that these rocks formed in lava

fl ows that reached the surface and did not solidify underground.

Absolute ages of the mare basalts can be found from the

radioactive atoms they contain (see Chapter 19), and these ages

range from about 2 to 4 billion years. Th ese ages confi rm that the

lava fl ows happened after the end of the heavy bombardment.

Th e highlands are composed of low-density rock containing

calcium-, aluminum-, and oxygen-rich minerals that would have

been among the fi rst to solidify and fl oat to the top of molten rock.

Mountains and examined Hadley Rille (see Figure 21-2). Apollo
missions 16 and 17 visited highland regions to sample older parts
of the lunar crust (look again at Figure 21-4). Almost all of the
lunar samples from these six landings are now held at the
Planetary Materials Laboratory at the Johnson Space Center in
Houston, although one has been embedded in a stained glass
window in the National Cathedral in Washington, DC. Th ese
lunar samples are a national treasure containing clues to the
beginnings of our solar system.

Moon Rocks

Many scientifi c measurements were made on the moon, but the
most exciting prospect was the return of moon rocks to Earth.
Analysis could reveal clues to the chemical and physical history
of the moon, the origin and evolution of Earth, and the condi-
tions in the solar nebula under which the planets formed.
Of the many rock samples that the Apollo astronauts carried
back to Earth, every one is igneous. Th at is, they formed by the
cooling and solidifi cation of molten rock. No sedimentary rocks
were found, consistent with the understanding that the moon
has never had liquid water on its surface. In addition, the rocks

MareMare

SerenitatisSerenitatis

Mare

Serenitatis

MareMare

CrisiumCrisium

Mare

Crisium

MareMare

TranquillitatisTranquillitatis

Mare

Tranquillitatis

MareMare

FoecunditatisFoecunditatis

Mare

Foecunditatis

MareMare

NectarisNectaris

Mare

MareMare Nectaris

NubiumNubium

Mare

Nubium

TychoTychoTycho

MareMare

HumorumHumorum

Mare

Humorum

OceanusOceanus

ProcellarumProcellarum

Oceanus

Procellarum

MareMare

ImbriumImbrium

Mare

Imbrium

PlatoPlatoPlato

KeplerKeplerKepler

CopernicusCopernicusCopernicus

Apollo 11 landed in

the lunar lowlands

in July 1969.

Apollo 17, the last Apollo

mission to the moon,

landed in the highlands

in December 1972.

■ Figure 21-4

Apollo 11, the first mission to the

moon, landed on the smooth surface

of Mare Tranquillitatis in the lunar

lowlands, and the horizon was straight

and level. When Apollo 17 landed at

Taurus–Littrow in the lunar highlands,

the astronauts found the horizon moun-

tainous and the terrain rugged. Landing

sites for the other Apollo missions

are shown on the photo of the moon.

(Moon: © UC Regents/Lick Observatory;

Astronauts: NASA)