The Solar System

444 PART 4^ |^ THE SOLAR SYSTEM

Th ose lava fl ows suggest volcanism, but only small traces of

past volcanic activity are visible from Earth. No major volcanic

peaks are visible on the moon, and no active volcanism has

ever been detected. Th e lava fl ows that created the maria

happened long ago and were much too fl uid to build peaks.

With a good telescope and some diligent searching you could

see a few small domes pushed up by lava below the surface, and

you could see long, winding channels called sinuous rilles

(■ Figure 21-2). Th ese channels are often found near the edges

of the maria and were evidently cut by fl owing lava. In some

cases, such a channel may once have had a roof of solid rock,

forming a lava tube. After the lava drained away, meteorite

impacts collapsed the roof to form a sinuous rille. Th e view

from Earth provides only hints of ancient volcanic activity

associated with the maria.

Lava fl ows and impact cratering have dominated the history

of the moon. Study Impact Cratering on pages 446–447

and notice three important points and fi ve new terms:

Impact craters have certain distinguishing characteristics,

such as their shape and the ejecta, rays, and secondary craters

around them.

Lunar impact craters range from tiny pits formed by micro-

meteorites to giant multiringed basins.

Most of the craters on the moon are old; they were formed

long ago when the solar system was young.

Meteorites strike the moon all the time, but large impacts

are rare today. Astronomers estimate that meteorites with diam-

eters of tens of meters strike the moon every few decades, but no

one has ever seen such an impact with certainty. Small fl ashes of

light have been seen on the dark side of the moon during showers

of meteors, but those impacts must have been made by very small

objects, perhaps only centimeters in size. No signifi cant change

has been seen on the moon since the invention of the telescope.

As you will learn in Chapter 25, large impacts do happen on the

moon and Earth, but they are quite rare, and nearly all of the

lunar craters seen through telescopes date from the solar system’s

youth.

Th e lunar features visible from Earth allow you to begin con-

structing a tentative hypothesis to explain the history of the moon.

Such a hypothesis provides a framework that will help you organize

all of the details and observations (How Do We Know?

21-1). As the moon formed, its crust would have been heavily

cratered by debris left over from the formation of the planets.

Sometime after the cratering subsided, lava welled up from below

the crust and fl ooded the lowlands, covering the craters there and

forming the smooth maria. You can locate a few large craters on the

maria such as Kepler and Copernicus in Figure 21-1, but note that

the bright rays around them show that they are relatively young,

because rays are darkened and erased by exposure to sunlight, solar

wind, and micrometeorite bombardment. Th e maria are only

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daylight and darkness, are sharp and black. Th ere is no air on the
moon to scatter light and soften shadows. Also, with even a small
telescope you could watch stars disappear behind the limb of the
moon—the edge of its disk—without dimming. Clearly, the moon
is an airless (and, therefore, soundless) world.
Th e surface of the moon is divided into two dramatically
diff erent kinds of terrain. Th e lunar highlands are fi lled with
jumbled mountains, but there are no folded mountain ranges
like the ones on Earth. Th is shows that the moon has no plate
tectonics. Th e mountains are pushed up by millions of impact
craters one on top of the other. In fact, the highlands are satu-
rated with craters, meaning that it would be impossible to form
a new crater without destroying the equivalent of one old crater.
In contrast the lowlands, about 3 km (2 mi) lower than the high-
lands, are smooth, dark plains called maria, Latin for “seas.” (Th e
singular of maria is mare, pronounced MAH-ray.) Th e fi rst tele-
scope observers thought these were bodies of water, but further
examination showed that the maria are marked by ridges, faults,
smudges, and scattered craters and can’t be water. Rather, the
maria are ancient lava fl ows that have apparently covered the
older, cratered lowlands.

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

Visual-wavelength image Much of the surfaceis covered with craters

on top of craters.

The dark, smooth areas

of the moon are called

seas (maria in Latin).

■ Figure 21-1

The side of the moon that faces Earth is a familiar sight. Craters have been
named for famous scientists and philosophers, and the so-called seas have
been given romantic names. Mare Imbrium is the Sea of Rains, and Mare
Tranquillitatis is the Sea of Tranquillity. There is, in fact, no water on the
moon. (Photo © UC Regents/Lick Observatory)