The Solar System

482 PART 4^ |^ THE SOLAR SYSTEM

lowlands were once fi lled by an ocean of liquid water. Th is is an
exciting hypothesis and will be mentioned again later in this
chapter when you consider the history of water on Mars.
Th e cratering and volcanism on Mars fi t with what you
already know of comparative planetology. Mars is larger than
Earth’s moon, so it cooled more slowly, and its volcanism has
continued longer. But Mars is smaller than Earth and less geo-
logically active, so some of its ancient cratered terrain has sur-
vived undamaged by volcanism and plate tectonics.
Th e Martian volcanoes are shield volcanoes with shallow
slopes, showing that the lava fl owed easily. As you learned from
Earth and Venus, shield volcanoes occur over hot spots of rising
magma below the crust and are not related to plate tectonics. Th e
largest volcano in the solar system is Olympus Mons on Mars
(see page 483). Th e shield volcano Mauna Loa in Hawaii is so
heavy it has sunk into Earth’s crust to form an undersea depres-
sion like a moat around a castle. Olympus Mons is much larger
than Mauna Loa but has not sunk into the crust of Mars, which
shows that the crust of Mars is much thicker than the crust of
Earth. You can see the evidence in ■ Figure 22-16.
Other evidence indicates that the Martian crust has been
thinner and more active than the moon’s. Valles Marineris is a
network of canyons 4000 km (2500 mi) long and up to
600 km (400 mi) wide (Figure 22-16). At its deepest, it is four
times deeper than the Grand Canyon on Earth, and it is long
enough to stretch from New York to Los Angeles. Th e canyon
has been produced by faults that allowed great blocks of crust
to sink. Later landslides and erosion modifi ed the canyon fur-
ther. Although Valles Marineris is an old feature, it does show
that the crust of Mars has been more active than the crusts of
the moon or Mercury, worlds that lack such dramatic
canyons.

Th e faults that created Valles Marineris seem to be linked at

the western end to a great volcanic bulge in the crust of Mars

called the Th arsis rise. Nearly as large as the United States, the

Th arsis rise extends 10 km (6 mi) above the mean radius of Mars.

Th arsis is home to many smaller volcanoes, but on its summit lie

three giants, and just off of its northwest edge lies huge Olympus

Mons (Figure 22-16). Th e origin of the Th arsis rise is not well

understood, but it appears that magma rising from below the

crust has pushed the crust up and broken through repeatedly to

build a giant bulge of volcanic deposits. Th is bulge is large

enough to have modifi ed the climate and seasons on Mars and

may be critical in understanding the history of the planet.

A similar uplifted volcanic bulge, the Elysium region, visible

in Figure 22-15, lies halfway around the planet. It appears to be

similar to the Th arsis rise, but it is more heavily cratered and so

must be older.

Th e vast sizes of features like the Th arsis rise and Olympus

Mons show that the crust of Mars has not been broken into hori-

zontally mobile plates. If a plate were moving over a hot spot, the

rising magma would produce a long chain of shield volcanoes

and not a single large peak. On Earth, the hot spot that creates

the volcanic Hawaiian Islands has punched through the moving

Pacifi c plate repeatedly to produce the Hawaiian-Emperor island

chain extending 7500 km (4700 mi) northwest across the Pacifi c

seafl oor (page 473). No such chains of volcanoes are evident on

Mars, so you can conclude that the crust is not divided into mov-

ing plates.

No spacecraft has ever photographed an erupting volcano on

Mars, but it is possible that some of the volcanoes are still active.

Lack of impact craters in the youngest lava fl ows in the Th arsis

region and the Elysium region show that the volcanoes may have

been active as recently as a few million years ago, which,

■ Figure 22-15

These globes of Mars are color

coded to show elevation. The

northern lowlands lie about

4 km (2.5 mi) below the south-

ern highlands. Volcanoes are

very high (white), and the

giant impact basins, Hellas and

Argyre, are low. Note the depth

of the canyon Valles Marineris.

The two Viking spacecraft

landed on Mars in 1976. Path-

inder landed in 1997. Rovers

Spirit and Opportunity landed

in 2004. (NASA)

Viking 2

Argyre

Elysium

Volcanoes

Olympus

Mons

TharsisTharsis

VolcanoesVolcanoes

Valles MarinerisValles Marineris

NorthernNorthern

lowlandslowlands

SouthernSouthern

highlandshighlands

Viking 1Viking 1

PathfinderPathfinder

Hellas

South Pole

Tharsis

Volcanoes

Valles Marineris

Northern

lowlands

Southern

highlands

Viking 1

Pathfinder

OpportunityOpportunityOpportunity

North Pole

Spirit