The Solar System

CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 483

geologically speaking, is only yesterday. Mars may still retain
enough heat to trigger an eruption, but the interval between
eruptions could be very long.

Finding the Water on Mars

Th e quest for water on Mars is exciting because water has been
deeply involved in the evolution of the planet, but it is also excit-
ing because life depends on water. If life managed to begin on
Mars, then the planet must have had water, and if life survives
there, water must be hidden somewhere on the desert planet.
Th e two Viking spacecraft reached orbit around Mars in
1976 and photographed exciting hints that water once fl owed
over the surface. As you have learned, liquid water cannot exist
on the surface now because it would boil away under the
extremely low atmospheric pressure, so the Viking photos were
evidence that water once fl owed on Mars and that conditions

there must have been quite diff erent long ago. More recent mis-

sions to Mars such as Mars Global Surveyor, which reached Mars

in 1997; Mars Odyssey (2001); Mars Express (2003); and Mars

Reconnaissance Orbiter (2005) have identifi ed numerous addi-

tional features related to water.

Two kinds of formations hint at water fl owing over the sur-

face. Outfl ow channels appear to have been cut by massive

fl oods carrying as much as 10,000 times the volume of water

fl owing down the Mississippi River (■ Figure 22-17a). In a mat-

ter of hours or days, such fl oods swept away landscape features

and left outfl ow channels. Th e number of craters formed on top

of the outfl ow channels show that they are billions of years old.

Th e valley networks look like meandering riverbeds that may

have formed over long periods (Figure 22-17b). Th e valley net-

works are also located in the old, cratered, southern hemisphere,

and they are also very old.

100 km

Sea level

Moat

Mauna Loa

Olympus Mons

10 km

Olympus

Mons

Ascraeus

Mons

Pavonis

Mons

Arsia

Mons

Olympus Mons is about 100 times

larger in volume than Mauna Loa,

the largest volcano on Earth.

Green area: The Tharsis

rise is a 10-km-thick

dome of lava flows.

■ Figure 22-16

High volcanoes and deep canyons mark the

surface of Mars. Olympus Mons, a shield vol-

cano, is much larger than the largest volcano

on Earth. In this false-color image, three

other volcanoes are visible. Those three vol-

canoes are also visible in the photo, along

with the canyon Valles Marineris, which

stretches as far as the distance from New

York to Los Angeles. (Four volcanoes: © Calvin

J. Hamilton, Columbia, Maryland; photo: NASA/

USGS)