The Solar System

490 PART 4^ |^ THE SOLAR SYSTEM

discover in the next chapters that low-mass moons are typically
irregular in shape, whereas more massive moons are more
spherical.
Images of Phobos reveal a unique set of narrow, parallel
grooves (see Figure 22-23). Averaging 150 m wide and 25 m
deep, the grooves run from Stickney, the largest crater, to an
oddly featureless region on the opposite side of the satellite. One
theory suggests that the grooves are deep fractures produced by
the impact that formed Stickney. Th e featureless region opposite
Stickney may be similar to the jumbled terrains found on Earth’s
moon and on Mercury. All these regions were produced by the
focusing of seismic waves from a major impact on the far side of
the body. High-resolution photographs show that the grooves
are lines of pits, suggesting that the pulverized rock material on
the surface has drained into the fractures or that gas, liberated
by the heat of impact, escaped through the fractures and blew
away the dusty soil.
Observations made with the Mars Global Surveyor’s infrared
spectrometer show that Phobos’s surface cools quickly from −4°C
to −112°C (from 25°F to −170°F) as it passes from sunlight into
the shadow of Mars. Solid rock would retain heat and cool more
slowly. To cool as quickly as it does, the dust must be at least a
meter deep and very fi ne. In most photos made by the spacecraft
camera, the dust blankets the terrain, but some photos show
boulders a few meters in diameter that are thought to be ejecta
from impacts.
Deimos looks even smoother than Phobos because of an
even thicker layer of dust on its surface (Figure 22-23). Th is
material partially fi lls craters and covers minor surface irregulari-
ties. It seems certain that Deimos experienced collisions in its
past, so fractures may be hidden below the debris.
Th e debris on the surfaces of the moons raises an interesting
question. How can the weak gravity of small bodies hold on to
fragments from meteorite impacts? Escape velocity on Phobos is
only 12 m/s. An athletic astronaut could almost jump into space.
Certainly, most fragments from impacts should escape, but some
do fall back and accumulate on the surface.

Deimos, smaller than Phobos, has a smaller escape velocity,

but it has more debris on its surface because it is farther from

Mars. Phobos is close enough to Mars that most ejecta from

impacts on Phobos will be drawn into Mars. Deimos, being far-

ther from Mars, is able to keep a larger fraction of its ejecta.

Phobos is so close to Mars that tides are making its orbit shrink,

and it will fall into Mars or be ripped apart by tidal forces within

about 100 million years.

Deimos and Phobos illustrate three principles of compara-

tive planetology that you will fi nd helpful as you explore farther

from the sun. First, some satellites are probably captured aster-

oids. Second, small satellites tend to be irregular in shape and

heavily cratered. And third, tidal forces can aff ect small moons

and gradually change their orbits. You will fi nd even stronger

tidal eff ects in Jupiter’s satellite system in the next chapter.

SCIENTIFIC ARGUMENT

Why would you be surprised if you found volcanism on Phobos

or Deimos?

This is another obvious argument, isn’t it? But remember, the pur-

pose of a scientifi c argument is to test your own understanding,

so it is a good way to review. In discussing Earth’s moon, Mercury,

Venus, Earth, and Mars, you have seen illustrations of the principle

that the larger a world is, the more slowly it loses its internal heat.

It is the fl ow of that heat from the interior through the surface into

space that drives geological activity such as volcanism and plate

motion. A small world, like Earth’s moon, cools quickly and remains

geologically active for a shorter time than a larger world like Earth.

Phobos and Deimos are not just small, they are tiny. However they

formed, any interior heat would have leaked away very quickly;

with no energy fl owing outward, there can be no volcanism.

Some futurists suggest that the fi rst human missions to Mars

will not land on the surface of the planet but will build a colony on

Phobos or Deimos. These plans are based on speculation that there

may be water deep inside the moons that colonists could use. Build

an argument based on what you know about water on Mars. What

would happen to water released in the sunlight on the surface

of such small worlds?

490 PART 4PART^4 || THETHE^ SOSOLARLAR^ SYSYSTESTEMM

What Are We? Earth-Folk

Space travel isn’t easy. We humans made it

to the moon, but it took everything we had

in the late 1960s. Going back to the moon

will be easier next time because the

technology will be better, but it will still be

expensive and will require people with

heroic talent to design, build, and fl y the

spaceships. Going beyond the moon will be

even more diffi cult.

Going to Mercury or Venus doesn’t seem

worth the effort. Mercury is barren and

dangerous, and the heat and air pressure on

Venus may prevent any astronaut from ever

visiting its surface. In the next two

chapters, you will discover that the Jovian

planets and their moons also are not places

humans are likely to visit soon. The stars

are so far away they may be forever beyond

the reach of human spaceships. But Earth

has a neighbor.

Astronomically Mars is just up the street,

and it isn’t such a bad place. You would

need a good spacesuit and a pressurized

colony to live there, but it isn’t impossible.

Solar energy and water are abundant. It

seems inevitable not only that humans will

walk on Mars but that they will someday live

there. We Earth-folk have an exciting future.

Eventually we will be the Martians.