CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 443
Th at’s one small step for [a] man...
one giant leap for mankind.
— NEIL ARMSTRONG, ON THE MOONBeautiful, beautiful. Magnifi cent desolation.
— EDWIN E. (BUZZ) ALDRIN JR., ON THE MOONI
f you had been the first person to step onto the surface of
the moon, what would you have said? Neil Armstrong
responded to the historic signifi cance of being the fi rst human
to step onto the surface of another world. Buzz Aldrin was sec-
ond, and he responded to the moon itself. It is desolate, and it is
magnifi cent. But it is not unusual. Many planets in the universe
probably look like Earth’s moon, and astronauts may someday
walk on such worlds and compare them with our moon.
In this chapter, you will use comparative planetology to
study the moon and Mercury, and you will continue following
three important themes of planetary astronomy: impact crater-
ing, internal heat fl ow, and giant impact-induced volcanism.
Th ese three themes will help you organize the fl ood of details
astronomers have learned about the moon and Mercury.
The Moon
Only people have stood on the moon, but planetary
astronomers know it well. Th e photographs, measurements, and
samples brought back to Earth paint a picture of an airless,
ancient, battered crust, and a world created by a planetary
catastrophe.
The View from Earth
A few billion years ago, the moon must have rotated faster than
it does today, but Earth is over 80 times more massive than the
moon (Celestial Profi le 3), and its tidal forces on the moon
are strong. Earth’s gravity raised tidal bulges on the moon, and
friction in the bulges has slowed the moon until it now rotates
once each orbit, keeping the same side facing Earth. A moon
whose rotation is locked to its planet is said to be tidally cou-
pled. Th at is why we always see the same side of the moon; the
back of the moon is never visible from Earth. Th e moon’s famil-
iar face has shone down on Earth since long before there were
humans (■ Figure 21-1).
Based on what you already know, you can predict that the
moon should have no atmosphere. It is a small world with an
escape velocity too low to keep gas atoms and molecules from
escaping into space. You can confi rm your theory with even a small
telescope. You see no clouds or other obvious traces of an atmo-
sphere, and shadows near the terminator, the dividing line between
21-1
Earth’s moon is about one-fourth the diameter of Earth. Its low density indicates
that it contains little iron, but the size of its iron core and the amount of
remaining heat are unknown. (NASA)Celestial Profi le 3: Th e Moon
Motion:
Average distance from Earth 3.84 105 km (center to center)
Eccentricity of orbit 0.055
Inclination of orbit to ecliptic 5.1°
Average orbital velocity 1.02 km/s
Orbital period (sidereal) 27.3 d
Orbital period (synodic) 29.5 d
Inclination of equator to orbit 6.7°Characteristics:
Equatorial diameter 3.48 103 km
Mass 7.35 1022 kg (0.0123 M⊕)
Average density 3.36 g/cm^3 (3.3 g/cm^3 uncompressed)
Surface gravity 0.17 Earth gravity
Escape velocity 2.4 km/s (0.21 V⊕)
Surface temperature 170° to 130°C (275° to 265°F)
Average albedo 0.07Personality Point:
Lunar superstitions are common. The words lunatic and lunacy come
from luna, the moon. Someone who is moonstruck is supposed to be a
bit nutty. Because the moon affects the ocean tides, many superstitions
link the moon to water, to weather, and to women’s cycle of fertility.
According to legend, moonlight is supposed to be harmful to unborn
children; but, on the plus side, moonlight rituals are said to remove
warts.