The Solar System

CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 457

around the sun (Figure 21-12). It would require another entire

orbit of Mercury for the sun to return to the noon position over-

head. So a full day on Mercury is two Mercury years long!

Th e complex tidal coupling between the rotation and revo-

lution of Mercury is an important illustration of the power of

tides. Just as the tides in the Earth–moon system have slowed the

moon’s rotation and locked it to Earth, so have the sun–Mercury

tides slowed the rotation of Mercury and coupled its rotation to

its revolution. Astronomers refer to such a relationship as a reso-

nance. You will see other such resonances as you continue to

explore the solar system.

Like its rotation, Mercury’s orbital motion is complex.

Recall from Chapter 5 that Mercury’s orbit is modestly eccentric

and precesses faster than can be explained by Newton’s laws but

at just the rate predicted by Einstein’s theory of general relativity.

Th e orbital motion of Mercury is taken as strong confi rmation of

the curvature of space-time as predicted by general relativity.

The Surface of Mercury

Because Mercury is close to the sun, the temperatures on

Mercury are extreme. If you stood in direct sunlight on Mercury,

you would hear your spacesuit’s cooling system cranking up to

top speed as it tried to keep you cool. Daytime temperatures can

exceed 700 K (800°F), although about 500 K is a more usual

high temperature. If you stepped into shadow on Mercury or

took a walk at night, with no atmosphere to distribute heat, your

spacesuit heaters would struggle to keep you warm. Th e surface

can cool to 100 K (-280°F). Nights on Mercury are bitter cold.

Don’t go to Mercury in a cheap spacesuit.

Nights are cold on Mercury because it has almost no atmo-

sphere. It borrows hydrogen and helium atoms from the solar

wind, and atoms such as oxygen, sodium, potassium, and calcium

have been detected in a cloud above the planet’s surface that has

such a low density that the atoms do not collide with each other.

Th ey just bounce from place to place on the surface and, because

of the low escape velocity, eventually disappear into space. Some

of these atoms are probably baked out of the crust, or possibly

produced by very low-level remnant volcanic venting.

In photographs, Mercury looks much like Earth’s moon

(■ Figure 21-13). It is heavily battered, with craters of all sizes,

including some large basins. Some craters are obviously old and

degraded; others seem quite young and have bright rays of ejecta.

However, a quick glance at photos of Mercury shows no large,

dark maria like the moon’s fl ooded basins.

When planetary scientists began looking at Mercury fl yby

photographs in detail, they discovered something not seen on the

moon. Mercury is marked by great curved cliff s called lobate

scarps (■ Figure 21-14). Th ese seem to have formed when the

planet cooled and shrank in diameter by a few kilometers, wrin-

kling its crust as a drying apple wrinkles its skin. Some of these

scarps are as high as 3 km and reach hundreds of kilometers

across the surface. Other faults in Mercury’s crust are straight and

If you fl ew to Mercury and landed your spaceship in the
middle of the day side, the sun would be high overhead, and it
would be noon. Your watch would show almost 44 Earth days
passing before the sun set in the west, and a total of 88 Earth
days would pass before the sun reached the midnight position. In
those 88 Earth days, Mercury would have completed one orbit

■ Figure 21-12

Mercury’s rotation is in resonance with its orbital motion. It orbits the sun
in 88 days and rotates on its axis in two-thirds of that time. One full day on
Mercury from noon to noon takes two orbits.

Imagine a mountain

on Mercury that

points at the sun. It is

noon at the mountain.

After one orbit, Mer-

cury has rotated 1.5

times, and it is mid-

night at the mountain.

The planet orbits and

rotates in the same

direction, counter-

clockwise as seen

from the north.

After half an orbit,

Mercury has rotated

3/4 of a turn, and it is

sunset at the

mountain.

As the planet

continues along its

orbit, rotation carries

the mountain into

darkness.

The Rotation of Mercury