The Solar System

CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 461

it fractured the crust and allowed lava fl ows to resurface wide

areas. Because this happened near the end of cratering, the

smooth plains have few craters.

Finally, the cooling interior contracted, and the crust broke

to form the lobate scarps. Lava fl ooding ended quickly, perhaps

because the shrinking planet squeezed off the lava channels to

the surface. Mercury lacks a true atmosphere, so you would not

expect fl ooding by water, but radar images show a bright spot

at the planet’s north pole that may be caused by ice trapped in

perpetually shaded crater fl oors where the temperature never

exceeds 60 K (–350°F). Th is may be water from comets that

occasionally collide with Mercury and deliver a burst of water

vapor. Similar water deposits may exist at the poles of Earth’s

moon.

Th e fourth stage in the story of Mercury, slow surface evolu-

tion, is now limited to micrometeorites, which grind the surface

to dust; rare larger meteorites, which leave bright-rayed craters;

and the slow but intense cycle of heat and cold, which weakens

the rock at the surface. Th e planet’s crust is now thick, and

although its core may be partially molten, the heat fl owing out-

ward is unable to drive plate tectonics that would actively erase

craters and build folded mountain ranges.

of its lower-density rock and left it a small, dense world with a
surprisingly large metallic core.
Like the moon, Mercury suff ered heavy cratering by debris
in the young solar system. Planetary scientists don’t know accu-
rate ages for features on Mercury, but you can assume that crater-
ing, the second stage of planetary development, occurred over
the same period as the cratering on the moon. Th is intense cra-
tering declined rapidly as the planets swept up the last of the
debris left over from planet building.
Th e cratered surface of Mercury is not exactly like that of the
moon. Because of Mercury’s stronger gravity, the ejecta from an
impact on Mercury is thrown only about 65 percent as far as on
the moon, and that means the ejecta from an impact on Mercury
does not blanket as much of the surface. Also, the intercrater
plains appear to have formed when lava fl ows occurred during
the heavy bombardment, burying the older surface, and then
accumulated more craters. Sometime near the end of cratering, a
planetesimal over 100 km in diameter smashed into the planet
and blasted out the great multiringed Caloris Basin. Only parts
of that basin have been fl ooded by lava fl ows.
Th e smooth plains contain fewer craters and may date from
the time of the Caloris impact. Th e impact may have been so big

SCIENTIFIC ARGUMENT

Why don’t Earth and the moon have lobate scarps?

Of course, this calls for a scientifi c argument based on comparative

planetology. You might expect that any world with a large metallic

interior should have lobate scarps. When the metallic core cools

and contracts, the world should shrink, and the contraction should

wrinkle and fracture the brittle crust to form lobate scarps. But

there are other factors to consider. Earth has a fairly large metallic

core; but it has not cooled very much, and the crust is thin, fl ex-

ible, and active. If any lobate scarps ever did form on Earth, they

would have been quickly destroyed by plate tectonics.

The moon does not have a large metal core. It may be that

the rocky interior did contract slightly as the moon lost its inter-

nal heat, but that smaller contraction may not have produced

major lobate scarps. Also, any surface features that formed early

in the moon’s history would have been destroyed by the heavy

bombardment.

You can, in a general way, understand lobate scarps, but now

add timing to your argument. How do you know the lobate scarps

on Mercury formed after most of the heavy bombardment was

over?

CHAPTERCHAPTER 2 211 || THETHE MOON AND MERCURY: COMPARING AIRLESS WORLDS MOON AND MERCURY: COMPARING AIRLESS WORLDS 461

What Are We? Comfortable

Many planets in the universe probably look

like the moon and Mercury. They are small,

airless, and cratered. Some are made of

stone; and some, because they formed

farther from their star, are made mostly of

ices. If you randomly visited a planet

anywhere in the universe, you might fi nd

yourself standing on a cratered moonscape.

Earth-like worlds are unusual, but

perhaps not rare. The Milky Way Galaxy

contains over 100 billion stars, and over

100 billion galaxies are visible with existing

telescopes. Most of those stars probably

have planets, and although many planets

look like Earth’s moon and Mercury, there

are also probably plenty of Earth-like worlds.

As you look around your planet, you

should feel comfortable living on such a

beautiful planet, but it was not always such

a nice place. The craters on the moon and

the moon rocks returned by astronauts show

that the moon formed as a sea of magma.

Mercury seems to have had a similar history,

so the Earth likely formed the same way. It

was once a seething ocean of liquid rock

swathed in a hot, thick atmosphere, torn by

eruptions of more rock, explosions of gas

from the interior, and occasional impacts

from space. The moon and Mercury show

that that is the way Terrestrial planets

begin. Earth has evolved to become your

home world, but mother Earth has had a

violent past.