The Solar System

CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 453

from the Imbrium impact (■ Figure 21-7), and by the Apollo 15
astronauts, who landed at the edge of the mare itself. Near the end
of the heavy bombardment, roughly 4 billion years ago, a planetes-
imal estimated to have been at least 60 km (40 mi) across (about
the size of Rhode Island) struck the moon and blasted out a giant
multiringed basin. Th e impact was so violent the ejecta blanketed
16 percent of the moon’s surface. After the cratering rate fell at the
end of the heavy bombardment, lava fl ows welled up time after
time and fl ooded the Imbrium Basin, burying all but the highest
parts of the giant multiringed basin. Th e Imbrium Basin is now a
large, generally round mare marked by only a few craters that have
formed since the last of the lava fl ows (■ Figure 21-8).
Th is story of the moon might suggest that it was a violent
place during the cratering phase, but large impacts were in fact
rare; the moon was, for the most part, a peaceful place even during
the heavy bombardment. Had you stood on the moon at that time
you would have experienced a continuous rain of micrometeorites
and much less common pebble-size impacts. Centuries might pass
between major impacts. Of course, when a large impact did occur
far beyond the horizon, it might have buried you under ejecta or
jolted you by seismic shocks. You could have felt the Imbrium
impact anywhere on the moon, but had you been standing on the
side of the moon directly opposite that impact, you would have
been at the focus of seismic waves traveling around the moon from
diff erent directions. When the waves met under your feet, the
surface would have jerked up and down by as much as 10 m. Th e
place on the moon opposite the Imbrium Basin is a strangely dis-
turbed landscape called jumbled terrain. You will see similar
eff ects of large impacts on other worlds.
Studies of our moon show that its crust is thinner on the side
facing Earth, perhaps due to tidal eff ects. Consequently, while

■ Figure 21-7

Apollo 14 landed on rugged terrain suspected of being ejecta from the
Imbrium impact. The large boulder here is ejecta that, at some time in the
past, fell here from an impact far beyond the horizon. (NASA)

■ Figure 21-8

Lava fl ooding after the end of the heavy bombardment fi lled a giant

multiringed basin and formed Mare Imbrium. (Don Davis)

Origin of Mare Imbrium

Archimedes

KeplerKepler CopernicusCopernicus

Four billion years ago,

an impact forms a

multiringed basin over

1000 km (700 mi) in

diameter.

Continuing impacts

crater the surface but

do not erase the high

walls of the

multiringed basin.

Impacts form a few

large craters, and,

starting about 3.8

billion years ago, lava

floods low regions.

Repeated lava flows

cover most of the inner

rings and overflow the

basin to merge with

other flows.

Impacts continue,

including those that

formed the relatively

young craters Coper-

nicus and Kepler.

lava fl ooded the basins on the Earth-ward side, it was unable to

rise through the thicker crust to fl ood the lowlands on the far

side. Th e largest known impact basin in the solar system is the

Aitken Basin near the moon’s south pole (Figure 21-6b). It is

about 2500 ki (1500 mi) in diameter and as deep as 13 kilome-

ters (8 mi) in places, but fl ooding has never fi lled it with smooth

lava fl ows.