The Solar System

576 PART 4^ |^ THE SOLAR SYSTEM

near limestone deposits, large amounts of carbon dioxide could

be released into the atmosphere and produce intense acid rain.

Geologists have located a crater at least 180 km (110 mi) in

diameter centered near the village of Chicxulub (pronounced

cheek-shoe-lube) in the northern Yucatán region of Mexico

(■ Figure 25-23). Although the crater is now completely covered

by sediments, mineral samples show that it contains shocked

quartz typical of impact sites and that it is the right age. Th e

impact of an object 10 to 14 km in diameter formed the crater

about 65 million years ago, just when the dinosaurs and many

other species died out. Most scientists now conclude that this is

the scar of the impact that ended the Cretaceous period.

Th ere are a number of major extinctions in the fossil record,

and at least some of these were probably caused by large impacts.

Large asteroid impacts on Earth happen very rarely from a

human perspective, but they happen often relative to geological

Mathematical models combined with observations create a
plausible scenario of a major impact on Earth. Of course, crea-
tures living near the site of the impact would die in the initial
shock, but then things would get bad elsewhere. An impact at sea
would create tsunamis (tidal waves) many hundreds of meters
high that would sweep around the world, devastating regions far
inland from coasts. On land or sea, a major impact would eject
huge amounts of pulverized rock high above the atmosphere. As
this material fell back, Earth’s atmosphere would be turned into
a glowing oven of red-hot meteorites streaming through the air,
and the heat would trigger massive forest fi res around the world.
Soot from such fi res has been found in the layers of clay laid
down at the end of the Cretaceous period. Once the fi restorms
cooled, the remaining dust in the atmosphere would block sun-
light and produce deep darkness for a year or more, killing off
most plant life. At the same time, if the impact site were at or

Infrared images

Visual Infrared

Visual image composite

Larger than

Earth

Fragments of

comet falling

toward Jupiter

Only 9 minutes after

one impact, the

fireball was brilliant

in the infrared.

Impact sites remained bright in the

infrared as the rotation of Jupiter

carried them into sight from Earth.

At visual wavelengths,

impact sites were dark

smudges that lasted for

many days.

Impact site just out

of sight as seen

from Earth

a

b

c

d

■ Figure 25-21

In 1992 Comet Shoemaker–Levy 9 passed within 1.3 planetary

radii of Jupiter’s center, well within its Roche limit, and tidal

forces ripped the nucleus into more than 20 pieces. The frag-

mented pieces were as large as a few kilometers in diameter

and spread into a long string of objects that looped away from

Jupiter and then fell back to strike the planet, producing mas-

sive impacts over a period of six days in July 1994. Note that

the impact fl ash in panel (c) is larger than Earth. (NASA)