The Astronomy Book

212

See also: Asteroids and meteorites 90–91 ■ The discovery of Ceres 94–99 ■

The composition of comets 207

S

o great was the contribution

of American geologist Eugene

Shoemaker to planetary

science that he is the only person

whose ashes have been sent to the

moon. He was a founding figure of

astrogeology, a science that uses

the techniques of geology to

investigate alien worlds.

Shoemaker’s early work hinged on

a crater in the Arizona Desert known

as Meteor Crater. Early European

settlers in nearby Canyon Diablo

believed it to be the vent of an

ancient volcano. Railroad engineers

passing through in the 1880s found

large iron-rich rocks strewn across

the desert, which suggested the

crater was caused by the impact of

a metallic meteorite. However, this

was dismissed because the volume

of debris around the rim largely

matched the volume of the crater

itself. It could not be a meteor

crater if there was no meteorite.

In 1903, Daniel Barringer, a

mining engineer, searched for the

iron meteor under the crater floor,

but to no avail. Not until 1960 did

Shoemaker find the evidence. The

crater contains shocked silica only

seen previously at nuclear bomb

test sites. These minerals cannot

have been made naturally by

volcanic forces: only the energy of a

meteorite traveling at 40,000 mph

(60,000 km/h) could have done it.

That energy had vaporized the

meteorite, which explained why

it was missing. Shoemaker had

provided the first proof that large

meteorites strike Earth, opening up

new possibilities for investigating

extraterrestrial objects. ■

METEORITES

CAN VAPORIZE

ON IMPACT

INVESTIGATING CRATERS

IN CONTEXT

KEY ASTRONOMER

Eugene Shoemaker

(1928–1997)

BEFORE

1891 American geologist

Grove Gilbert suggests that

the moon’s craters are the

result of meteorite impacts.

1891 Mineralogist Albert

E. Foote produces the first

geological description of

the crater.

AFTER

1980 American physicist

Luis Alvarez proposes that a

worldwide stratum of shocked

(pressurized) quartz between

the Cretaceous and Tertiary

periods indicates a huge

impact, which led to the

mass extinction of dinosaurs.

1994 Comet Shoemaker–

Levy, codiscovered by

Eugene Shoemaker, impacts

Jupiter and is observed by

the Galileo spacecraft en

route to the planet.

Not going to the moon and

banging on it with my own

hammer has been the biggest

disappointment in life.

Eugene Shoemaker