CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 575
asteroid about 30 m in diameter, perhaps one-tenth the mass of
the Barringer impactor. Th e models indicate that an object of
this size with moderate material strength would have fragmented
and exploded at just about the right height to produce the
observed blast. Th is conclusion is consistent with modern studies
of the Tunguska area showing that thousands of tons of pow-
dered material with a composition resembling carbonaceous
chondrites are scattered in the soil.Big Impacts
Th ere are some very big craters in the solar system, for example
on the moon (page 446), that show what can happen when a
full-sized asteroid or comet collides with a planet. Also, Earthlings
watched in awe in 1994 as fragments from the nucleus of Comet
Shoemaker–Levy 9 (abbreviated SL-9) slammed into Jupiter and
produced impacts equaling millions of megatons (that is, trillions
of tons) of TNT (■ Figure 25-21). Note that the Shoemaker in
Shoemaker–Levy refers to Carolyn and Eugene Shoemaker who
co-discovered the comet with David Levy. Eugene Shoemaker is
the person whose analysis showed that Barringer Crater in
Arizona is an impact crater.
As you know, Jupiter does not have a solid surface, so SL-9
did not leave any permanent craters, but astronomers have found
chains of craters on other solar system objects that seem to have
been formed by fragmented comets (■ Figure 25-22). Evidently
events like the SL-9 collision with Jupiter have occurred many
times in the history of the solar system.
What would happen if an object the size of SL-9, or even
larger, were to hit Earth? Sixty-fi ve million years ago, at the end
of the Cretaceous period, over 75 percent of the species on Earth,
including the dinosaurs, became extinct. Scientists have found a
thin layer of clay all over the world that was laid down at that
time, and it is rich in the element iridium—common in meteor-
ites but rare in Earth’s crust. Th is suggests that an impact
occurred that was large enough to have altered Earth’s climate
and caused the worldwide extinction.atmosphere. On the other hand, a dense iron-rich meteorite
would be so strong that it would have survived to reach the
ground and would have formed a large crater. Th erefore, the
most likely candidate for the Tunguska object seems to be a stony
Trees blown downBeltway
Trees charredProposed path of meteorWashington, D.C.15 km■ Figure 25-20
The 1908 Tunguska event in Siberia destroyed an area the size of a large
city. Here the area of destruction is superimposed on a map of Washington,
D.C., and its surrounding beltway. In the central area, trees were burned; in
the outer area, trees were blown down in a pattern away from the path of
the impactor.