Far from Earth, between Mars and Ju

Far from Earth, between Mars and Jupiter there
is a ring composed of countless millions of rocks
of varying sizes. This is the asteroid belt, and
deep inside it there is a group of rocks known
as the Baptistina asteroid family. At one time
these rocks were joined together, forming a
single asteroid approximately 105 miles (170 km)
in diameter, but about 160 million years ago that
body was shattered in a collision. In uenced by
the gravitational attraction of neighboring bod-ies, little by little the Baptistina fragments left
the asteroid belt, some on orbits that brought
them close to Earth. About 109 million years
ago a fragment about 2.5 miles (4 km) across
struck the Moon, forming the 53-mile- (85-km-)
diameter crater Tycho. About 65 million years
ago a fragment about six miles (10 km) across
struck Earth.
In 1979 Walter Alvarez, Luis W. Alvarez, Frank
Asaro, and Helen V. Michel published in volume
11 of Geological Society of America: Abstracts with
Programan account of their discovery of traces
of two metals, iridium and osmium, in a thin layer
of clay at Gubbio, Italy, and the implications of
that discovery. The following year they told the
story again in Sciencein a paper entitled “Extra-terrestrial Causes for the Cretaceous—Tertiary
Extinction.” Iridium and osmium are very rare
in the rocks of the Earth’s crust but much more
common in extraterrestrial dust and other mate-rial that has survived from early in the formation
of the solar system. The Alvarez team found that
the Gubbio clay layer contained about 30 times
more of the two metals than the material above
and below that level, and the clay was dated at
about 65 million years old. Later, similar enrich-ment was discovered in clays of the same age
at Stevns Klint, Denmark, and Woodside Creek,
about 25 miles (40 km) from Wellington, New
Zealand. The enrichment was global in extent,
and the most plausible explanation for its pres-ence was that it had arrived from space. Relat-ing calculations of the total amount of iridium
and osmium with the average concentration
in extraterrestrial material led the scientists to
conclude that the enrichment resulted from the
impact and destruction of an extraterrestrial
body about six miles (10 km) in diameter that
struck the Earth 65 million years ago traveling at
about 12 miles per second (20 km/s). An impact
of that magnitude would leave a large crater,
but several years passed before the crater was
discovered, beneath Chicxulub, on the Yucatán
Peninsula, in Mexico.
The boundary between the end of the Creta-ceous and beginning of the Paleogene periods,
dated at 65.5 million years ago, is marked by
the disappearance of fossils of a wide range of
animals that are present before the boundary
but absent after it. A mass extinction of animals
occurred at that time, but until the Alvarez dis-covery its cause had been a mystery. Today most
geologists and paleontologists accept that the
impact of a body from outer space, releasing
an amount of energy equivalent to the detona-tion of about 100 trillion (10
12
) tons of TNT, was
responsible. The letters K(from the German
MODERN CATASTROPHISM—AND THE DEATH
OF THE DINOSAURS
(continues)