What Happened in Brief
According to abundant geological evidence, an asteroid
roughly 10 km (6 miles) across hit Earth about 65 million years ago.
This impact made a huge explosion and a crater about 180 km (roughly 110
miles) across. Debris from the explosion was thrown into the
atmosphere, severely altering the climate, and leading to the extinction
of roughly 3/4 of species that existed at that time, including the
dinosaurs. Many asteroids of this type are now known; their orbits pass
through the inner solar system and cross Earth's orbit. Some of these
could potentially hit Earth in the future. Most, but not all are smaller
than the one that hit us 65 million years ago.
How Was the Impact Event Discovered?
In the late 1970s, a team of geochemists headed by Luis Alvarez and his son Walter Alvarez, both with connections to the University of California at Berkeley, was
studying chemical changes in soil layers corresponding to breaks in the
fossil record. In the soil layer that separates the Mesozoic Era from
the Cenozoic Era, dating 65 My ago, they found an excess of the element
iridium, which is common in meteorites. Meteorites are believed to be
fragments of asteroids. Therefore, the Alvarez team theorized that an
asteroid had hit Earth at this time, and that the debris ejected from
the explosion were spread in the soil layer.
For about ten years, this theory was extremely
controversial. However, compelling evidence has accumulated to support
the theory.
Evidence for the Impact Event
There are now many lines of evidence to prove that a relatively large impact happened 65 My ago.
- The iridium excess in the 65 My-old soil layer has been confirmed at many points around the world.
- The same soil layer contains grains of quartz that were
deformed by high shock pressures, as would occur in a giant explosion.
(The deformation is a microscopic structure called "twinning," in the
crystals).
- The same soil layer contains enough soot to correspond to
burnding down all of the forests of the world. This suggests that
massive fires were touched off at the time of impact.
- The same soil layer, especially around the Gulf of Mexico,
contains massive deposits of tumbled boulders, as would be generated in a
large tsunami, or "tidal wave." The geographic distribution of tsunami
deposits suggest the impact was in the Caribbean area.
- After a decade of searching, scientists in 1990 identified
the crater associated with this material. It is no longer visible on the
surface of the Earth, but is buried under sediments. It straddles the
coast of Yucatan. It is revealed by mapping the strength of the gravity
field over that area, and by drilling; it has been dated to 65 My old.
- Astronomers have charted numerous asteroids that cross
Earth's orbit. From studies of orbit statistics, it is estimated that
asteroids of 10 km size can hit the earth roughly every 100 My or so --
which fits with the idea that we actually did get hit 65 My ago by an
object this size. (Smaller hits are much more common).
What Happened During the Impact?
Asteroids hit Earth typically at
high speeds of 16 to 32 km/sec (10-20 miles/sec). During the impact, the
kinetic energy in the asteroid (or energy of motion) is converted to
explosive energy, blowing debris of dust, soil, and rocks not only into
the atmosphere, but out into space, where it fell back into the top of
the atmosphere. Early calculations in the 1980s (using in part ideas
worked out by Carl Sagan and his colleagues) showed that so much dust
entered the high atmosphere that the Earth was shrouded in a dust layer
that blocked sunlight for several weeks or months. This would have
killed some plants, disrupting the food chain.
Later calculations (especially by Jay Melosh at the
University of Arizona) indicated that for the first few hours after the
impact, rocky debris would have fallen back into the high atmosphere,
creating a storm of glowing fireballs in the sky. The radiant energy
from these would have heated the surface to boiling temperatures for
some minutes, and would have been enough to kill many animals and plants
on the surface. However, in regions of heavy rainstorms or snowstorms,
these organisms would have survived the first few hours. Sea creatures
would have been buffered from effects in the first hours, but plankton
on the surface might have died out over the weeks of darkness,
decreasing the food supply for small fish, which affected the bigger
fish, and so on.
These examples show how hard it is to predict the exact
effects of the impact. Many species who lived on the surface (such as
dinosaurs) might have been decimated in hour or weeks. Species who lived
in burrows, or hibernated (like some mammals) might have survived. This
may explain why mammals replaced giant reptiles after the impact. Tiny
primitive mammals may have emerged from their dens, to find that their
giant reptile competitors were mostly gone.
In the late 1970s, a team of geochemists headed by Luis Alvarez and his son Walter Alvarez, both with connections to the University of California at Berkeley, was
studying chemical changes in soil layers corresponding to breaks in the
fossil record. In the soil layer that separates the Mesozoic Era from
the Cenozoic Era, dating 65 My ago, they found an excess of the element
iridium, which is common in meteorites. Meteorites are believed to be
fragments of asteroids. Therefore, the Alvarez team theorized that an
asteroid had hit Earth at this time, and that the debris ejected from
the explosion were spread in the soil layer. 
