The evidence for an impact by an enormous asteroid or comet comes from tiny particles of dust.
Evidence: In the late 1970s a group of researchers led by physicist Luis Alvarez and geologist Walter Alvarez made an important discovery. While analyzing rock samples from 65 million years ago, the father-son team measured exceptionally high concentrations of a rare element, iridium.
Interpretation: The element iridium is rare in Earth's crust but is more abundant in the interior of our planet and in certain asteroids and comets. The Alvarez team argued that the iridium-rich layer of dust was the fallout from a massive asteroid or comet that vaporized upon impact with Earth, blasting huge quantities of dust and debris into the atmosphere.
Conclusion: The consequences of a major impact--reduced sunlight, short-term cooling, acid rain, huge tsunamis, global forest fires and long-term greenhouse warming--would have been devastating to life on Earth. Because the iridium-rich layer corresponded with the mass extinction 65 million years ago, the Alvarez team concluded that the impact triggered the extinction. In 1991, other investigators proposed that a crater in Mexico, called Chicxulub, was the likely site of this impact.
Collision With a Comet
Huge asteroids and comets don't collide with our planet very often, so scientists can't easily observe the effects of a major impact. But in 1994, researchers got a glimpse of what can happen when a sizeable comet crashes into a planet--in this case, Jupiter. The fiery results offered clues to how devastating the ancient impact on Earth might have been.
The comet named Shoemaker-Levy 9, already shattered into many pieces, slammed into Jupiter in a series of impacts. Many of the fragments were between one and three kilometers (0.6 and 1.9 miles) across in size. The multiple impacts sent fireballs high above Jupiter's atmosphere and left dark scars so large our own planet would have fit inside.
What happened in the first few minutes after the impact 65 million years ago? Thanks to powerful computers, physicists have created virtual models of the collision, which released as much energy as 300 million nuclear bombs. This model, created by astrophysicist Galen Gisler and colleagues, shows the size, direction and temperature of the impact debris.