Comets and Asteroids
Learning Objectives
Comets
A comet is a body composed of ice and rock that is in a long elliptical orbit around the Sun.
The ice portion of the comet is composed of frozen gases.
As a comet nears the Sun the "solar wind" (ionized particles from the Sun) forces gas out from behind the comet in a long "tail". The tail is illuminated by the Sun and can put on a spectacular show. The tail always points away from the Sun.
Asteroids
Asteroids are rocky or iron bodies that orbit the Sun. Most asteroids can be found in the Asteroid Belt, an area of space between Mars and Jupiter with a high concentration of asteroids.
Asteroids come in all sizes. A meteor is an asteroid that has entered the Earth's atmosphere, a meteorite has struck the ground.
It is believed that asteroids are the remains of a planet that never formed or a planet that was destroyed.
Asteroids in the asteroid belts occur in distinct bands. Two bands of concern are the near Earth asteroids and the Earth crossing asteroids. Near Earth asteroids orbit close to the Earth's orbit. Earth crossing asteroids have orbits that cross Earth's orbit.
Asteroids strike the Earth regularly. Impact scars are not readily visible because they are obliterated through geologic processes. Modern geologic methods allow scientists to image the rocks below the surface. When scientists look below the sediment that covers the Earth, impact craters become apparent and their abundance on the Earth's surface becomes evident.
Meteor Crater, Arizona is a famous impact site. It is only 50,000 yrs old and has not been heavily eroded.
The Chesapeake Bay was impacted by a meteor 35 million yrs ago. It is not evident because it lies under hundreds of feet of sediment.
It has been theorized that the dinosaurs became extinct as the result of a meteor impact. Evidence seems to support that theory.
Effects of a meteor impact:
As a meteor enters the Earth's atmosphere it encounters friction and starts to burn. Meteors are referred to as "shooting stars" because they often glow as they streak across the sky. If the meteor is larger than a basketball it will create a sonic boom that can be heard. If the meteor is large enough it will survive its trip though the atmosphere and strike the Earth's surface. The larger the meteor the larger and more far reaching the effects.
Initially a meteor will:
1. Create a crater by pushing the rocks outward and downward. The crater can be up to 100 times larger than the meteor that created it.
2. Debris is thrown high into the atmosphere.
3. Extreme temperatures and pressures will vaporize and melt some of the meteor and surrounding rock material. This will fall back to the surface as rounded glassy bead-like objects called tektites.
4. Shock waves are generated that pass through the atmosphere and can be destructive.
5. If the meteor strikes in the ocean a tsunami can form.
6. Extreme temperatures can cause wildfires.
If the meteor is large enough the effects will be felt world-wide:
1. Dust thrown up into the atmosphere blocks sunlight. This cools the climate and causes the cessation of photosynthesis, the food chain collapses.
2. Rock material thrown into the atmosphere causes acid precipitation.
Evidence for a meteor strike:
Many impact structures are so badly eroded that it is often necessary to look for other evidence that a meteor impacted.
1. Shocked quartz: Quartz grains near the impact site will have characteristic fractures running through them.
2. The presence of tektites.
3. Evidence of wildfires.
4. As a crater forms the slides collapse inward forming a series of faults outward from the crater.
5. Extensive debris field surrounding the suspected impact site. The debris field will consist of jumbled rock debris of all types.
