Meteors and Meteorites

Meteorite Facts

• Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta (picture 6); a few probably come from comets. A small number of meteorites have been shown to be of Lunar (15 finds) or Martian (12) (picture 5) origin.

• One of these Martian meteorites, known as ALH84001 (picture 10), is believed to show evidence of early life on Mars.

• Though they are just boring looking rocks, they are extremely important in that we can analyze them carefully in our labs. Aside from the few kilos of moon rocks brought back by the Apollo and Luna missions, meteorites are our only physical evidence about the universe beyond the Earth.

• Meteorites are classified as:
  • carbonaceous chondrites: very similar in composition to the Sun less volatiles; similar to type C asteroids (picture 4);
  • ordinary chondrites: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets (picture 1, above);
  • irons: primarily iron and nickel; similar to type M asteroids (picture 3);
  • stony irons: mixtures of iron and stony material like type S asteroids;
  • achondrites: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites (picture 2)

• A very large number of meteoroids enter the Earth's atmosphere each day amounting to several hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites.

• The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

• A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona (picture 7). It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far (see below).

• A more recent impact occured in 1908 in a remote uninhabited region of western Siberia known as Tunguska. The impactor was about 60 meters in diameter and probably consisting of many loosely bound pieces. In contrast to the Barringer Crater event, the Tunguska object completely disintegrated before hitting the ground and so no crater was formed. Nevertheless, all the trees were flattened in an area 50 kilometers across. The sound of the explosion was heard half-way around the world in London.

• There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in 300,000 years on average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disasterous consequences.

• The impact of a comet or asteroid about the size of Hephaistos or SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago.

• Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like the one 180 km in diameter near Chicxulub in the Yucatan Peninsula (picture 8); once per 100 million years seems like a reasonable guess.

Pictures

1. Chondrite Meteorite 186k gif

2. Achondrite Meteorite 178k gif

3. (above) Iron Meteorite 193k gif

4. part of the Allende fall, a carbonaceous-chondrite 30k jpg

5. Martian meteorite 212k gif

6. a piece of 4 Vesta 78k jpg; 376k gif; caption

7. Barringer Crater 308k gif; 107k jpg; 2000k tif

8. Chicxulub Crater gravimetric "image" 103k gif (caption)
9. a fireball captured on video 20k gif (caption)

10. ALH84001: evidence of life on Mars? 64k jpg (more info)

More about Meteorites

• from LANL
• from JPL
• from RGO
• What is a Meteorite? by Paul Sipiera
• near-Earth objects from RGO
• Micrometeorites from LANL
• Meteorite and Impacts Advisory Committee, includes info and more images
• an essay on Antarctic meteorites and Antarctic Meteorite News
• International Meteor Organization
• Gary Kronk's Meteor Showers page
• the Dutch Meteor Society -- info about current meteor showers (and more)
• SNC Meteorites
• Mars Meteorites from Ron Baalke at JPL (lots of images!)

  Impacts

• Asteroid and Comet Impact Hazards from NASA Ames
• the Spacewatch Project
• The Impact Catastrophe (ExInEd hypercard stack)
• Essay about the K-T Event by Calvin Hamilton of LANL
• The Probability of Collisions with Earth (also from LANL)
• text and pictures of Terrestrial Impact Craters (also from LANL)
• a teacher's guide about Cosmic Collisons by Sally Stephens of PASP
• Comet Busters by Peter Tyson (from Technology Review)
• Meteorite! Magazine Home Page
• Orbit Diagrams for various Near-Earth objects and comets
• tables of past and future close approaches to the Earth and Potentially Dangerous NEOs
• nice atlas of Terrestrial Impact Structures (with maps and images)
• another list of identified impact craters on Earth (plain text)
• Catastrophism by Philip Burns

Open Issues

• Can we trace the origin of the various types of meteorites to a parent asteroid?
• Was it a comet or an asteroid that caused the Chicxulub crater in the Yucatan (and probably caused the extinction of the dinosaurs)?
• Will Spacewatch retain enough funding to finish its job of identifying all the potentially dangerous near-Earth asteroids?
• Will we find more pieces of Mars in Antarctica?

Note

It is well worth contemplating how we reached this moment of discovery. More than 4 billion years ago this piece of rock was formed as a part of the original crust of Mars. After billions of years it broke from the surface and began a 16 million year journey through space that would end here on Earth. It arrived in a meteor shower 13,000 years ago. And in 1984 an American scientist on an annual U.S. government mission to search for meteors on Antarctica picked it up and took it to be studied. Appropriately, it was the first rock to be picked up that year -- rock number 84001.

Today, rock 84001 speaks to us across all those billions of years and millions of miles. It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered. Its implications are as far-reaching and awe-inspiring as can be imagined. Even as it promises answers to some of our oldest questions, it poses still others even more fundamental.

We will continue to listen closely to what it has to say as we continue the search for answers and for knowledge that is as old as humanity itself but essential to our people's future.