A Kitchen Table Comet

Comets formed long ago at the very edge of the Solar System, beyond the orbit of Pluto! In this region, chemicals we think of as gases are so cold they are frozen solid. In this demonstration you will work with solid carbon dioxide, also known as dry ice.

Comets are mostly made of frozen water, frozen carbon dioxide, a few other frozen gases like ammonia and methane, dust and rocks, and carbon-rich organic material. In fact, some cometary material contains amino acids, the building blocks of life here on Earth.

While real comets are many miles across and contain complex chemicals, they can still be thought of as dirty snowballs. You can build a model comet from some readily available materials.

The ingredients for a six-inch comet are: *Dry ice is available from local supermarkets - we've found it at Albertson's here in the Valley.

Other materials to have on hand:

Instructions:
  1. Open one garbage bag and use it to line the mixing bowl,
  2. Place water in mixing bowl,
  3. Add sand or dirt,
  4. Add organic material, stir well,
  5. Place dry ice inside 3 garbage bags which have been placed inside each other,
  6. Crush dry ice into powder with hammer or mallet, making sure there are no lumps,
  7. Add dry ice powder to the rest of the mixture while stirring vigorously,
  8. Continue stirring and adding dry ice powder until mixture becomes slushy,
  9. Gather garbage bag around mixture and shape it as you would a snowball,
  10. Unwrap the comet from the garbage bag and place it on display for everyone to see!
The comet is reasonably safe to touch since it is mostly water ice.

You should see (and hear) carbon dioxide escaping from the comet at various spots on the surface. This is due to solid carbon dioxide turning directly to a gas, a process called sublimation. (A solid is said to sublimate when it turns directly to a gas.) This behavior is seen on real comets, and these jets of gas can actually push a comet enough to change its orbit around the Sun.

Just as a real comet is warmed when it passes near the Sun, your comet will warm and the carbon dioxide will sublimate. After an hour or more, the comet will become a crater-filled lump of dirty ice. When a real comet passes the Sun many times it can become very fragile and eventually break into many pieces. The rocks and dust left over can hit the Earth's atmosphere and form meteors, also known as shooting stars.

Here are a few links related to comets:
Comets at the Nine Planets website
SpaceWeather.com has news on comets visible in the sky
Comet Observation Home Page at JPL



Questions/comments? Contact us at astro.info@asu.edu