Lab IVa -- Comets

Comets: Characterizing a Comet's Nucleus

Objective

To identify the primary constituents and physical characteristics of the nucleus of a comet; to make a "model" comet.

Introduction

Comets are some of the most spectacular objects in our solar system. The large, bright, fuzzy object that one sees through a telescope or occasionally with the naked eye is an immense, diffuse cloud of gas and dust. This cloud surrounds the tiny, hard object called the cometary nucleus. Heating of the nucleus by sunlight as the comet moves in close to the Sun (inside the "snow line") causes the ices in the comet to become gaseous. These gases spew away from the comet carrying small particles of cosmic dust with them.

When comets are well beyond the snow line, well beyond the orbit of Jupiter, they do not have a well defined coma, and definitely not any tail. But, whether the comet is just a dirty snowball freezing at the outer reaches of the solar system, or displaying a spectacular coma and tails when whizzing around in the inner solar system, we still call it a "comet". What we are observing is a comet at various levels of activity.

From the Cosmic Cookbook:

A Comet Recipe

Place in a large mixing bowl:

9 billion tons of water ice
800 million tons of dry ice (carbon dioxide)
50 million tons of ammonia ice
Carbon monoxide ice to taste
140 million tons of carbonaceous particles
9 million tons of silicate sand
A pinch of cosmic seasonings (sodium, potassium, calcium, iron, copper, etc.)
Miniscule amount of organic material

Combine all ingredients and stir well. Place in a freezer and freeze at a temperature of -200 C until hard.

Yield: One 10 billion ton cometary nucleus

A Comet Recipe
Place in a large mixing bowl:
9 billion tons of water ice 800 million tons of dry ice (carbon dioxide) 50 million tons of ammonia ice Carbon monoxide ice to taste 140 million tons of carbonaceous particles 9 million tons of silicate sand A pinch of cosmic seasonings (sodium, potassium, calcium, iron, copper, etc.) Miniscule amount of organic material
Combine all ingredients and stir well. Place in a freezer and freeze at a temperature of -200 C until hard.
Yield: One 10 billion ton cometary nucleus

A More Reasonable Comet Recipe

2 cups water (H₂O)
2 spoonfuls of carbon (charcoal)
dash of ammonia (NH₃)
drop of organic material
pinch of salt (NaCl)
pinch of sand (Silicates)
2 cups crushed dry ice (frozen carbon dioxide, CO₂)

Combine all ingredients, except the dry ice, in a pail lined with heavy plastic.
Use a wooden spoon to mix all ingredients.
Wearing gloves, add the dry ice and stir until the mixture becomes stiff.
Carefully lift the plastic by the corners and squeeze almost closed just above the comet.
Using two hands (thus more than one person is needed), carefully, but forcefully form the comet into a ball through the plastic.
Place on table and unwrap the newborn comet.

Yield: One 15-cm cometary nucleus

A More Reasonable Comet Recipe
2 cups water (H₂O) 2 spoonfuls of carbon (charcoal) dash of ammonia (NH₃) drop of organic material pinch of salt (NaCl) pinch of sand (Silicates) 2 cups crushed dry ice (frozen carbon dioxide, CO₂)
Combine all ingredients, except the dry ice, in a pail lined with heavy plastic. Use a wooden spoon to mix all ingredients. Wearing gloves, add the dry ice and stir until the mixture becomes stiff. Carefully lift the plastic by the corners and squeeze almost closed just above the comet. Using two hands (thus more than one person is needed), carefully, but forcefully form the comet into a ball through the plastic. Place on table and unwrap the newborn comet.
Yield: One 15-cm cometary nucleus

Instructor's
Initials

Questions

Why are comets made out of ices?
What do you notice about the "ingredients" of a comet?
Why are comets easily broken apart by Jupiter or the Sun?
How did some or most of our water get on Earth?
If they were the same size, what would make a larger crater: a meteorite or a comet? Why?

Physical characteristics of the nucleus of a comet

Let's now consider each of four different characteristics of a comet separately. For each particular characteristic of the nucleus, list a terrestrial object (ANYTHING found on Earth) that also shares that characteristic. Then, list a solar system object that the nucleus resembles.

Property Terrestrial Object Solar System Object

Albedo

Density

Shape

Mechanical Strength

Reasonable comet recipe adapted from "recipe" by Dennis Schatz, Pacific Science Center, Seattle, WA.