This lab was adapted from JPL Education’s “Create a Comet Lab”

Overview

It is believed that the water Earth was born with largely evaporated during its hot and turbulent formation. The rest may have been delivered by comet impacts. In this lab, we will explore the composition of comets, and the role they played in making Earth suitable for life to evolve.

Science Question

Where do comets come from, what are they made of, and how are they responsible for our living planet?

Materials

• Dry ice (10 lbs for the class)
• Insulated container to hold the dry ice
• Thick gloves
• Safety goggles
• Hammer (to crush the ice)
• Large trash bag to contain mess
• One plastic/styrofoam bowls
• Water (100 ml)
• Dirt/potting soil (70 ml)
• Dark corn syrup (100 ml)
• Vinegar (50 ml)
• Rubbing alcohol (1 tbsp)
• Hairdryer (just one)
• Flashlight or Heat Lamp (just one)
• Timer

Astronomy

Out in the far reaches of the solar system, beyond the orbit of Neptune, lies the Kuiper Belt (the ring of objects which contains Pluto) and the Oort Cloud (the sphere of dust and ice which extends almost ⅓ of the way out to the nearest star). The Kuiper Belt and the Oort cloud are both full of large, dirty iceballs which we know as comets.

Comets have a small core known as the “nucleus” which is usually only a few km across. The reason we can see them is because when the approach the sun, the solar wind causes gas to evaporate off of them in a big, bright cloud called a “coma”. The coma can be as large as the planet Jupiter!

Comets are happy when they live very far away from us, but sometimes gravitational interactions can fling them into the inner solar system. When this happens, they develop the pair of glowing tails which we usually associate with seeing them. The dust tail always trails directly behind the comet. The gas tail always points away from the sun. Sometimes these are close to the same direction, but usually they are not.

It is believed that comets may have delivered some, but not all, of Earth’s water in the first few hundred millions of years of the solar system. We do know that much of the water Earth was born with evaporated during its hot formation. But the ratio of “heavy water” (water whose hydrogen atoms have a neutron) to regular water in the ocean does not match the ratio observed in comets. The most likely explanation is that some of Earth’s water is from comets, and some of Earth’s water may have been hidden deep inside the Earth during its formation and later was brought to the surface by mantle circulation.

Comets also contain the organic molecules which make up life, such as amino acids and hydrocarbons. There is still much to learn about how the presence of comets in a solar system contributes to the development and evolution of life.

Activity 

1. Put on your safety goggles and cold-weather gloves. Do not take the goggles off until the comets are safely disposed of at the end.
2. Use a scale to measure the combined mass of your bowl and plastic bag. Record this mass.
3. Line your bowl with a plastic bag
4. Add 100 ml water to your bowl.
5. Add 70 ml of dirt.
6. Add 100 ml of corn syrup.
7. Add 50 ml of vinegar.
8. Add a tablespoon of rubbing alcohol—not too much, otherwise it will have an anti-freeze effect.
9. Mix the ingredients together.
10. Add the crushed/powdered dry ice to the mixture.
11. Cover the mixture with the top of the plastic bag and use your gloved hands to mix the dry ice with the rest of the materials. It is essential to cover the bowl because random bits of water and ice and dirt will be shooting out of the bowl. Make sure to let the gaseous CO2 come out of the bag.
12. You should feel a clump forming through the bag. If not, carefully uncover the bowl and add a little more water. Repeat until you feel the materials clump together. If your comet falls apart, that’s ok, real comets fall apart in space all the time. This may take a few minutes.
13. Put your bowl, bag, and comet on the scale and weigh them. Subtract the mass of the bowl and bag you measured earlier to obtain the weight of your comet. Record this, and the time that you weighed it as well on the table.
14. Your professor will have a flashlight and a hairdryer. When you have your comet assembled and weighed, hold it in your gloved hands in front of the flashlight/hairdryer (an analog for the sun) and observe the direction of the evaporating materials. Make one “orbit” of the flashlight/hair dryer to get a sense of where the comet tail goes as it approaches and recedes from the sun. Make sure part of your orbit gets pretty close to the ‘sun’, like a real comet.
15. Continue to weigh your comet every five minutes on the scale and fill out the table with your data, being careful to subtract the mass of the bowl and bag each time.

In the space below, draw a graph with time on the horizontal axis and comet mass on the vertical axis. If your comet was exposed to the “sun” in that time period, note where that occurs.

Questions

1. Describe the behavior of the evaporating materials as you orbit the flashlight/hair dryer.
2. Does this behavior mimic the dust tail, or the gas tail of a real comet?
3. If the nucleus of a comet is only a few km across, what makes them visible in the sky?
4. Draw a circle 1 inch across to represent the sun. Then, draw a stretched-out (“elliptical”) oval where the sun is close to one side of the oval. Draw eight evenly-spaced dots along the oval to represent possible locations of a comet. For each dot, draw a shaded gray dust tail pointing in the right direction, and an outline of the gas tail, also pointing in the right direction.