Dunking the Planets
In this 30-minute demonstration, children compare the relative sizes and masses of scale models of the planets as represented by fruits and other foods. The children dunk the "planets" in water to highlight the fact that even a large, massive planet — such as Saturn — can have low density. They discuss how a planet's density is related to whether it is mainly made up of rock or gas.
This activity establishes the relationship between density and composition. It should be conducted before Heavyweight Champion: Jupiter! in order for the children to better distinguish the concepts of size, weight, and mass. These concepts involve more advanced science than previous activities in Jupiter's Family Secrets, and they explore more deeply the science of the Juno mission and the rich information it will return to us. Facilitators who choose to undertake this activity should have a firm grasp of the scientific basis so that misconceptions are not introduced to the children.
What's the Point?
For each group of 10 children:
- One of each of the following fresh fruits and other foods (listed from largest to smallest):
- (5 1/2"-wide) large mango or potato
- (4 1/2"-wide) large unpeeled orange, coconut, or cantaloupe
- (2"-wide) plum
- (2"-wide) kiwi or lime (not a lemon)
- (1/2"-wide) small grape
- (1/2"-wide) large blueberry
- (1/4"-wide) pea or navy bean
- (1/5"-long) uncooked orzo pasta
- 1 (18" wide x 8" deep or larger) bowl, tub, or small wading pool
- A sink or other access to water
- Optional: 1 golf ball or ball bearing
- Optional: 1 ping-pong ball or a marble that is similar in size to the ball bearing
1. Present the fruits and other foods to the children and explain that you will use them to model the physical properties of the planets. Discuss one important property of the planets represented by this model: size.
- What's a model?
We use models to help us represent objects and systems so that we can study and understand them more easily. By "a scale model" in this case, we mean a model that has smaller parts but parts that are relatively the same size and distance to each other as the real planets, dwarf planets, asteroid belt, and Sun.
- What are the names of the planets in the solar system?
Starting with those closest to the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- Which planets are the biggest? Which are the smallest?
As the children answer, give them the fruit or other object that represents each planet and ask them to stand at the front of the room.
2. Optional: Have the children investigate the difference between size and mass by comparing a golf ball and a similarly sized ping-pong ball. Invite them to predict which is more massive before allowing them to hold them.
- How do the sizes of the two balls compare? They're about the same size.
- Which is more massive? The golf ball.
- Is that what they predicted, based on looking at their sizes? Why would they be different? Answers may vary. Encourage discussions about how the golf ball has more mass packed into the same volume, i.e. it is more dense, and help the children discover the concept of density as it relates to the planets as the activity progresses.
Facilitator's Note: Children may not know the difference between mass and weight, so it is important to explore the concept of mass as you introduce this activity. Mass is the amount of matter an object contains and is an intrinsic property of that object — its mass does not change depending on its location, temperature, or any other function of its environment. Weight depends in part on the gravitational pull experienced by the object; an object's weight does depend on its location. An object may weigh less on the Moon, but it still has the same mass as it does on the Earth.
3. Discuss which of the models of planets has the most mass and which has the least.
4. Invite the children to group the different models of the planets based on their estimated masses. The groups may include " heavy" and "light," or a range of estimated masses. Invite them to record their estimates in their journals.
5. Invite the children to describe the different planets' compositions.
6. In their journals, have the children record their notes and make predictions: Which of the fruit "planets" will float and which will sink if placed in water? Have them consider, in general, which is more dense: rock or gas? Which planets are more dense: the rocky, inner planets or the gaseous outer giants?
7. Begin the density experiment. First, invite the children to state their predictions about what will happen when "Saturn" is placed in the water.
8. Invite the child holding "Saturn" to place it in the bowl filled with water, and then continue with the other planets.
9. Compare the properties of the fruits and food to the planets with the children, as a model.
Facilitator's Note: The hydrogen and helium that make up much of Saturn are compressed to a density more like that of a liquid or even a solid for much of the planet. However, the planet's overall density is still lighter than water. That does not detract from the enormous amount of mass that the planet has; it is the second most massive planet in our solar system.
Help the children compare the models of the planets to the characteristics of the planets.
- In this activity, which planet was the least dense? Saturn.
- Were the giant planets all less dense than water? No, only Saturn was. Why not? The giant planets are made of mostly gas, but they are made of a LOT of gas! The gases are squished, or compressed, to a greater density, and even act like liquids or even solids.
- The giant planets are made mostly of gas. What are Mercury, Venus, Earth, and Mars made of? Rock.
- In general, what can we say about what a planet is made of from its density? The giant planets are less dense overall because they are made mainly of gases, and the inner rocky planets are more dense because they are made mainly of rock.
- If we weighed our models, which planet would weigh the most (and has the most mass)? Jupiter.
- The least? Mercury.
- Which planets were the biggest? Jupiter, followed by Saturn. Smallest? Mercury.
Ask the children to write, in their own words, the relationship between mass, size, and density in their journals.
Reiterate that a planet's density is determined by a combination of its size and mass, and that we can't measure an object's density just by weighing it. Jupiter has the most mass and is very big, but is denser than Saturn. The Juno mission to Jupiter will help scientists map the densities of the planet's deepest layers in order to better understand their composition and structure.