Differentiation Demonstration: Moon
In this 15-minute demonstration or 30– to 45-minute activity, children, ages 8 to 13, observe a model of planetary differentiation, the organization of planetary interiors into layers of different densities. Materials of different densities are mixed in a bottle and allowed to separate into layers. The simple model illustrates how the Moon's interior became organized into a distinct core, mantle, and outer crust from the debris of the Giant Impact.
What's the Point?
- Like Earth, the Moon's materials separated into distinct layers of different densities: a dense metallic core, a rocky mantle of intermediate density, and a rocky crust that is comprised of the least dense materials.
- Planetary differentiation is the process by which planetary interiors organize into layers of different densities.
- The Moon was originally a mixture of materials ejected by the giant impact that created it; a major period of planetary differentiation occurred early in its history.
- Models — such as those the children are using here — can be tools for understanding the natural world.
For the group, or for each group of 3 to 4 children:
- 1 clean, clear plastic 2-liter soda or water bottle with a cap and with the label removed
- 1 1/3 cup (a little over 1 pound) small aquarium gravel
- 5 drinking straws, all of one color (not clear)*
- 57 oz. (7 1/8 cups) water
- 1 funnel
- Measuring cups
- Three clear plastic cups
For the facilitator:
*Select a straw color that is distinct from the gravel color.
- Prepare a Differentiation Demonstration bottle. If the group is large, consider making more than one Differentiation Demonstration bottle before you start, so that the children can examine what happens in smaller groups.
- Cut the straws into 1/4" sections to make about140 pieces for each bottle.
- Using the funnel, pour the gravel, water, and straw sections into the 2-liter bottle. Seal the bottle tightly!
- Put a few straw sections in one of the clear cups. Put some water in another cup. Put some gravel in the third cup.
- Alternatively, have groups of children create their own differentiation bottles. While messy, the children enjoy this activity.
1. Invite the children to gather in a central location to share their ideas about the interiors of Earth and the Moon.
- Is the Moon — or Earth — composed of only one, or several different, layers? The Moon and the Earth each are composed of several layers.
- What are the names of those layers? Crust, mantle, and core.
- How might these layers be different?
- Why might the Moon and Earth have layers?
2. Share with the children how the Moon and Earth and the other inner rocky planets formed. A little over four and a half billion years ago, dust and gas and tiny particles, mostly made of iron and rock, were in a disk surrounding our early Sun in the region where the inner, rocky planets of Mercury, Venus, Earth, and Mars eventually would be. This material began to clump together, or accrete. With time, the small clumps grew larger and larger. The largest ones swept up the smaller clumps (through gravitational attraction), eventually becoming the planets of the inner solar system. These planets did not have any layers — all the materials that made them were mixed up together.
3. Show the materials in the cups to the children, and share that you have all of these ingredients mixed together in a big bottle.
- What do the children think will happen when you mix them together?
- Will they stay mixed?
- Will some sink to the bottom? Which? Why?
- Will some float? Which? Why?
4. Show the children the Differentiation Demonstration bottle. Shake the bottle vigorously — be careful as it is heavy and takes vigorous shaking, and then set it on the table.
- What happens? The materials separate. The gravel sinks, the straw sections float.
- Were the children's predictions correct?
Invite the children to consider how this model tells the story of the Moon's formation.
- What might the model represent when the materials have settled? How might this relate to what we may see inside the Moon? The layers in the bottle represent the layers in the Moon.What might this model represent in the story of how the Moon developed? How all of the materials that made up the Moon or Earth first started — all mixed together.
- Why does the gravel sink and the straw sections float? The materials in the water have different densities. The "heavier" gravel sinks because it is more dense than the water, and the "lighter" straw sections float because they are less dense than the water. Sometimes some of the materials get "caught" in the wrong layers. This also happens inside planets.
Density is a measure of how much mass is contained in a specified volume of a material (density = mass/volume). Density refers to how tightly that material is packed together.
Mass and weight are different. Mass is a measure of how much material is in an object. The mass of an object — a child, a toaster, a bottle of water — does not change, no matter where the object is. A child will have the same mass on Earth, the Moon, or in space. That child's weight will change on different planets however, because weight depends on mass and gravity.
- How does this model help us understand what might have happened inside the Moon and Earth to create layers? The materials separated. The most dense materials (mostly iron and nickel) sank to the center of the planets to make their cores. The least dense materials make up the crusts, and the material in between makes up the mantles.
The Moon is smaller than the Earth and cooled more quickly. Iron sank to form the core of the Moon, but an ocean of liquid rock — a magma ocean — crystallized more slowly over the course of 50-100 million years to produce the mantle and crust. Differentiation within the magma ocean produced the features we still see on the Moon today. Less dense minerals floated slowly to the surface, forming a light grey crust of anorthosite rock. More dense minerals sank out quickly into the mantle and lower crust. These more dense minerals later sourced, in part, the basalt lavas that later erupted from lunar volcanos and filled in low-lying areas, like impact basins. We see the basalt plains on the Moon today as the dark areas, or maria.
August 27, 2010