Explore - Mars: Inside and Out! - Setting the Scene

Cooling Planets


Cooling Planets is an optional 10 to 15 minute discussion in which older children, ages 10 to 13, discover, through inquiry-based dialogue, which planet is hotter on the interior — Mars or Earth! The children consider the effect of size (volume) on the cooling rate of objects and, based on extrapolations, interpret the cooling histories of the inner, rocky planets of our solar system.

An alternative to this demonstration, more suitable for classroom-style settings, is to have children perform the experiment in small groups. They will collect data, graph the information, and interpret the results. This version of the activity can be found at Cooling Planets Experiment.

What's the Point?

  • All inner, rocky planets were very hot in the early stages of development; they have been cooling since that time.
  • Smaller planets cool faster than larger planets because smaller planets have a larger surface area to volume ratio.
  • The stage of cooling of a planet plays an important role in the geologic activity of that planet.
    • Earth, a large planet, is hot and has active volcanos and plate tectonics at its surface. In its interior, motion in its liquid outer core generates a magnetic field. This magnetic field shields Earth's surface from the charged particles of the solar wind, protecting our atmosphere and surface.
    • The Moon, a small satellite of Earth, has cooled completely and no longer has active volcanism or a magnetic field.
    • Mars, intermediate in size between Earth and the Moon, has not cooled completely, and has few, possibly still active, volcanos.


For each child:

For the Group:

  • Two like objects of different sizes or volumes, such as 1/2 and 2 liter soda bottles or quart and 1/2 gallon milk containers. The containers need to be made of the same material and be the same shape. There needs to be at least a 50% difference in volume between the large and small container.
  • Access to warm water

For the Facilitator:


  • If you are using objects to aid in the discussion, place them in a central location that is clearly visible to all the children.


1. Ask the children to imagine the Earth and Mars when they were first becoming planets. Share with them that the solar system was a very messy place. Lots of big and small asteroids were flying around, smashing into planets. Sometimes these rocky asteroids "got stuck" to the planets, helping them to grow. When materials in space slam together to form a planet it is called accretion. Having lots of things run into a planet heats it up. Both Mars and Earth — and Mercury, the Moon, and Venus - were very hot when they first formed. All of these planets have been slowly cooling since they first formed.

  • Have all the planets cooled the same amount — are they all the same temperature on the inside today?
  • What might control how much a planet has cooled on the inside? Answers will vary, but may include what it is made of or its size. Some children may say "how close a planet is to the Sun will control its temperature; remind them that you are thinking about how warm the planet is on the inside, not the surface where the Sun shines.
  • What evidence might a planetary scientist use to suggest that a planet is still warm? Answers will vary, but may include whether or not a planet has active volcanos.

2. Invite the children to think as planetary scientists to determine how a planet's size affects the rate at which it cools after it forms. Share with the children the objects of different sizes and invite them to examine them.

  • What is the same — or different — about them?
  • Do the objects have different shapes? No
  • Are they made of different materials? No
  • Do they have different sizes? Yes
  • If one of these represents the planet Earth and one is Mars, which is which? The larger one is Earth and the smaller one is Mars.

3. Fill the containers with warm water to the same level relative to the size.

  • Which container will cool fastest, the large container or the small container? The small container will cool fastest. Just like a small bowl of soup will cool faster than a big bowl of soup, or a small cupcake will cool faster than a big sheet cake.
  • How might this experiment relate to how planets cool? Small ones cool faster than big ones.
  • Which is smaller, Earth or Mars? Mars.
  • Which would they expect to have cooled more since they formed, Earth or Mars? Mars, because it is smaller.

4. Share the image of Earth, Mars, and the Moon with the children. The planets are to scale; their sizes are correct with respect to one another.

  • Which has cooled the most? The Moon. The least? Earth. Why? Because the Moon is smaller than the others and small things cool faster than large things. The inside of the Moon has completely cooled.
  • Is Mars completely cool on the inside? What evidence might we have that Mars is still a "little" warm? Mars, like Earth, has volcanos, although fewer of them. Volcanos show that a planet is still hot enough — or was recently — to melt rock that erupts at the surface and makes volcanos. Scientists are not sure if Mars' volcanos are active because they have never observed one actually erupting. Planetary scientists interpret that they could have erupted within the last 10 million to 100 million years, 10,000 to 100,000 years. Others claim longer, based on how broken down — weathered — the features look and how cratered their surfaces are.
  • Is there any evidence that Earth is still hot, based on their earlier investigations? Yes, Earth has volcanos (and a protective magnetic field and plate tectonic activity).
  • Why might Mars have fewer volcanos, and an interior with no molten layer, compared to Earth with its many volcanos and molten outer core? Mars has cooled more than Earth.


Review with the children what they have just learned about the volume and cooling rates of planets, drawing connections between interior heat and volcanism. Invite them to record their observations in their GSI Journals.

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