Geologic Scene Investigator: Inside Mars
Cooling Planets Experiment


1. Invite the children to examine the two containers and let them know that you will fill them both with warm water at the same temperature.

  • What do the children think will happen to the temperature of the hot water in each of the containers after several minutes?
  • What do you predict will cool more quickly, the larger or the smaller container?
  • Will it cool a lot more quickly, or just a little more quickly?

2. Share with the children that they will be taking the temperatures of the two containers every 5 minutes for a period of 30 minutes, recording the temperatures, and graphing them.  Have each group designate a timer, temperature reader, recorder, and grapher. Allow them to become familiar with their instruments. Emphasize that it is important to get the readings on the 5-minute mark. Remind them that the water is HOT and they need to be very careful when getting the temperature.

3. Fill the containers of each team about 2/3 full. Have them take a temperature immediately. Continue timing, measuring, and recording for 30 minutes.

4. As they get into the routine of collecting and recording the data, discuss how they should prepare their graph.

  • What are they measuring? Temperature. 
  • Over what time interval? 5 minutes.
  • For how long? 30 minutes.
  • What should be on the Y-axis (side axis)? Temperature.
  • What should the range of temperature be? As it is cooling, from the warmest temperature to the temperature of the cool water. The water will not get colder than the temperature of the room.
  • What should be on the X-axis (bottom axis)? Time.
  • What should the range of time be? From the start to 30 minutes.
  • What kind of graph might be best for this activity? A line graph will best show the results, with a dot or x for every measurement and a line connecting them indicating the change in temperature between readings.
  • What should they include on their graph? A title, labels for their axes, and a key or legend indicating which line is for which container..

5. Have the groups continue to collect their data and graph it.

6. At the end of 30 minutes, invite the children to wrap up their data collecting and graphing and to discuss their results with their team.  They should see about a 10° temperature difference in the two containers.

  • What was the temperature of large container at the start? At the end of 30 minutes?
  • What were the starting and ending temperatures of the small container?
  • Which container cooled more quickly, the large or small one? The small container cooled more quickly.
  • Were their predictions correct?

7. As a group, discuss their findings.

  • How might this experiment relate to planets?  If all the planets started at the same time at the same hot temperature, what happens to them through time? They cool.
  • Which cool fastest? The smaller planets.
  • Which is smaller, Earth or Mars? Mars.
  • Which would they expect to have cooled more through time, Earth or Mars? Mars.
  • What do volcanos tell scientists about the conditions inside a planet? That it is still hot enough – or was recently - to melt rock and create magma that erupts at the surface and makes volcanos.
  • Is there any evidence that Earth is still hot, based on their earlier investigations? Yes, Earth has volcanos.
  • Is there any evidence that Mars is still hot, based on their earlier investigations? Yes, Mars has volcanos.
  • Are there any other differences in the volcanos on Earth and Mars? Earth has many more volcanos, but Mars' few volcanos are larger.
  • Do Earth and Mars have active volcanos – volcanos that have recently erupted? Earth does. Scientists are not sure if Mars' volcanos are active because they have never observed one actually erupting.  Scientists interpret that they could have erupted within the last 10 million to 100 million years, based on how broken down – weathered – the features look and how cratered their surfaces are.
  • Based on the demonstration, why might Mars have fewer volcanos compared to Earth?  Mars has cooled more than Earth.

In Conclusion

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.



  • Have a sufficient amount of hot water available for all of the groups at the beginning of the activity.

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.


Cooling Planets is a 60 minute inquiry activity in which children, ages 8 to 13, take temperature readings from large and small containers of hot water, and graph the measurements to determine how volume affects cooling. They use this information to interpret the cooling histories of the different sizes of the inner, rocky planets of our inner solar system.

This demonstration involves very hot water and requires adult supervision at all times.


For each child:

  • One GSI Investigator Journal Mars Inside and Out or One GSI Investigator Journal Part II: Inside Mars
  • One pencil

For each group of 3 to 4 children:

  • One 2-liter container
  • One 1-liter container

Clear soda bottles with the tops cut off work, as do other types of containers; the two containers must be the same shape, thickness, and material

  • 3 liters of very hot water 176 to 194 F (80-90 C)  
  • Two thermometers that measure from boiling point to room temperature
  • Watch or clock (a stopwatch or timer works well)
  • Graph paper

For the Facilitator:

  • Background Information