Education and
Public Engagement
at the Lunar and Planetary Institute
Explore! Mars: Inside and Out

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?


For each child:

For the Group:

For the Facilitator:



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.

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.

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

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.


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.