Directions

7. Have children apply their understanding of habitability factors by using the craft materials to construct or draw a cross-section of a planet whose surface is hostile to life but whose sub-surface environment a mile or two down can support life.

• Remind children that the "What Makes a World Habitable" sheet can help them develop a suitable sub-surface environment.
• Give them 10–15 minutes to create their planet.

8. Reconvene the group and process their models/drawings. Call out a habitability factor. Go around the room and have each team describe in turn where this factor is found on its model or drawing. Do this with all five factors. After reviewing all five factors, ask teams to share any special features of their worlds.

9. Discuss the following questions:

• If life exists beyond Earth in our solar system, is it more likely to be life that lives on or below the surface? (Below)
• What size organism is best suited to life far beneath the surface? (Single-celled life)
• Why is a hot interior necessary for life far underground? (Heat circulates water. It can also melt and circulate minerals, bringing them close enough to organisms to be useful.)
• What might become a problem for life far below the surface? (Becomes too hot for life.)
• In the solar system, which planets or moons do astrobiologists think are promising places for finding life (or signs of past life)? (Mars and Jupiter's moons Europa & Ganymede)

Follow-Up Questions

• Research life found below the surface, such as at sea-floor vents, subterranean caves, and in rock layers a mile or two below the Earth's surface.
• Find out more about the search for extraterrestrial life on Mars and on Jupiter's moons.