Key Ideas and Terms

6-1 Comparing terrestrial planets and moons shows distinct similarities and dramatic differences in appearance

• Comparing planets to Earth helps astronomers understand the solar system.
• Venus is particularly difficult to study because it is covered in a thick layer of clouds.
• Planet interiors are invisible to telescopes.

6-2 Many terrestrial world surfaces are dominated by impact craters revealing the age of underlying processes

• Circular impact craters result when one object collides with the solid surface of a terrestrial planet or a satellite.
• Water ice exists in some craters near the poles that have walls permanently shielded from the Sun.
• The smaller the terrestrial world, the less internal heat it is likely to have retained from its formation, and, thus, the less geologic activity it will display on its surface. The less geologically active the world, the older and hence more heavily cratered its surface.
• The rate of impact crater formation has decreased dramatically since the early years of the solar system.

6-3 Tectonics and volcanism influence surface features

• Mars’s surface is characterized by relatively smooth northern lowlands and jagged southern highlands.
• Venus has a relatively thin crust and radar evidence suggests no active plate tectonics.
• Mars has a thick, rigid crust resulting in a rift valley stretching across one side of the planet.
• Mars’s Olympus Mons is a giant shield volcano formed by layer after layer of magma being deposited.
• Jupiter’s satellite Io has virtually no impact craters, as it is constantly being smoothed over by active volcano flows.

6-4 Atmospheres surrounding terrestrial planets vary considerably

• Mercury, the Moon, and Mars are easier to study than Venus, which is obscured by its thick atmosphere.
• Venus’s atmosphere causes an extremely high surface temperature due to a runaway greenhouse effect.
• Mars’s atmosphere experiences a runaway icehouse effect where CO₂ is pulled from the atmosphere, lowering the temperature even more.

6-5 Evidence exists for water in locations besides Earth

• Frozen water on Mars has been revealed by landers drilling into the surface.
• Frozen water on the Moon has been revealed by studying ejected plumes created by crashing spacecraft into shadowed crater walls.
• Jupiter’s satellite Europa may harbor a subsurface ocean that causes ice rafts to move across the surface.