Venus, the nearest planet, is a great disappointment through the telescope because of its impenetrable cloud cover. Mars is more tantalizing, with dark markings and polar caps.

Early in the twentieth century, it was widely believed that the “canals” of Mars indicated intelligent life there. Mars has only 11% the mass of Earth, but Venus is nearly our twin in size and mass.

runaway greenhouse effect — the process by which the greenhouse effect, rather than remaining stable or being lessened through intervention, continues to grow at an increasing rate. tectonic — geological features that result from stresses and pressures in the crust of a planet; tectonic forces can lead to earthquakes and motion of the crust.

Be able to explain why it’s difficult to learn about Venus from Earth-based observation alone; describe the history of our interest in Mars before the Space Age; compare the basic physical properties of Earth, Mars, and Venus, including their orbits.