How did these vast valleys on Mars form? (This is a shaded relief map; the colors represent elevation.)

The last two or three billion years of the histories of the small inner planets (like the Moon and Mercury) are dominated by contraction, whereas the late histories of the small icy satellites of Saturn and Uranus are dominated by expansion like that evidenced on Uranus’ moon Ariel. Why?

What are the circular features with dark centers?

This crater on the Moon is probably ________.  

Which of the following factors best explains the number of impact craters on Earth? (Select all that apply.)

What is the most likely cause of the sinuous feature that extends across the following radar image of Venus?

Which planetary bodies have or had a hydrologic system involving a liquid on the surface (not from lava eruptions)?

Which of the following is true about Europa?

Large amounts of water (as gas or liquid) on Venus ________.

What is the sinuous feature that extends across this image of the surface of Venus?