Answer: A shield volcano on Mars
Notes: Scientists discovered frost on Mars' tallest volcano, Olympus Mons, indicating an active water cycle. Olympus Mons, a 3.5-billion-year-old shield volcano in Mars' western hemisphere, is the highest point on Mars and the largest volcano in the solar system. It stands 16 miles (24 km) high, about three times taller than Mt. Everest, and spans 340 miles (550 km), larger than Hawaii's Mauna Loa. Its summit caldera, with overlapping pits, is 50 miles (80 km) wide.
