31. What is the largest canyon?
The Grand Canyon is billed as the world's largest canyon system. Its main branch is 277 miles (446 kilometers) long. But let's compare. Valles Marineris on Mars extends for about 3,000 miles (4,800 kilometers). If added it to a U.S. map, it would stretch from New York City to Los Angeles. In places this vast scar on the Martian surface is 5 miles (8 kilometers) deep.
32. What is the deepest canyon in the United States?
Over the eons, the Snake River dug Hell's Canyon along the Oregon-Idaho border. It is more than 8,000 feet (2.4 kilometers) deep. In contrast, the Grand Canyon is less than 6,000 feet deep -- a bit more than a mile.
33. Is Earth the largest rocky planet in the solar system?
Just barely! Earth's diameter at the equator is 7,926 miles (12,756 kilometers). Venus is 7,521 miles (12,104 kilometers) wide. Mercury and Mars, the other two inner rocky planets, are much smaller. Pluto is rocky, too, but it's comparatively tiny (and some say it is not a planet at all).
34. How many of Earth's volcanoes are known to have erupted in historic time?
About 540 volcanoes on land are known. No one knows how many undersea volcanoes have erupted through history.
35. Is air mostly oxygen?
Earth's atmosphere is actually about 80 percent nitrogen. Most of the rest is oxygen, with tiny amounts of other stuff thrown in.
Venus is almost as big as Earth. Despite sweltering heat at the surface, its clouds might support life, some scientists say.
36. What is the highest waterfall in the United States?
Yosemite Falls in California is 2,425 feet (739 meters).
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