Mars takes after Earth in many ways. It is only about half the size of Earth, but its similar geology, thin atmosphere, and the presence of water make it seem more like home. Its day is almost as long as Earth's, but it takes about two Earth years to orbit the Sun. Mars is tilted on its axis, so it experiences seasons.
Mars has the tallest volcano in our solar system — about 22 kilometers tall (almost 14 miles high). [Compare this to Hawaii's Mauna Loa at 9 kilometers (5.5 miles) tall measured from the sea floor.] Some of the volcanos on Mars have been recently active. However, its surface temperatures are cold — –125°to –23°F (–87° to –5°C) — and the planet is very dry. The atmosphere is thin and composed mostly of carbon dioxide.
There is no liquid water present at the surface, but robotic explorers have discovered frozen water in the subsurface and in its polar ice caps, which are comprised of frozen carbon dioxide and water ice. There is evidence that Mars had flowing water and oceans at its surface during its early history, perhaps until about three and a half billion years ago.