View a QuickTime movie of Mars created from Hubble Space Telescope (HST) pictures taken during Martian summer.
	Compare Earth and Mars.

For a place we’ve never been to in person, we know a lot about Mars—and we’re learning more all the time. A day is about the same length on Mars—24.6 hours—as a day on Earth. But a Martian year is 687 days! Both Mars and Earth are “rocky” planets, relatively close to the Sun (not gaseous like Jupiter or distant like Pluto). The two planets have similar features, such as valleys and mountains and polar caps, but the processes that shape the land are very, very different. Mars is smaller than Earth—with just over half of Earth’s diameter—and Martian gravity is only three-eighths as strong as Earth’s. What we don’t know for sure yet is whether or not there’s ever been liquid water—or life—on Mars.

Viewing Mars and Earth from their surfaces and from space gives us a glimpse at how different—and similar—these two planets can be. See how the Big Blue Marble and the Red Planet measure up to one another by viewing a comparison of Mars and Earth. Want to find out more about our planetary neighbor? Link to the top ten Mars pictures of all time, and watch a QuickTime movie of Mars created with pictures from the Hubble Space Telescope.

	Investigate canyon formation on Mars.

How did Mars’s canyons form?
Mars’s largest canyon system, Valles Marineris, is a giant crack in the Martian crust. Planetary scientists don’t know for sure how the canyons formed, but signs point to three physical forces that are well-known on Earth: heat, cold, and gravity. Take a look at an artist’s rendering of this scientific theory to learn more.

	Olympus Mons

Why are mountains so much bigger on Mars?
The largest mountains on Mars are all volcanoes. Since Mars has no plate tectonics, the planet’s crust doesn’t move and the volcanoes pretty much remain where they are, instead of moving with plates of crust and being “recycled” over time like they are on Earth. Mars’s crust is much thicker than Earth’s, which means it’s strong enough to support more massive structures. And there’s no active, long-term erosion on Mars from rain or flowing rivers and streams like there is on Earth. Mars’s lower gravity helps the volcanoes get—and stay—large, too.

	An artist’s rendering of the Martian sky

What color is the Martian sky?
Everyone expects the sky on Mars to be red, but if all the dust were removed it would be very dark blue. With all the tiny, reddish yellow dust particles suspended in the atmosphere, scientists think that it would actually look yellowish green. The sky’s color shifts throughout the day as sunlight scatters through the dusty air.

	An artist’s rendering of a foggy Martian morning

Are there clouds in the Martian sky?
When Mars, with its elliptical orbit, is farthest from the Sun, there are water-ice clouds in the sky. The ice clouds—which look like high, cirrus clouds on Earth—form at night, and gradually burn off during the day. As temperatures decrease, “polar hoods” of ice cloud obscure the poles. Sometimes in the cold seasons, bluish carbon dioxide clouds even form! And there’s often icy morning ground fog in the canyons year-round.