Mars is the fourth planet from the Sun. It is about 1.5 times as far from the Sun as Earth is.

Mars is about 6,780 kilometers (4,212 miles) across, which is about half the width of Earth. Mars looks red because there is rusty iron in its ground.

Mars is a cold, dry, desert world. Temperatures on Mars can drop to about minus 153 degrees Celsius and reach about 20 degrees Celsius on a warm day.

Air pressure at the surface of Mars is only about one hundredth of Earth's, about 6 millibars. The air on Mars is about 95 percent carbon dioxide. People cannot breathe the air on Mars.

A day on Mars, called a sol, lasts about 24 hours 40 minutes. A year on Mars is 687 Earth days, which is about 669.6 Mars days (sols).

Olympus Mons on Mars is the biggest volcano in the Solar System. It rises about 25 kilometers (16 miles) above the plains around it, much taller than Mount Everest.

Valles Marineris on Mars is the biggest canyon in the Solar System. It is a canyon system about 4,000 kilometers (2,500 miles) long, long enough to stretch across the United States.

Mars has two small moons, named Phobos and Deimos. The two moons of Mars are lumpy, like potatoes, not round like our Moon. The Mars moon Phobos circles Mars about three times every Mars day, in under 8 hours.

In 1997 the Mars Pathfinder lander carried Sojourner, the first robot rover to drive on Mars. The Mars rover Sojourner weighed about 10 kilograms (23 pounds).

Long ago, liquid water was on the surface of Mars. The delta in Jezero Crater is a fan of ancient river mud and sand that holds evidence of a lake that once filled the crater.

Words to know

• sol: a Mars day
• millibars: air pressure units
• delta: fan of mud
• canyon: deep valley