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. That 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. They can reach about 20 degrees Celsius on a warm day. Today, water on Mars is found mostly as ice. It is near the poles. Long ago, liquid water was on the surface of Mars.

The air on Mars is about 95 percent carbon dioxide. People cannot breathe the air on Mars. Gravity on Mars is weaker than on Earth. It is only about 38 percent as strong. So you would weigh much less there. A day on Mars is called a sol. It lasts about 24 hours 40 minutes.

A year on Mars lasts 687 Earth days. That is almost twice as long as a year on Earth. Mars is tilted about 25 degrees, close to the tilt of Earth. So it has seasons. Mars has two small moons, named Phobos and Deimos. They are lumpy, like potatoes, not round like our Moon. Astronomer Asaph Hall found them in 1877. Phobos circles Mars about three times every Mars day, in under 8 hours.

Olympus Mons on Mars is the biggest volcano in the Solar System. It is wider than 600 kilometers at its base. That is wide enough to cover a whole U.S. state like Arizona. Valles Marineris on Mars is the biggest canyon in the Solar System. It is about 4,000 kilometers (2,500 miles) long. That is long enough to stretch across the United States.

In 1976 Viking 1 became the first American spacecraft to land safely on Mars. In 1997 the Mars Pathfinder lander carried Sojourner. It was the first robot rover to drive on Mars. Curiosity is a robot rover about the size of a small car. A small helicopter named Ingenuity was the first aircraft to fly on another planet. It flew on Mars.

Words to know

• sol: a Mars day
• gravity: a pull
• lumpy: not round
• rover: a robot car
• lander: a landing craft

The short version

Mars is the fourth planet from the Sun. It looks red because there is rusty iron in its ground. Mars is a cold, dry, desert world, and people cannot breathe the air on Mars. Long ago, liquid water was on its surface, but today water is found mostly as ice, near the poles.

Quick facts

• Mars is the fourth planet from the Sun.
• Mars looks red because there is rusty iron in its ground.
• Mars has two small moons, named Phobos and Deimos.
• Olympus Mons on Mars is the biggest volcano in the Solar System.
• A day on Mars, called a sol, lasts about 24 hours 40 minutes.