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. Olympus Mons on Mars is the biggest volcano in the Solar System. 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.

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, 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. Mars has about 38 percent of Earth's gravity, a bit more than one third.

A day on Mars, called a sol, 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. They are not round like our Moon. The moon Phobos circles Mars about three times every Mars day, in under 8 hours.

In July 1965 Mariner 4 made the first successful flyby of Mars. It took the first close-up pictures of Mars. Mariner 4 sent back 21 pictures of Mars in 1965. 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. The rover Curiosity landed in Gale Crater in August 2012. It went to learn whether Mars was ever a place where tiny living things could have lived.

Curiosity is a robot rover about the size of a small car. A box on the Perseverance rover called MOXIE separated oxygen from carbon dioxide in the Mars air. A small helicopter named Ingenuity was the first aircraft to fly on another planet. It flew on Mars. Ingenuity flew 72 times on Mars, far more than the five test flights it was built for.

Words to know

• sol: a Mars day
• flyby: a close pass
• lander: a landing craft
• rover: a robot car
• lumpy: not round