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.

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; the Tharsis Rise is a huge bulge in the surface of Mars, about 4,000 kilometers wide and 10 kilometers high, near the equator.

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; Valles Marineris was made by the crust stretching and cracking, with walls collapsing in landslides, while the Grand Canyon on Earth was carved by a river.

Curiosity landed in Gale Crater in early August 2012 to assess whether Mars could ever have been a place for life. Curiosity landed with a new method, the sky crane, because the rover was too heavy and big for airbags. Curiosity showed that Gale Crater was once a lakebed with water and the chemical ingredients that could have supported life.

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. Long ago, liquid water was on the surface of Mars.

Scientists conclude that the solar wind and the Sun's radiation stripped away most of the air that Mars once had. Mars has no global magnetic field, so the solar wind hits its upper air directly; NASA's MAVEN orbiter studies how the solar wind and the Sun act on the upper air of Mars, to learn what happened to its once-thick atmosphere.

InSight studied the inside of Mars. Its seismometer listened to marsquakes, and the waves told scientists how big the core is. The core of Mars is large: InSight's marsquake data show it has a radius of about 1,840 kilometers; the rocky crust of Mars is about 10 to 50 kilometers thick. At the InSight landing site, the crust is 15 to 47 kilometers thick.

In 1976 Viking 1 became the first American spacecraft to land safely on Mars. The Viking landers searched Mars soil for life in 1976; the results were mostly negative but not a clear answer.

A small helicopter named Ingenuity was the first aircraft to fly on another planet. It flew on Mars; flying on Mars is very hard because the air there is only about one percent as dense as Earth's. Ingenuity flew 72 times on Mars, far more than the five test flights it was built for.

In the 1890s Percival Lowell, who founded Lowell Observatory in Arizona, was sure the lines some astronomers drew on Mars were artificial canals. In 1965 Mariner 4 flew over where canals had been drawn and saw none.

Words to know

• seismometer: quake listener
• lakebed: old lake floor
• canals: dug waterways
• crust: rocky outer layer
• delta: fan of mud