Mars is the fourth planet from the Sun, a rocky world about 6,780 kilometers (4,210 miles) across. That figure is its mean diameter, the width of a ball with the same volume. Mars is a little more than half as wide as Earth, and it is often called the Red Planet.

Size

NASA gives Mars's mean radius as 3,389.5 kilometers, so its mean diameter is 6,779 kilometers. Mars is slightly flattened. It is about 6,792 kilometers wide at the equator and 6,752 kilometers from pole to pole, a difference of 40 kilometers.

Mars has about one-tenth of Earth's mass, and gravity at its surface is about 38% as strong as on Earth. From its lowest point to its highest, the surface spans about 30 kilometers (19 miles) in height. Although Mars is only about half as wide as Earth, its surface has nearly the same area as all of Earth's dry land.

Size comparisons

Earth is almost twice as wide as Mars. It would take about 6.6 Mars-sized balls to fill the same volume as Earth.
Mars is almost twice as wide as the Moon, which is 3,475 kilometers (2,159 miles) across.
Jupiter's moon Ganymede, the largest moon in the Solar System and the next smaller object in the explorer, is 5,262 kilometers (3,270 miles) across. That is a little more than three-quarters of the width of Mars.
Venus, 12,104 kilometers (7,521 miles) across, is about 1.8 times as wide as Mars.
If Earth were shrunk to the size of a basketball, Mars would be a ball about 13 centimeters (5 inches) across. At its closest to Earth, it would be about 1 kilometer (0.6 miles) away.

The Red Planet

Mars looks red because iron minerals in its rocks, soil and dust have rusted. Close up, the surface also shows browns, golds and tans. Wind lifts the reddish dust into the air, and dust storms sometimes cover much of the planet.

The atmosphere is thin and made mostly of carbon dioxide. At the surface, its pressure is less than 1% of the air pressure on Earth. Temperatures range from about 20 °C (70 °F) down to about −143 °C (−225 °F).

Canyons, volcanoes and water

Valles Marineris, the largest canyon in the Solar System, is about 3,870 kilometers (2,400 miles) long, more than half the width of Mars. It is up to 600 kilometers (370 miles) wide and 9.3 kilometers (5.8 miles) deep. Mars also has the largest volcano in the Solar System, Olympus Mons.

Mars was once wetter and warmer, with a thicker atmosphere. Ancient river valleys, deltas and lakebeds show that water flowed on its surface, and huge floods may have swept across it about 3.5 billion years ago. Today the air is too thin for liquid water to last long on the surface. In the polar regions, water remains as ice close beneath the surface.

Two small moons

Mars has two small, potato-shaped moons, Phobos and Deimos. They may be captured asteroids. Phobos, the larger and closer of the two, is slowly moving toward Mars. In about 50 million years it will crash into the planet or break apart, perhaps leaving a dusty ring.

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