Mars: Facts About the Red Planet

Mars is the fourth planet from the Sun and one of the most intensively explored worlds beyond Earth. Its rust-colored surface, polar ice caps and evidence of ancient rivers and lakes have made it a central target in the search for environments that may once have supported life.

Known as the Red Planet, Mars is a cold, dry world today. However, its landscapes preserve evidence of a much warmer and wetter past, while water ice remains at its poles and beneath parts of the surface.

What Is Mars?

Mars is a terrestrial planet composed mainly of rock and metal. It orbits between Earth and Jupiter at an average distance of about 228 million kilometers (142 million miles) from the Sun.

It is the second-smallest planet in the solar system after Mercury. Although Mars resembles Earth in several ways—including its solid surface, day length and changing seasons—its atmosphere is far thinner and its climate is much colder.

Mars Quick Facts

Why Is Mars Called the Red Planet?

Mars appears reddish because its surface rocks and dust contain iron minerals that have oxidized, producing compounds similar to rust on Earth. Fine particles of this iron-rich dust cover much of the planet and are frequently lifted into the atmosphere by winds.

The surface is not uniformly red. Darker gray and brown regions appear where winds have removed some of the dust and exposed volcanic rock beneath it. From Earth, however, the widespread oxidized material gives the planet its distinctive orange-red appearance.

A Small Rocky Planet With a Layered Interior

Mars is approximately half as wide as Earth, with a diameter of 6,792 kilometers (4,220 miles). It has about 11% of Earth’s mass, while its surface gravity is only 38% as strong. A person weighing 100 pounds on Earth would therefore weigh approximately 38 pounds on Mars.

Like Earth, Mars is divided into a crust, mantle and metallic core. Its crust is composed mainly of volcanic basalt, while the mantle contains silicate rock. At the center is a core rich in iron, sulfur and other lighter elements that remains at least partly molten. Unlike Earth, Mars no longer generates a global magnetic field, although magnetized regions of its ancient crust preserve evidence that it once did.

The Surface of Mars

Mars has an ancient, heavily cratered surface shaped by impacts, volcanism, wind and flowing water. Its southern hemisphere is dominated by older, elevated highlands, while much of the northern hemisphere consists of younger, smoother lowlands. This difference is known as the Martian hemispheric dichotomy.

The planet contains some of the solar system’s most extreme terrain. Olympus Mons is the largest known volcano, rising roughly 22 kilometers (14 miles) above the Martian reference level. Valles Marineris is a vast canyon system extending for more than 4,000 kilometers (2,500 miles), or almost one-quarter of the way around Mars.

Other prominent landscapes include the enormous Hellas impact basin, the Tharsis volcanic plateau, extensive dune fields and layered polar caps. These and other landmarks are covered in greater detail in our guide to the most interesting surface features on Mars.

Mars Has a Thin and Cold Atmosphere

Mars has an extremely thin atmosphere composed of about 95% carbon dioxide, with smaller amounts of nitrogen, argon, oxygen and water vapor. Atmospheric pressure at the surface averages less than 1% of that at sea level on Earth and varies considerably with elevation and season.

This tenuous atmosphere retains very little heat, producing large temperature differences between day and night. Temperatures can briefly rise above freezing near the equator during summer days but fall far below −100°C (−148°F) during the coldest polar nights. The thin atmosphere also provides little protection from radiation or incoming meteoroids.

Dust Storms, Clouds and Martian Weather

Mars experiences powerful weather systems despite its thin atmosphere. Winds regularly lift fine surface dust, producing local storms that can expand across entire regions. On rare occasions, dust spreads around almost the whole planet and can remain suspended for weeks or months, obscuring the surface and reducing the sunlight available to solar-powered spacecraft.

The planet also develops towering dust devils, thin clouds of water ice and occasional carbon-dioxide ice clouds at high altitudes. Although Martian winds can reach high speeds, the atmosphere is so tenuous that they exert much less force than winds of the same speed on Earth. Seasonal freezing and sublimation of carbon dioxide at the poles also alter atmospheric pressure over the course of a Martian year.

Water on Ancient and Modern Mars

Dry river valleys, lakebeds, deltas and water-altered minerals show that liquid water once flowed across the Martian surface. Billions of years ago, Mars had a thicker atmosphere and a warmer climate capable of supporting rivers and long-lived lakes, although scientists continue to investigate how extensive and persistent these wet conditions were.

Most water on Mars today is frozen within the polar caps or buried beneath the surface, with smaller amounts present as atmospheric vapor and frost. The planet’s low pressure and cold climate prevent exposed liquid water from remaining stable for long: it generally freezes, evaporates or passes directly into vapor. Whether small quantities of salty liquid water occasionally persist underground remains unresolved.

Could Life Have Existed on Mars?

Ancient Mars appears to have possessed several conditions associated with habitability, including liquid water, essential chemical elements and sources of energy. Rovers have also detected organic molecules in Martian rocks, but these carbon-based compounds can form through non-biological processes and are not evidence that life existed.

No mission has discovered confirmed signs of past or present Martian life. The modern surface is cold, dry and exposed to damaging radiation, making it extremely hostile to organisms. If life ever developed on Mars—or survives there today—its most likely refuge may be in protected environments beneath the surface.

A Day Like Earth’s but a Year Almost Twice as Long

A solar day on Mars, known as a sol, lasts 24 hours, 39 minutes and 35 seconds—only slightly longer than a day on Earth. This similarity allows Mars missions to organize their operations around a recognizable day-and-night cycle.

Mars takes 687 Earth days to orbit the Sun. Its axis is tilted by about 25 degrees, close to Earth’s 23.4-degree tilt, so the planet experiences spring, summer, fall and winter. However, the seasons last almost twice as long and vary considerably in length because Mars follows a more elliptical orbit than Earth. Southern summers are shorter and warmer, while northern summers are longer and cooler.

Phobos and Deimos

Mars has two small, irregularly shaped moons: Phobos and Deimos. Both were discovered by American astronomer Asaph Hall in 1877 and are named after the mythological sons of Ares, the Greek counterpart of the Roman god Mars.

Phobos is the larger and closer moon, with an average diameter of about 22 kilometers (14 miles). It completes an orbit in less than eight hours and is gradually spiraling toward Mars. In tens of millions of years, it may either break apart to form a temporary ring or collide with the planet.

Deimos is only about 12 kilometers (7.5 miles) wide and takes roughly 30 hours to orbit Mars. The moons resemble dark asteroids, but their exact origin remains uncertain; they may be captured objects or debris produced by a major impact on early Mars.

Spacecraft Exploration of Mars

Mariner 4 completed the first successful flyby of Mars in 1965 and returned the first close-range images of another planet. The Soviet Mars 3 lander reached the surface in 1971 but transmitted for only seconds. NASA’s Viking 1 and Viking 2 then achieved the first fully successful Mars landings in 1976.

Mars Pathfinder delivered the small Sojourner rover in 1997, followed by Spirit and Opportunity in 2004, Curiosity in 2012 and Perseverance in 2021. Orbiters have mapped the planet, monitored its atmosphere and searched beneath the surface, while landers and rovers have examined its rocks, climate and history of water.

Perseverance is exploring Jezero Crater and storing rock samples for possible return to Earth. It also carried Ingenuity, which made the first powered flight on another world in 2021 and completed 72 flights before its mission ended in 2024. Mars has additionally been explored by missions from Europe, India, China and the United Arab Emirates.

How to Find and Observe Mars

Mars is easily visible without optical aid when it is favorably positioned and resembles a bright orange-red star along the zodiac. Its brightness changes dramatically as the distance between Earth and Mars varies. The planet is easiest to observe around opposition, which occurs approximately every 26 months, when Earth passes between Mars and the Sun.

Binoculars emphasize its color but usually show it only as a bright point. A telescope with an aperture of about 4 inches (100 millimeters) can reveal a small disk and, under good conditions, one of the white polar caps. Dark surface markings such as Syrtis Major may also become visible when Mars is relatively close to Earth.

Mars is a demanding telescopic target because it appears small even near opposition. A 6- to 8-inch telescope, high magnification and steady atmospheric conditions provide a better chance of seeing surface detail. Observing repeatedly over several nights also allows the planet’s rotation and changing features to become apparent.

Who Discovered Mars?

Mars is visible without a telescope and has been known since prehistoric times, so it cannot be credited to a single discoverer. Babylonian, Egyptian, Greek, Chinese and other early astronomers recorded its movement against the background stars.

Galileo Galilei made some of the earliest telescopic observations of Mars in 1610. During the 17th century, Christiaan Huygens mapped the dark feature now called Syrtis Major and used its movement to estimate the planet’s rotation period. Giovanni Domenico Cassini later produced a similarly accurate estimate and documented one of the polar caps.

Detailed maps followed during the 19th century. Giovanni Schiaparelli’s observations in 1877 included features he called canali, meaning channels. Its mistranslation as “canals” helped create the mistaken idea that Mars contained artificial waterways constructed by an intelligent civilization.

Why Mars Is Named After the Roman God of War

The Romans named the planet after Mars, their god of war, because its reddish appearance suggested blood and battle. The god’s Greek counterpart was Ares, although other ancient cultures developed their own names and associations for the conspicuously red planet.

Mars also gave its name to the month of March, while the planet’s association with Tuesday survives in Romance-language names such as the French mardi and Spanish martes.