Introduction

The ancient Romans named Mars for their god of war because its reddish hue resembled blood. This association was shared by other civilizations; for instance, the Egyptians knew it as "Her Desher," or "the red one." Today, it remains familiarly known as the "Red Planet," a nickname derived from the iron minerals in the Martian soil that oxidize, or rust, giving the surface its distinctive color.

Exploration

Mars has been scrutinized more intensely than any other planet beyond Earth. Our observation of the Red Planet dates back over 4,000 years to ancient Egypt. Today, a robust international fleet of robotic orbiters, landers, and rovers studies Mars from every angle.

• On the ground: NASA's next-generation Perseverance rover – the largest, most advanced rover NASA has sent to another world – touched down on Mars on Feb. 18, 2021. Two other NASA spacecraft already are at work on the surface. NASA's Curiosity rover is exploring Mount Sharp in Gale Crater. NASA's InSight, a stationary lander, is probing the interior of Mars from a site on a flat smooth plain called Elysium Planitia


• In orbit: NASA has three spacecraft in orbit: Mars Reconnaissance Orbiter, Mars Odyssey, and MAVEN.


• International missions: The Hope orbiter from the United Arab Emirates arrived at Mars on Feb. 9, 2021. China’s Tianwen-1 mission arrived on Feb. 10, 2021, and it includes an orbiter, a lander, and a rover. ESA manages the ExoMars Trace Gas Orbiter and Mars Express missions. India's first Red Planet spacecraft – the Mars Orbiter Mission (MOM) – has been orbiting Mars since 2014.

Mariner 9: NASA's Mariner 9 achieved a historic milestone by beating the Soviet Mars 2 to the Red Planet, becoming the first spacecraft to orbit another planet. It successfully mapped 85% of the Martian surface and transmitted over 7,000 images, revealing iconic features like Olympus Mons, Valles Marineris, and the moons Phobos and Deimos.

Size and Distance

Mars, with a radius of 2,106 miles (3,390 kilometers), describes a world about half the size of Earth. To visualize: if Earth were the size of a nickel, Mars would be roughly the size of a raspberry.

Orbiting at an average distance of 142 million miles (228 million kilometers), Mars sits 1.5 astronomical units (AU) from the Sun. Light from the Sun requires 13 minutes to travel this distance.

Orbit and Rotation

Mars completes one rotation every 24.6 hours, a period remarkably similar to an Earth day (23.9 hours). These Martian days are termed "sols." A Martian year spans 669.6 sols, equivalent to 687 Earth days.

With an axial tilt of 25 degrees, Mars closely mirrors Earth's 23.4-degree tilt. This similarity means Mars experiences distinct seasons, though they are longer due to its extended orbit. Unlike Earth's evenly divided seasons, Martian seasons vary in length because of the planet's elliptical orbit.

In the northern hemisphere, spring is the longest season (194 sols), while autumn is the shortest (142 sols). Winter lasts 154 sols, and summer extends for 178 sols.

Formation

Mars formed approximately 4.5 billion years ago, coalescing from swirling gas and dust to become the fourth planet from the Sun. About half the size of Earth, it shares the terrestrial characteristics of a central core, rocky mantle, and solid crust.

Structure

At its heart, Mars possesses a dense core of iron, nickel, and sulfur, with a radius between 930 and 1,300 miles (1,500 to 2,100 kilometers). Encasing this is a rocky mantle 770 to 1,170 miles (1,240 to 1,880 kilometers) thick. Above lies a crust rich in iron, magnesium, aluminum, calcium, and potassium, ranging from 6 to 30 miles (10 to 50 kilometers) deep.

Surface

While famously red due to iron oxidation (rust) in its regolith and dust, Mars exhibits a palette of colors including brown, gold, and tan. This rusted dust is frequently swept into the atmosphere, tinging the planet red from afar.

Despite being half Earth's diameter, Mars boasts a surface area nearly equal to Earth's dry land. Its landscape is dramatic, shaped by volcanoes, impact craters, and atmospheric erosion.

The planet hosts Valles Marineris, a canyon system stretching over 3,000 miles (4,800 kilometers)—spanning the distance from California to New York. It is up to 200 miles (320 kilometers) wide and 4.3 miles (7 kilometers) deep, dwarfing Earth's Grand Canyon.

Mars also features Olympus Mons, the largest volcano in the solar system. Rising three times higher than Mount Everest, its base covers an area comparable to New Mexico.

Evidence points to a watery past, with ancient river networks, deltas, and lakebeds. Floods likely washed over the planet 3.5 billion years ago.

Today, water exists primarily as ice in polar regions and subsurface deposits, with some briny water seasonally flowing on slopes. The thin atmosphere prevents liquid water from persisting on the surface.

Atmosphere

The Martian atmosphere is tenuous, composed primarily of carbon dioxide, nitrogen, and argon. This composition creates a hazy, reddish sky due to suspended dust. The thin air provides little shield against meteorites and asteroids.

Temperatures fluctuate wildly, from a mild 70°F (20°C) to a freezing -225°F (-153°C). Heat escapes rapidly; standing at the equator at noon, your feet might feel spring-like warmth (75°F/24°C) while your head experiences freezing winter cold (32°F/0°C).

Powerful winds can trigger planet-wide dust storms that obscure the surface for months.

Potential for Life

Current scientific consensus deems Mars unlikely to host living organisms today. Research is instead focused on detecting signs of ancient life that may have thrived when the planet was wetter and warmer.

Moons

Two small moons, Phobos and Deimos, orbit Mars. Likely captured asteroids, their low mass prevents gravity from shaping them into spheres, leaving them potato-shaped.

Named after the horses of Ares (the Greek god of war), Phobos means "fear" and Deimos "panic."

Phobos, the larger inner moon, is scarred by deep grooves and craters. It is slowly spiraling inward and is destined to crash into Mars or shatter in about 50 million years.

Deimos, half the size of Phobos, orbits much farther out. Its surface appears smoother, blanketed in loose dirt that fills its craters.

Rings

Mars possesses no rings. However, it is theorized that in 50 million years, the destruction of Phobos could form a dusty ring system around the planet.

Magnetosphere

Lacking a global magnetic field today, Mars leaves its surface exposed to solar radiation. However, pockets of the southern hemisphere's crust remain highly magnetized, a fingerprint of a global magnetic field that existed 4 billion years ago.