Mars

Mars is the fourth planet from the Sun.¹ It is a dusty, cold, desert world with a very thin atmosphere.¹ This dynamic planet has seasons, polar ice caps, extinct volcanoes, canyons, and weather.¹ It is a terrestrial planet.¹ Two small moons, Phobos and Deimos, circle it.¹

Mars is one of the most explored bodies in the solar system.¹ It is the only planet where rovers have been sent to roam the landscape.¹ The ancient Romans named the planet for their god of war because its reddish color was reminiscent of blood.¹ The Egyptians called it "Her Desher," meaning "the red one".¹ Even today it is frequently called the "Red Planet" because iron minerals in the Martian dirt oxidize, or rust, causing the surface to look red.¹

NASA missions have found evidence that Mars was much wetter and warmer, with a thicker atmosphere, billions of years ago.¹ Scientists do not expect to find living things currently thriving on Mars.¹ Instead, they are looking for signs of life that existed long ago, when Mars was warmer and covered with water.¹ That search now runs through ancient rocks and minerals that could only have formed in liquid water.¹

With a radius of 3,390 kilometers (2,106 miles), Mars is about half the size of Earth.¹ If Earth were the size of a nickel, Mars would be about as big as a raspberry.¹ The planet orbits at an average distance of 228 million kilometers (142 million miles) from the Sun, or 1.5 astronomical units.¹ ESA states the distance as 227,940,000 kilometers, or 1.52 times that of Earth.² From this distance, it takes sunlight 13 minutes to travel from the Sun to Mars.¹

As Mars orbits the Sun, it completes one rotation every 24.6 hours, which is very similar to one day on Earth.¹ Martian days are called sols, short for "solar day".¹ ESA gives the Martian day as 24 hours 37 minutes.² A year on Mars lasts 669.6 sols, which is the same as 687 Earth days.¹ ESA records the Martian year as 669 sols or 687 Earth days.²

Mars' axis of rotation is tilted 25 degrees with respect to the plane of its orbit around the Sun.¹ Earth, by comparison, has an axial tilt of 23.4 degrees.¹ Like Earth, Mars has distinct seasons, but they last longer than seasons on Earth.¹ The seasons vary in length because Mars' orbit is elliptical.¹ Spring in the northern hemisphere is the longest season at 194 sols, and autumn in the northern hemisphere is the shortest at 142 days.¹

When the solar system settled into its current layout about 4.5 billion years ago, Mars formed as the fourth planet from the Sun.¹ Like its fellow terrestrial planets, it has a central core, a rocky mantle, and a solid crust.¹ The core at its center is between 1,500 and 2,100 kilometers (930 and 1,300 miles) in radius, and it is made of iron, nickel, and sulfur.¹ The rocky mantle is between 1,240 and 1,880 kilometers (770 and 1,170 miles) thick, and the crust is 10 to 50 kilometers (6 to 30 miles) deep.¹

Mars looks reddish due to oxidization — rusting — of iron in the rocks, regolith, and dust.¹ This rusting is why the planet is called the "Red Planet".¹ While Mars is about half the diameter of Earth, its surface has nearly the same area as Earth's dry land.¹

Mars is home to the largest volcano in the solar system, Olympus Mons.¹ It stands more than 25 miles (40 kilometers) tall from base to summit, and its base covers an area as large as the state of Arizona.¹ It dwarfs Earth's highest peak, Mount Everest, which rises 29,029 feet, or 8,848 meters, above the Earth's surface.¹ The planet's Valles Marineris canyon — the largest in the solar system — extends about 3,870 kilometers (2,400 miles) in length, spans about 600 kilometers (370 miles) across at its widest, and descends about 9.3 kilometers (5.8 miles) at its deepest, more than 10 times longer and wider than Earth's Grand Canyon.¹ Mars has no global magnetic field today, but areas of the southern-hemisphere crust are highly magnetized, indicating traces of a magnetic field from 4 billion years ago.¹

Mars has a thin atmosphere made up mostly of carbon dioxide, nitrogen, and argon gases.¹ ESA measures the air as 95.32% carbon dioxide, 2.7% nitrogen, 1.6% argon, and 0.13% oxygen.² Atmospheric pressure at the surface is 6.35 mbar — less than one hundredth of Earth's atmospheric pressure.²

To our eyes, the Martian sky would be hazy and red because of suspended dust, instead of the familiar blue tint seen on Earth.¹ The tint comes from dust suspended in the thin air.¹ The sparse atmosphere does not offer much protection from impacts by meteorites, asteroids, and comets.¹

The temperature on Mars can be as high as 20 degrees Celsius (70 degrees Fahrenheit).¹ It can be as low as about -153 degrees Celsius (-225 degrees Fahrenheit).¹ ESA gives the average as -55 degrees Celsius, from -133 degrees Celsius at the winter pole to +27 degrees Celsius during summer.² Standing on the equator at noon, it would feel like spring at your feet (24 degrees Celsius) and winter at your head (0 degrees Celsius).¹

Occasionally, winds on Mars are strong enough to create dust storms that cover much of the planet.¹ After such storms, it can be months before all of the dust settles.¹ A planet-wide storm of this kind blanketed Opportunity's location in June 2018, and the rover never communicated with Earth again.³

Mars appears to have had a watery past.¹ The surface preserves ancient river valley networks, deltas, and lakebeds.¹ Rocks and minerals on the surface could only have formed in liquid water.¹ Some features suggest that Mars experienced huge floods about 3.5 billion years ago.¹

There is water on Mars today, but the atmosphere is too thin for liquid water to exist for long on the surface.¹ Water is found as water-ice just under the surface in the polar regions.¹ It also exists as briny, salty water that seasonally flows down some hillsides and crater walls.¹

Early in its mission, Curiosity's scientific tools found chemical and mineral evidence of past habitable environments on Mars.⁴ The rover also detected the largest organic molecules found on Mars to date — decane, undecane, and dodecane — in a drilled rock sample called "Cumberland".⁴ Perseverance explored the history of geology, water, and habitable environments in Jezero crater and on the crater rim.⁵ In September 2025, after a rigorous yearlong peer review, the journal Nature published validated results that Perseverance's "Sapphire Canyon" sample from the rock nicknamed "Cheyava Falls" contains potential biosignatures.⁵ These are clues that suggest past life may have been present, but they require more data or further study before any conclusions about the absence or presence of life.⁵

Mars has two small moons, Phobos and Deimos, that may be captured asteroids.¹ They are potato-shaped because they have too little mass for gravity to make them spherical.¹ Asaph Hall discovered Deimos on one August night in 1877, after his wife Angelina urged him on, and he found Phobos six nights later.⁶ Hall named the moons for the mythological sons of Ares, the Greek counterpart of the Roman god Mars: Phobos means fear and Deimos means dread.⁶ The moons are among the smallest in the solar system.⁶

Phobos, the innermost and larger moon, is heavily cratered with deep grooves on its surface.¹ Phobos measures 27 x 22 x 18 kilometers and orbits about 6,000 kilometers above Mars.² Deimos measures 15 x 12 x 11 kilometers and orbits about 20,000 kilometers above Mars.² Deimos is about half as big as Phobos and orbits two and a half times farther away from Mars.¹ Phobos whips around Mars three times a day, while the more distant Deimos takes 30 hours for each orbit.⁶

No known moon orbits closer to its planet than Phobos, which circles only 6,000 kilometers (3,700 miles) above the Martian surface.⁶ In 1971, NASA's Mariner 9 spacecraft got a much better look at the two moons from its orbit around Mars.⁶ The dominant feature on Phobos, it found, was a crater 10 kilometers (6 miles) wide — nearly half the width of the moon itself.⁶ It was given Angelina's maiden name: Stickney.⁶ Phobos is gradually spiraling inward, drawing about 1.8 meters (six feet) closer to the planet each century; within 50 million years it will either crash into Mars or break up and form a ring.⁶

Like Earth's Moon, Phobos and Deimos always present the same face to their planet.⁶ Both are lumpy, heavily cratered, and covered in dust and loose rocks.⁶ They are among the darker objects in the solar system.⁶ The moons appear to be made of carbon-rich rock mixed with ice.⁶ Phobos has only 1/1,000th as much gravitational pull as Earth, and a 150-pound (68 kilogram) person would weigh two ounces (68 grams) there.⁶

NASA's Mariner 4 became the first spacecraft to fly by Mars and captured the first close-up views of another planet.⁷ Its closest approach came at 08:00:57 p.m. EDT on July 14, 1965 (01:00:57 UT July 15).⁷ The TV camera began taking the first of 21 images at a range of 9,846 kilometers (6,118 miles).⁷ In 1971, Mariner 9 became the first spacecraft to orbit another planet, mapping 85% of the Martian surface.⁸ That same year, the Soviet Mars 3 lander claimed a technical first with a survivable landing, but contact was lost seconds after it touched down.⁹

Viking 1 was the first of a pair of complex deep-space probes designed to collect evidence on the possibility of life on Mars.⁹ It made the first truly successful landing on Mars and returned the first images from the surface.⁹ The lander set down on July 20, 1976 at 22.483 degrees north latitude and 47.94 degrees west longitude.⁹ On July 28, 1976, its robot arm scooped up the first soil samples from the Martian surface.⁹

Mars Pathfinder landed on July 4, 1997 and delivered the first-ever robotic rover, Sojourner, to the Martian surface.¹⁰ It entered the atmosphere with the help of a parachute and cushioned the impact with a giant system of airbags — the first time this technique had been used.¹⁰ The twin Mars Exploration Rovers, Spirit and Opportunity, landed on January 3 and January 24, 2004, in search of answers about the history of water on Mars.³ Spirit concluded its mission in 2010, and Opportunity last communicated with Earth on June 10, 2018, as a planet-wide dust storm blanketed its location.³

Curiosity, the largest and most capable rover ever sent to Mars when it launched, was launched on November 26, 2011 and landed on August 6, 2012.⁴ The rover landed in Gale Crater and set out to answer whether Mars ever had the right environmental conditions to support microbes.⁴ Perseverance was launched on July 30, 2020 and landed on February 18, 2021 in Jezero Crater.⁵ As of October 2024, it had driven over 30 kilometers (18.65 miles) and collected 24 samples of rock and regolith, as well as one air sample.⁵ Strapped to its belly for the journey was the Mars Helicopter, Ingenuity, which completed 72 historic flights and became the first aircraft to achieve powered, controlled flight on another planet.⁵

Mars is about half the diameter of Earth, yet its surface has nearly the same area as Earth's dry land.¹ The table below sets the two worlds side by side, using the figures stated in the cited sources.² The moons row marks a clear difference: Mars has two small moons, Phobos and Deimos.¹