Mars

Mars is the fourth planet from the Sun, covered in red dust. Its day length and seasons resemble Earth's, and with traces of water that flowed long ago, it's the planet where the search for signs of life is most active.

What kind of planet is it?

Mars is the fourth planet, orbiting at an average distance of about 1.52 AU from the Sun. It's about 6,779 km across, just over half Earth's diameter, and its surface gravity is about 0.38 times Earth's. Someone whose bathroom scale reads 50 kg on Earth would see about 19 kg on Mars.

Mars is a rocky planet with an iron-rich core, a rocky mantle and a crust. Its soil and dust contain lots of iron oxide (rust, basically), which gives it a reddish color and its nickname, the Red Planet.

Mars has some of the most spectacular landforms in the Solar System. Olympus Mons is a volcano about 22 km high, more than twice the height of Mount Everest, and Valles Marineris is a canyon system about 4,000 km long and up to 7 km deep. Dried-up riverbeds, deltas and old lake beds suggest that liquid water once flowed across the surface long ago.

Mars rotates once in about 24 hours 37 minutes, almost the same as Earth. Its axis is tilted about 25.19°, so it has four seasons like Earth, but since its year lasts about 687 days, each season is nearly twice as long as ours.

Atmosphere and weather

The Martian atmosphere is about 95% carbon dioxide, but it's so thin that the surface pressure is less than 1% of Earth's. With so little air to trap heat, the average temperature is a frigid -63°C (-81°F), and temperatures swing widely within a single day. At midday in equatorial summer it can approach 20°C, but at night it can plunge below -70°C.

Mars's signature weather is the dust storm. These range from small dust devils to huge storms that cover the entire planet for weeks or months. A planet-wide dust storm in 2018 blotted out the sunlight and ended the mission of the solar-powered rover Opportunity.

Both poles have white polar caps. Water ice is topped with dry ice (frozen carbon dioxide), and as the seasons change, the dry ice freezes and then turns back into gas, making the caps grow and shrink. Several spacecraft have also confirmed that large amounts of ice are buried underground.

Moons and rings

Mars has two small moons. They were discovered in 1877 by American astronomer Asaph Hall and named Phobos and Deimos after the two sons of Ares, the Greek god of war. Both are lumpy, potato-shaped bodies, and scientists are weighing two ideas: that they're captured asteroids, or that they formed from debris blasted out by a big impact.

• Phobos: The larger of the two, about 22 km across on average. It circles about 6,000 km above the Martian surface once every 7 hours 40 minutes or so, so from Mars it rises in the west and sets in the east twice a day. It's slowly creeping closer to Mars and is expected to break apart or crash into the planet in tens of millions of years.
• Deimos: Smaller, about 12 km across on average, orbiting farther out about once every 30 hours.

Mars has no rings.

Exploration

More spacecraft have visited Mars than any other planet beyond Earth. There have been plenty of failures, but we've learned a great deal.

• Mariner 4 (USA, 1965): Flew past Mars and sent back the first close-up photos. Many people were surprised to see a surface riddled with craters.
• Mariner 9 (USA, 1971): The first spacecraft to orbit another planet. It gave the first detailed views of the giant volcanoes and canyons, and Valles Marineris is named after it.
• Viking 1 and 2 (USA, 1976): Landed on the surface, took photos and ran experiments to look for life in the soil.
• Mars Pathfinder and Sojourner (USA, 1997): The little rover Sojourner became the first to roll across the Martian surface, studying rocks.
• Spirit and Opportunity (USA, landed 2004): Found many pieces of evidence that water existed in the past. Opportunity kept going for more than 14 years on a mission planned to last about 90 days.
• Curiosity (USA, landed 2012): Has been studying traces of an ancient lake and organic matter in Gale Crater.
• Perseverance (USA, landed 2021): Is collecting rock samples in Jezero Crater, the site of an ancient delta. The small helicopter that came along, Ingenuity, made the first powered flight on another planet in 2021.

Many other countries have joined in too, including India's Mangalyaan (entered orbit 2014), the United Arab Emirates' Hope (2021), and China's Tianwen-1 with the Zhurong rover (2021).

Fun facts

• Sunsets on Mars are bluish, because fine dust in the atmosphere scatters blue light toward the direction of the Sun more effectively.
• A Martian day is called a sol and lasts about 24 hours 40 minutes. Rover teams sometimes live on Mars time for a while.
• Earth and Mars come closest about every 2 years and 2 months, which opens a good launch window for sending spacecraft.
• Mars lacks the global magnetic field and thick atmosphere that Earth has, so far more ultraviolet light and cosmic radiation pour down on its surface.

See it in Planetica

In Solar System view, Mars appears on the fourth orbit, just outside Earth. Its position is calculated from J2000 orbital elements for today's date, so you can instantly see whether Mars is nearby on Earth's side or far away on the other side of the Sun. When Earth and Mars are side by side, that's when Mars really does shine brightly in the night sky.

Pick Mars and fly in to see its two moons, Phobos and Deimos, circling with it. The real moons are only about 10 to 20 km across and would be nearly invisible at true scale, so they're enlarged on screen. Size, gravity, day length, axial tilt, average temperature and orbital period are based on real values.

The surface patterns are created by Planetica's planet generator. It isn't a map showing the real locations of Olympus Mons or Valles Marineris, so enjoy it as a way to soak up the mood of the Red Planet.

Other planets

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• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune