Mars
Mars is the fourth planet from the Sun and the closest planet to Earth that we can easily visit. It is smaller than Earth, much colder, and covered in rusty red dust. That dust is why it's nicknamed the Red Planet. Humans have been fascinated by Mars for thousands of years, and right now there are several robots driving around on its surface.
- Position 4th planet from the Sun
- Distance from Sun 228 million km 1.5 times further than Earth
- Diameter 6,779 km about half of Earth
- Day length 24h 37m almost the same as Earth
- Year length 687 days 1.9 Earth years
- Moons 2 Phobos and Deimos
Where Mars sits
Mars orbits the Sun at an average distance of about 228 million km. That's one and a half times further out than Earth. Because Mars's orbit isn't a perfect circle, the distance between Earth and Mars changes a lot. At their closest, every 26 months or so, the two planets pass each other at approx. 54 million km apart. At their furthest, they're nearly 401 million km apart, over seven times further. That's why missions to Mars are usually launched in tight six-week "launch windows" every 26 months, when the journey is shortest.
Mars vs Earth
How does the Red Planet compare to home?
Mars is about half the diameter of Earth. If Earth were a basketball, Mars would be a softball.
If you weighed 50 kg on Earth, on Mars you'd weigh just 19 kg. You could jump three times as high.
A day on Mars (called a sol) is almost exactly the same length as on Earth, just 37 minutes longer.
A year on Mars is nearly twice as long as a year on Earth, so kids on Mars would have a birthday every two Earth years.
Mars is much colder than Earth. Summer days near the equator can reach 20°C, but nights at the poles can plunge to −140°C.
Phobos and Deimos are both tiny, lumpy, and probably asteroids that Mars captured long ago.
Why is Mars red?
If you visited Mars, the first thing you'd notice is the colour. The whole planet is covered in a layer of fine dust, and that dust is rust. Iron in Martian rocks reacted with oxygen in the early atmosphere billions of years ago, slowly turning everything red. The dust gets everywhere: into the sky, onto solar panels, into machinery. NASA's Spirit rover died in 2010 partly because dust covered its solar panels.
Mars's giant landscapes
Mars holds two of the most extreme records in the whole solar system.
Olympus Mons, the biggest mountain
Olympus Mons is a giant shield volcano that stands 22 km tall, nearly three times the height of Mount Everest. Its base is approx. 600 km across, almost as big as the entire country of France. If you tried to climb it, the slope is so gentle (only two to five degrees) you might not even notice you were on a mountain at all. From the summit you wouldn't be able to see the bottom because the mountain is so wide it disappears over the horizon.
Valles Marineris, the longest, deepest canyon
Valles Marineris stretches more than 4,000 km across the Martian equator, as long as the entire United States. In places it's up to 7 km deep, nearly five times deeper than the Grand Canyon. If you put it on Earth, it would reach from Los Angeles to New York.
How did Valles Marineris form?
It wasn't carved by a river. It probably opened up billions of years ago when the whole Tharsis region of Mars bulged upwards (pushed by giant volcanoes like Olympus Mons), cracking the crust along its eastern flank. Wind, landslides and possibly ancient water then shaped it over time. Some parts of it might have once held lakes.
Polar ice caps
Both Martian poles are covered in caps of ice. The ice is a mixture of water ice and frozen carbon dioxide ("dry ice"). The caps shrink in summer and grow in winter, just like on Earth, except on Mars the CO2 turns directly from solid to gas without melting first, like the dry ice in a Halloween fog machine.
Moons of Mars
Mars has two moons, both named after the Greek gods of fear and dread: Phobos ("fear") and Deimos ("terror"). They're tiny. Phobos is only approx. 22 km across and Deimos just 12 km. They're not round like our Moon; they look more like potatoes, and they're probably captured asteroids rather than "real" moons that formed alongside the planet.
Phobos orbits Mars so fast (every 7.5 hours) that from the Martian surface you can see it rise in the west, cross the sky in about four hours, and set in the east. It does this twice a day.
Was there ever water on Mars?
Today, Mars's surface is too cold and its atmosphere too thin for liquid water to last. Water would either freeze solid or boil away almost instantly. But the evidence is everywhere that Mars used to be very different.
- Dry riverbeds wind across the surface, branching like rivers do on Earth.
- Ancient shorelines trace what look like the edges of long-vanished lakes and possibly even a northern ocean.
- Minerals like clay and gypsum, which only form in water, are common in older rocks.
- Polar ice caps still hold huge amounts of frozen water today.
- Underground ice has been mapped across much of the planet.
- Recurring slope lineae are dark streaks that appear on cliff faces in summer. They may be salty water briefly seeping out and quickly evaporating.
Mars was probably warm, wet and Earth-like for the first billion years of its life. Then something went wrong. Possibly the loss of its global magnetic field allowed the solar wind to strip its atmosphere away, and the planet slowly froze.
Could anything live there?
If Mars was once warm and wet, it's perfectly possible that life started there, just as it did on Earth around the same time. It might even still be there, hiding underground where temperatures are warmer and there's liquid water.
That's exactly what NASA's Perseverance rover is looking for right now in Jezero Crater, an ancient river delta where lakebed sediments are perfect for preserving fossilised microbes. The rover is collecting rock samples to be picked up by a future mission and brought back to Earth for analysis in the 2030s. This will be the first sample return from another planet.
Robots on Mars
Mars is the only planet, other than Earth, where humans have successfully landed working machines. Over 20 missions have reached the surface from Russia, the US, Europe, China, India and the UAE. Some highlights:
- Viking 1 and 2 (1976) were the first successful landers and sent the first photos from the surface.
- Sojourner (1997) was the first rover, the size of a microwave oven.
- Spirit and Opportunity (2004) were designed for 90 days but kept going for years. Opportunity drove over 45 km before a giant dust storm killed it in 2018.
- Curiosity (2012) is the size of a small car and still going, climbing Mount Sharp.
- Perseverance and Ingenuity (2021): Perseverance is the rover; Ingenuity was a tiny helicopter, the first aircraft to fly on another planet. It made 72 successful flights between 2021 and 2024.
- Zhurong (2021) was China's first Mars rover, successful on the first try.
Could humans live there?
Mars is the most plausible place for humans to visit and possibly even live on, but it's extremely hostile to life as we know it. The main challenges:
- You can't breathe the air. Mars's atmosphere is 95% carbon dioxide and less than 1% as thick as Earth's. Step outside without a pressure suit and you'd be unconscious in seconds.
- It's cold. Even on a "warm" summer day at the equator, nights drop below −70°C.
- Radiation is intense. Mars has no global magnetic field, so the surface receives much more solar and cosmic radiation than Earth. Long-term colonists would need to live underground or under thick shielding.
- The gravity is weak. Years in 38% gravity would weaken human bones and muscles, and we don't yet know how badly it would affect children born there.
Despite all of this, NASA, SpaceX and other agencies are seriously planning crewed missions for the 2030s and 2040s. The first humans to set foot on another planet are probably already alive today.