Mars: The Red Planet

The world that glows red in our sky

If you look at the right part of the night sky, you may spot a steady point of light with a clear reddish-orange tint. That is Mars, the fourth planet from the Sun and our neighbour just beyond Earth. People have watched it for thousands of years, and many cultures linked its blood-red colour to war — the Romans named it after their war god, Mars.

Mars is the planet most like Earth in some ways, and the one humans dream of visiting most. It has seasons, polar ice caps, towering volcanoes and deep canyons. Yet it is also a harsh, frozen desert with almost no air. Understanding why Mars turned out so different from Earth tells us a great deal about how planets work — and about how lucky we are to live on a world that stayed warm and wet.

Why Mars is red

Mars's nickname comes from a simple piece of chemistry. Its surface is covered in fine dust rich in iron oxide — exactly the compound we call rust. When iron is exposed to oxygen, it turns reddish-brown, whether it is an old nail in your garden or the soil of an entire planet. Long ago, iron in the Martian rocks reacted and rusted, and dust storms spread that rusty powder everywhere.

So Mars is, in a sense, a rusty world. Its sky even looks butterscotch or pinkish during the day, because sunlight scatters off the dust floating in the thin air — the opposite of Earth's blue sky.

A small, cold, thin-aired world

Mars is about half the width of Earth and much less massive, so its gravity is only about 38% of ours — you could jump nearly three times as high there. Its smaller size matters in a surprising way. Smaller planets cool down faster inside, and as Mars's interior cooled, it lost the churning that once gave it a protective magnetic field.

Without that shield, the solar wind — a constant stream of particles from the Sun — slowly stripped away most of Mars's atmosphere over billions of years. Today the air is more than 100 times thinner than Earth's and is mostly carbon dioxide. There is far too little oxygen to breathe.

The thin air and Mars's greater distance from the Sun make it bitterly cold. The average temperature is around minus 60°C, though a summer day near the equator can briefly climb above freezing. With such low air pressure and cold, liquid water cannot last on the surface — it freezes or boils straight into vapour. That single fact shapes everything about Mars today.

Giant volcanoes and a canyon to dwarf all others

For a small planet, Mars has some enormous landscapes — partly because it is small and quiet.

Olympus Mons is the tallest volcano in the Solar System, rising about 22 kilometres above the plains. That is roughly two and a half times the height of Mount Everest, and its base is about as wide as the whole country of Poland. It grew so huge because Mars has no shifting crust to move the surface off the hot spot below — so lava piled up in the same place for millions of years.

Valles Marineris is a system of canyons stretching over 4,000 kilometres — about as long as the entire United States is wide, and in places several kilometres deep. The Grand Canyon on Earth would fit inside one small side-branch of it. It probably formed as the crust cracked and pulled apart when a nearby bulge of volcanoes swelled the surface.

Two tiny moons

Mars has two small moons, named Phobos and Deimos — Greek for "fear" and "dread", the companions of the war god. They are nothing like our round, bright Moon. Both are tiny, dark and lumpy, shaped more like potatoes, and are probably captured asteroids pulled in by Mars's gravity. Phobos orbits so close and so fast that it crosses the Martian sky twice a day, and it is slowly spiralling inwards — in tens of millions of years it may break apart into a ring or crash into the planet.

Searching for water — and for life

The biggest question about Mars is whether anything ever lived there. We have good reason to ask. Orbiting spacecraft have photographed dried riverbeds, deltas and lake basins, and rovers have found minerals like clays and salts that only form in water. Billions of years ago, when Mars had a thicker atmosphere, it seems to have had flowing rivers and standing water — exactly the conditions life needs.

Most of that water did not all vanish. Huge amounts are now locked up as ice in the polar caps and frozen underground. If microscopic life ever started on early Mars, traces of it might still be hidden in ancient rocks.

That is why robots have been sent to look. The Perseverance rover, which landed in 2021, is exploring Jezero Crater — an ancient lake and river delta — drilling rock cores and sealing them in tubes. The hope is that a future mission can bring those samples back to Earth, where powerful laboratories could search them for fossil-like clues. Perseverance even carried a small helicopter, Ingenuity, which became the first machine to fly in the air of another planet.

Try it yourself: find Mars in the real sky

You can spot Mars with your own eyes — no telescope needed.

1. Check a stargazing app or website for when Mars is visible from where you live. It is not up every night, because Earth and Mars move around the Sun at different speeds.
2. Look for its colour. Unlike the white twinkle of most stars, Mars shines with a steady, slightly orange or salmon light. Planets generally do not twinkle as much as stars.
3. Track it over a few weeks. Mark its position against nearby stars on a simple sketch. You will see it slowly drift — the word "planet" comes from the Greek for "wanderer", because planets shift while the stars stay fixed.
4. Use binoculars if you have them. They will not show much detail on Mars's small disc, but steadying them will sharpen its colour and brightness.

When Mars and Earth are closest, roughly every two years, Mars becomes one of the brightest objects in the night sky — a great time to look.

Want to explore further? Meet Mars's neighbours in Planets of the Solar System, and see how scientists study distant worlds in Telescopes and Studying Space.