Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. Facts from NASA, ESA and published studies; the pictures are not to scale.
Tap, drag and slide.
Slide to zoom from the orange-red dot in the evening sky all the way to the planet.
Zoom from a point of light to the disk. The colour you see from Earth is the colour of the dusty surface.
Slide to zoom in on the red dot. What is it?
Leave the nail out for a long time. Then switch the water off and try again. Which way rusts faster?
Iron + oxygen gives iron oxide; water speeds the reaction up. Compare wet and dry.
Slide the time. Does the nail get rusty? Now turn off the water.
The speeds are made up to show the idea; real rusting takes much longer.
Start a dust storm and watch the rusty dust paint the planet. Then clean it and try again.
Global storms (about once every three Mars years on average) spread a thin, uniform veneer of oxidized dust.
Tap Dust storm. Watch the planet turn red!
Drag the drill down. What colour is the rock under the dust?
Drill through the oxidized coating into the basalt-like rock below, as Curiosity did in Gale crater.
Pull the drill down. What colour is inside?
Layer thickness not to scale.
Slide through time, from ancient wet Mars to the dry red desert of today.
Scrub from an early, wetter Mars (ferrihydrite forming with cool water) through field loss and atmospheric stripping to today.
A simplified timeline.
Pick a planet, then lower the Sun. How are the colours on Mars different from Earth?
Compare Earth, where air scatters blue light across the sky, with Mars, where fine dust keeps blue light close to the setting Sun.
Slide the Sun down. What colour is a sunset on Mars?
Colours are a simple sketch, not a photo.
Mars is a planet. In the evening sky it looks like a little orange-red dot. People long ago called it "the red one".
Have you seen an old bike with orange spots? That is rust. Rust happens when iron meets air and water. The rocks on Mars have lots of iron in them. Over a very, very long time, the iron turned rusty.
The rusty rocks crumbled into dust, finer than flour. The wind blew the dust all over Mars. Sometimes a dust storm covers the whole planet! The dust settles on everything, like sugar on a cake.
Mars is not red all the way through. A rover dug a little hole in a rock, and the dust inside was grey. The red is just a thin coat on top.
Want more? See how planets are made, why Saturn has rings and how gold is made. ← More explainers
Mars is red because it is rusty. Its rocks, soil and dust are rich in iron, and over a very long time that iron reacted with oxygen and turned into iron oxide, the same family of minerals as the rust on an old bike.1 Winds grind the rusty rock into fine dust and blow it all over the planet, so from far away the whole of Mars looks red.13
Two surprises make the story even better: the red is only a thin coat (when rovers drill into many rocks, the powder underneath is grey), and a recent study suggests Mars did most of its rusting long ago, when it still had liquid water.37
Mars is bright enough to see without a telescope, so people have watched it since ancient times.2 Its warm orange-red glow stands out among the white stars. The ancient Romans named it after their god of war because the colour reminded them of blood, and the ancient Egyptians called it "Her Desher", which means "the red one".1 The name "Red Planet" is thousands of years older than the science that explains it.
Mars is on average about 225 million kilometres (140 million miles) from Earth12, and a spacecraft takes about seven months to get there.13 It is about half as wide as Earth, and it is a cold, dusty desert with a very thin atmosphere made mostly of carbon dioxide.1
On Earth, iron rusts when it meets oxygen and water. The iron atoms join with oxygen to make a crumbly, reddish-brown iron oxide, and because rust flakes off, fresh iron underneath keeps rusting.14 Mars is packed with iron: its crust contains iron along with magnesium, aluminium, calcium and potassium, and its core is made of iron, nickel and sulfur.1 Here is the chain that turned a whole planet red:
In the explainer above, the nail toy shows the basic chemistry: with water, the nail rusts fast; without water, it barely changes. The speeds in that toy are made up to make the idea visible; real rusting takes much longer.
| Rust on Earth | Red on Mars | |
|---|---|---|
| Where the iron is | Nails, bikes, cars, and iron-rich soils | Rocks, soil and dust across the whole planet1 |
| What it reacts with | Oxygen and water in the air14 | Oxygen, and water long ago3 |
| What we see | Red-brown crust that flakes off | A thin rusty dust coat over grey rock9 |
| What spreads it | Not much: it stays put | Wind and dust storms5 |
The Martian dust is very fine and a little electrostatic, so it clings to everything it touches, much like packing peanuts. Rover engineers know this well: dust coats rovers and settles on solar panels.5
Every Mars year, moderately big dust storms cover areas the size of continents and last for weeks. About once every three Mars years (roughly five and a half Earth years), on average, a storm grows until it wraps around the entire planet.5 After such a storm, it can be months before all the dust settles.1 Even small dust devils, just a few feet across, move dust around.5 Martian storm winds reach only about 100 km/h (60 mph), and the air is only about 1% as dense as Earth's, so the wind pushes gently, but it has had billions of years to do the job.5
As the lead scientist of a recent study put it, "As Martian winds spread this dust everywhere, it created the planet's iconic red appearance."3
No. The red is only skin-deep. When NASA's Curiosity rover drilled holes about 6 centimetres deep into rock in Gale crater, the powder that came out was grey, in striking contrast with the dusty red surface.9 The rocks Curiosity has studied are basaltic in composition, meaning they are made from the same kind of material as dark volcanic basalt on Earth.6
Even the surface itself is not one flat red. Up close, NASA describes colours such as brown, gold and tan.1 The red is what wins when you look at the whole planet from far away, because the fine rusty dust is everywhere, including up in the air. In the drill toy above, the thickness of the red layer is exaggerated so you can see it; the real coat of dust is far thinner compared with the rock.
For a long time, the leading idea was that Mars' red colour came mostly from hematite, a dry form of iron oxide that could form slowly in a cold, dry world.411 A study published in the journal Nature Communications (Valantinas and colleagues, 2025) suggests a different main culprit: ferrihydrite, a water-rich iron oxide.37
The team compared data from orbiters (NASA's Mars Reconnaissance Orbiter and ESA's Mars Express and Trace Gas Orbiter) and from rovers with a Martian dust they made in the lab.3 The difference matters because ferrihydrite has water built into its structure and forms with cool water, at lower temperatures than hematite.34 If the study is right, Mars rusted early, while liquid water was still around, and the red dust we see today is a leftover from that wetter time.311 Scientists say the idea can be tested for certain once rock samples collected by the Perseverance rover are studied in labs on Earth.3
Billions of years ago Mars looked very different. It has ancient river valleys, deltas, lakebeds and minerals that only form in liquid water, and some features suggest huge floods about 3.5 billion years ago.1 The Perseverance rover landed in Jezero crater, where it found signs of a lake, delta and river system.15 ESA's Mars Express orbiter has traced the history of water across the globe, too.17
So what happened? Mars has no global magnetic field today, although magnetized rocks show it had one about 4 billion years ago.1 Measurements from NASA's MAVEN spacecraft show that the solar wind and radiation stripped most of Mars' atmosphere away into space.16 With thin, cold air, liquid water cannot last on the surface; today Mars' water is mostly ice under the ground near the poles, plus some salty seeps.1 The time slider in the explainer is simplified: it squeezes billions of years into a few steps.
The dust does one more trick: it colours the sky. To our eyes, the daytime Martian sky would look hazy and reddish, a butterscotch colour, because of the dust hanging in the air.1 At sunset, it flips. The sky around the setting Sun turns blue-grey, the opposite of Earth.10
Why? The fine dust particles let blue light pass through more efficiently than red and yellow, so blue light stays close to the Sun's part of the sky, while the reds and yellows are scattered widely. The effect is strongest at sunset, when sunlight travels a longer path through the air. NASA's Curiosity rover photographed exactly this blue sunset in Gale crater.8 On Earth it works the other way: our air molecules scatter blue light all over the sky, which is why our daytime sky is blue (see why is the sky blue). The sky colours in the sunset toy are a simple sketch, not exact photo colours.
The colour of Mars is a history book. The rust tells us Mars has lots of iron, the dust tells us about its winds, and the kind of rust may tell us when water flowed. That is why scientists care so much: a planet that once had lakes might once have been a place where life could exist.317 Curious how Mars formed in the first place? See how planets form, or find out how rockets get us there. More space stories: why Saturn has rings and how gold is made.
Mars is red because its rocks and soil contain a lot of iron, and that iron has rusted into iron oxide. Winds grind the rusty rock into fine dust and spread it over the whole planet, so from far away Mars looks red.
Because it looks reddish, even to the naked eye. The ancient Romans named it after their god of war, and the ancient Egyptians called it 'Her Desher', meaning 'the red one'. Today we know the colour comes from rusty iron minerals in its dust.
Up close, the surface shows many colours: brown, gold, tan and rusty red. Under the thin layer of dust, many rocks are dark grey. When NASA's Curiosity rover drilled into rock, the powder it brought up was grey.
No. The red is a thin coat of rusty dust. The rock underneath is mostly dark volcanic rock similar to basalt, and deep inside Mars has a rocky mantle and an iron-rich core.
No. The red is the colour of rust, not heat. Mars is a cold desert: temperatures range from about 20 degrees Celsius on a warm day to about minus 153 degrees Celsius.
In the daytime the Martian sky looks hazy butterscotch or reddish because of the dust in the air. At sunset the sky around the Sun turns blue-grey, the opposite of Earth, because fine dust lets blue light through toward the Sun's direction.
Probably. A recent study suggests much of the red dust is ferrihydrite, a kind of rust that forms with cool water. That would mean Mars rusted long ago, when it still had liquid water, before it turned into the dry desert we see today.