Why is Mars red?
Because its dust is rusty. Iron in Martian rock has oxidized, and a 2025 study suggests the dust is mostly ferrihydrite, an iron oxide that contains water and probably formed when Mars was wetter.
Quick Answer
Mars looks red because its surface is covered in fine dust rich in iron oxide, the same family of compounds that gives rust its color. Iron in Martian rock reacted with oxygen and water in some form, and over billions of years, wind eroded the material into dust and spread it across the planet. Dust kicked into the atmosphere helps make Mars look red from far away. For years, scientists thought the red mineral was hematite, an iron oxide that forms in dry conditions. In 2025, a team led by Brown University and the University of Bern reported in Nature Communications that when they combined spacecraft observations with new laboratory experiments, a water-bearing iron oxide called ferrihydrite fit the dust best. Ferrihydrite forms quickly in cool water, which suggests the dust formed early in Mars’s history, when the planet was wet, before it dried into the cold desert it is today.
Key Facts
- The color
- Iron oxide in the surface dust, like rust
- How it spread
- Wind has carried the dust across the planet over billions of years
- New idea (2025)
- The dust is mostly ferrihydrite, an iron oxide that contains water
- Up close
- Parts of Mars look butterscotch, golden, tan or brown
Two ideas about Mars’s red mineral
| Hematite (the older idea) | Ferrihydrite (the 2025 study) | |
|---|---|---|
| What it is | An iron oxide without water | An iron oxide that contains water |
| How it forms | Slowly, in dry conditions | Quickly, in cool water |
| What it suggests | Mars has been dry for a long time | Early Mars was wet before it dried out |
Source: Valantinas et al., Nature Communications (2025). Scientists continue to study the question.
Rust on a planet
The iron in Martian rock reacted with oxygen and water in some form, much like rust forming on Earth. Over billions of years, the rusty material was eroded into fine dust, and winds still spread it around the planet today. When storms kick the dust high into the air, it also helps tint the sky.
A twist: the rust may be wetter than we thought
Scientists long believed the red mineral was hematite. The 2025 study, led by Brown University’s Adomas Valantinas, combined data from spacecraft orbiting Mars and rovers on its surface with lab experiments, and found that ferrihydrite matched the dust best. Because ferrihydrite forms quickly in cool water, it points to a wetter early Mars.
Not all red
Mars isn’t red everywhere. Up close, depending on the minerals nearby, landscapes can look butterscotch, golden, brown or tan, and a few look slightly greenish.
Myth vs. Fact
MythMars is red because it’s hot.
FactMars is cold. Its color comes from iron oxide in its dust, not from heat.
MythMars is red all over.
FactClose up, its landscapes can look butterscotch, golden, tan, brown or even slightly greenish.
The Bottom Line
Mars is red because its dust is rich in iron oxide, a kind of rust. A 2025 study suggests that rust holds water, which would mean it formed when Mars was wetter.