The reason Mars is called the "Red Planet" is that its reddish hue can be detected even without any instruments by using just our naked eyes from Earth. This color of Mars is due to the presence of iron oxide or commonly called rust that covers the rocks, soil, and dust of the planet. However, Mars' coloration is not only due to chemistry but also geology, iron oxidation, dust transport in the thin atmosphere of Mars, and the likelihood that there was water on its surface at some point in time. In this paper, we will investigate how the planet became red and what role does iron oxide play here.

Observations Throughout History and the Name "Red Planet"

For millennia now, Mars has been distinguished among planets in the sky due to its distinctive red appearance. For the Ancient Egyptians, the planet was called "Horus the Red", referring to its celestial significance. In Babylonian cosmology, Mars was connected with the god of war, destruction, and disease, Nergal. For the Romans, the planet received its name from the Roman war god Mars since the planet's red appearance brought associations with wars and battles.

Mars has fascinated skywatchers for thousands of years. Its distinctive reddish appearance made the planet visually different from the other wandering objects in the night sky.

With the advent of telescopes, people were able to conduct close observations of the planet's surface. Among those astronomers who studied Mars using the telescope was Galileo Galilei. Later Christiaan Huygens and other astronomers observed surface features and noted that the planet has an unusual red appearance. Still, these observations were unable to explain why the planet has a reddish color.

Only with the development of space exploration technologies has the answer appeared. Orbiters, landers, and rovers found out that unlike some artificial painting on the planet, Mars does not have natural red color. Instead, its surface is covered with a thin layer of oxidized rocks and dust containing iron, making the whole planet look red. In other words, Mars is the "Red Planet" thanks to its rusted surface.

Mars' Surface: Basic Information

First, it should be noted that Mars is a terrestrial rocky planet; that means that its surface is solid and consists of rock and mineral matter.Gravity plays a fundamental role in shaping planetary bodies, influencing their formation, structure, and geological evolution. Unlike a gas planet, Mars is covered by a thick crust. It is filled with regolith—a combination of dust, soil, sand, and rocks fragments formed during billions of years due to meteorite impacts, volcanic activity, and weathering.

The main constituent of the Martian crust is basalt – the volcanic rock rich in iron and magnesium. Basalt is created as a result of cooling of molten lava. It can also be observed on Earth, particularly, at the bottom of the oceans. Fresh basalt is dark gray or even black despite popular red coloring of Mars.

This information is critical because Mars got red color not because of original rocks, but because of their weathering. Basaltic rocks are able to react with oxidants and decompose to form red-colored dust. This dust covers the majority of the Martian surface now.

The Chemistry of Red Color on Mars – Iron Oxide

First of all, the reason why Mars is red is the existence of iron oxide or rust. One of such compounds that is present in Mars soil is hematite which has the chemical formula Fe₂O₃. The reactions of oxidation lead to compounds of iron which usually have reddish, orange or brown colors.

For example, here on Earth people know how iron starts to rust when it contacts oxygen and water. There are no seas or oceans on Mars anymore and there is very thin atmosphere there. However, iron-bearing minerals in the basaltic rocks were changed for billions of years due to oxidation. Iron oxides started to be disintegrated and became dust which covered the planet after millions of years with help of winds.

From iron-bearing rock to reddish dust. Oxidized iron minerals in Martian rocks and regolith contribute strongly to the planet's characteristic reddish appearance.

This iron oxide covers all kinds of rocks, plains, craters, sand dunes on Mars. Part of this mineral remains in atmosphere and causes a reddish color of Martian sky. Therefore, the red color is not created by red rock layer but by iron oxide which covers the whole surface of the planet.

How Was the Surface of Mars Rusted?

Firstly, the ruddiness of Mars started because of its volcanic past. Numerous eruptions on the planet resulted in the formation of many basalt rocks – dark rocks containing many iron compounds, such as olivine and pyroxene. However, while fresh basalt is typically gray or black, the iron contained in these compounds can turn into something else with the help of oxidation.

The climate on early Mars was much warmer and wetter compared to modern times, so there might have been flows of water running around on its surface. These flows interacted with rocks on the planet and caused the breakdown of the aforementioned compounds. Oxygen released from water, as well as some other processes on the surface and in the atmosphere, helped oxidize iron, as did ultraviolet radiation, which could lead to disintegration of molecules.

A thin veil of iron-rich dust covers much of the Martian surface. Although Mars appears red from space, many of its underlying volcanic rocks are considerably darker.

Many different oxidizing agents have been proposed: hydrogen peroxide, as well as oxygen-bearing molecules in ancient atmosphere or near the surface. In any case, the process took place gradually for billions of years, and the precise mechanism of the formation of ruddy color is unknown yet.

Weathered rocks formed plenty of iron oxides, which turned into fine dust. It was spread all over the planet because of winds and global dust storms, covering rocks, plains, and bottoms of craters. So, the ruddiness is mostly a thin layer of iron-rich dust on the surface of the planet, not the color of all rocks beneath.

Why is the Color of Martian Sky Pink or Butterscotch?

The reason for the reddish color of the surface of Mars is linked with the color of the planet's sky. As has already been mentioned, the atmosphere on Mars is very thin and is filled with lots of dust. This dust is regularly lifted up by winds and dust devils and other dust storms from the surface of the planet. Consequently, these particles of dust scatter sunlight creating pink, tan, or butterscotch color in the daytime sky.

Sunset in Gale Crater. Dust suspended in Mars' thin atmosphere changes the way sunlight is scattered, producing a distinctive appearance of the Martian sky. Credit: NASA/JPL-Caltech/MSSS

The color of the sky on the Red Planet is determined by the size and composition of dust particles in the atmosphere. Dust on Mars consists of iron oxides that reflect light and provide red and yellow hues. As a result of this fact, the planet looks redder from the outer space due to the fact that the dust is evenly distributed across the planet's atmosphere.

Moreover, the sunrises and sunsets on Mars are very different from what we see on Earth. Close to the position of the Sun during sunset and sunrise, the sky turns blue, while the surrounding area stays reddish or golden. The fact is that because of special scattering of light from suspended particles of dust, blue light is concentrated in the area of the Sun, whereas on Earth, because of different scattering, our sunsets are red or orange.

Therefore, the same iron-containing dust that gives red color to Mars's surface provides unique atmosphere appearance of this planet.

Myths Concerning Mars

The vivid red color of Mars has caused a number of myths and misconceptions to arise about this alien planet. Let us examine and debunk these common misconceptions:

Myth 1

Mars consists of red rock only

Fact: Despite the popular belief that Mars is the red planet, it has a diverse composition of surface elements. Underneath its red coating, Mars has a crust composed largely of dark volcanic basalt containing high concentrations of iron and magnesium. Most of the reddish coloring is the result of a thin layer of iron oxide covering the surface.

Myth 2

Mars is red because it is hot

Fact: Mars is incredibly cold. Its average temperature is around −60 °C. The planet has nothing to do with heat as such. The red color of Mars is the effect of iron oxidation just like rust on Earth's iron objects.

Mars is not uniformly red. Dark volcanic rocks, lighter minerals, ice and dust create a much more varied surface than its nickname suggests.

Myth 3

Mars is burning

Fact: There is no fire on the planet. The red coloration is the effect of iron oxidation, i.e., a chemical reaction of iron with some other substances that contain oxygen. As opposed to combustion, oxidation is a slow process that does not require fire or excessive heat.

Myth 4

The whole planet is red

Fact: Mars is far from being monochromatic. Apart from a variety of rocks, there are many other elements in the landscape of Mars. Black volcanic rocks, dark sand dunes, white ice caps, dusty plains, and mineral deposits are some examples of diversity breaking the red color of the planet. The red coloration depends on the amount of iron oxide in each particular spot.

Thus, Mars is better perceived as a dark rocky planet covered in rust-colored dust.

Scientific Evidence from Space Missions

It is known that the reddish color of Mars is caused by iron-bearing minerals, and scientific evidence has been found in orbit, at the planet's surface, and during laboratory experiments on Earth.

Evidences from Orbit

Spacecraft in orbit measure Mars's spectra using spectroscopy which detects minerals based on their absorption and reflection of certain electromagnetic waves. The CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) on board the NASA Mars Reconnaissance Orbiter identified iron-bearing minerals such as hematite along with minerals formed in watery environments on Mars in the past.

The OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) on board the Mars Express mission by ESA found ferric oxides across Mars. It shows that iron oxides can be found not only in Mars's crust and lava flows but also as extremely small particles of dust on the planet's surface. Such nanoparticles are particularly important because they have a very strong effect on the reddish color of Mars.

Findings of Rovers

Surface missions provide a direct evidence.

Hematite-rich “blueberries” on Mars. These spherical formations examined by NASA's Opportunity rover provided important evidence of mineral formation associated with ancient water activity. Credit: NASA/JPL-Caltech/Cornell/USGS

Curiosity studies rocks and soil in Gale Crater identifying iron-bearing minerals and analyzing how they changed in ancient environment. In addition, earlier missions such as Pathfinder and Spirit analyzed iron-rich soil and dust at the Martian surface while the latest mission Perseverance also investigates the mineralogical and chemical history of the planet's surface.

Laboratory Evidence

Laboratory experiments help to recreate the dust of Mars from basaltic materials when oxidizing them in conditions similar to the conditions on Mars. Such experiments confirm the fact that finely divided iron oxides cause the reddish color of the planet.

According to new studies, ferrihydrite, a poorly crystalline, hydrated iron oxide, is probably a significant iron-bearing mineral of Martian dust. Its spectrum is consistent with the orbiting, surface and laboratory measurements of iron oxides on Mars, but the dust of Mars can include several types of iron oxides instead of one.

In general, spectroscopy from orbit, rover findings and laboratory experiments provide sufficient evidence that the reddish color of Mars results from the oxidation of iron-containing minerals forming fine planet-wide dust.

NASA CRISM Orbiter

Identified hematite and iron-bearing minerals across Mars surface and subsurface formations

ESA OMEGA Spectrometer

Detected ferric oxides and nanoparticles throughout Martian crust and atmosphere

Opportunity Rover

Discovered hematite-rich "blueberries" at Meridiani Planum indicating ancient water interactions

Curiosity & Perseverance

Analyze iron minerals and their oxidation states in Gale Crater and beyond

Red Color Implications

The red color of Mars has geological and chemical value that is of relevance to scientific research and space exploration. This characteristic raises some problems in terms of future missions but also brings a number of advantages.

Water Presence on Mars

Hydration of iron oxides can occur in different ways, but recently scientists found out that most of the red dust on Mars consists of ferrihydrite – hydrated iron oxide. It usually forms when water interacts with materials rich in iron. This fact proves that the planet had a wet period in its geological past.

The presence of the red dust adds weight to the theory of the existence of an ancient Martian river system, lakes with sediments and minerals that were formed in water. Of course, the red color does not indicate the presence of the entire global ocean or any signs of life on Mars, but gives additional arguments for the existence of ancient wet Mars.

Atmospheric Composition

In addition, the oxidation process shows information about the composition of the ancient Martian atmosphere. In order for iron to get oxidation, there should be oxygen-bearing compounds or other oxidizers in the surrounding air. This implies that the chemical composition of Mars was different in the ancient period compared to the current one, and probably included water, atmospheric interactions, UV rays, and oxidizers.

Analyzing all chemical processes scientists try to recreate the transformation of the planet into the cold and desert one with the thin atmosphere. Besides, the atmosphere and surface evolved together. In fact, as the atmosphere has changed, the weathering of rocks began. At the same time, chemical signs of weathering remained in the Martian regolith.

Signs of Habitability

The formation of hydrated iron oxides can help in searching for the signs of ancient habitability. If ferrihydrite formed in cool liquid water, then at least some regions on Mars had favorable conditions for the existence of microbes in their geological past. It does not imply that there was life on Mars but indicates the places and periods which were better than the current.

Iron oxides help to choose promising sites for exploration. Deposits of minerals related to the interaction with water can store the information about the environment of Mars and even biological activity in rocks and sediments.

Human Explorers Face the Challenges

In addition, the red dust on Mars can create engineering and medical problems for the future astronauts. This dusty atmosphere may enter habitable zones and damage mechanisms and also contain harmful substances like silica, perchlorates, and nanoparticles of iron oxides. Now NASA is working on establishing safety limits and ways of protection from Martian dust.

However, the iron-rich regolith can become a resource for explorers. Minerals containing iron can be used in construction and manufacturing while oxygen-bearing compounds in the soil can be useful in developing oxygen production systems. The technologies of the in-situ resource utilization presuppose the use of local resources instead of those which are brought from Earth.