Mars is a brilliant and intense red in appearance. It’s so red that if you saw it in the night sky you might think it was on fire. This name comes from the Greek word for “burning”: Mars. So why is Mars often called the Red Planet? Red is the color of light with the longest wavelength.
We can see visible light from blue through to red, with green and orange falling somewhere in between. The smaller wavelengths of light are referred to as ultraviolet (UV) rays. But, we cannot see them directly because our atmosphere filters them out as a defense against UV radiation which can harm us.
At its surface, Mars is covered with fine iron-rich dust that reflects longer wavelengths such as red and orange more efficiently than shorter ones like blue or violet.
If we were standing on Mars, we would probably be able to see more blues and greens than we do now from Earth. Because they would be transmitted through that atmosphere rather than reflected at us by the Martian soil.
Is Mars Actually Red?
Of course, Mars is red, but this is only true when we view it from the perspective of our planet. If we were on Mars, it would appear blue-green, like the oceans on Earth. This is because the red dust on Mars scatters the blue light from the Sun, leaving the redder light for us to see.
If you were able to see Mars in a low orbit, you would see it as red, like from the surface of Earth. The Martian dust is very fine and is easily lifted by the Martian winds, traveling high above the ground where it can be seen by orbiting spacecraft. From above, the dust appears orange or red, reflecting the sunlight.
The Meaning of “Red” in Science
Most people probably associate the word “red” with the color red. It’s true that in everyday language we use the word “red” to describe a color, but in science, the word “red” has a different meaning.
The word “red” in science refers to a wavelength of light that has the longest wavelength of the visible light spectrum. So, red light has a longer wavelength than blue or violet light.
Detecting Dust on Mars
When we look at images of Mars, we can see very fine dust coating the surface. This dust is red and is made up of tiny iron-rich particles. This iron-rich dust may be the result of a collision between Earth and Mars billions of years ago when Earth was still covered in a primordial ocean and Mars was still volcanological active.
The iron-rich dust that covers Mars is easily disturbed by the Martian winds and by space weather, particularly the charged particles from the Sun. These winds carry dust up into the Martian atmosphere, where it is detected by NASA Orbiters and other ground-based observatories.
How Does the Atmosphere Influence Color?
The Martian atmosphere is very thin and overwhelmingly composed of carbon dioxide. This gas is transparent to most wavelengths of light, but some short-wavelength light, including ultraviolet, is absorbed by the CO2 molecules in the atmosphere.
The dust in the Martian atmosphere scatters light both forward and backward, including ultraviolet light. Ultraviolet light is absorbed by CO2, which re-emits it as infrared light, which is invisible to the human eye. This re-emitted light is reddened by the scattering effect, which is known as Rayleigh scattering.
The blue light is unaffected by the scattering and passes through the Martian atmosphere to the surface below. The redder light that is scattered back upwards towards orbiting spacecraft is what astronomers see as the characteristic Martian color.
When was Mars Called the Red Planet?
Mars was first observed by humans in the 2nd millennium BC, with records of its appearance found in Sumerian, Babylonian, Chinese, and Egyptian civilizations. The ancient civilizations of Asia and the Mediterranean all had their names for the planet.
The Babylonians called the planet Nergal, which they associated with death and disease. The Chinese called it Ao, which translates as “disease”, while the Greeks named it Ares, after their god of war.
Summary
The red color of Mars comes from the iron-rich dust that covers the planet’s surface. This dust is easily disturbed by the Martian winds and by space weather, particularly the charged particles from the Sun. The Martian atmosphere is very thin and composed mostly of carbon dioxide, which scatters light both forward and backward.
This scattered light is reddened by the scattering effect and is what astronomers see as the characteristic Martian color. The ancient civilizations of Asia and the Mediterranean all had their names for the planet. But Mars is a brilliant and intense red in appearance.
It’s so red that if you saw it in the night sky you might think it was on fire. This name comes from the Greek word for “burning”: Mars.
