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The short answer to ”why is Mars red” is that the planet is covered in rust. Iron oxide to be exact. The iron oxide is simply a dust that covers the entire surface in varying depths. That dust is picked up by wind and carried into the atmosphere where it reflects the red part of the light spectrum and, viola, Mars is the Red Planet.
The short answer does not explain where all of the iron oxide comes from, though. There is a larger percentage of iron on the Martian surface than there is on other planets. The exact source is unknown, but many scientists believe that it came from the volcanoes that used to erupt all over the planet. When the Sun’s radiation and the solar wind break down water molecules that are in the Martian atmosphere they interact with, and oxidize, the iron. That process would take billions of years to cover the planet. Scientists have two theories about how the red dust became so thick and spread so widely across the surface.
The first theory harkens back to the first 50-500 million years of the Solar System. NASA scientists and others around the world believe that Mars was a wet and warm planet in the early stages of its evolution. They base that belief on minerals, features, and other data gleaned from spacecraft in orbit and on the ground. The early abundance of water would have jump started the oxidation process. Minute dust particles would have been picked up as the surface water evaporated and then deposited around the planet when it rained.
A newer, less widely held theory is based on data sent back by the Pathfinder mission. The spacecraft found that the Martian soil contained far more iron than its rocks, suggesting that some of the iron on the surface came from meteorites. That led JPL’s Albert Yen to theorize that water did not have to be present for the meteorite-borne iron to oxidize. To test his theory, he used a 100 mg piece of labradorite(a common mineral in Martian soil) and subjected it to gases common to the Martian atmosphere, chilled it to average Martian temperatures, and bombarded it with ultraviolet light equal to the exposure on Mars. After a week, they analyzed the sample for negatively charged oxygen molecules(superoxides) that are capable of causing iron oxidization when there is no water and found them as predicted. The problem with the theory is that the process would take much longer than the existence of the Solar System to cover the entire planet, so it is possible that both theories are correct to some degree and combined to create the reddish hue we see today.
Many astrobiologists do not want Dr. Yen to be correct because it would refute the belief that liquid water existed on Mars, setting back their own theories. Whatever the mechanism, Mars appears red and iron oxide causes it.
We have written many articles about Mars on Universe Today. Here’s an article about a one-way, one-person trip to Mars, and here’s another about how scientists know the true color of planets like Mars.