Which planet appears red

It orbits the Sun at an average distance of million km, half as far again as the Earth, so human visitors would find it very cold. Although summers near the equator can be quite warm, the average temperature is 63 degrees Celsius below zero - similar to winters in Antarctica.

The nights are also bitterly cold. A planet with very little oxygen producing huge amounts of an element requiring oxygen is not so easy to explain. The one thing it does tell us is that at some point in history, there was a lot more oxygen on Mars. Currently all of the water on Mars is in the form of ice.

This can be seen on the surface of the north polar ice cap. Research carried out in suggested that billions of years ago Mars once had a lot of liquid water on its surface, as much as is found in our Arctic Ocean! This would mean that water vapour would be present in the atmosphere. This vapour would have been broken down by the atmosphere into Oxygen and Hydrogen. The combination of iron and oxygen in the atmosphere caused the reaction that created iron oxide, giving us the rusty dust clouds that float around.

However, the entire planet is not red. Some regions look bright red, while others will appear black because not everything is covered in iron oxide dust. When the Phoenix Lander drilled just a few centimeters below the iron oxide rich surface, the ground was brown. Because shorter wavelengths of light, like violet and blue, are scattered more by the molecules in the atmosphere, the blue photons appear to come from all directions.

This can also happen on Earth when the air is heavily polluted or covered in smoke. Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. For sure, some sort of weathering gradually rusted the iron on Mars. But did the ancient rainstorms that are thought to have occurred on a young, wet Mars rust the iron by pounding the regolith with oxygen atoms freed from water molecules?

Or, did the oxidation happen gradually over billions of years, as sunlight broke down carbon dioxide and other molecules in the atmosphere, producing oxidants such as hydrogen peroxide and ozone? Or, as a group of Danish scientists suggested in , have Martian dust storms slowly rusted the iron, by crumbling the quartz crystals that also exist in the regolith and leaving their oxygen-rich surfaces exposed?

Because no one yet knows the right explanation, the color of Mars is, in a sense, still a mystery. But however its surface rusted, the compound iron III oxide appears red because it absorbs the blue and green wavelengths of the light spectrum while reflecting the red wavelengths.