Why is the ‘Red Planet’ red?

The colour of Mars

Photo: Pixabay

Mars is known as the Red Planet.

We were pretty sure even before the first lander grounded on Mars, the red colour of the planet is due to a lot of iron oxide in the rocks and sands making up its surface.

Some recent research suggests the form of iron oxide making the “Red Planet” red required both oxygen and water. We know there was a lot of water on the Martian surface billions of years ago, and the prospect of oxygen suggests life.

To get an idea of what might have happened on Mars we should look at the early history of our planet.

Our planet formed along with the other planets of the Solar System about 4.5 billion years ago. As soon as the Earth had cooled enough for water to accumulate on its surface, the processes leading to life got under way. However, from our point of view, the conditions were hostile. The atmosphere consisted of gases like hydrogen, carbon dioxide, methane, ammonia and a collection of hydrocarbons. There was no oxygen.

The first living things were single-celled and microscopic. They evolved to have an oxygen-free (anaerobic) lifestyle and flourished in those early oceans. However, their existence bore the seeds of their own demise because they released oxygen as a by-product of their existence. Some of the oxygen remained dissolved in the oceans and the rest escaped into the atmosphere, starting to change it to what we have today, here on Earth.

When the Earth formed, it received a lot of iron. In the oxygen-free, early oceans a lot of this iron ended up dissolved in the water. Iron forms two main classes of compounds. When there is not much oxygen around, the iron forms ferrous compounds, which are green. If left exposed to oxygen and water, these transition into ferric compounds, which are white or brown, and are often insoluble in water. Rust is a ferric compound.

As the oxygen concentration rose, soluble ferrous compounds became insoluble ferric compounds, such as red ferric oxide (rust). These drifted down to the seabed and were incorporated into the sediments. Those sediments eventually became huge thicknesses of red, iron-rich rocks, which today are present all over the world, which underlines the global nature of the event.

Some of these rocks were eroded away by water and weather, and the result incorporated into new, younger red rocks. Oxygen is a very reactive element and life forms using it had more energy available than the anaerobic life forms, so today, the Earth is dominated by oxygen breathers.

If the red iron compounds responsible for the Red Planet being red were formed in the presence of water and oxygen, then maybe Mars' young days were like the youth of our planet. There were oceans and rivers; we can see widespread evidence of this today. The whole planet is covered by a red desert, which suggests the deposition of iron-rich, red material was almost global. In which case, could it be that in Mars' young, oxygen-free oceans, life formed as it did here, and produced the Martian equivalent of our global oxygenation event?

However, then things changed. Mars is a smaller world than ours, and cooled faster. The core solidified and the flows of liquid iron and nickel forming the dynamo driving the magnetic field shut down.

The disappearance of the Martian magnetic field allowed the atmosphere to be scoured away by the solar wind, leaving the cold, almost airless desert we see today. However, if Mars' youth was like our world's, the evidence of copious ancient life must be there somewhere, waiting for us to find it.

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Before dawn, Saturn is visible low in the southeast and Venus low in the east. In the evening, Mars lies low in the west. The Moon will reach its last quarter on June 18. On June 20, the Sun will reach the northernmost point in its yearly travels, marking the summer solstice.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.