Why is the Red Planet red?

That red planet in the sky was named Mars because red seemed the right colour for the god of War.

As soon as the telescope came along, we realized that Mars is red because it is covered in desert. Deserts are mostly made of sand, particles of quartz.

This mineral is normally colourless, and is coloured red or brown by the presence of iron. This is actually telling us a lot of important things about Mars' early history.

If you have ever travelled the south-western United States or explored the coast of southwest England, you must have noticed the red deserts and red rocks.

The red hills and deserts of Jordan are so Mars-like they were used as the setting for the movie The Martian.

What is this telling us?

The rock in question is called sandstone, and consists of grains of iron-stained quartz cemented together. This iron-stained quartz is mostly billions of years old.

It has been incorporated into rocks, which have been weathered and eroded back to sand, and then incorporated into new red sandstone rocks. The interesting issue is where the original iron-stained quartz came from.

When the Earth was young, some 3.5 billion years ago, when the first living things were swimming around in ancient oceans, the Earth's atmosphere was unbreathable to us. It contained no free oxygen.

There was carbon dioxide, nitrogen and methane. The oceans contained huge amounts of dissolved iron compounds. That iron did not rust because there was no oxygen to rust it. It is very likely Mars was built from the same ingredients, and had a similar atmosphere.

Water, carbon dioxide, methane and the other things in the young oceans were ideal for plant growth; just add sunlight. About 3.5 billion years ago single-celled plants, such as algae, filled the Earth's oceans, consuming carbon dioxide and releasing oxygen.

The availability of oxygen and water led to the rusting of the iron compounds in the sea and exposed iron minerals on land. The produce was iron oxide, which is red. The thin rain of rust to the bottom of the oceans, lakes and rivers led to the formation of red sandstone.

This was then eroded down by the weather to form red sand. Some of this was compacted into new rocks, and possibly eroded down again. So those red sands are the result of the oxygenification of the Earth's oceans and atmosphere.

On our world this is a product of living things. What about Mars?

Recent research supports the idea that the atmosphere of the young Mars was like the Earth's, and then oxygen appeared in its atmosphere and oceans, leading to all the iron rusting out, colouring the rocks and desert, giving us the Red Planet.

Then, though, over time the atmosphere was lost to space, leaving us with the cold, dry, desert world we see today. The big question is what produced the oxygen. Maybe as on Earth, single-celled plants appeared in Mars' oceans, releasing oxygen.

Another idea is that water vapour, reaching the upper atmosphere, was broken down by solar radiation into hydrogen and oxygen.

The hydrogen wandered off into space, leaving the oxygen. So we cannot look at the red deserts and just conclude "life.”

One thing that is giving us some hope about there being life on Mars is the presence of methane in the planet's atmosphere. There is methane in Earth's atmosphere too, produced by living things.

Methane cannot last long in an oxygen atmosphere, and it needs to be continually topped up. The low concentration of oxygen in Mars' atmosphere will remove methane. The thin atmosphere allows intense ultraviolet radiation to reach the Martian surface. This also destroys methane.

What is topping up the supply on the Red Planet? Is this evidence of the existence of living Martians, or is something else going on?

• Mars is high in the southwest after dark.
• The Moon will reach Last Quarter on the 5th.

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