Mars was too cool to lose its red colour. Credit: © NASA

Why is Mars so much rustier than the Earth? The red planet has more than twice as much iron oxide in its outer layers as our own, yet most planet scientists reckon the two bodies were formed from the same materials.

David Rubie and colleagues from the University of Bayreuth, Germany, say they have an answer: the intense heat inside the early Earth was enough to convert a lot of iron oxide into molten metallic iron, which seeped down into the planet to form a huge liquid core.

Mars never achieved the temperatures needed for this process simply because it is smaller, they say. This left more iron oxide in the upper layers of the planet, which led to its distinctive russet hue and relatively puny iron core.

"Our model shows that the planets could have formed from the same material and then evolved to their present compositions and internal structure," says Rubie.

To reach their conclusions, the team used a hydraulic press to squeeze a sample of iron, nickel and oxygen to more than 175,000 times atmospheric pressure while heating it up to 2,400 °C. These experiments, published today in Nature1, helped them to understand how oxygen and iron would have behaved in the planets' early magma oceans.

Crimson tide

About 4 billion years ago, the newly formed Earth and its neighbouring rocky planets were hammered by intense meteor bombardment. This would have melted the planets' surfaces into oceans of magma. Rubie estimates that on Earth, these seas of molten rock were about 1,800 km deep. Below that lay a solid mantle and, finally, a core of molten iron.

The Earth is almost twice the diameter of Mars and is ten times more massive. This means that the bottom of the Earth's magma ocean would have been under much more pressure than that on Mars, simply because there was more material pressing down from above.

On Earth, the pressure would have raised the magma's temperature to more than 3,200 °C, at which point iron oxide readily converts into metallic iron and dissolved oxygen. The liquid iron would then have rained downwards through the magma, creating a relatively large core and leaving a paltry 8% iron oxide in the outer mantle. The process was probably complete within the first 30 million years of the planet's life, says Rubie.

But on Mars, the magma ocean would not have reached more than about 2,200 °C. In such an ocean, iron oxide would have been perfectly stable. Rubie's experiments predict that these conditions would leave behind a solid outer mantle that contains about 18% iron oxide, precisely matching observations of martian geology. This also explains why the martian core makes up a much smaller fraction of planet mass than the Earth's core does.

"I do not know of any other explanation for Mars's rustiness," says John Murray, a planetary scientist at the Open University in Milton Keynes, UK. He adds that as most theories of planet formation assume that Mars and Earth started off with the same materials, an explanation of how they became so different is fundamental to understanding how our solar system evolved.