why is Mars rustier than the Earth? This question has been raised by many experts, as both the planets were formed from the same materials. More importantly, the red planet has more than twice as much iron oxide in its outer layers as the Earth.

David Rubie and colleagues from the University of Bayreuth, Germany, have found 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.