Talking about her cover story on what iron-loving elements are telling geologists about the Earth’s deep past, Alexandra Witze likens these rare metals to time travelers. They can tell you, she says, what was happening more than 4.5 billion years ago, during the first 50 million years of our planet’s existence. By then the Earth’s molten interior had begun to settle into its current layer cake form: a dense, solid inner core surrounded by an outer liquid core — both rich in iron and metals such as gold, platinum, ruthenium and others that tend to form alloys with iron. The scarcity of these metals in the outer layers of the planet — the mantle and crust — make them precious to us.
Their high melting points and other properties help them resist