Their high melting points and other properties help them resist change, allowing geoscientists to use them as fingerprints that mark events in the distant past. With new, more precise analytic techniques, scientists can now measure the amounts of these iron-loving metals relative to other elements to deduce what happened to them over eons of time. These traces are found in some very old rocks, Witze reports (SN: 8/6/16, p. 22), such as 3.8-billion-year-old deposits in Greenland. But the metals also show up as ancient time capsules in younger rock. Studying these traces reveals the imperfect mixing of the mantle and can provide insight into outstanding questions, such as why amounts of these metals differ in the mantles of the moon and Earth.