SSS 2011-09-08
时间：2019-01-08 作者：英语课 分类：Scientific American(九)月
英语课
This is Scientific American’s 60-Second Science. I’m Karen Hopkin. This will just take a minute.
Thar’s gold in them thar hills—and we may have meteorites to thank. Because it appears that a rain of meteors nearly 4 billion years ago peppered the Earth’s exterior with precious metals. So says a study in the journal Nature.
When Earth was forming, molten iron sank to its center, creating the planet’s core. That iron carried with it a slew of iron-loving metals, including gold and platinum. In fact, the Earth’s core is packed with enough precious metals to cover the entire planet four meters deep.
But not all that glitters is down at the planet’s center. Today there’s enough gold in the Earth’s rocky shell to please prospectors—and to puzzle planetary scientists: why is that gold not down in the core, too?
The answer, it seems, is that it wasn’t even on the Earth until after the core formed. Isotope concentrations in rock samples of different ages indicate that the composition of the Earth’s mantle changed after the planet was blasted with meteors 3.9 billion years ago. That bombardment coated our home with a thin veneer of riches. Which means that the gold in them thar hills is older than the hills.
Thanks for the minute. For Scientific American’s 60-Second Science. I’m Karen Hopkin.