"We had to find a rock that had just enough iron to record a magnetic signature, but not so much that it would be affected by later chemical changes," Tarduno says.

“我们必须找到一块岩石，它含铁量恰恰好能够用来记录磁信号，却又不至于多到后来受到化学变化的影响”，塔都诺说。