The trick to finding the iron beneath hundreds of kilometers of rock is its effects not only on the motions of an orbiting satellite but also on the motions of the planet itself. (sciencemag.org)
And there was no clear geographic pattern to the diversity: Two populations living close together could be as different from each other as from one hundreds of kilometers away. (sciencemag.org)
The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. (science.sciencemag.org)