Were the Earth a crystal ball, you might gaze 2,900 kilometers down to its outer core with a telescope. The Earth, though, is frustratingly opaque — to light. Most knowledge of the planet’s internal structure comes from studying seismic waves, which give a kind of ultrasound image. Inferences about Earth’s internal chemistry rely on the elements found in near-surface rocks, meteorites and the sun. Recently, geoscientists have developed a new tool for probing the Earth’s innards. Borrowing a page from astrophysics, they are using the curious subatomic particles known as neutr...