Where on Earth is the Crust?

Astronomers confidently speak of the chemistry of stars, galaxies and even interstellar space, but curiously, we are grossly ignorant of the composition of our own planet. “Whole Earth” scientists work under an extreme handicap compared with investigators in most other disciplines. Physicists, astronomers and biologists can formulate general theories and design experiments on the basis of many examples. By contrast, there is a single Earth, the result of a unique experiment, and we have direct access only to its outer portions. (Another obvious “singular” object of scientific study is the universe itself.)