Rock Doc

I think the most memorable single day of all my years as a student was the afternoon I got to examine Moon rocks in graduate school.

Rocks here on Earth are exposed to water throughout their existence, and water acts to break down mineral grains on a tiny scale. If you look at thin slices of rock under a microscope – a normal activity for geologists like me – you see this tiny breakdown at work. To use a technical term, the mineral grains appear “cruddy” because they are breaking down to minerals like clay due to the presence of water.

What impressed me so much about the Moon rocks we looked a...