Planetary scientists have known since 1959 — when the Soviet Union’s Luna 3 spacecraft returned the first images of the Moon’s farside — that our satellite’s two hemispheres bear little resemblance to each other. The quest to understand this dichotomy took a big step forward in 2012 when NASA launched its Gravity Recovery and Interior Laboratory (GRAIL) mission. The twin space probes, named Ebb and Flow, measured the Moon’s gravitational field with extraordinary accuracy. Scientists on the ground then used this data to build a high-resolution map of the Moon’s crust. The observations show that the farside crust is roughly twice as thick (about 37 miles [60 kilometers]) compared with the nearside (which ranges between 12 and 19 miles [20 and 30km]).
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