Peering into Past: What Happened to the Moon 3.6 Billion Years Ago

Invaluable data can at times be overlooked or not fully exploited when first collected. Striking conclusions can often be drawn on the basis of a specific analysis many years later. The Apollo 11 - 17 missions (1961-1972) provided detailed information on lunar basalts which make it possible to measure the iron grains in basalts from microphotographs of thin sections. Analysis of the average size of these grains (D) differed as a function of the age of these basalts dated between 3.9 and 3.4 billion years ago, revealed that D increased 1.5 fold, therefore the gravity acceleration g decreased 1.5 fold. The intriguing conclusion can only be that the size of the Moon increased, its mass decreased, or both these factors have changed.