How would you date a moon rock?

• A. By the ratio of radioactive potassium to argon
• B. By carbon dating
• C. By uranium-lead dating
• D. By ratio of helium to hydrogen

Answer: (A)

Dating rocks is about tracking how radioactive atoms decay into stable ones at a known rate. Potassium-40 decays to argon-40, a gas, and when a rock forms the argon is initially scarce but begins to accumulate as potassium-40 continues to decay. By measuring the amount of potassium-40 left and the amount of argon-40 present, you can calculate how long it has been since the rock solidified. This method fits moon rocks well because many lunar samples are volcanic and contain potassium, so potassium-argon dating yields ages for rocks that are billions of years old. Carbon dating wouldn’t work here since it dates organic material and only up to about 50,000 years. Uranium-lead dating is another method for certain minerals, but potassium-argon dating is particularly well suited for lunar rocks. So dating moon rocks is done by the ratio of potassium-40 to argon-40.