Lunar highlands carry about ten times as many craters as the maria, yet they are only about 4.2 billion years old against 3.8 billion. What must be true?

1. Counting craters dates a surface only if the cratering rate has stayed steady. 2. Radioactive dating puts the highlands at about 4.2 billion years and the maria at about 3.8 billion. 3. That is a gap of roughly 0.4 billion years, yet the highlands carry ten times the craters. 4. A steady rate would need the highlands to be ten times older than the maria. 5. Ten times 3.8 billion is 38 billion years, which is longer than the universe has existed. 6. An impossible conclusion means the assumption of a steady rate has to go. 7. So the rate was much higher before 3.8 billion years ago, a period called the heavy bombardment. 8. The maria are cratered too, only far more lightly, so calling them uncratered misreads the counts. _Source: OpenStax Astronomy (CC BY 4.0), section 9.3 Impact Craters_