For example, although geologists determined unambiguously from photographs that the maria are younger than the highlands, they did not know their absolute age, the age in years.
Rocks also provide key tests to hypotheses.
For instance, the maria were thought to be covered with lava flows, but we did not know for sure until we collected samples from them. Also, no method can accurately determine the chemical and mineralogical composition of a rock except laboratory analysis. Most important, samples provide surprises, telling us things we never expected. The highlands provide the best example of a geological surprise, and one with great consequences for our understanding of what Earth was like 4.5 billion years ago. Strange as it may seem, the first highland rocks were collected during the first lunar landing, the Apollo 11 mission, which landed on a mare, Mare Tranquillitatis.