A lunar rock brought to Earth nearly half a century ago is revealing new information about the moon’s complex history.

NASA’s Apollo 17 mission left the moon to return to Earth 49 years ago Tuesday (Dec. 14) and humanity hasn’t been back to our natural satellite since. In a new study, researchers examined a moon rock collected by astronauts during Apollo 17. By measuring the composition of the rock, designated “troctolite 76535,” scientists have found patterns that point to a 20-million-year cooling period during the moon’s history, defying previous understanding of lunar evolution.