The researchers analyzed seven  collected during the Apollo 12, 15 and 17 lunar missions. They also examined one lunar meteorite — a rock that a cosmic impact knocked off the moon, which later crashed on Earth.

The researchers focused on ratios between different isotopes of oxygen in the rocks. Isotopes of an element have differing numbers of neutrons from one another — for instance, oxygen-16 has eight neutrons in its nucleus, while oxygen-17 has nine. [Violent Birth of the Moon Explained (Infographic)]

The ratio of oxygen-17 to other oxygen isotopes typically gets smaller the bigger planets and moons get. This means that different planets and moons usually have distinct isotope signatures.

However, the researchers found that Earth and the moon have indistinguishable oxygen isotope ratios, within 5 parts per million when it comes to oxygen-17.

"As we continue to improve our ability to make measurements, the moon and Earth continue to be more and more alike isotopically," Young told Space.com.