: A new study has revealed a very slight difference in the proportion of oxygen-17 present in Earth rocks compared with Moon rocks. The difference may support the theory that the Moon formed following a collision between Earth and another large body 4.5 billion years ago. The slightly higher concentration of the isotope in the Moon rocks would suggest that the rock originated somewhere other than Earth. However, previous measurements found no significant difference in the concentration of the isotope. Despite the increased precision of the new measurement, some scientists argue that the difference is not large enough to be sure that the rocks are truly different. In fact, the concentration of 17
O is known to vary between different lunar rocks, so critics are suggesting that isotopes of other elements be compared instead.