New Evidence From the Moon

Even while scientists were evaluating Opik's theory, the first physical evidence from the moon had become available. From 1969 to 1972, Apollo astronauts landed on the moon six times, collecting more than 2,000 rock and soil samples. Researchers who analyzed the samples found that the specimens shared a unique characteristic: The relative amounts of oxygen isotopes (atoms of the same element that differ in atomic weight) in the moon rocks proved to be almost identical to those found in the Earth's mantle, a thick layer of hot rock between the Earth's outer core and its crust. The relative amounts of these isotopes are believed to have varied in different parts of the solar system, so the oxygen-isotope ratios strongly implied that the moon and Earth's mantle had a common origin.

The moon rocks were also extremely dry, whereas a great many rock types and minerals on Earth contain water. Compared with Earth rocks, there were very few other substances in moon rocks that, like water, are volatile (vaporize easily).

Finally, the dating of radioactive isotopes in the lunar samples told the scientists how long it had been since minerals in the samples had solidified from a molten state. (A radioactive isotope is one that decays spontaneously to become another substance. For example, an isotope of hafnium decays to form an isotope of tungsten.) By comparing the relative number of such isotopes, scientists can tell the age of the sample. The dating process showed that all or most of the moon had been molten at one time, solidifying by 4.4 billion years ago. This finding, in particular, convinced most scientists that the moon could not be the primordial body that Urey thought had been captured by the Earth.

Thus, studies of the moon rocks clearly showed that the moon could not be an intact, or nearly intact, remnant from the earliest days of the solar system. The evidence suggested that the moon was born from the Earth's mantle by a process that released enormous heat. The heat would have driven off the volatiles and vaporized the rock.