Three Ways to Make a Moon
Hypotheses abound on how the Moon came to be. The giant-impact hypothesis, sometimes called the Big Splash, is illustrated in the figure at the left. This hypothesis suggest that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, about 20 to 100 million years after the Solar System coalesced.
Three major ideas are: 1. Co-formation Hypothesis; 2. Capture Hypothesis; and 3. Collision Hypothesis.
- Co-Formation Hypothesis. Moons like Jupiter’s Galilean satellites probably formed out of the same primordial gas and dust that formed the planet. You can think of the Jupiter system much like a scaled-down solar system.
- Capture Hypothesis: Some moons are on highly eccentric, highly inclined, or even retrograde (Neptune’s Triton) orbits, or they may have suspiciously aspherodical shapes (Mar’s Deimos). These are likely captured asteroids.
- Collision Theory: Our Moon is likely the rarest type, formed out of the ejecta from a major collision between Earth and another planet-sized body.
According to the reigning hypothesis, about 4.5 billion years ago, shortly after Earth had accreted down into a sphere from its little slub of circumsolar material, another newborn planet, still shaky on its feet, slammed obliquely into Earth with terrifying force.
That “giant impactor,” Theia, who in Greek mythology was mother to the goddess of the Moon, is thought to have been roughly the size of Mars and to have been pulverized in the encounter, along with a good chunk of proto-Earth. From that fiery cloud of all-Theia and part-Earth, the scenario goes, our Moon soon condensed.
The impactor hypothesis made sense and comported with computer models, but hard evidence for it proved elusive. If the Moon were partly the offspring of a non-Earth body—Theia—there should be chemical fingerprints attesting to the foreign parentage. Astronomers who have analyzed a wide array of extraterrestrial material have determined that the many residents of the Solar System differ measurably in their isotope ratios, the forms of the chemical elements they carry. (Heavy oxygen or light? Titanium with more neutrons or fewer?) But when researchers checked the isotope content in rocks from the moon, the ratios looked identical to rocks on Earth. Where were the traces of Theia?
Now it looks as if the evidence has arrived. In 2014, Daniel Herwartz, a geochemist working at the University of Göttingen in Germany and his colleagues reported in the journal Science that they had detected isotopic ratios of oxygen in lunar rocks that were unlike the forms of oxygen found on Earth. It is, Dr. Herwartz said, “the difference between Earth and Moon predicted by the impact theory.”
 See “Three Ways to Make a Mooon,” Astronomy (43, 5, May 2015, p. 20
 See Kenneth Chang, “If a ‘Big Whack’ Made the Moon, Did it Also Knock the Earth on Its Side?” New York Times (November 4, 2016); Natalie Angier, “The Moon Comes around Again,” The New York Times (September 7, 2014).