The idea that our Moon originated by collision of a large space body with Earth1 has much shakier foundations than its almost universal acceptance might suggest. Before 2012 no published giant-impact model completely supported that hypothesis. All attempts to explain isotopic identity of Apollo samples and Earth’s mantle left too much alien impactor material in the resulting Moon.2
In 2012 a new mechanism became the basis for the first successful giant-impact models.3 That mechanism allowed early Earth at the moment of impact to be spinning near its approximately 2.7-hour limit of rotational stability. With that added energy, the impactor could blast part of the mantle into an orbiting cloud, a future moon polluted only by acceptably small traces of the impactor. The new mechanism of lunar–solar tidal resonance transfer, a descendant of George Darwin’s original lunar origin by tidal interaction,4 then reduced angular momentum to near-modern values.
The new models have the same initial conditions as fission models of the 1960s—namely, a very rapidly spinning, partially segregated, early Earth. Elimination of excess angular momentum by the new mechanism removes once fatal objections to fission hypotheses. In these revitalized models late-stage core segregation increased the rotation rate beyond stability limits5 to separate single or multiple parts of the mantle as lunar precursors. Subsequently a combination of tidal transfer; late-stage, backward-directed impacts; magnetic braking; drag in the solar wind; or escape of a silicate atmosphere,5 other volatiles, small debris, or other moonlets reduced angular momentum to near-modern levels.
Distinctions between these contrasting hypotheses are already blurred. Authors of the new model note that it “blends aspects of the original impact hypothesis … and the fission hypothesis.”3 In reality giant impact is an unnecessary complication. Reinstated fission models could include possibilities for separation into two bodies, multiple moonlets, or equatorial fragmentation and reassembly. All avoid celestial dynamic baggage and impactor contamination while invoking the same rapid rotation and momentum-reduction mechanism as the recent giant impact models. Perhaps our quest to answer one of mankind’s oldest questions should expand to include a simpler hypothesis.
