For now, our anomalously large Moon is spinning away from us at a variable rate of 3.8 centimeters per year. But, in fact, the Earth and Moon may be on a very long-term collision course — one that incredibly some 65 billion years from now, could result in a catastrophic lunar inspiral.
“The final end-state of tidal evolution in the Earth-Moon system will indeed be the inspiral of the Moon and its subsequent collision and accretion onto Earth,” Jason Barnes, a planetary scientist at the University of Idaho, told me
We can’t be sure, yet, though, whether or not the Earth-Moon system would survive the Sun’s Red Giant Phase, says Barnes. That is, when some six billion years from now our Sun runs out of nuclear fuel; its core becomes a burned-out remnant white dwarf; and, its outer layers expand outward beyond Earth orbit.
Although there is much disagreement on the timing of such a lunar endgame inspiral, most theorists do agree on the timing of the Moon’s formation. While there are alternative ideas about the Moon’s formation and/or capture in Earth orbit, most theorists think that some 4.5 billion years ago, our Moon formed from the coalescence of matter ejected after a Mars-sized impactor slammed head-on into our nascent Earth.
And it’s been receding from Earth ever since.
This recession is due to tidal friction caused by the effects of the Moon’s gravitational tides on the Earth, Barnes told me. In fact, the rate at which the Moon recedes is a function of Earth’s rotation and the dissipation of tidal energy in Earth’s oceans, says Barnes. This happens primarily in shallow seas of 100 meters or less, he says.
As a result, the timing of the Moon’s ongoing recession is hard to precisely predict . That’s because, as Barnes points out, the Earth goes through glacial and interglacial cycles, causing the area of shallow seas to change as sea level rises and falls.
And over longer geologic time, Earth’s tectonic plates also rearrange themselves, which changes the amount of tidal dissipation and thus the rate of the Moon’s recession.
Researchers can put a precise figure on the current rate of recession as a result of retro reflector experiments the Apollo missions left on the lunar surface.
“We bounce lasers off of these and other retro reflectors on the Soviet Lunokhod rovers to measure the distance to the Moon precisely,” said Barnes. “Using these, we’ve directly measured how fast the Moon is receding.”
However, Barnes says eventually the Earth’s rotation will keep slowing until it’s the same as the Moon’s orbital period. At that point, the Moon will stop moving outward, he says. Assuming the Earth-Moon system has not been swallowed by a Red Giant Sun by that point, any remaining solar tides would soon sap energy from the Earth-Moon system. And the Moon would start creeping inwards, towards Earth.
“Eventually, it would get so close that it would spiral inward, dissipating its orbital kinetic energy in a spectacular collision and merger with the Earth,” said Barnes.
Could our future progeny geo-engineer their way around such a crash?
With the Sun having been a remnant white dwarf for tens of billions of years, the Earth would be frozen solid and would have been that way for billions of years, says Barnes.
“But if there were someone hanging out here in a heated dome or something, the energy released in the merging would re-melt the Earth into a magma ocean,” said Barnes.
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