What Will Happen to the Moon in the Far Future? From Boiling Oceans to a Fiery End
Featured image: [The Moon as seen from Earth, with an artist’s overlay showing its gradual recession; credit: NASA/ESA]
The Moon formed tens of millions of years after Earth when a Mars-sized object named Theia slammed into the young planet, blasting debris into orbit that coalesced into our celestial companion. Initially just 20,000 kilometers away — appearing roughly 10 degrees across in Earth’s sky, about the size of a fist at arm’s length — the Moon has been drifting outward ever since.
Today the Moon averages 380,000 kilometers from Earth and recedes at about 4 centimeters per year. Tidal forces drive this process: the Moon’s gravity pulls on Earth’s oceans, creating a bulge, and because Earth rotates faster than the Moon orbits, that bulge leads ahead of the Moon and pulls it forward into a higher orbit. The same interaction slows Earth’s rotation by roughly 2 milliseconds per century.
This much is well understood. But what happens next, on timescales of billions of years, involves a less certain — and more dramatic — story.
When the Oceans Boil
In roughly 1 billion years, the Sun, gradually brightening as helium ash accumulates in its core, will make Earth so hot that the oceans will boil away entirely. Without oceans, the primary mechanism of tidal dissipation vanishes. The Moon’s recession and Earth’s rotational slowing effectively grind to a halt.
Earth itself may eventually become tidally locked to the Moon, always showing the same face, with the Moon fixed in the sky for any given location. But this end state may never actually be reached, because a much larger event will intervene first.
The Red Giant Phase
In 6 to 7 billion years, the Sun will run out of hydrogen fuel in its core and swell into a red giant, expanding past the orbit of Mercury and possibly Venus. Whether the Sun will literally engulf Earth remains debated, but the outcome is effectively the same. Both Earth and the Moon will be completely incinerated and vaporized as the Sun’s outer atmosphere reaches them.
After the Sun Dies
After the red giant phase, the Sun will shed roughly half its mass and become a white dwarf, a dense Earth-sized cinder of carbon and oxygen. At this point, solar tides — currently about half as strong as lunar tides — could become the dominant gravitational influence.
Over tens of billions of years, white dwarf tides could destabilize the Moon’s orbit. The Moon might be flung away from Earth entirely, or it could spiral inward and collide with its former host. The exact outcome remains uncertain.
“This won’t occur for tens of billions of years, which in the potentially infinite future of the universe is like a single tick of the cosmic clock,” writes astronomer Phil Plait in Scientific American. “Better enjoy your view of the moon now while you can.”
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