Is Earth's Moon going to turn into a ring?

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Karen P.L. Hardison eNotes educator| Certified Educator

In about 5 billion years from now, when the Sun enters its expansive red giant phase [after fusion has been exhausted in the core and begins in the hydrogen shell surrounding the core leading to increased pressure, decreased temperature, increased density (and heat), increased luminosity and expansion up to or more than 100 times the Sun's original size], yes, the Sun's expansion will deteriorate the Moon into exploded bits that will form a ring around the Earth.

The Sun's mutation into a red giant provides ... ensure the Moon ends its days the way it began; as a ring of Earth-girdling debris. ... "the Moon's orbit to decay. ... [until]  the gravity holding it together is weaker than the tidal forces acting to pull it apart," ...[and the] Moon will be torn to pieces and ... scattered ... [in] a ring of debris above Earth's equator.

As the Sun moves [very very slowly] toward its red giant phase, two forces will be acting on the Moon to cause its deterioration into a ring of debris. One of the forces is the expansive heat and density of the red giant Sun. As the Moon and giant Sun come into nearer proximity, the gravitational drag exerted by the Sun will cause the Moon's orbit to be interfered with, thus its orbit will disintegrate. As this disintegration of the Moon's orbit occurs, it will descend closer and closer to Earth. When it reaches the level of disequilibrium called the Roche limit (11,470 miles above Earth), where Earth's tidal forces are greater than the Moon's gravitational forces, the tidal forces will overpower the Moon's gravitational forces and it will no longer have enough gravitational force to hold together. Without sufficient equilibrium to hold together through its gravitation, the Moon will deteriorate and become a ring of debris.

The other force that will affect this deterioration is the conflict between Earth tides and the Moon's proximity to Earth. Every year, the Moon moves away from Earth by 1.6 inches (4 centimeters) because of this conflict. The Moon's gravitational force acts to raise Earth's tides: Earth's ocean tides are getting increasing higher across the planet each year. Earth responds by attempting "to drag ahead" of the Moon, to outrun the Moon, so to speak. This conflict between the Moon's force and the rising tides causes the Moon to drop away from Earth (due to less complementary Earth gravitational force to keep the orbital location stable). Consequently, a more distant Moon causes less reciprocal complementary force on Earth's spin, thus, Earth's spin is slowing: Earth is spinning less quickly causing an elongated Earth-Sun rotation period.

The result of this second force of gravitational-tidal conflict is that by the time the Sun is a red giant, the Moon will be much further away from Earth than it is now. The Moon's Earth-orbit will then be about 47 days (compare to today's approximate 30 day orbit). Additionally, the slowing Earth spin will change Earth's Sun-orbit to an equal time of approximately 47 days. Thus the change to Earth-Moon proximity puts the Moon much closer to the Sun as it expands, thus contributing to its speedier approach to the Roche limit and to its deterioration into a ring of debris.