Why Moon looks younger than its true age of 4.5B years

Why Moon looks younger than its age of 4.5B years
19 Dec 2024

The Moon has been Earth's close companion, yet researchers have struggled to pinpoint exactly when it formed.

According to a new study published in Nature, the Moon may have experienced a remelting phase owing to Earth's early gravitational pull.

This process, called "tidal heating," is said to have happened around 4.35 billion years ago—hundreds of millions of years earlier than previously estimated.

Tidal heating is currently seen in Jupiter's moon Io, which is volcanically active due to Jupiter's gravitational influence.

Moon's remelting phase: A volcanic spectacle
Volcanic activity

Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, drew parallels between the Moon's remelting phase and Io's current state.

He suggested that during this period, our Moon resembled Io for similar reasons.

"There would have been volcanoes jetting off all over the place," Nimmo stated.

This volcanic activity would have made the Moon appear younger than its actual age.

How did the Moon formed?
Cosmic collision

The Moon is thought to have formed when a Mars-sized object collided with the proto-Earth, during an era of extreme energy and chaos.

The impact would have liquefied Earth and vaporized portions of it, resulting in the formation of our Moon.

Covered by a magma ocean, it had to cool down and solidify into rocks over time.

Moon rocks reveal clues to lunar history
Lunar history

Lab analyzes of moon rocks returned by Apollo astronauts show that most samples are about 4.35 billion years old.

This means they formed some 200 million years after the solar system came into being.

However, simulations of the solar system's evolution suggest an earlier emergence for the Moon, creating a gap between physical evidence and theoretical models.

Resolving the lunar age conundrum
Age conundrum

Nimmo and his team proposed a solution to the lunar age conundrum.

They suggested that after its formation, the Moon cooled down and later reheated due to gravitational interactions with Earth.

This reheating would have caused parts of the lunar rock to remelt around 4.35 billion years ago, aligning with the ages of lunar rocks brought back by Apollo astronauts.

Nimmo mentioned that there was continuous melt generation leading to volcanic eruptions on the lunar surface.

Moon's remelting phase and its aftermath
Aftermath

Unlike Io, which looks yellow due to sulfur abundance, the Moon would have stayed gray during its remelting phase.

The molten rock would have been bright red and flowing lava could have filled up old craters. This could explain why the Moon has fewer craters than expected.

As it kept moving away from Earth, this heating would have stopped and it would have cooled down again.