Did Earth’s Gravity Give the Moon a Volcanic Makeover?

The Mystery of the Moon’s Age

The Moon’s true age remains one of its enduring mysteries. Samples collected from its surface suggest it formed about 4.35 billion years ago, roughly 200 million years after the solar system began to take shape.

However, this timeline raises questions among scientists. During the solar system’s chaotic early days, collisions between debris and planetary bodies led to the formation of planets. By the 200-million-year mark, most of this debris had already been absorbed into larger celestial objects. As a result, many researchers who model the solar system’s evolution find it unlikely that a massive collision, such as the one thought to have formed the Moon, could have occurred so late in the process.

A “Volcanic Facelift” for the Moon

In an “idea paper” published on December 18 in Nature, UC Santa Cruz Professor Francis Nimmo and his co-authors propose a possible explanation for this discrepancy: that the Moon underwent a “remelting” 4.35 billion years ago due to the tidal pull of Earth causing widespread geological upheaval and intense heating. They say this remelting would have “reset” the age of the lunar rocks —masking the Moon’s true age with what could be likened to a volcanic facelift.

“We predict that there shouldn’t be any lunar rocks that are older than 4.35 billion years because they should have experienced the same resetting,” said Nimmo, a professor of Earth and planetary sciences. “Because this heating event was global, you shouldn’t find rocks anywhere on the Moon that are significantly older than that.”

The Moon as a Cosmic Timekeeper

The Moon has fascinated humanity for millenia, and in the last few centuries, people started to wonder how—and when—the Moon formed. One of the reasons for sending astronauts to the Moon was to answer this question. The Moon also serves as a luminous stepping stone to comprehending more distant objects. But, if we can’t nail down the age of the Moon, how can we be confident about the precise age of anything beyond it?

The Moon is thought to have been born from a collision between the early Earth and a Mars-sized protoplanet—the last giant impact in our planet’s history. The timing of this event has been estimated by dating lunar-rock samples presumed to have crystallized from the lunar magma ocean that existed post-impact, placing the Moon’s age at about 4.35 billion years old.

Discrepancies in Lunar Age Estimates

However, this age fails to account for several discrepancies with thermal models and other pieces of evidence, such as the ages of some zircon minerals on the lunar surface, which suggest that the Moon could be up to 4.51 billion years old.

Nimmo and his colleagues hypothesize that a remelting event driven by the Moon’s orbital evolution could account for the frequent occurrence of approximately 4.35-billion-year-old rocks—such as those collected by the U.S. Apollo mission and others—rather than the first solidification of the lunar magma ocean.

Tidal Heating and a Global Reset

For their paper, the authors used modeling to show that the Moon may have experienced sufficient tidal heating to cause this remelting approximately 4.35 billion years ago, which could “reset” the apparent formation age of these lunar samples.

Tidal heating is a process where the gravitational forces between two celestial bodies cause internal friction that leads to intense heating. For the Moon, this effect was likely more pronounced in its early history when it was closer to Earth. According to the latest models, during certain periods in its early years, the Moon’s orbit would have been unstable, causing it to experience intense tidal forces from Earth that could have led to significant heating events, dramatically altering the Moon’s geology.

Lessons from Io and Lunar Geology

The research team draws parallels between this hypothetical heating event on the Moon and the current volcanic activity observed on the DOI: 10.1038/s41586-024-08231-0

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