It is a striking paradox: while the overwhelming majority of lunar rocks that have been dated fall around 4.35 billion years, both orbital dynamics and evidence provided by zircon crystals suggest that the Moon actually formed considerably earlier-between 4.51–4.53 billion years. The mismatch has long polarised planetary scientists into two factions-those trusting radiometric ages from samples returned by Apollo and those whose simulations demand an older Moon. A new hypothesis offers a resolution rooted in a process more normally associated with Jupiter’s volcanic moon Io: tidal heating.
Tidal heating is a consequence of gravitational forces between two bodies flexing and deforming one of them, thereby creating internal friction and heat. In the early history of the Moon, when it was orbiting much closer to Earth at just 16-22 Earth radii, these forces would have been far stronger than today. Modelling the orbital evolution of the Moon does show that during certain unstable phases, its eccentricity could spike, and tidal stresses would strengthen. The resulting heat would be enough to cause partial or even global remelting of the lunar crust and upper mantle. Such an event would “reset” isotopic clocks in minerals like zircon, erasing older ages and producing a uniform 4.35 billion-year signature in most samples.
High-precision U-Pb dating of Apollo zircons has yielded discrete age clusters within a narrow 4-million-year window around 4.336 Ga, consistent with a short-lived, planet-wide magmatic pulse. Thermal models indicate that tidal heating in this late planetary tidal regime would have generated up to heat fluxes of 3–30 W/m², enough to cycle the whole crust through the melting regime in less than a million years. In intrusive-dominated realms, conductive heating due to the repeated injection of melt would raise temperatures above the zircon closure temperatures and reset the radiometric ages. The same process explains the isotopic equilibrium between various types of lunar rocks, such as mare basalts, ferroan anorthosites, and KREEP-rich samples, which are derived from a depth range from tens to more than one thousand kilometres.
The analogy with Io is strong. NASA Juno mission observations show that Io’s volcanoes are distributed in patterns dictated by tidal dissipation, where internal heating is modulated by a feedback between the generation of melt and rheology. On Io, tidal flexing driven by Jupiter and its other moons creates an average surface heat flux of ~2.24 W/m², supporting more than 400 active volcanoes. For the early Moon, similar but transient conditions could have powered extensive volcanism, resurfacing impact basins and obscuring evidence of older cratering.
This would account for the relative scarcity of ancient basins compared with the expectations from models of bombardment. The hypothesis is also consistent with the constraints that result from the modelling of orbital instability. During the early phase of its outward migration, the Moon’s semi-major axis would, in fact, have run through resonant configurations that amplify tidal heating. Estimates of Earth’s tidal dissipation factor, Qₑ, during this molten phase are values much larger than today’s ~300 but less than Jupiter’s ~ 30,000, in agreement with expectations for a partially molten silicate body.
Ages of the zircons place the LPT heating event approximately 80-180 Myr after the origin of the Moon and hence support an origin time close to 4.5 Ga. Recent sample returns by the Chang’e 6 mission will, for the first time, test this hypothesis with materials sampled from the far side’s South Pole-Aitken basin; if those, too, cluster at 4.35 Ga, it would reinforce the case for a global remelting event. Significantly older, unreset zircons would instead be indicative of less pervasive tidal heating, perhaps confined to specific hemispheres or crustal domains.
The Chang’e 6 samples further provide a missing link in the crater-counting chronology between 3.2 and 2.0 Ga, adding new calibration points for impact flux models on both hemispheres. This model integrates geochemical dating, orbital mechanics, and tidal heating physics into a coherent picture to reframe the apparent youth of lunar rocks as a geological illusion. Thus, the Moon would be one of the first survivors of such giant impacts in the solar system, with its true age recorded only in rare, unaltered zircons, small time capsules that somehow escaped the tidal furnace.
