The formation of the Moon is widely attributed to a catastrophic event known as the Giant Impact Hypothesis. This scenario proposes that the proto-Earth collided with a Mars-sized protoplanet named Theia approximately 4.5 billion years ago. The enormous energy released by this impact ejected a massive cloud of superheated, vaporized rock and metal into orbit around Earth. This orbiting material then rapidly coalesced to form our planet’s only natural satellite.
Establishing the Broad Timeline
Initial attempts to date the Moon relied on rocks returned by the Apollo missions between 1969 and 1972. Scientists used radiometric dating techniques on these lunar samples to establish a general timeframe for the Moon’s existence. Analysis of the oldest pristine lunar rocks, such as those found in the lunar highlands, provided an upper limit for the Moon’s age.
One key method involved Uranium-Lead (U-Pb) dating, applied to tiny, highly stable mineral crystals called zircons. The oldest lunar zircons collected have provided ages nearing 4.46 billion years. This date marks when the Moon’s crust first solidified after the impact, not the impact itself. The collision must have occurred shortly before this solidification, placing the event firmly within the first 100 million years of the Solar System’s history.
Refining the Date: Isotopic Signatures
To pinpoint the exact timing of the collision, scientists turned to short-lived radioactive chronometers that act as high-precision geological clocks. The most informative of these is the Hafnium-Tungsten (Hf-W) isotope system, which has a short half-life of just 8.9 million years. This system is effective because the two elements behave differently when metal and silicate separate, a process defining the formation of a planetary core.
Hafnium is a lithophile element, preferring to bond with the silicate rock that makes up a planet’s mantle. Tungsten, however, is moderately siderophile, meaning it tends to bond with iron and sink into the core during planetary differentiation. If a planet’s core forms while the parent isotope, Hf-182, is still active (within the first 50 to 60 million years of the Solar System), the mantle retains the hafnium. As the Hf-182 decays to the daughter isotope, W-182, the mantle develops a measurable excess of W-182.
Analysis shows that the Moon’s silicate mantle has a slight but distinct excess of W-182 compared to the bulk silicate Earth. This isotopic difference is interpreted as a signature left by the Giant Impact, which acted as a major core-forming event for the Earth-Moon system. The Hf-W data strongly suggests the impact occurred relatively early, within about 50 million years after the initial formation of Solar System solids. This places the collision at an age of approximately 4.518 billion years ago, though some models still permit a range up to 150 million years after the Solar System’s start.
The Immediate Aftermath: Planetary Transformation
The immense energy unleashed by the impact fundamentally transformed both the proto-Earth and the resulting Moon. The collision generated enough heat to completely melt the outer layers of the Earth, creating a global Magma Ocean that extended hundreds of miles deep. This molten state allowed for a rapid and complete chemical separation, or differentiation, of the planet’s interior.
The heaviest elements, particularly iron and nickel, quickly sank to form Earth’s final, dense core, a process that the impact accelerated dramatically. Simultaneously, the ejected debris cloud, which was a mixture of vaporized material from both Theia and the Earth’s mantle, began to cool and condense in orbit. The new Moon then coalesced from this ring of molten rock and vapor in a remarkably short time, possibly in as little as a few hours or years. This rapid formation process ensured the Moon was born as a molten body, which quickly differentiated into its own core, mantle, and crust.