Recent analysis of some of Earth's oldest rocks found in Western Australia is providing compelling new evidence that supports the Giant Impact Hypothesis for the formation of the Moon. This research offers a unique chemical "fingerprint" linking the composition of the early Earth and the Moon, shedding light on the dramatic collision event billions of years ago.
The Western Australian Anorthosites
The focus of the study, led by the University of Western Australia (UWA), is on 3.7-billion-year-old magmatic anorthosite rocks from the Murchison region of Western Australia. These rocks are among the oldest known surviving pieces of Earth's crust.
Rock Type Significance: Anorthosites are a type of igneous rock rich in plagioclase feldspar. While they are extremely common on the Moon, they are rare on Earth, suggesting a deep geological connection between the two bodies.
Analytical Method: Researchers used fine-scale analytical methods to isolate and study feldspar crystals within these anorthosites. These crystals act as tiny time capsules, recording the isotopic "fingerprint" of Earth's ancient mantle.
Evidence for the Giant Impact Hypothesis
The prevailing theory for the Moon's origin is the Giant Impact Hypothesis, which posits that a Mars-sized planet, often named Theia, collided with the early Earth about 4.5 billion years ago. The debris from this high-energy impact then coalesced to form the Moon.
Compositional Link: The UWA team compared the isotopic composition of the Australian anorthosites with measurements of lunar anorthosites collected during NASA's Apollo missions.
Consistent Starting Composition: The comparison revealed a consistent starting composition for both the Earth and the Moon, dating back to around 4.5 billion years ago. This strong match suggests that the Moon formed from material that was already mixed and homogenized with the early Earth's material following a massive, high-energy impact.
Support for the Theory: This consistency provides robust support for the Giant Impact Hypothesis, indicating that the Earth and the material that formed the Moon shared a common origin in the aftermath of the collision.
Insights into Early Earth and Continental Growth
The research also provided valuable information about the formation of Earth's continents.
Late Continental Growth: The findings suggest that the Earth's continents began to grow relatively late in the planet's history, around 3.5 billion years ago, which is approximately one billion years after the planet itself formed. This challenges some conventional ideas about the rate and timing of early crustal growth.
Ancient Mantle Record: The analysis of the feldspar crystals and their isotopic records provides a unique window into the ancient mantle of the Earth, the layer of molten rock beneath the crust.
The study on these 3.7-billion-year-old Australian anorthosites represents a significant step forward in understanding the co-evolution of Earth and its Moon, confirming a violent birth event that left an indelible chemical signature on both worlds.