The Moon's Fiery Birth: A Cosmic Mystery Deepens
Planetary scientists continue to debate the details of the Moon's formation, particularly the size of the impactor called Theia that struck Earth around 4.51 billion years ago. The leading hypothesis is a giant impact, but models conflict with geochemical data from Apollo lunar samples.
The Impactor's Size and the Evidence
- Estimated size of Theia ranges from a Mercury-sized object to half the size of present-day Earth.
- Hydrodynamic models suggest a larger impactor better explains the chemical similarity between Apollo Moon rocks and Earth's olivine-rich volcanic basalts.
- The initial Moon was a glowing magma ball that needed to cool before minerals could form.
- The Genesis rock, collected by Apollo 15 in 1971, is 4.46 billion years old and composed mostly of plagioclase, indicating a global magma ocean.
Simulating the Moon's Interior
- Wim van Westrenen (Vrije Universiteit Amsterdam) recreates high-pressure and high-temperature conditions inside the Moon in the lab.
- The lab achieves temperatures over 1700 °C and pressures of 250,000 Earth atmospheres.
- The maximum internal pressure of the Moon is about 50,000 Earth atmospheres, allowing researchers to simulate lunar interior conditions.
The Core Conflict in the Models
- Classical numerical simulations predict the Moon's composition should be very different from Earth, but observed Moon rocks are chemically similar to Earth.
- In the classic giant impact scenario, most silicate in the Moon should come from Theia, not Earth, but that is inconsistent with geochemical evidence.
- Two competing scenarios have emerged: (1) a small, fast, high-angle impactor hitting a nearly complete Earth, or (2) a half-formed Earth being impacted by another half-Earth sized object, with the Moon forming from mixed debris.
"All the classical simulations predict that the Moon should have a very different chemical composition from what we see."
"Every human can see the Moon, but not everyone realizes that its formation is directly linked to our own planet's history."
— Wim van Westrenen, Vrije Universiteit Amsterdam