LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK

Area of Science:

• Planetary Science
• Geochemistry
• Astrophysics

Background:

• The Moon's formation is theorized to involve a giant impact creating an Earth-orbiting disk.
• Lunar and Earth mantle compositions are similar, but lunar samples show significant volatile element depletion.
• Evaporative escape mechanisms do not fully explain this volatile depletion under expected disk conditions.

Purpose of the Study:

• To investigate the cause of volatile element depletion in the Moon.
• To reconcile the Moon's composition with giant impact and accretion models.
• To explain why lunar rocks are depleted in volatile elements compared to Earth's mantle.

Main Methods:

• Combined dynamical, thermal, and chemical modeling.
• Simulated the evolution of an Earth-orbiting disk following a giant impact.
• Modeled melt accretion onto the growing Moon and Earth.

Main Results:

• Volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt to Earth.
• Inner disk melt becomes volatile-rich only after cooling, by which time lunar accretion had largely ceased.
• The Moon's expanding orbit halted accretion from the inner disk before key volatile condensation.

Conclusions:

• The Moon's volatile depletion is likely due to the cessation of accretion from the inner disk before volatile condensation.
• This mechanism explains volatile-poor lunar samples, with the degree of depletion depending on internal mixing.
• The study provides a new model for lunar formation and composition, addressing a key geochemical puzzle.