JA

9.4 Moon's Origin

1. Introduction to Lunar Origin Challenges

  • Understanding the Moon's origin is challenging for planetary scientists due to extensive knowledge, yet the Moon is both tantalizingly similar to Earth and frustratingly different.

2. Early Hypotheses for Moon's Origin

  • Learning Objective: Describe the top three early hypotheses of the formation of the Moon. #### 2.1. Fission Theory

    • Premise: The Moon was once part of Earth but separated early in their history.

    • Problems: Modern calculations show spontaneous fission is impossible. It's difficult to explain how a Moon made of terrestrial material could have developed its distinct chemical differences.
      #### 2.2. Sister Theory

    • Premise: The Moon formed together with (but independent of) Earth, similar to many moons of outer planets.

    • Problems: With increasing knowledge of the Moon, this idea became less fitting.
      #### 2.3. Capture Theory

    • Premise: The Moon formed elsewhere in the solar system and was later captured by Earth.

    • Problems:

    • No known mechanism for early Earth to capture such a large moon without substantial energy loss; spacecraft use retro-rockets for orbit insertion.

    • A captured object would likely have an eccentric orbit, not the nearly circular orbit of our Moon.

    • Too many compositional similarities, particularly identical fractions of major oxygen isotopes, to support a completely independent origin.

3. The Giant Impact Hypothesis

  • Learning Objective: Summarize the current "giant impact" concept of how the Moon formed.

    • Context: Developed to resolve contradictions presented by earlier theories and growing evidence of large, planetary-mass objects in the inner solar system during planet formation.

    • Mechanism: Envisions Earth being struck obliquely by an object approximately one-tenth Earth's mass (about the size of Mars).

    • This impact would disrupt much of Earth, ejecting a vast amount of material into space.

    • Computer simulations indicate several percent of Earth's mass could be ejected.

    • Ejected material would primarily be from the stony mantles of Earth and the impacting body, not their metal cores.

    • This ejected rock vapor cooled and formed a ring of material orbiting Earth, which subsequently condensed into the Moon.

    • Solutions Offered by the Hypothesis:

    • Absence of metals: Easily understood as the Moon's raw material is derived primarily from the mantles of Earth and the projectile.

    • Lack of volatile elements: Most volatile elements would have been lost during the high-temperature phase following the impact.

    • Similarities with Earth: By forming the Moon primarily from terrestrial mantle material, similarities such as identical abundances of various oxygen isotopes are explained.

    • While not definitively proven, this hypothesis offers potential solutions to most major problems concerning the Moon's chemistry.