Consider two competing models for the formation of Earth's Moon. Model A proposes that the Moon formed from debris ejected after a Mars-sized object impacted the early Earth. Model B suggests that the Moon formed much earlier, as a co-accretion body alongside Earth from the same primordial disk of material.
Which of the following findings would pose the greatest challenge to Model B?
Correct: B
Model B posits co-accretion from the same primordial disk, which would imply similar bulk chemical compositions for Earth and the Moon. If the Moon's chemical composition were strikingly similar to Earth's mantle (Choice A), it would strongly support Model A (giant impact), as the ejected debris from such an impact would primarily come from Earth's mantle. However, the question asks what would challenge Model B. Model B implies forming from the same disk, which might suggest a similar overall composition including the core. A crucial challenge to Model B, which suggests co-accretion, would be evidence that contradicts its formation from the 'same primordial disk.' If the Moon's iron-poor core (Choice B) is significantly different from Earth's iron-rich core, it suggests different formation conditions or material, thus challenging the idea of co-accretion from *exactly* the same initial material. The giant impact hypothesis (Model A) explains this difference well, as the impactor's core would mostly merge with Earth's, leaving the Moon to form primarily from Earth's mantle and the impactor's mantle, which are iron-poor relative to the core.