Question

Recent observations suggest that the Moon has a very small, solid core that is less than \(3 \%\) of its mass. In comparison, the Earth's core is about \(33 \%\) of its mass. Explain why this difference might exist. (Hint: Recall the model for Moon formation that we presented in Chapter 1.) (D)

Short answer

The difference in core compositions between the Moon and Earth can be explained by the Giant Impact Hypothesis. This model suggests that a Mars-sized body, Theia, collided with early Earth, and the resulting debris eventually formed the Moon. During this process, the heavier iron and nickel-rich core materials from Theia mostly fell back onto Earth, leaving the Moon with a smaller core consisting primarily of lighter silicate materials. As a result, the Moon's core constitutes only about 3% of its mass, while Earth's core makes up approximately 33% of its mass.

Step-by-step solution

Outline the different models of Moon formation

There are several models of Moon formation that have been proposed over the years. Some of these include the fission hypothesis, the capture hypothesis, and the impact hypothesis (also known as the Giant Impact Hypothesis). It is important to get acquainted with these models in order to better understand which model better explains the difference in core compositions.

Explain the currently leading model of Moon formation

The Giant Impact Hypothesis is the currently leading model for Moon formation. According to this model, a Mars-sized body, known as Theia, collided with the early Earth about 4.5 billion years ago. The debris from this impact eventually coalesced to form the Moon. This model is supported by evidence from, for example, isotopic similarities between the Earth and Moon rocks and computer simulations of the impact.

Explain why the leading model accounts for the difference in core compositions

The Giant Impact Hypothesis could explain the difference in core compositions of the Moon and the Earth. When the Moon was formed from the debris of the impact, it is likely that a large portion of the heavier iron and nickel-rich material from Theia's core fell back onto the Earth, leaving the Moon with a mantle consisting primarily of lighter silicate materials. This lack of heavy core materials would mean that the Moon's core is much smaller in proportion to its mass when compared to the Earth's core.

Provide a concluding summary

In conclusion, the difference in core compositions between the Moon and Earth can be explained by the Giant Impact Hypothesis. The Moon formed from the debris resulting from the collision between early Earth and Theia, with heavy core materials falling back onto the Earth. This resulted in the Moon having a smaller core, constituting approximately 3% of its mass, as compared to Earth's core, which constitutes about 33% of its mass.

Key concepts

Giant Impact Hypothesis

The Giant Impact Hypothesis presents a dramatic and violent origin story for our Moon, captivating both scientists and non-scientists alike. According to this theory, about 4.5 billion years ago, a Mars-sized body named Theia crashed into the early Earth with a force of cosmic proportions. The aftermath of this colossal collision was a cloud of debris, part of which slowly coalesced under gravity to form what we now see as our Moon.

This hypothesis not only offers a plausible explanation for the Moon's creation but also for the distinct differences in the core compositions of Earth and its satellite. The celestial crash would have resulted in the Earth's core absorbing much of Theia's metallic core, leaving a deficit of such materials in the resultant moon dust. This leaves us with a Moon that, unlike Earth, boasts a diminutive core - a telltale remnant of its tumultuous birth.

Lunar Geology

Diving into the realm of lunar geology, we uncover that the Moon is so much more than just a silvery celestial body lighting up our night sky. Its surface is composed of vast maria—dark basaltic plains—and lighter highlands, canvased with craters, the evidence of a long history of meteorite impacts. Lunar rocks obtained during the Apollo missions provide pivotal clues about the Moon's composition, supporting the Giant Impact Hypothesis.

As for the lunar core, it's intriguingly small and solid, distinct from the larger, partially molten core of Earth. This disparity is a geological signature of the Moon's formation, echoing the impact that would have flung lighter silicate materials into orbit, while heavier iron-rich elements were left behind or absorbed by Earth. The tireless study of lunar geology helps us piece together this cosmic puzzle, connecting the Moon's present features to their formative past.

Earth's Core Composition

Now let's travel deep beneath our feet into the iron-clad heart of our planet, unraveling the mysteries of Earth's core composition. The inner core of the Earth is a solid sphere composed primarily of iron and nickel, surrounded by a molten outer core of similar composition. This ferrous center plays a vital role in generating Earth's magnetic field and accounts for about 33% of the planetary mass.

Understanding why Earth retains such a hefty core while the Moon's is much smaller provides profound insights into the formational differences between these two celestial neighbors. During the Giant Impact, the heavy metallic core of Theia likely merged with Earth's, fortifying it with an additional iron contingent. The result? A stark contrast in core size and composition that speaks volumes about the Moon's violent origins and underscores the integral role of Earth's core in shaping the geological and magnetic phenomena we observe today.