Question

(a) Recent observations suggest that the Moon has a very small, solid core that is less than \(3 \%\) of its mass. In comparison, 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.) (b) The Moon has virtually no magnetosphere. Why? (Hint: Remember what causes Earth's magnetic field.)

Short answer

(a) The Moon's smaller core percentage compared to Earth's is due to its formation through a collision between Earth and a Mars-sized object, which ejected mixed debris containing elements of both Earth and the impacting object into Earth's orbit. The ejected debris had a lower percentage of core material, and most heavier materials sank back into Earth, resulting in the Moon's core being less than \(3 \%\) of its mass, while Earth's core is about \(33 \%\) of its mass. (b) The Moon has virtually no magnetosphere because it lacks the necessary convection currents required to generate a magnetic field. Unlike Earth's molten outer core, the Moon's core is believed to be solid, meaning there is no liquid metal flow to create electric currents within the core.

Step-by-step solution

(a) Understanding the Moon formation model

To explain why the Moon has a much smaller core percentage than Earth, we need to recall the Moon formation model discussed in Chapter 1. According to the model, the Moon was formed through a collision between Earth and a Mars-sized object, which caused molten debris to be ejected into Earth's orbit. This debris eventually coalesced to form the Moon. This process would result in material from both the Earth and the Mars-sized object, including their cores, being mixed into the ejected debris.

(a) Comparing the core compositions of Earth and Moon

Since the Moon formation model suggests that the Moon is made up of elements from both Earth and the Mars-sized object, it would have a different composition than Earth. The percentage of core material in the ejected debris would be lower than in the Earth, resulting in a smaller core in terms of percentage for the Moon. Moreover, most of the heavier materials, like those found in the core, would have sunk back into the Earth due to gravity. Therefore, the Moon's core is less than \(3 \%\) of its mass, while Earth's core is about \(33\%\) of its mass.

(b) Earth's magnetic field cause

To answer why the Moon has virtually no magnetosphere, we need to recall the cause of Earth's magnetic field. Earth's magnetic field is primarily caused by the convection currents of its molten outer core, which consists of iron and nickel. As the liquid metal flows in response to heat, it generates electric currents, which in turn create a magnetic field.

(b) Applying Earth's magnetic field cause to the Moon

The Moon has a much smaller core, as established in part (a). Besides its small size, the Moon's core is also believed to be solid and not in a molten state like Earth's outer core. This means that the Moon lacks the necessary convection currents required to generate a magnetic field, as there is no liquid metal flow to create electric currents within its core. Thus, the Moon has virtually no magnetosphere.

Key concepts

Moon's core composition

The Moon's core is remarkably smaller compared to Earth's core. Studies suggest that it accounts for less than 3% of its total mass, while Earth's core is about 33%. Why such a significant difference? The explanation lies in the Moon's formation. Scientists widely accept the giant-impact hypothesis, which proposes that the Moon formed after a colossal collision between the young Earth and a Mars-sized celestial body.

This impact led to debris being ejected into Earth's orbit, ultimately forming the Moon. Now, during such a chaotic event, the heavier elements, typically found in a planet's core like iron and nickel, are more likely to remain within the planet due to their density and gravity's pull. As a result, the debris that formed the Moon had a smaller proportion of dense core materials compared to Earth, explaining its smaller core by comparison.

Magnetosphere

The magnetosphere is the region around a planet dominated by its magnetic field, shielding the surface from the solar wind. The Earth has a strong magnetosphere, thanks to its molten outer core, which facilitates the creation of electric currents through convection movements.

These currents generate Earth’s magnetic field, which extends outward and forms our protective magnetic bubble. Without this shield, life as we know it would be exposed to harmful solar and cosmic radiation.

The Moon, in contrast, has virtually no magnetosphere. This absence is linked to its small and mostly solid core, lacking the convection currents necessary for generating a magnetic field. Consequently, without these electric currents, the Moon cannot form a significant magnetosphere like Earth.

Earth's magnetic field

Earth's magnetic field is a crucial feature, generated by a phenomenon known as the geodynamo. This process occurs in the liquid outer core of the Earth, composed mainly of iron and nickel. As these conductive metals flow, they move heat away from the inner core, creating convection currents.

These currents, coupled with the rotation of the Earth, result in electric currents which then produce a magnetic field. This magnetic field extends far into space, creating the magnetosphere. For Earth, this process is vital, as it not only helps protect the planet from solar radiation but also aids in navigation systems for many species, including migratory birds and sea turtles.

Core formation model

Understanding a celestial body's core involves exploring its formation. In the case of the Moon, the core formation model is rooted in the giant-impact hypothesis. This model demonstrates that a massive collision can determine the composition and size of a celestial body's core.

The Moon was formed from debris ejected into space from the impact between early Earth and another large body. Since the debris included materials from the outer layers of both bodies, the resulting Moon ended up with a smaller proportion of dense core elements.

This model of core formation explains why the Moon has a smaller core than Earth. It highlights how initial formation events can drastically influence internal structures and composition. Similarly, studying Earth's core formation also leans on understanding such primordial impacts and their lasting implications on planetary composition.