APS 105: Formation of the Earth and Differentiation

Vocabulary practice flashcards covering the accretion, chemical composition, interior structure, and differentiation of Earth based on APS 105.

Planetesimals

Lumpy objects composed of rock and metal, ranging from $1$ to $100\,\text{km}$ in size, which orbited the Sun at approximately $1\,\text{AU}$ and served as the building blocks of Earth.

Earth's Formation Requirements

To account for Earth's total mass of $6 \times 10^{24}\,\text{kg}$, approximately $300{,}000$ spherical planetesimals of $100\,\text{km}$ diameter (each with a mass of $2 \times 10^{19}\,\text{kg}$ at a density of $5{,}000\,\text{kg/m}^3$) had to collide and accrete.

Magma Ocean

A deep layer of molten rock covering the exterior of the accreting Earth, generated as collisions with larger incoming bodies became increasingly energetic and violent.

Primitive Core

The cooler interior center of the early accreting Earth, comprised of the initial unmelted planetesimals before intense impact heating melted the outer layers.

Condensation Sequence

The temperature-dependent distribution of solid materials across the solar nebula, dictating that only rock and metal could condense as solids near Earth's orbital distance of $1\,\text{AU}$.

Snow Line

The boundary in the solar nebula beyond which water ice could freeze solid, producing planetesimals composed of approximately two parts ice to one part rock by mass (or seven parts ice to one part rock by volume).

Feeding Zone

The specific region of the protoplanetary disc from which a growing planet gathers its constituent material during accretion.

Jupiter's Role in Earth's Water Delivery

The gravitational disturbance caused by Jupiter's massive gravity, which stirred the orbits of outer solar system bodies and hurled volatile-rich, icy planetesimals into the dry inner solar system.

Phoebe

A captured icy moon of Saturn imaged by the Cassini spacecraft, regarded as a modern representative of primitive planetesimals from the outer solar system.

Volatile-Depleted

The state of asteroids and meteorites that have lost icy elements (such as oxygen, carbon, and nitrogen in compounds like $\text{NH}_3$, $\text{CH}_4$, and $\text{H}_2\text{O}$) due to heating and gas escape into space.

Earth's Bulk Density Discrepancy

Surface rocks exhibit densities of only $2.2$ to $3.0\,\text{g/cm}^3$, whereas Earth's average density is $5.5\,\text{g/cm}^3$, necessitating an interior core composed of dense metal like iron ($\sim 8\,\text{g/cm}^3$).

Earth's Core Structure

A dense central feature identified by seismometers, consisting of an outer molten liquid sphere with a $3{,}400\,\text{km}$ radius and an inner solid sphere with a radius of approximately $1{,}200\,\text{km}$, primarily composed of iron with some nickel.

Earth's Primary Chemical Composition

The four main elements that constitute nearly the entire Earth: iron ($35\%$), oxygen ($30\%$), silicon ($15\%$), and magnesium ($13\%$), with all other elements comprising the remaining $9\%$.

Differentiation

The gravitational separation of materials by density in a molten planet, causing heavy molten metals (predominantly iron) to sink to form the core while lighter rocky silicates rise to form the mantle and crust.

Siderophile Elements

"Iron-loving" elements—such as iridium, platinum, and osmium—that chemically prefer dissolving in molten iron over molten rock and sank into Earth's core during differentiation, making them rare at the surface.

Iridium

A siderophile element that is extremely depleted in Earth's crust due to core differentiation, making its presence in surface rocks a primary diagnostic tracer for extraterrestrial meteoritic material.