APS 105: Formation of the Earth

Vocabulary flashcards covering the accretion, heating, chemical composition, and core differentiation of the Earth from Hampton University's APS 105 lecture.

Planetesimals

Solid bodies ranging from $1\text{ to }100\,\text{km}$ in size that formed in the solar nebula and served as the gravitational building blocks that accreted to form terrestrial planets.

Accretion

The growth process in planetary formation where solid dust grains and planetesimals collide, stick together, and gravitationally assemble into full-sized planets.

Magma Ocean

A layer of molten rock covering the growing Earth under a thin crust, produced when increasingly energetic planetesimal impacts melted major portions of the planet once it grew to about $1/4$ of its final size.

Condensation Sequence

The thermodynamic sequence that governs which chemical elements and mineral compounds can condense from gas into solid grains based on temperature and distance from the Sun.

Snow Line

The boundary distance in the solar nebula beyond which temperatures dropped sufficiently low for water and other volatile compounds to freeze into solid ice.

Hydrated Silicates

Rock-forming silicate minerals containing chemically bound water, which formed in the nebular zone near Mars alongside dry rock and metal.

Phoebe

A moon of Saturn that represents an icy planetesimal originating from beyond the solar system's snow line.

Asteroid Eros

A rocky asteroid that represents a surviving example of the dry, rocky planetesimals that formed near $1\,\text{AU}$.

C1 Carbonaceous Chondrites

A class of primitive meteorites whose condensable element abundances match the solar photosphere relative to $10^6$ silicon atoms, reflecting primordial cosmic composition.

Bulk Density of Earth

The average density of the whole Earth ($5.5\,\text{g/cm}^3$), calculated by dividing Earth's mass ($6 \times 10^{27}\,\text{g}$) by its volume ($1.1 \times 10^{27}\,\text{cm}^3$), which requires an interior core composed of dense metal ($8\,\text{g/cm}^3$).

Seismometer

An instrument used to measure the transit and reflection of seismic waves through the planetary interior, which allowed geophysicists to detect and measure the dimensions of Earth's core.

Earth's Core

The dense metallic center of the Earth consisting of a liquid iron outer sphere with a radius of about $3{,}400\,\text{km}$ and a solid inner core with a radius of about $1{,}200\,\text{km}$.

Bulk Elemental Composition of Earth

The overall chemical makeup of the whole Earth, dominated by Iron ($35\%$), Oxygen ($30\%$), Silicon ($15\%$), Magnesium ($13\%$), and other elements ($9\%$).

Differentiation

The density-driven separation process where dense molten iron sinks through a planetary magma ocean to assemble a central core while lighter silicates float upward to form the mantle and crust.

Siderophile Elements

Elements that have a strong chemical affinity for molten iron over molten rock (e.g., Iridium, Platinum, and Osmium) and therefore sank into Earth's metallic core during differentiation.

Planetesimals Near 1 AU

Early building blocks that formed in Earth's immediate orbital vicinity, composed of approximately $2/3$ dry rock and $1/3$ metal.

Planetesimals Near Mars

Nebular building blocks that condensed near Mars's orbit, composed of approximately $1/3$ hydrated rock, $1/3$ dry rock, and $1/3$ metal.

Planetesimals Beyond the Snow Line

Outer solar system bodies formed in cold environments, consisting of approximately $3/4$ ice and $1/4$ rock and metal.

Gravitational Stirring by Jupiter

The orbital scattering effect caused by Jupiter's massive gravity that propelled icy, volatile-rich planetesimals from beyond the snow line inward toward the warm terrestrial habitable zone.

Kinetic Impact Heating

The conversion of impact kinetic energy ($\frac{1}{2} M V^2$) into thermal energy, which heated the early Earth from a cold body ($< 200\,\text{K}$) to a molten state ($\sim 1{,}600\,\text{K}$).