Formation and Differentiation of the Earth

Vocabulary practice flashcards covering the accretion, thermal evolution, composition, and differentiation of the Earth from Hampton University's APS 105 lecture notes.

Planetesimal

A solid object ranging from $1\,\text{km}$ to $100\,\text{km}$ in size that formed in the protoplanetary disk and served as the building block for the terrestrial planets.

Accretion

The growth process in which solid grains and planetesimals collide, stick together, and gravitationally assemble to form larger bodies and eventually planets.

Estimated Number of Earth Planetesimals ($N_{pl}$)

Approximately $300{,}000$ spherical planetesimals of $100\,\text{km}$ radius (mass $\approx 2 \times 10^{19}\,\text{kg}$ each with density $5000\,\text{kg}\,\text{m}^{-3}$) required to account for Earth's total mass of $6 \times 10^{24}\,\text{kg}$.

Planetesimal Collision Heating

The thermal energy resulting from impact kinetic energy ($\frac{1}{2}MV^2$); for two $100\,\text{km}$ planetesimals colliding at $1\,\text{km/s}$, the energy is $10^{26}\,\text{Joules}$, raising their temperature by approximately $1.5^\circ\text{C}$.

Magma Ocean

A deep layer of molten rock covering the Earth capped by a thin crust, produced when impacts grew energetic enough to melt large chunks of the planet once it reached about $1/4$ of its final size.

Early Earth Thermal Evolution

The progression of internal temperature as Earth accreted: cold ($< 200\,\text{K}$) at radius $R = 1/8$, warm ($\sim 500\,\text{K}$) at $R = 1/4$, hot ($\sim 1000\,\text{K}$) at $R = 1/2$, and molten ($\sim 1600\,\text{K}$) at $R = 1$.

Condensation Sequence

The temperature-dependent progression of materials forming solids at varying radial distances from the Sun: refractory metals and rocks condense close in, hydrated silicates condense farther out, and volatile ices condense only beyond the snow line.

Snow Line

The solar system boundary distance beyond which temperatures were cold enough for water and other volatile compounds to condense into solid ice.

Jupiter's Role in Earth's Water Delivery

Gravitational perturbations by Jupiter that scattered icy planetesimals from beyond the snow line inward toward the warm habitable zone, delivering water to planets where it would have otherwise vaporized.

Near-Earth Planetesimal Composition

Planetesimals formed near $1\,\text{AU}$ consisting of approximately $2/3$ dry rock and $1/3$ metal.

Near-Mars Planetesimal Composition

Planetesimals formed in the region of Mars consisting of approximately $1/3$ hydrated rock, $1/3$ dry rock, and $1/3$ metal.

Outer Solar System Planetesimal Composition

Planetesimals formed beyond the snow line consisting of approximately $1/4$ rock and metal and $3/4$ ice.

Asteroid Eros

A rocky asteroid that exemplifies an inner solar system leftover planetesimal composed predominantly of rock.

Phoebe (Moon of Saturn)

An icy body originating beyond the snow line that represents the volatile-rich planetesimals that formed in the outer solar system.

Asteroids

Leftover solar system bodies containing some of the oldest mineral grains, altered by subsequent impacts, melting, and flowing water, which serve as the parent bodies for most meteorites.

C1 Carbonaceous Chondrites

Primitive meteorites whose abundances of condensable elements (normalized to $10^6$ silicon atoms) match the solar photosphere, demonstrating an approximately cosmic composition while lacking volatile ice-forming elements like carbon and nitrogen.

Earth's Mean Density Discrepancy

The observation that Earth's bulk density of $5.5\,\text{g/cc}$ (mass of $6 \times 10^{27}\,\text{g}$ divided by volume of $1.1 \times 10^{27}\,\text{cc}$) significantly exceeds surface rock densities of $2.2\text{ to }3.0\,\text{g/cc}$, implying a dense interior composed of roughly half iron ($8\,\text{g/cc}$) and half rock ($2.5\,\text{g/cc}$).

Earth's Core

A dense central metallic region discovered via seismic wave reflections, consisting of an outer liquid iron sphere of radius $\approx 3{,}400\,\text{km}$ and an inner solid core of radius $\approx 1{,}200\,\text{km}$.

Bulk Elemental Composition of Earth

The overall chemical breakdown of the whole Earth: Iron ($\text{Fe}$) at $35\%$, Oxygen ($\text{O}$) at $30\%$, Silicon ($\text{Si}$) at $15\%$, Magnesium ($\text{Mg}$) at $13\%$, and other elements at $9\%$.

Differentiation

The compositional separation process in a molten planet where dense liquid iron sinks to the center through lighter silicate molten rock, producing a distinct metallic core and rocky mantle.

Siderophile Elements

Elements with a high chemical affinity for molten iron over molten silicate rock (such as iridium, platinum, and osmium) that dissolved into the iron layer and sank into Earth's core, leaving the crust and mantle depleted of them.

Lunar Core Deficiency

The Moon's characteristic of having an exceptionally small core and being composed almost entirely of rock, demonstrating that it formed differently from terrestrial planets.