Ch. 2.4 Planetary Differentiation and Heat

What is planetary differentiation?

Separation of materials by density and chemical affinity, producing layered interiors (core, mantle, crust—>chemical properties)

Were planets originally layered or homogenous?

Likely homogenous —> heat + gravity caused differentiation into layers

What drives planetary differentiation?

Density, chemistry, and heat

What is released during core formation that accelerates into differentiation?

Gravitational potential energy —> converted into heat

What are the main material groups in planets?

Metals, silicates, ices, atmophiles

Which elements belong to the metals group and where do they go?

Fe (Iron), Ni (Nickel), Co, (Cobalt) —> very dense so they sink to the core

Which elements form silicates and where do they end up?

O, Si, Al, Ca, Na, K —> less dense so they end up in the mantle & crust

What are ices and when are they solid?

H2O, CH4, N2—> solid only at low temp’s (volatiles)

What are atmophiles and where are they found?

H, He, Ne, C, O —> form gases —> atmospheres

How does heat enable differentiation?

Heat weakens “solid” rock under pressure —> allows more dense rock to sink through less dense rock

What are the three methods of heat transfer?

Conduction, Convection, Radiation

Conduction

• Happens in rigid solids

• Heat is passed atom-to-atom by vibration

• Example: a metal spoon handle getting hot after sitting in soup.

• In planets: heat moves slowly through the lithosphere (rigid outer shell)

Convection

requires material that can flow

• Fluids (like molten magma or water), OR 

• Weak solids under high temp/pressure (like the mantle)

Even though the mantle is “solid rock”, over millions of years, it flows like putty —> so it convects

Example: boiling water (the rolling motion) or Earth’s mantle circulation 

Radiation

• Heat transfer by infrared light

• doesn’t need matter

• Example: heat from the sun warming Earth

• In Planets: important at the surface (Earth radiates heat into space), but not effective inside solid planets because radiation doesn’t move well through dense rock.

Which heat transfer process is most efficient in planetary interiors?

Convection

What is accretionary heating?

Collisions of debris or rock into a planetary object convert kinetic energy —> thermal

Example: Getting your ears pierced and then your ear gets warm

How does core formation release heat?

• Accretion (collisions) heated the early Earth

• Core formation added extra heating because iron sinking released gravitational energy

• This is one reason the early Earth was partly or fully molten, fueling volcanism and outgassing

What is radiogenic heating?

Heat from radioactive decay (long-lived U, Th, K; short-lived isotopes in early stages).

How does solar energy affect planets?

Drives surface systems but too weak to affect interiors

What is T-Tauri heating?

Strong early solar winds/magnetic fields induce currents in inner planets —> surface melting (controversial, early only)

What is tidal heating? 

Flexing from gravitational pulls —> frictional heating

Example: Io’s volcanic activity from Jupiter’s pull

Why do small planets cool faster?

Larger surface area-to-mass ratio → radiate heat quickly

Why do large planets stay active longer?

Retain heat longer → extended geologic activity