• Purdue University

• EAPS 105

EAPS-10500: The Planets
Exam 2: Study Guide (Andrew Johnson)
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Unit 4: Heating and Cooling
Part 1: Heating Up
1. Consequence of a planet or moon being hot inside.
• Unit 4: Slide 3.
o There are many consequences of planets & moons being hot inside.
▪ Causes differentiation (denser components sink to core).
▪ A hot, rotating, conductive interior can produce a magnetic field.
▪ A hot interior can cause crustal plate tectonics.
▪ A hot interior can produce volcanism.
o Astronomical objects with hot interiors are usually pretty large.
▪ Planets, large moons, large asteroids, etc.
2. The state (solid/liquid) of each layer in the Earth.
• Unit 4: Slide 15.
o Crust (Surface): SOLID
o Mantle: SOLID
▪ However, the mantle is very hot, and often acts like a “semisolid”. It
can still flow like a fluid.
o Outer Core: LIQUID
▪ The outer core is hot enough to melt itself into a liquid.
o Inner Core: SOLID
▪ The inner core is actually HOTTER than the outer core, but the
immense pressures at the inner core raise its melting temperature,
preventing the inner core from melting into a liquid.
3. How Earth’s internal temperature compares to that of the surface of the Sun.
• Unit 4: Slide 15.
o The inner core is nearly as hot as the surface of the Sun.
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4. What happens to atoms when they heat up.
• Unit 4: Slides 4 – 5.
o “Temperature” is a measurement of the average kinetic energy of the atoms
within a certain substance.
▪ In other words, it is a measurement of how much the atoms of a
substance are vibrating.
o Heating atoms to a higher temperature makes them vibrate more.
▪ At “absolute zero” (0 K), atoms cease to vibrate.
• Absolute zero is a thermodynamic idealization (it is not
possible). All atoms have some amount of irreducible motion.
5. How accretion leads to heat.
• Unit 4: Slides 6 – 7.
o “Accretion” is the process of a bunch of smaller objects assembling into a
larger object (like during the formation of a planet).
▪ When two moving objects collide with each other, their kinetic
energies (energies of motion) get converted into deformation, and
then into thermal energy (heat).
• In planetary accretion, there are lots of collisions. This causes
the young planet to be very hot.
6. How core formation leads to heat.
• Unit 4: Slide 8.
o Young planets have melted interiors. The dense materials (like iron) sink
through the melted interior, down into the core. “Differentiation”.
▪ Dense, sinking materials collide with each other once they reach the
core. These collisions deform the core, producing internal heat.
o Heat of core formation amounts to only 10% of heat from accretion.
▪ Slower collisions in core formation. Less deformation = less heat.
7. What is meant by primordial heating.
• Unit 4: Slide 8.
o “Primordial” means EARLY.
▪ Heating due to accretion & core formation are primordial, because
they happened EARLY during the planet’s formation.
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8. The primary source of Jupiter’s moon Io’s internal heat.
• Unit 4: Slides 9 – 10.
o “Tidal heating” occurs when gravitational interactions between two bodies
cause one of them to continuously deform.
▪ The Jovian moon Io orbits Jupiter in an ellipse. The gravitational tug
on Io from Jupiter varies continuously as it orbits closer and further
away from the planet.
• This causes Io to elongate as it approaches Jupiter, and shorten
as it moves away. This results in tidal heating.
o Tidal heating on Io is the reason why its mantle has
completely melted. It is the most volcanologically active
body in the Solar System. > 400 active volcanoes.
9. Why Lord Kelvin’s calculation the age of the Earth did not work.
• Unit 4: Slide 11.
o Lord Kelvin did not watch the HBO miniseries Chernobyl.
▪ He failed to account for radioactive decay.
• During Kelvin’s time, nobody knew that unstable radioactive
isotopes decay into lighter elements and produce heat.
• Radioactive decay within the Earth adds to its other primordial
heat sources.
10. How radioactive decay produces heat.
• Unit 4: Slides 12 – 13.
o Radioactive decay happens when the atomic nucleus (which contains
protons and neutrons) of an unstable atom decays into a more stable one.
▪ The unstable atom will throw off hadrons (protons or neutrons) and
become a lighter element (known as a “daughter product”). These
protons and neutrons cause collisions that produce heat.
• Stable atom: Positively-charged protons within a nucleus
want to repel each other, but they are successfully held
together by the “strong nuclear force (gluons)”.
• Unstable atom: There are either too many or too few hadrons
in the nucleus, creating a force imbalance. The atom will decay
into a lighter, more stable element, losing energy through
radiation in the process.
11. What contributes to the current internal temperature of the Earth.
• Unit 4: Slides 13 – 14.
o Today, the two main radioactive elements that decay and produce heat
within the Earth are uranium (isotope-238) and thorium (isotope-234).
o Today, Earth’s internal heat is caused by:
▪ 50% radioactive decay (238U, 234Th)
▪ 50% primordial heat sources (accretion, core formation).
o Tidal heating on the Earth is negligible.
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12. Why smaller planets cool faster than larger ones.
• Unit 4: Slides 24 – 25.
o An object’s “surface-area to volume ratio (SAtVR)” is equal to:
▪ (Object’s Surface Area) / (Object’s Volume).
▪ For a sphere (which is the ideal shape of a planet), this ratio reduces
to SAtVR = (3 / r), where r is the radius of the sphere (planet).
• Objects with a HIGH SAtVR cool faster.
o SMALL planets have a HIGHER SAtVR than LARGE
planets, and therefore, cool faster.
o Smaller planets also have less primordial heat, because they took less
collisions to form, and have smaller cores.
o Smaller planets also have less radioactive material to heat them up.
13. Why Pluto is still hot inside.
• Unit 4: Slide 16.
o The reason why Pluto remains hot inside is unknown.
▪ Likely isn’t primordial heat. Collisions are slower farther away from
the Sun, because orbits are slower out there (Kepler’s 3rd Law).
▪ Pluto does not have an abundance of radioactive materials.
▪ Pluto’s Moon Charon has a nearly circular orbit, so it does not exert
much tidal heating on Pluto.
▪ The collision that formed Charon was likely not recent, so Pluto’s
internal heat is probably not a result of that.
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Part 2: Cooling Down
14. The mediums by which conduction, convection, and radiative heat transfer move
heat.
• Unit 4: Slides 18 – 26.
o Conduction is the transfer of heat through solid material.
o Convection is the transfer of heat through fluid motion.
o Radiative heat transfer is the process by which heat is transferred through
electromagnetic waves (light / photons) through a transparent medium.
▪ Only takes place at the surface of a planet / star. The only means by
which planets and stars lose their heat to space.
15. How conduction transfers heat.
• Unit 4: Slide 19.
o Conduction transfers heat through solids by the spread of vibrations.
▪ Recall that the hotter atoms are, the more they vibrate.
16. How convection transfers heat.
• Unit 4: Slides 20 – 21.
o Convection transfers heat through fluid motion.
▪ Materials expand and become less dense when heated.
▪ Less dense materials are more buoyant, and they rise.
▪ Rising hot volumes of material lose heat to the coolant medium,
causing it to contract and sink.
▪ The process then repeats itself.
o An example of a convection process is the interior of a lava lamp.
17. The difference between radioactive decay and radiative heat transfer.
• Unit 4: Slide 23.
o Radiative heat transfer is the TRANSFER of heat through EM waves.
o Radioactive decay is the GENERATION of heat through the decay of
unstable radioisotopes.
18. How each layer within the Earth transfers heat.
• Unit 4: Slide 26.
o The Earth’s surface transfers heat to space through radiative heat transfer.
o The Earth’s lithosphere (crust and upper mantle), which is solid, transfers
heat through conduction.
o The Earth’s mantle, which is a solid that behaves like a fluid, transfers heat
mostly through convection (not conduction). Don’t be fooled.
▪ Remember that some solids can behave like fluids.
o The Earth’s outer core, which is a liquid, transfers heat through convection.
o The Earth’s inner core, which is a solid, transfers heat through conduction.
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Part 3: Plate Tectonics
19. What lithosphere is.
• Unit 4: Slide 28.
o The Earth’s “lithosphere” is the strong outer shell of the planet.
▪ It consists of the crust and the uppermost mantle.
o The lithosphere is broken into fragments, called “tectonic plates”.
20. What asthenosphere is.
• Unit 4: Slide 29.
o The Earth’s “asthenosphere” is the weaker, hotter part of the mantle that
underlies the lithosphere.
▪ The asthenosphere is “plastic,” and its fluidity allows the lithosphere
to “float” on top of it, and even move around (“plate tectonics”).
• Although the lowermost mantle is even hotter than the
asthenosphere, the higher pressures down there prevent the
lower mantle from flowing around as much.
21. What drives plate tectonics.
• Unit 4: Slides 30 – 32.
o Plate tectonics is driven by convection in the mantle.
o The Earth’s surface is broken up into about a dozen tectonic plates.
▪ The tectonic plates “float” on top of the fluid-like asthenosphere.
▪ These plates slide past, under, and over each other.
• The motions of these tectonic plates give rise to earthquakes
and volcanism.
22. What a mid-ocean ridge is.
• Unit 4: Slides 32 – 34.
o “Mid-ocean ridges” are underwater mountain ranges in the middle of the
oceans. They are located at oceanic tectonic plate margins.
▪ At mid-ocean ridges, two oceanic tectonic plates are spreading apart
from one another. “Divergent” plate margin.
• Hot, fluid-like mantle material rises up and seeps into the gap
between the plates, forming new crust along the ridge.
• This new crust then spreads outwards in both directions, away
from the mid-ocean ridge. “Seafloor spreading.”
o The Mid-Atlantic Ridge has been spreading new crust for 180 million years.
▪ The crust that makes up the ocean basins of the Earth is relatively
young (< 200 million years old), compared to the continental crust,
which is over 1 billion years old.
o New crust is created at mid-ocean ridges (divergent plate margins).