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Lecture

9.1-Connecting Planetary Interiors and Surfaces


Department
Astronomy & Astrophysics
Course Code
AST101H1
Professor
C.B.Netterfield

Page:
of 3
CHAPTER NINE: PLANETARY GEOLOGY
9.1 Connecting Planetary Interiors and Surfaces
-the great differences in the present day appearance of the 5 terrestrial planets must be the result of
changes that have occurred through time
-volcanoes, earthquakes, asteroid or comets slamming into Earth, erosion by wind rain or ice, etc.
-Mercury and Moon Æ scars of their battering during the heavy bombardment
Ædensely covered by craters
-Venus Æ bizarre bulges and odd volcanoes dot the surface of Venus
-Mars Æ has the solar system·s largest volcanoes and a huge canyon
Æ numerous features that appear to have been shaped by running water
-Earth Æ similar to all those on the other terrestrial worlds including a unique layer of living organisms
that covers almost the entire surface
Inside of Terrestrial Planets
-our most detailed information comes from seismic waves, vibrations that travel both through Earth·s
interior and along its surface after an earthquake
Layering by Density
-Core Æ the highest-density material
Æ consists primarily of metals such as nickel and iron
Æ resides in the centre of the core
Æ consists of two distinct regions; a solid inner core and a molten outer core
-Mantle Æ rocky material of moderate density
Æ mostly minerals that contain silicon, oxygen and other elements
Æ forms thick mantle that surrounds the core
-Crust Æ the lowest-density rock, granite and basalt
Æ forms the thin crust, essentially representing the world·s outer skin
-Mercury has a large core
Æformed in the region of the solar nebula where the planetesimals should have been most metal-
rich
Æ may have also suffered a giant impact that blasted away much of its original mantle and crust
-Moon has a small core
Æa small core is expected if the Moon really did form from debris blasted out of Earth·s outer
layers since that debris would have contained very little high-density metal
-all worlds underwent differentiation at some time in the past, which means all these worlds must once
have been hot enough inside for the interior rock and meta to melt and separate by density
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Layering by Strength: The Lithosphere
-rocks are the very definition of strength, but rocks can deform and flow
-weak gravity of a small object is unable to overcome the rigidity of its rocky material, so the shape
stays
-strong gravity of a larger object can overcome the strength of solid rock, slowly deforming and
molding it into a spherical shape (pulling slow)
-Lithosphere: a planet·s outer layer that consists of relatively cool and rigid rock
-floats on warmer, softer rock beneath
-generally encompasses crust and part of the mantle
-a thin lithosphere is brittle and can crack easily
-a thick lithosphere is much stronger and can prevent volcanic eruptions and creation of mountain
ranges
-smaller planets tend to have thicker lithospheres
Geological Activity
-used to describe how much ongoing change occurs on their surfaces
-in contrast to Earth, the Moon and Mercury have virtually no geological activity
-Earth has things like volcanic eruptions, earthquakes, erosion
-interior heat is the primary driver of geological activity
-volcanoes can erupt on Earth because Earth is quite hot inside
-the Moon does not have any active volcanoes because its interiors is too cool to melt rock and push
it to the surface
How Interiors Get Hot
-sunlight is the primary heat source for the surfaces of the planets, but virtually n one of the solar
energy penetrates more than a few metres into the ground
Heat of Accretion
-accretion deposits energy brought in by colliding planetesimals
-an incoming planetesimals has a lot of gravitational potential energy and as it approaches, its
gravitational potential energy converts into kinetic energy, causing it to accelerate
-much of the kinetic energy is converted to heat, adding to the thermal energy of the planet
Heat from Differentiation
-differentiation also converts gravitational potential energy into thermal energy
-the process of denser materials sinking and less dense materials rising adds mass to the planet·s core
and reduces the mass of outer layers.
-must be a mix of materials of different density and materials inside must be able to flow
-much of the planets· mass effectively moves inward Æ losing gravitational potential energy
-the friction converts the lost gravitational potential energy to thermal energy, heating the interior
Heat from Radioactive Decay
-radioactive decay affects terrestrial worlds because the rock and metal planetesimals that built them
contained radioactive isotopes
-when radioactive nuclei decay, subatomic particles fly off, colliding with neighbouring atoms and
heating them
-the only one that continues to heat the terrestrial planets
to this day
-the other two happened when the planets were young
How Interiors Cool Off
Convections
-transports heat upward when hot material expands and
rises while cooler material contracts and falls
-hot rock rises and cooler rock falls in a mantle convection
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cell
-most important process in Earth·s deep interior
Conductions
-process in which hot material transfers heat to cooler material through contact
-occurs through microscopic collisions of many individual atoms or molecules
-carries heat through the rigid lithosphere to the surface
Radiation
-at the surface, energy is radiated into space
-an object can also lose heat through radiation
-convection and conduction transports interior heat toward a planet·s surface, and through radiation,
loses heat to space
Role of Planetary Size (more later)
-Size is the single most important factor in planetary cooling
-a large planet can stay hot inside much longer than a small one
-Moon and Mercurys· interiors probably cooled off way before, causing their lithospheres to thicken
and mantle convection was confined to deeper and deeper layers
-and now they are geologically ´deadµ (no more heat-driven geological activity)
-can trace interior heat, heat transport and geological activity back to planetary size
Magnetic Fields
-some planetary interiors create magnetic fields (ie. Earth)
-Earth·s magnetic field is generated by a process more similar to that of an electromagnet, (the
magnetic field arises as a battery forces charged particles to move along a coiled wire)
-Earth has no battery, but charged particles move with the molten metals in its liquid outer core
-internal heat Æ liquid metals rises and falls (convection) Æ Earth·s rotation twists and distorts the
convection pattern = the result: electrons in the molten metals move within Earth·s outer core,
generating Earth·s magnetic field.
-basic requirements
1-an interior region of electrically conducting fluid, such as molten metal
2-convection in that layer of fluid
3-at least moderately rapid rotation
-Moon; no ²Mars; no ²Venus; probably has a molten core layer, but not enough convection to generate a
magnetic field ²Mercury; an enigma Æ possess a measurable magnetic field despite its small size
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