Download planetary geology

10/12/2011
Planetary Geology
The Terrestrial Planets
The Jovian Planets
Recall…
• Formation of the planets
– Materials condense out of the protoplanetary disk
• Heavy materials closer to the Sun, lighter materials
outside the frost line
– Small grains stick together to form planetesimals
– Planetesimals collide and grow
• Only the most massive survive
– Large planetesimals outside the frost line begin
to accumulate gases/ices
• Why the large disparity?
Planetary Geology
• All terrestrial surfaces must have started out
roughly the same.
– All made out of rocky materials that condensed
out of the solar nebula
– Only difference is temperature
• Planetary surfaces are not unchanging.
– Earthquakes, erosion, impacts
– (Only terrestrial planets)
The Terrestrial Planets
Terrestrial Surfaces
Planetary Geology
• The study of how these planetary surfaces
came to be is called planetary geology
– geo for Earth
– Planetary for other worlds (including moons)
Terrestrial Interiors
• The terrestrial planets look very different on
the outside, but what about the inside?
– Most geological processes take place beneath
the surface
– Look here for clues
• For Earth, most information about the
interior comes from seismic waves.
– Vibrations that travel through the interior and
along the surface after an Earthquake
– Use seismic waves and a few other clues to build
a picture
Terrestrial interiors
• The terrestrial planets all have layered
interiors.
• Earth’s core has two distinct regions
– Liquid outer core
– Solid inner core
Layering by Density
• We can understand this layering by
considering the density of the materials
– Metals are the most dense
– Materials in the mantle are of middling density
– Materials in the lithosphere are the least dense
• This separation by density is called
differentiation and gives us a clue to the
histories of the planets.
– All of the terrestrial planets had to have been hot
enough in their past to be entirely molten.
Terrestrial Interiors
• Comparisons of the interiors of planets can
also give us clues.
• The composition of the solar nebula was
roughly homogeneous during the epoch of
planetary formation.
– Smaller planets should have smaller cores
proportional to their size.
Terrestrial Interiors
Terrestrial Interiors
• Comparisons of the interiors of planets can
also give us clues.
• The composition of the solar nebula was
roughly homogeneous during the epoch of
planetary formation.
– Smaller planets should have smaller cores
proportional to their size.
– Mercury and the Moon don’t follow the rule.
Layering by Strength
• Planetary interiors today are almost entirely
solid.
• It is also useful to categorize the layers of
planets by strength.
– Not all rock is the same strength.
• Rock is similar to silly putty
– Flows slowly over time, breaks when pulled
sharply
– Flows more easily when warmer
Layering by Strength
• A planet’s outer layer is composed of cool,
rigid rock called the lithosphere.
– Notice that the thickness of the lithosphere is
dependent on the size of the planet.
• Beneath the lithosphere is the warmer,
partially molten mantle.
– Warmer = softer
– The lithosphere essentially floats on the mantle
• At the very center is the solid, metallic core.
– Why is it solid?
Why are Big Worlds Round?
• The ability of rock to deform and flow can
also explain the shapes of the planets
• Notice that small moons and asteroids are
potato-shaped.
• The higher gravity
of larger worlds slowly
deforms the rock into
a sphere.
– 1 billion years for 500 km
– Faster for larger bodies.
Geological Activity
• Geological activity describes the ongoing
process of changes to a planet’s surface.
– Different planets have varying amounts of
geological activity.
• Interior heat is the primary driver of
geological activity.
Interior Heating
• Heat must have come from somewhere.
– Not the Sun
• Three processes for heating interiors
– Accretion
– Differentiation
– Radioactive decay
• Accretion and differentiation deposited heat
very early
– Radioactive decay heats the interior today
Interior Heating
• The combination of these methods of
heating explains the layering of planetary
interiors.
– Accretion heated the surfaces of the planets to
the point of melting.
– Differentiation began, which released more heat.
– This along with heat from radioactive decay
allowed interiors to differentiate.
Cooling Off
• All of this heat must go somewhere, so how
do planets cool off?
– Convection
• Transfer of energy through movement
– Conduction
• Transfer of energy through contact
– Radiation
• Transfer of energy by photons
• Convection is the most important cooling
process for the Earth
Cooling Down
• The ongoing
process of
convection creates
convection cells.
– Material progresses
slowly through the
mantle (1cm/year)
– Stops at the
lithosphere
Cooling Down
• The biggest factor in the cooling rate is the
size of the planet.
– Volume increases as the cube of the radius
– Surface area increases as the square of the
radius
• The larger the planet, the large the volumeto-surface-area ratio.
– Smaller planets cool off more slowly.
• Once a planet’s interior cools off enough to
halt convection, it is geologically “dead”.
Cooling Down
Magnetic Field
• Interior heating is
also related to
magnetic fields.
• Earth’s magnetic
field resembles a
bar magnet or
electromagnet.
Magnetic Field
• Electrons in molten metal are carried along
by convection
• Rotation of Earth distorts the convection
pattern
• Result is that electrons in the molten metal
move like electrons in an electromagnet.
Magnetic Field
• Three requirements
– Interior with electrically conducting fluid (either
liquid or gas), such as molten metal
– Convection
– At least moderately rapid rotation
• Earth is the only terrestrial planet that meets
all three requirements.
– Moon and Mars: no or little convection
– Venus: slow rotation
– Mercury: magnetic field despite small size and
slow rotation
Next time…
Quiz 6
1) Energy in the radiative zone of the Sun is carried
by
a)
b)
c)
d)
Bulk motions of gas
Photons
The magnetic field
Sunspots
2) The primary factor for the cooling rate of a planet
is
a)
b)
c)
d)
Rate of convection
Size of the planet
Thickness of the lithosphere
Size of the core