Lecture 22
Terrestrial Planets
Mercury
Venus
Earth
What are they like? Why?
Mars
Terrestrial planets are mostly made of rocky
materials (with some metals) that can deform
and flow over time.
Likewise, the larger moons of the Jovian planets
are made largely of icy materials (with some
rocks and metals) that can deform and flow.
The ability to deform and flow leads every object
exceeding approximately 500 km in diameter to
become spherical under the influence of gravity.
Early in their existence, the Terrestrial
planets and the large moons had an
extended period when they were
mostly molten.
The heating that led to this condition
was caused by impacts, where the
kinetic energy of the impacting
material was converted to thermal
energy.
Today, the interiors of planets are heated mainly by
radioactive decay.
The heating to a molten state, and
subsequent cooling, had important
repercussions for:
 interior structure of the planet,
 surface features,
 atmosphere,
 magnetic fields,
 presence/absence of water.
Differentiation – the
process by which
gravity separates
materials according to
their densities
Denser materials sink,
less dense material
“float” towards top
Differentiation during the molten phase
resulted in the formation of three distinct
density zones within each terrestrial world:
Core - contains metals (e.g., iron, nickel)
Mantle – intermediate layer with rocky material
(sometimes partially semi-molten)
Crust – lowest-density rocks (surface)
Terrestrial planets have metallic
cores (which may or may not be
molten) & rocky mantles
Earth (solid inner,
molten outer core)
Earth’s interior structure
Mercury (solid core)
Differentiated Jovian moons have
rocky cores & icy mantles
Europa
Io
Ganymeade
Callisto
The Lithosphere…
Layer of rigid rock (crust plus upper mantle) that
floats on softer (mantle) rock below
While interior rock is mostly solid, high pressures
and stresses can cause rock to deform and flow
(think of silly putty)
This is why we have spherical planets/moons
The interiors of the terrestrial planets slowly
cool as their heat escapes.
This cooling gradually makes the lithosphere thicker
and moves molten rocks deeper.
Larger planets take longer to cool, and thus larger
planets:
1) retain molten cores longer
2) have thinner, and thus weaker, lithospheres
The stronger (thicker) the lithosphere, the less
geological activity the planet exhibits.
Planets with cooler interiors have thicker
lithospheres.
lithospheres of the Terrestrial planets:
Stresses in the lithosphere lead to “geological
activity” (e.g., volcanoes, mountains,
earthquakes, rifts, …) and, through outgassing, leads to the formation and
maintenance of atmospheres.
Cooling of planetary interiors (energy
transported from the planetary interior to the
surface) creates these stresses
Convection - the transfer
of thermal energy in which
hot material expands and
rises while cooler material
contracts and falls (e.g.,
boiling water).
Convection is the main cooling process for
planets with warm interiors.
Larger planets stay hot longer.
Earth and Venus (larger) have continued to cool
over the lifetime of the solar system  thin
lithosphere, lots of geological activity
Mercury, Mars and Moon (smaller) have cooled
earlier  thicker lithospheres, little to no
geological activity
Under what circumstances can differentiation occur in a
planet?
red) The planet must have a molten interior.
blue) The planet must be made of both metal and rock.
orange) The planet must be geologically active, that is,
have volcanoes, planet-quakes, and erosion from
weather.
green) The planet must have a rocky surface.
Under what circumstances can differentiation occur in a
planet?
red) The planet must have a molten interior.
Which internal energy source is the most
important in continuing to heat the
terrestrial planets today?
red) differentiation
blue) tidal heating
orange) accretion
green) radioactivity
Which internal energy source is the most
important in continuing to heat the
terrestrial planets today?
green) radioactivity
Heat escapes from the planet's surface into
space by thermal radiation. Planets
radiate almost entirely in the wavelength
range of the
red) infrared.
blue) visible.
yellow) radio.
green) ultraviolet.
orange) none of the above
Heat escapes from the planet's surface into
space by thermal radiation. Planets
radiate almost entirely in the wavelength
range of the
red) infrared.
Side effect of hot interiors - global
planetary magnetic fields
Requirements:
1. Interior region of electrically conducting
fluid (e.g., molten iron)
2. Convection in this fluid layer
3. “rapid” rotation
Earth fits
requirements
Venus rotates too
slowly
Mercury, Mars &
the Moon lack
molten metallic
cores
Sun has strong
field
Planetary Surfaces
4 major processes affect planetary surfaces:
Impact cratering – from collisions with asteroids
and comets
Volcanism – eruption of molten rocks
Tectonics – disruption of a planet's surface by
internal stresses
Erosion – wearing down or building up
geological feature by wind, water, ice, etc.
Impact Cratering: The most common
geological process shaping the surfaces of
rigid objects in the solar system (Terrestrial
planets, moon, asteroids)
Volcanism
Volcanoes help erase impact craters
Volcanic outgassing:
source of atmospheres
and water
Erosion: the breakdown and transport of rocks and
soil by an atmosphere.
 Wind, rain, rivers, glaciers contribute to erosion.
 Erosion can build new formations: sand dunes,
river deltas, deep valleys).
 Erosion is significant only on planets with
substantial atmospheres.
Tectonics: the action of internal forces and
stresses on the lithosphere leading to the
creation of surface features & geological activity.
Tectonics can only
occur on planets
with convection in
the mantle (Earth
& Venus today,
some icey Jovian
moons)
Tectonics…
raises mountains
creates huge valleys (rifts) and cliffs
creates new crust
moves large segments of the
lithosphere (plate tectonics)
Tectonic plates
divergent plate boundary
(plates move away from each
other).
 Atlantic Ocean
 Great Rift Valley in Africa
 Valles Marineris (Mars)
Portion of Valles
Marineris on Mars
It was created by
tectonic stresses
during formation of the
Tharsis Bulge
convergent plate boundary with subduction : plates move
towards each other & one slides beneath the other.
 Nazca plate being subducted under the South American plate
to form the Andes Mountain Chain.
 Island arc system
convergent plate boundary without subduction : plates
move towards each other and compress.
 Formation of Himalayas.
Plates sliding past each other: earthquakes,
valleys, mountain building
Half of the world’s volcanoes surround the Pacific plate
Tectonic plates