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Planetary Geology
Earth and other terrestrial worlds
Chapter 9
Planetary Geology
• Geology is the science that deal with Earth’s
physical structure, it history and processes
that act on it.
• An extension of this science is Planetary
geology, the extension of the study to other
solid bodies in the solar system
What are the terrestrial planets like inside?
Most of the terrestrial planet interiors are divided into three layers:
• Core
Mainly consist of high density material such as iron and nickel
• Mantle
This is the layer that surrounds the core. It is rocky material that consist of mineral
that contains silicon, oxygen and other elements
• Crust
This is the outer most layer. It is lowest-density rocky material such as granite and
basalt ( a common form of volcanic rock)
The interior of the planets are layered because the material was melted. The heavier
material sank towards the interior. Gravity was the force that drag the heavy
material and left the lighter material to remain at the top.
This process is called differentiation
All the terrestrial planets went trough the process of differentiation
The Earth’s metallic core consist of two distinctive regions: A solid inner core and a
molten (liquid) outer core
Interior structure of the terrestrial planets
Why is the Earth Differentiated?
•
•
•
Begins by accretion
– 4.6 Billion years ago (age of Sun)
Differentiation - The Earth melts
– 4.5 Billion years ago
Crustal Formation
– 3.7 Billion years ago
1,500 K
2,000 – 2,500 K
Melt iron
1/3 mass of Earth falls to
the center in the form of
molten iron  5000 K
(molten Earth)
What causes differentiation and geological activity
• Heat of accretion
As planetesimals collide and form a planet, the kinetic energy from the
collision is transformed in heat which add to the thermal energy of the
planet
• Heat from differentiation
In the process of differentiation, the dense material sink and the lighter
material rises. The gravitational potential energy is transformed into
thermal energy by friction
• Heat from radioactive decay
The material that built the planets contain radioactive isotopes of
elements such as uranium, potassium and thorium. These elements decay
into lighter elements releasing energetic subatomic particles that collide
with the material heating it.
Heating mechanisms for a terrestrial planet
Cooling mechanisms for the interior of a
terrestrial planet
Convection.
Hot material expands and rises
while cooler material contract
and falls
Conduction.
Transfer of heat from hot
material to cooler material
through contact
Radiation.
A planet will lose heat to space
through thermal radiation. A
planet acts like a black body
and emit radiation (light) .
Because of their low
temperature, planets radiate
primarily in IR
How do we know what is inside a planet?
The only planet for which we have information about the interior
is the Earth. But the information does not come from direct
sampling of the interior of the Earth
Earth’s radius is 6,378 km. Drilling only can go not more than a
few km (16 km).
It is possible to sample only a small upper layer of the crust!
For Earth, much of the information about the interior comes
from seismic waves (waves generated during an earthquake)
Seismic waves have been used to study the interior of the Moon
using monitoring stations left by the Apollo astronauts on the
surface
Seeing inside the Earth
• Drill a hole
– Petroleum geologists usually
drill to ~ 6 km
– Deepest hole: Kola Super
deep borehole (~ 12 km)
– Deepest we can go ~16
km
P (pressure) and S (shear) Wave analogy
How do we know anything about the interior of the Earth?
Seismology!
• Earth’s interior structure is probed by
studying how seismic waves travel
through it (we can only drill so far! – 16
km).
• Earthquakes generate seismic waves.
•P waves (pressure) can travel through
the liquid core but they are deflected by
the core
•S waves (shear) travel in the mantle
but not through the core
• Waves are reflected and refracted by
different materials and travel through
these materials at different speeds
(higher density material →faster speed).
The Earth interior
Using seismic wave and computer models it
is possible to model the interior
Earth radius = 6,378 km
• Mantle - 3000 km thick (80% of planet volume).
• Crust - 15 km thick (8 km under ocean - 20-50 km
under continents.
• Density and temperature increase with depth.
• High central density suggests the core is mostly nickel
and iron.
• There is a “jump” in density between the mantle and
the core caused by different materials.
• No jump in density between inner and outer core
because material is the same and just goes from liquid
to solid.
• The temperature in the core is about 5,000K and the
density about 12 g/cm^3
•(Reference: Density of water is 1 g/cm^3 or 1000
kg/m^3)
The surface area-to-volume ratio
Mathematical Insight 9.1
For a spherical object (planet), r is the radius of the
sphere:
Surface area = 4 π r²
Volume = 4/3 π r³
Surface area-to-volume ratio = Surface area/ volume
= (4 π r²) / (4/3 π r³)
= 3/r
The radius appears in the denominator. Larger bodies
have a lower surface area- to-volume ratios.
If two objects start with the same internal temperature, the
larger body cool off slower than the smaller body
Parameters that affect the geological history of a planet
Shaping the planetary surface
Process that shape a planetary surface:
• Impact cratering
Creation of bowl-shape impact craters by asteroids and comets striking
the surface
• Volcanism
Molten rocks or lava coming from the plantet’s interior in an eruption
• Tectonics
The disruption of the planet’s surface by the internals stresses
• Erosion
Wearing down or building up of features by effect of wind, rain, water and
ice
Shaping the crust: Impacts on Earth
Barringer meteor Crater
Located near Winslow,
Arizona
The best preserved impact
crater
Diameter 1.2 km
Age ~ 50,000 yr
Estimated impactor size
30-50 m
Estimated speed of
impactor ~ 26,000 miles/hr
Shaping the crust: Volcanoes
Mt. St Helen before and after 1980 eruption
Mauna Loa – 1984
Length ~ 75 miles
Covers ~ half Big Island of Hawaii
33 eruptions since 1850s
Shaping the crust: Cordon Caulle Volcano (Chile). June, 2011
Shaping the crust: Cordon Caulle volcano (Chile). June 2011
Shaping the Crust: Tectonics
• The ocean floors are continually moving, spreading from the
center, sinking at the edges, and being regenerated.
• Convection currents beneath the plates move the crustal
plates in different directions.
• The source of heat driving the convection currents is
radioactivity deep in the Earths mantle
Shaping the Crust: Erosion and Deposition
• Erosion (breaking down) and deposition (building up) require
the presence of a fluid (gas or liquid)
• Water, rain, wind cause erosion
River erosion created the
Grand Canyon
Deposition from the Mississippi
river created Louisiana
wetlands.
Geology of the Moon and Mercury
Mercury’s Surface
• Surface similar to the moon, large
number of impact craters!
•Old surface
•No indication of plate tectonics
•Craters have a flat bottom and have
thinner ejecta rims than lunar craters due
to higher gravity on Mercury than the
moon
•Craters not as dense as on the moon filled by volcanic activity
• Not dark features like the “maria” on
the Moon
• Caloris Basin, evidence of a large
impact crater. It is the largest crater on the
planet, about 1/2 Mercury radius
Mariner 10 image (Flyby in 19741975)
Mercury, an image of half of Caloris Basin
Image taken by Mariner 10
Mercury’s Surface
• Cliffs are seen on the surface
•This features are not seen on
the moon
•They appear to be about 4
billion years old
•They are not the result of
plate tectonics
•Probably the result of the
surface cooling, shrinking and
splitting at this time
•Some are several hundred
km long and has high as 3 km
high
•Cliffs may have formed
when tectonic forces
compressed the crust
Mercury
A recent image (false colors) taken by the Messenger spacecraft
The Messenger
spacecraft is at the
present in orbit around
Mercury. It went into
orbit in 2011
It is the first and only
spacecraft to orbit this
planet
A high resolution image of Mercury taken by the
Messenger mission
A detailed view of craters in high resolution image of
Mercury taken by the Messenger mission
Lunar Geological History
Moon surface
Energy from the formation caused at least the outer few kilometers to melt
(deep ocean of molten rock)
This took place about 3-4 billions years ago.
Molten rocks flooded the largest impact craters. Maria (singular mare) are
large impact craters flooded by lava. The dark color comes from dense, iron
rich basalt that rose from the lunar mantle
Lunar Surface , large scale features
Lack of atmosphere and water preserves surface features
Maria, singular Mare (younger) –
Highlands (older) – crust material
mantle material (maria means
• Elevated many km above maria
“seas’)
• Aluminum rich, low density (2,9
• Maria - darker areas resulting
g/cm3).
from earlier lava flow
• Basaltic, relatively iron rich, high
density (3,3 g/cm3).
Lunar Highlands
Highlands - light, rough,
heavily cratered
Older areas compared to
maria
Heavy cratered compared
with maria
Lunar Lowlands
Lowlands – dark, smooth
Maria
Basalt – fine grained dark
igneous rock rich in iron
and magnesium (stuff
that sank in magma
ocean)
Few hundred meters thick
Younger than the high
lands
Less craters compared
with highlands
The formation of Mare Humorum
Lunar maria and mountain ranges
Caucasus Mountain
↓
← Mare
Serenitatis
← Apollo 11
Mare Tranquillitatis
Moon Volcanism: Maria (Dark Areas)
Mare Imbrium
SW Mare Imbrium
The volcanism took placeafter the impacts – most 3 – 1 billion years ago
Rilles
Aristarchus Plateau
Marius Hills
Mountains
Montes Tenerife
Montes Appeninus
(Mons Huygens 5.5 km)
Montes Alpes- Mons Blanc 3.6 km
The Alpine valley is the long feature
to the left of the image
Lunar Erosion
Lunar Craters - caused by meteoroid impacts
• Pressure to the lunar surface heats the rock and deforms
the ground.
• Explosion pushes rock layers up and out.
• The ejecta blanket surrounds the crater
•It forms radial features around some craters. This are
called rayed craters
• In some cases, the material compressed bounces back
and form a crater with a central peak
• Craters can be up to 100 km in diameter
• A new 10 km crater is formed every 10 million years
• A new 1m crater is formed each month
• A new 1cm crater is formed every few minutes!
Cratering on the Moon
• The rate of cratering on the moon is determined from the known ages of the highland and
maria regions.
• The Moon (and solar system?) experienced a sharp drop in the rate of meteoritic
bombardment ~ 3.9 billion years ago.
• The rate of cratering has been roughly constant since that time.
• Only a few craters appear on the maria, showing that they are younger
•The highlands have a large concentration of craters
Tycho Crater ~ 100 MY old
Rays
Fresh rays, young feature
85 kilometers across
Wall
Floor
Rim
Central Peak
Lunar Impact Basins
Imbrium Rim
Orientale Basin (Far side of the Moon)
Big, frequent impacts until 3.8 billion years ago
Impact events continue on all moons and planets today but at a much lower rate!
Geology of Mars
• Main geological features
• Evidence of flow of water
Major Martian
geographical feature
Tharsis Bulge
Valles Marineris
•Roughly the size of North America
•Lies on the equator - 10km high
•The Grand Canyon would fit into one of its
side "tributary" cracks
•NOT caused by water - probably due to
stretching and cracking when Tharsis bulge
formed
•Less heavily cratered = Young (2-3
billion yrs)
• Valley that extends one-fifth of the way
around the planet(!) at the equator
•Up to 120 km across and 7 km deep
Map from
Mars Global
Surveyor 2001
Valles Marineris
120 km across and at least 2,000 km long
Mars - Telescopic Exploration
Giovanni Schiaparelli (1835-1910)
•Mapped bright and dark regions
•Saw polar caps which changed with seasons
•Surface colors appeared to change - plant life?
•Identified long narrow features he called them
canali (grooves, channels). It was translated as
canals…
Percival Lowell (1855 -1916)
•Built an observatory in 1894 in Flagstaff AZ to
study Mars (now Lowell Observatory)
•He purchase and used a 24” Alvan Clark
refractor for his observations
• He thought that canals were used by a
civilization to bring water from the poles to the
equatorial desert . He argued that canal were the
work of an intelligent civilization on Mars
•Images taken by the Mariner and Viking space
craft about a century later (1960”s) prove that
there were no canals
Map of Mars by Schiaparelli (1988)
Volcanism on Mars
• The largest volcanoes in the solar system are found in Mars
•Shield volcanoes
•None are known to be currently active but eruptions occurred 100
million years ago
Olympus Mons on Tharsis slope
Its base is about 600 km across. Its
peak stands about 26 km above the
average surface level (about three
time as high as Mount Everest)
Mars has a surface gravity only 40
percent that of Earth, and its volcanoes
rise roughly 2.5 times as high because of
this.
Liquid Water on Mars?
Yes
About 4 billion years ago Mars had a thicker
atmosphere, warmer surface, and liquid
water.
– Massive flow long ago
Some recent evidence of minor water flow
Runoff channels
•Found in southern highlands
•Extensive river systems (like Earth)
•Carried water from highland to valleys
Mars
Earth
Outflow channels
•Caused by flooding
•Found at the equator
•Formed about 3 billions year ago
A high resolution image of a Mars runoff channel
Evidence of flow of liquid water
Ridges of crater suggest the impact debris was muddy. This
crater was probably made by an impact on icy ground
More evidence of flow of liquid water on Mars
Another image showing the erosion caused by flow of
water in Mars.
The Oraibis crater is about 32 km across. The bottom is filled with
sediments.
Image was taken by the Mars Express in May, 2011
What happened to the water?
• Liquid water (from runoff channels) froze into permafrost (water ice just below
surface) and polar caps about 4 billion years ago
• After ~1 billion years, volcanic activity heated the surface and melted permafrost
• Flash floods created outflow channels
• Volcanic activity slowed after that and the liquid water refroze
• Mars Global Surveyor in 2000 revealed “gullies” along the insides of craters evidence for more recent existence of liquid water?
Evidence of salty water running down the slopes of rims of
craters (gullies).
Salty water may stay in liquid phase for a short time due to its lower freezing point
Polar Lander Phoenix
(Phoenix landed on Mars on May 25, 2008)
More evidence of frozen water under the surface
A trench carved by the
scoop of Phoenix
lander (about 10 cm
deep).
The white material
inside the trench is
water ice that melted
(or sublimated) soon
after being exposed
Mars most recent mission: rover Curiosity
•Curiosity landed on
Mars on August 5th,
2012 inside the Gale
crater
•The area may have
been flooded by fastmoving water
•A good site to look for
possible organic
material which may be
indication of fossil
bacterial life
An image send by Curiosity (left) showing evidence of flowing of
liquid water. Comparison with deposit of rocks in a dry river bed
on Earth
The Face on Mars
In 1976, Viking orbiter 1 images revealed a mountain that looked somewhat
like a human face
•Commented on in a NASA press release
•Immediately seized by tabloids
•Taken by some as proof that an
advanced civilization existed on Mars
•More likely just how our minds find
familiar patterns
•Below is a high resolution image taken
a few years ago by the Mars Orbiter
spacecraft
Geology of Venus
The thick cloud cover of the planet completely hide the surface of Venus.
No visual images from the surface of the planet are available.
All the images shown here (except for the image from Venera) are radar images .
Radar: Radio waves are emitted towards the surface , they bounce back and are
received by the spacecraft
Magellan spacecraft (1990-1994) used radar to map the surface of Venus. This
were the first look at the surface of the planet.
Major geological features
 Impact craters
Venus has only a few impact craters. Most of the ancient craters have been erased.
The few craters are large, there are no small craters suggesting that small bodies may burn
in the thick atmosphere before they reach the surface.
Geology of Venus
Major geological features
 Volcanic and tectonic features
Evidence of volcanism is clear. Several volcanoes were imaged by the radar.
There is evidence of lava flow, lava plains and volcanic mountains. The radar
images do not show active volcanoes or eruptions. The presence of sulfuric
acid clouds suggest that outgassing is replenishing the atmosphere of sulfur.
Sulfur dioxide is removed from the atmosphere by chemical reaction with rock.
This suggest that outgassing must still occur at least in geological time scales
(millions of years)
 Weak erosion
The images returned by the Venera mission does not show evidence of erosion. Since the
temperature is high, there is no precipitation. Wind must be very slow at the surface.
 No evidence of plate tectonics. All the surface seems to have the same age
everywhere. Radar maps of the surface doesn’t show the types of plates we find on the
Earth.
Venus’s Surface
Radar (radio waves) echoes reveal the surface topology (Magellan spacecraft)
There are no direct images of the surface. The planets is always
covered by thick clouds. Only radio waves can penetrate the clouds
•Elevated “continents” make up 8% of the surface (25% on Earth)
•Mostly rolling plains with some mountains (up to 14 km)
•No indication of plate tectonics
•Buckled and fractured crust with numerous lava flows
Venus’s Surface: Volcanoes and Craters
Images from the Magellan spacecraft (1990-1994)
•Volcanoes resurface the planet every
~300 million years
•Shield volcanoes are the most
common (like Hawaiian Islands)
• A caldera (crater) is formed at the
summit when the underlying lava
withdraws
•Largest volcanic structures are called
coronae - upwelling in the mantle which
causes the surface to bulge out - not a fullfledged volcano.
•Usually surrounded by other volcanoes
•Venus is thought to still be volcanically
active today. Magellan radar images did not
show any active volcano.
Radar image of a volcano with lava flow taken by
Magellan spacecraft
A 3-D view of three volcanoes taken by Magellan
spacecraft (radar images)
Radar images taken by Magellan of “pancake” volcanoes
An image taken by the soviet spacecraft Venera. The spacecrafts
Venera landed on the surface of Venus in 1970s
• The spacecraft had a very short life time on the surface, survived only an hour
before the heat damaged the equipment.
•No other spacecraft have landed on the surface of Venus after the Venera
•Little evidence of erosion - young surface
•Rocks are basaltic and granite
• The thick cloud cover makes Venus seem like a heavily overcast day on Earth
all the time!
The unique geology of Earth
• Plate tectonics
• How was the earth surface shaped by
plate tectonics?
Overview of the Earth
•
•
•
•
•
Dense rocky composition
Evolving surface < 600 Myr old
Atmosphere mostly Nitrogen and oxygen
Oceans of Water
Magnetic Field
Mass
6 x 1024 kg
Radius
6400 km
Density
5.5 g/cm3
Surface gravity
9.8 m/s2
Escape Speed
11 km/s
The Earth Interior
• The Earth is very dense: 5.5 g/cm3
– Reference: water 1 g/cm3
– Normal Rocks 2-4 g/cm3
– Pure Iron 7.8 g/cm3
The Earth’s interior is
differentiated (distinct layers –
densest sink, lightest floats)
 At some point in our past,
Earth melted
 bombardment of debris
 radioactivity
The Earth Interior
Layer
Comp
Type
Thickness
Temp
Core
Iron & Nickel
Solid/liquid
3500 km
>6000 K
Mantle
Silicates
Plastic
2900 km
1300 – 3800 K
Crust
Granite & Basalt
Solid
10-1100 km <1000 K
Plate boundaries
The motion of the Earth surface
•
•
•
•
•
•
The plate tectonic theory explain how the plates moves and drift as they
“float” on top of the mantle (Asthenosphere)
The theory was proposed in 1912 by the German scientist Wegener. The
idea was that continents drift across the surface of the Earth.
But at that time, nobody knew about a mechanism that will allow the
continents to move.
But today there is more evidence to back up the theory and it is widely
accepted.
The discovery of mid-ocean ridges and the seafloor spreading in the
1950’s provided more evidence.
Today the continental motion is explained by convection, driven by heat
released by the Earth interior
Seafloor spreading
Convection pushed the material up into the ridges in the ocean floor
separating the two plates
Subduction pushes denser seafloor crust material under layers of less
dense continental crust
California’s San Andreas fault
• The San Andreas fault
is the boundary where
two plates are sliding
sideways
• It is very active
• The dates mark the
most recent earthquakes
• The lines in the inset
show how far the two
sides of the fault shift
moved in an earthquake
Where do we find volcanoes?
An example are the Hawaiian islands
Shield volcanoes are
formed by “Hot Spots”.
As the plate moves
over the “hot spot”
multiple volcanoes are
formed.
Stratovolcanoes typically where one
plate descends beneath an adjacent
plate.
Hawaiian Island hot spot
Geological history of the terrestrial planets
Continental Drift
Past present and future of Earth’s continents
About 200 millions years ago all the continents were together in
a single “supercontinent” called Pangaea
Production of magnetic fields in planets
Creation of magnetic field in planets:
An interior of electrically conducting fluid (Liquid). Charged particles move with the molten or
conductive material
Convection in that layer
Rapid rotation
The Earth meet all the requirements
Venus has a very slow rotation rate (-243 days), no magnetic field
Mars may retain enough heat but not enough to have convection
Mercury is a mystery: It has some magnetic field but has a slow rotation (58.6 days). It is possible that
it has a molten core with convection (or the magnetic field could be relics of ancient magnetic field
frozen in the core?)
How are the planetary magnetic fields
generated?
– metallic liquid regions plus rotation of planet
– Jupiter & Saturn: liquid metallic
– Uranus & Neptune: near surface convecting
ices?
Earth Magnetic Field
• Earth’s magnetic field protect us from energetic particles in the solar
wind and cosmic rays.
• These particles do not strike directly the surface, they are trapped in the
magnetic field lines.
Auroras
• Some charged particles from the solar wind get trapped in the Earth’s
magnetic field lines. They will spiral toward the magnetic poles and
precipitate in the atmosphere releasing energy .
• That energy excite the atoms in the upper part of the atmosphere and
they emit light. The emission is line emission. The colors of the aurora
depends on the atoms being excited.