Download Terrestrial Planets

Earth and the
Geology of
the
Terrestrial
Planets
(Bennett et al. Ch. 9)
Major Ideas In This Chapter
●
Terrestrial planets looked (largely) the same when they
were formed. Differences due to geological processes.
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Geological activity is driven by internal heat
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Planetary size plays a large role in retaining heat
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Distance from the Sun, rotation affects erosion
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Crater density can indicate surface age
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Earth has a unique geology
Terrestrial Planets
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Compared to Jovian planets:
–
Smaller size/mass
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Large “core” to atmosphere ratio
–
Higher density
–
Closer to Sun and closer together
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Warmer
–
Few or no moons
–
No rings
(NASA)
Planetary Surfaces and Interiors
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Terrestrial planets + Moon were similar when young
–
Subjected to heavy bombardment
–
Differences due to processes that occurred after formation
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Understanding the surface features: planetary geology
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Processes in the interior drive activity at the surface
Your book uses “terrestrial worlds” to refer to the
terrestrial planets + the Moon.
(from Bennett et al.)
(from Bennett et al.)
How Do We Learn About
Planetary Interiors?
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Average density determinations
Local gravity variations as measured with artificial satellites
Magnetic fields: molten core/convection
Lava flow: internal composition
Earthquakes: internal structure
Earthquakes: Seismic Waves
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Earthquakes generate vibrations
–
Typical wavelength ~ several
km
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Reconstruct interior
Two types of waves:
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P-waves: compressional waves
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S-waves: shear waves
S-waves cannot pass through
liquid
–
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Earth's interior has liquid layer
(from Bennett et al.)
Monitoring also done on the Moon
(from Morrison and Owen)
Interior Structure of Terrestrial
Planets
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crust
Density stratification
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Core
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Mantle
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–
iron, nickel
Earth has liquid outer core
core
Rocky layer (minerals with
silicon, oxygen, ...)
Crust
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mantle
granite, basalt
lithosphere
Interior Strength
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crust
Most of earth's interior: solid rock
–
Rock varies in strength
–
Can deform and flow
Lithosphere
Below lithosphere: higher T →
rock flows easier
core
Lithosphere “floats” on the soft
rock below
mantle
Thickness important
lithosphere
How does lithosphere thickness affect volcanic eruptions/mountain
formation?
Why Layering?
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Differentiation
–
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Gravitational separation of materials with different densities
Interiors were hot initially → rock/metal molten
Why are planets round?
Phobos and Deimos
(the moons of Mars)
(NASA)
Planetary Interiors
(from Bennett et al.)
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We expect smaller planets to have smaller cores
–
Mercury?
–
Moon?
Small planets = thicker lithospheres
What Drives Geological Activity?
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Heat
–
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In general: bigger = more heat
How do we heat?
–
Accretion
–
Differentiation
–
Radioactivity
Which of these processes is still taking place in terrestrial planets?
What about sunlight?
These processes result in the core/mantle/crust structure
What Drives Geological Activity?
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How do we cool?
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Convection
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Conduction
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Radiation
Example: Earth:
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Convection in interior (flowing solid rock)
–
Above lithosphere, too rigid to flow—conduction takes
over
–
At surface: radiation
What Drives Geological Activity?
(from Bennett et al.)
Planetary Size
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Larger planets remain hotter longer
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Mercury/Moon
●
–
Cooled quickly (~ 1 billion years)
–
Lithosphere thickens, mantle convection stops
–
Geologically dead
Venus
–
●
Similar in size to Earth, so probably still active
Mars
–
Cooled more—unclear if the deep interior is still
convecting
Cooling Terrestrial Planets Interiors
Total store of heat is proportion to the planet's volume,
Energy is only lost through the surface—rate of energy
loss is proportion to the surface area of the planet,
Cooling time is related to the total amount of heat/energy
stored / rate of energy loss (volume to surface ratio)
Planetary Cores and Magnetic
Fields
●
●
Magnetic fields are generated in some planets
What is needed to generate a
magnetic field?
B-field
Mercury
Venus
Earth
Moon
Mars
yes
no
yes
no
no
why?
large metal core (despite slow rotation)
rotation too slow
molten rock
cooled off
no metallic core or cooled
Planetary Cores and Magnetic
Fields
(from Bennett et al.)
Shaping Surfaces
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Impact Cratering
–
More small than large craters
–
All terrestrial planets had impacts
–
Impact at 40,000 to 250,000 km/h
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Craters are circular
D ~ 10x impactor size
Depth ~ 10-20% diameter
Sometimes: central peak
Tycho crater on
the Moon ( NASA)
(from Bennett et al.)
Impact Craters
Shaping Surfaces
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Volcanism
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The eruption of molten lava onto surface
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Magma rises: lower density / trapped gases / squeezed
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Result depends on how easily lava flows
(from Bennett et al.)
Shaping Surfaces
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Volcanism (cont.)
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Volcanic plains and shield volcanoes made of basalt (high
density, but runny)
●
all terrestrial planets and some Jovian moons show volcanic
plains or shield volcanoes—basalt common
–
Stratovolcanoes made of lower-density rock—rare outside
of Earth.
–
Volcanoes outgas atmospheres
●
Atmospheres of Venus, Earth, and Mars, and Earth's
oceans came from outgassing
Solar System Volcanos
The Culann Patera volcano on Jupiter's
moon Io (Galileo Project, JPL, NASA)
Olympus Mons on Mars—the
largest volcano in the solar
system (Mars Global Surveyor Project,
MSSS, JPL, NASA)
Shaping Surfaces
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Tectonics
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Surface changes due to forces acting on lithosphere
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Most tectonic features arise from mantle convection
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●
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Compression features
Cracks and valleys
Fractured lithosphere → plate tectonics
(from Bennett et al.)
Shaping Surfaces
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Erosion
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Breakdown/transport of rock
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Glaciers
Rivers
Wind
...
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Erosion can build (sand dunes, river deltas, ...)
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Erosion makes sedimentary rock
Effect of Planetary Properties
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Volcanism/Tectonics
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Requires internal heat → planetary size matters
●
Which planets had volcanism/tectonics initially?
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Moon/Mercury already cooled
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Earth large → still active
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Venus similar to Earth → still active?
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Mars should be cooler inside, much less activity than past
Effect of Planetary Properties
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Erosion
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Requires weather (wind, rain, ...)
–
How does planetary size affect an atmosphere?
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Distance from Sun (how does this affect things?)
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Rotation (why?)
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Moon/Mercury: no atmosphere → no erosion
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Mars: thin atmosphere → little erosion
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Venus/Earth: thick atmospheres
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Earth cooler: oceans form. Still lots of erosion.
Venus slow rotator: little erosion
(from Bennett et al.)
(from Bennett et al.)
Impact Craters and Age
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All planets impacted during
heavy bombardment
–
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Old surface = high crater
density
Age ~ 4.4 billion years
Maria
–
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highlands
Lunar highlands
–
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maria
Age ~ 3.0 – 3.9 billion years
Heavy bombardment ended ~4
billion years ago
Impact history on moon applies
to other planets
Crater counts → geological age
Apollo 16 image of the moon (mostly far
side) (NASA)
Geology of the Moon and Mercury
Apollo 16 image of the moon
(mostly far side) (NASA)
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MESSENGER image of Mercury
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1
&gallery_id=2&image_id=143
Credit: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington
Many craters
Cooled long ago—little recent tectonic/volcanic activity
No atmospheres—no erosion
Ancient volcanic features—active when young
Terrestrial Planets
(from Bennett et al.)
Mercury
Venus
Earth
Moon
Mars
Mass
0.055 M
0.815 M
1.0 M
0.012 M
0.107 M
Radius
0.382 R
0.949 R
1.0 R
0.272 R
0.533 R
Density
5.43 g cm-3
5.25 g cm-3
5.52 g cm-3
3.34 g cm-3
3.93 g cm-3
Overview of the Moon
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Smallest of the terrestrial
worlds
Heavily cratered highlands
Smooth maria: lava plains
Some tectonic features
No erosion
Geologically dead today.
Crater counts, calibrated on
the Moon, allow us to
determine geological age
Apollo 15 image of Mare Imbrium (NASA;
http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=863)
Geology of Moon
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Highlands: bright, heavily cratered
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Maria: smooth, dark regions
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Craters should be roughly uniform—what happened in Maria?
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Lava very runny—lack of water/trapped gases
(NASA/Apollo 17)
(Luc Viatour/Wikipedia)
Geology of Moon
(from Bennett et al.)
Geology of Moon
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Tectonic features found in
maria
–
Contraction during
cooling (graben)
Apollo 15 image of Mare Imbrium (NASA;
http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=863)
High iron content in the Maria
Source: Clementine Project database
Maria are composed of basalt. Lunar basalt has a higher iron content than on earth.
(NASA/LPI)
Lunar far side is at higher elevation than Earth-facing side. Why?
Today's Moon
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No geological activity
Major impacts infrequent
No wind/weather
Micrometeorite impacts break up surface rock into powder
Apollo 11 footprint.
(NASA; Apollo 11, AS11-40-5878)
Overview of Mercury
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Lots of craters (lower density than
Moon)
Volcanic resurfacing
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Small lava plains
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Cliffs and shrinking of planet
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Geologically dead
MESSENGER image of Mercury
http://messenger.jhuapl.edu/gallery/sciencePhotos/image
.php?page=1&gallery_id=2&image_id=143
Credit: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington
Scarp radial to Caloris Basin
(NASA/Mariner 10)
Mercury
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Innermost planet
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No activity
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No atmosphere
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Hot on day side, cold on night
side (100 K)
Rotates 3 times for every two
orbits
Unusually high density
Distance from Sun: 0.39 AU
radius: 0.38 R⊕
mass 0.055 M⊕
average density: 5.43 g cm-3
composition: rocks, metal
moons: 0
Mercury as imaged by the MESSENGER
spacecraft
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=7543
Credit: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington
Mercury from Earth
http://www.nasa.gov/mission_pages/solar-b/solar_015.html
Geology of Mercury
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Lots of impact craters = old
surface
Lower crater density than
some lunar regions
–
Resurfacing by lava flows?
–
Small lava plains
–
Volcanically active when
young
MESSENGER image of Mercury
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1
&gallery_id=2&image_id=143
Credit: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington
Mare Orientale (Moon) as imaged by
NASA's Lunar Orbiter 4 (NASA/Lunar Orbiter 4)
• Caloris Basin - largest impact
crater on Mercury (1300 km)
– Few craters inside—occured near
end of heavy bombardment
• Similar feature seen on the western
edge of the Moon—Mare Orientale.
Caloris Basin (Mercury)
imaged by Mariner 10 (NASA)
“This enhanced-color image of
Mercury, ... shows the great
Caloris impact basin, visible in this
image as a large, circular, orange
feature in the center of the picture. The
contrast between the colors of the
Caloris basin floor and those of the
surrounding plains indicate that the
composition of Mercury’s surface is
variable. Many additional geological
features with intriguing color signatures
can be identified in this image. For
example, the bright orange spots just
inside the rim of Caloris basin are
thought to mark the location of volcanic
features....”
Credit: Image produced by NASA/Johns Hopkins
University Applied Physics Laboratory/Arizona State
University/Carnegie Institution of Washington. Image
reproduced courtesy of Science/AAAS.
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1&gallery_id=2&image_id=193
Mercury’s Surface
Scarp radial to Caloris Basin
(NASA/Mariner 10)
Jumbled Terrain Opposite
Caloris Basin on Mercury
(NASA/Mariner 10)
Disturbance from Caloris Basin impact propagated
throughout Mercury.
Mercury’s Surface
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Mercury's surface shows many
scarps or cliffs.
–
Unique to Mercury—not seen
on Moon
–
Some are 3 km in height
–
Associated with shrinking of
the crust of Mercury
●
how?
A 300 km long scarp or cliff on
Mercury's surface (NASA/Mariner 10)
Mercury’s Surface
(from Bennett et al.)
Mercury’s Volcanism
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Impact cratered spurred
volcanism—spreading basalt
Smooth plains evidence of
later volcanism
Young craters stand out
because of prominent rays
–
“This NAC image ... acquired during MESSENGER's
third flyby of Mercury ... show large areas of Mercury's
surface that appear to have been flooded by lava. In this
view, craters are visible that have been nearly filled with
lava, leaving only traces of their circular rims.
MESSENGER images have revealed that the smooth
plains in this region of Mercury's surface are quite
extensive.... After the Mariner 10 mission, there was
some controversy concerning the extent to which
volcanism had modified Mercury's surface. Now
MESSENGER results, including color composite images
, evidence for pyroclastic eruptions, and images of vast
lava plains (such as shown here) have demonstrated
that Mercury was indeed volcanically active in the past.”
Credit: NASA/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.
php?page=1&gallery_id=2&image_id=372
Mercury Today
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Geologically active longer than Moon
Volcanic/tectonic activity ended ~ 1 billion years after
formation
Dead today
Overview of Venus
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Thick clouds
Major volcanic and tectonic
activity
Relative lack of craters
Erosion not important
Remains geologically
activity
(Magellan Spacecraft, Arecibo Radio Telescope, NASA)
Venus
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Earth's twin
Rotates opposite direction from
orbit
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Very slow rotation period
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Thick atmosphere—surface hidden
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Extreme greenhouse effect
(average temperature 740 K)
Surface shows evidence of activity.
Distance from Sun: 0.72 AU
radius: 0.95 R⊕
mass 0.82 M⊕
average density: 5.24 g cm-3
composition: rocks, metal
moons: 0
Venus in UV as imaged by the Pioneer
Venus Orbiter (NASA)
Phases of Venus
Missions to Venus
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Soviet “Venera” probes explored Venus.
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Venera 3 – 6 were descent probes
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Venera 4: atmosphere CO2
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High atmospheric pressure, Failed before reaching surface
Venera 7: new design for higher P
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Confirmed 90 bar surface pressure and 740 K surface
temperature
Surface of Venus as imaged by
Venera 14. The compressibility
of the surface was supposed to
be measured by a probe
extending from the lander, but
instead it measured the
compressibility of the lens cap.
(this copy from
http://nssdc.gsfc.nasa.gov/photo_gallery/photogaller
y-venus.html)
Craters on the Venusian Surface
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Magellan: ~ 1000 impact craters
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Sizes: 2 – 280 km
●
–
No really large craters / impact basins
–
Venus surface only ~750 million yr old (heavy bombardment?)
No really small craters (why?)
The triple crater Stein—
likely produced by the
incoming body breaking
apart in the Venusian
atmosphere (NASA/Magellan)
Magellan radar image of
craters Danilova,
Aglaonice, and Saskia
(NASA)
Craters on the Venusian Surface
Computer generated 3-dimensional
perspective view of the "crater farm" on
Venus, consisting of the 37.3 km
diameter Saskia in the foreground
(28.6S,337.1E), 47.6 km Danilova
(26.35S,337.25E) to the left, and 62.7
km Aglaonice to the right (26.5S,340E).
The image was created by
superimposing Magellan images in
topography data, and coloring is based
on Venera 13 and 14 Lander images.
(Magellan press release P-39146)
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/mgn_p39146.html
Volcanic Features
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Volcanism important
Lava plains and volcanic
mountains
–
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Some steep volcanoes
–
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Runny lava
Thicker lava
Probably still active
–
No eruptions observed
–
Cloud composition
Pancake domes
—stratovolcanoes
(thick lava)
(NASA/Magellan)
Shield volcanoes (thinner lava)
http://pds.jpl.nasa.gov/planets/captions/venus/sapasmon.h
tm
(NASA/Magelilan)
Volcanic Features
“The hot surface of Venus shows clear
signs of ancient lava flows. Evidence of
this was bolstered by the robot
spacecraft Magellan, which orbited
Venus in the early 1990s. Using imaging
radar, Magellan was able to peer
beneath the thick perpetual clouds that
cover Earth's closest planetary neighbor.
Picture above, lava apparently flowed
down from the top of the image and
pooled in the light colored areas visible
across the image middle and bottom.
The lava cut a channel across the
darker ridge that runs horizontally
across the image center. The picture
covers about 500 kilometers across.”
http://apod.nasa.gov/apod/ap040323.html
(Magellan Project, JPL, NASA)
Volcanic Features
NASA/JPL-Caltech/ESA
“... volcanoes on Venus appeared to erupt between a few hundred years to 2.5 million years ago. This
suggests the planet may still be geologically active, making Venus one of the few worlds in our solar
system that has been volcanically active within the last 3 million years.
The evidence comes from the European Space Agency's Venus Express mission, which has been in orbit
around the planet since April 2006. The science results were laid over topographic data from NASA's
Magellan spacecraft. Magellan radar-mapped 98 percent of the surface and collected high-resolution
gravity data while orbiting Venus from 1990 to 1994.
Scientists see compositional differences compared to the surrounding landscape in three volcanic regions.
Relatively young lava flows have been identified by the way they emit infrared radiation. These
observations suggest Venus is still capable of volcanic eruptions.”
http://www.jpl.nasa.gov/news/news.php?release=2010-121
The “Tick” Crater
(Magellan/NASA)
A volcano on the surface of Venus with ridges along its perimeter that give it
an insect appearance
Tectonics
Animation of the
surface of Venus
based on NASA
Magellan data (Calvin J.
Hamilton)
Magellan probe radar image of
Venus. Here we see through
the clouds. Red: high
elevations (mountains); blue:
low elevations (valleys) (Magellan
Spacecraft, Arecibo Radio Telescope, NASA)
One hemisphere of Venus is dominated by long bands—these
are compression features in the crust.
Tectonic Features of Venus
●
●
Created via tension or
compression in the crust
No well-defined tectonic
plates.
Lakshmi plains: cracks (spaced 1 – 2 km apart)
created by stretching in one direction and
compressing in the perpendicular direction.
(NASA/Magellan)
Coronae
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●
●
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Aine Corona (200 km diameter)
(NASA/Magellan)
Concentric circular/oval
features 100s – 1000s
km across
No counterpart on Earth
Origin: Mantle plumes
Volcanoes found near
coronae.
Geology of Venus
(from Bennett et al.)
Erosion on Venus
●
●
●
Thick atmosphere, but slow rotation: little erosion
Water cannot be in ice or liquid form: little erosion
Surface images show little evidence of erosion
Venera 13 Lander image of the surface of Venus, showing a composition
similar to basalt on Earth (NASA)
Plate Tectonics on Venus
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●
No evidence for plate tectonics on Venus
–
Surprising (why?)
–
Dominant reshaping force for Earth
Venus: surface uniformly repaved ~ 750 million years
ago
–
Tectonic/volcanic activity
–
Earth's surface: different ages in different locations
–
Perhaps plate tectonics was active before repaving?
Why no plate tectonics?
–
thicker/stronger lithosphere?
–
High surface T → water in crust/mantle backed out
Overview of Mars
●
All 4 geological processes
present
●
Largest volcanoes in SS
●
Large tectonic canyons
●
Impact craters
●
Erosion features
–
Water still in polar ice
caps and underground
ice.
–
Perhaps liquid water
underground?
Mosaic of Viking 1 images of Mars
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=2050
(NASA)
Mars
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Much smaller than Earth
●
Two small moons
●
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●
●
Surface shows ancient volcanoes,
large canyon
Polar caps of CO2 ice and water ice
Evidence that liquid water was once
present
Atmosphere is very thin
Distance from Sun: 1.52 AU
radius: 0.53 R⊕
mass 0.11 M⊕
average density: 3.93 g cm-3
composition: rocks, metal
moons: 2 (captured asteroids)
Mosaic of Viking 1 images of Mars
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=2050
(NASA)
Terrestrial Planets
(from Bennett et al.)
Mercury
Venus
Earth
Moon
Mars
Mass
0.055 M
0.815 M
1.0 M
0.012 M
0.107 M
Radius
0.382 R
0.949 R
1.0 R
0.272 R
0.533 R
Density
5.43 g cm-3
5.25 g cm-3
5.52 g cm-3
3.34 g cm-3
3.93 g cm-3
Map of Martian Surface
Topography from the
orbiter Mars Global
Surveyor. Spatial
resolution is 15 km at
the equator. (NASA/JPL)
Great differences
between the Northern
and Southern
hemispheres remains
a mystery
●
North
●
South
–
Primarily low-lands
–
Primarily highlands
–
Relatively low crater count
–
Relatively high crater count
–
Younger surface
–
Older surface
Martian Craters
• 1000s of impact craters
• Fluidized ejecta craters
Viking 1 image of Yuty a “flower” or
rampart crater. The ejecta features here
were likely shaped by subsurface ice
melted upon impact, giving it the fluidized
appearance (NASA;
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=824)
Viking of Arandas a 28-km “pancake” crater.
(NASA)
Impact Basins
●
Largest—Hellas basin
–
1800 km in diameter
–
6 km deep
–
Interior covered by dust
(NASA)
Martian Volcanoes
●
Volcanism important in shaping
surface
–
●
Dominates in Northern highlands
Several large shield volcanoes
–
Olympus Mons largest
●
●
●
Base is larger than Arizona
3x higher than Mt. Everest
Rimmed by cliff (as high as 6 km)
Why can mountains grow so high on
Mars?
Olympus Mons: the tallest
mountain in the solar
system
(Mars Global Surveyor Project, MSSS, JPL, NASA)
Martian Volcanoes
●
●
●
Tharsis Bulge
–
Olympus Mons + several other
shield volcanoes
–
~ 4000 km across
–
How did it form?
Still active?
–
Interior heat?
–
Impact crater density → inactive
for 10s of millions of years
Martian volcanoes may erupt
again
–
Lithosphere thickening, dead
within billions of years
(Malin Space Science Systems, MGS, JPL, NASA)
Tectonics and Valles Marineris
●
●
●
Tectonic features (not
plate tectonics)
Largest: valley system
Valles Marineris
–
~ 1/5th of the planet's
equator
–
4x deeper than Grand
Canyon
Not caused by water or
lava
–
Likely connected to
Tharsis Bulge
(Image Credit: NASA/MOLA Science Team/O. de Goursac-A. Lark)
Water on Mars
●
●
●
Erosion features present
–
Some features carved by water.
–
Rain? Underground source of
water? ...
Is there liquid water on the surface
of Mars now?
Water features?
–
Mars was wet in the past
–
Warmer T, higher P
–
Crater count → 2-3 billion years
old
Martian riverbed imaged by Mars Global Surveyor
http://apod.nasa.gov/apod/ap980205.html
Credit: MGS Project, JPL, NASA
Overview of Earth
●
●
●
Plate tectonics
Volcanism, tectonics,
erosion all important
Erosion more important
than on any other planet
(Apollo 17/NASA)
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/a17_h
_148_22727.html
Earth
●
●
●
Only planet known to host life
Temperature allows for liquid
water
Moon is unusually large
Distance from Sun: 1.0 AU
radius: 1.0 R⊕
mass 1.0 M⊕
average density: 5.52 g cm-3
composition: rocks, metal
moons: 1
Earth and Moon as imaged by the Galileo
spacecraft (3.9 million miles away)
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=1879
(NASA)
from Astronomy Picture of the Day
The Moon transiting Earth as seen from 31 million miles away. This image
was taken by the EPOXI mission (Deep Impact spacecraft).
(Donald J. Lindler, Sigma Space Corporation, GSFC,Univ. Maryland, EPOCh/DIXI Science Teams)
Terrestrial Planets
(from Bennett et al.)
Mercury
Venus
Earth
Moon
Mars
Mass
0.055 M
0.815 M
1.0 M
0.012 M
0.107 M
Radius
0.382 R
0.949 R
1.0 R
0.272 R
0.533 R
Density
5.43 g cm-3
5.25 g cm-3
5.52 g cm-3
3.34 g cm-3
3.93 g cm-3
Geology of Earth
●
●
Earth exhibits all the processes we've discussed
–
Largest terrestrial planet—interior still warm
–
Atmosphere + rotation rate + surface temperature =
erosion
–
Volcanic outgassing created atmosphere
Surface is shaped primarily by plate tectonics
–
Next chapter: plate tectonics important for climate too.
–
Important for life!
Plate Tectonics
●
Lithosphere fractured
into plates
–
Plates float on
mantle
–
Plates move over,
under, around one
another
–
~ few centimeters /
year movement
(USGS; http://pubs.usgs.gov/gip/dynamic/slabs.html)
How Do We Know Earth's
Surface Moves?
●
Today: GPS
●
Other evidence:
–
Past continental motion
●
●
Continents seem to “fit together”
Rocks and fossils similar where Africa and S. America join
–
Sea floor spreading
–
Difference in sea and continental crust
(from Bennett et al.)
(USGS; http://geomaps.wr.usgs.gov/parks/pltec/pangea.html)
Past Continental Motion
●
●
Mid-ocean ridges indicate
seafloor spreading
Continents move due to
underlying mantle
convection
Mid-atlantic ridge
(NOAA)
Seafloor vs. Continental Crust
●
Seafloor crust thinner, denser, younger than continental crust
●
Seafloor crust
●
–
5-10 km thick
–
Primarily basalt (high density, runny when molten)
–
< 200 million years old (crater counts/radiometric dating)
New seafloor is constantly emerging
●
(from Bennett et al.)
Continental crust
–
20-70 km thick
–
Lower density rock (granite)
–
Some parts of continental crust ~
4 billion years old
Seafloor Recycling
●
Subduction: seafloor pushed under less dense continental crust
–
Trenches form
–
Subducted seafloor heats up, melts, erupts upward
–
Low density material melts first—creates continental crust
●
Stratovolcanoes form
(from Bennett et al.)
Seafloor Recycling
●
Mid-ocean ridges: new sea floor created by rising mantle
–
About 2 sq. km of new seafloor produced at ridges each year
–
Enough to replace seafloor every 200 million years.
(from Bennett et al.)
Continent Building
●
Continental crust not recycled—built over billions of years
–
Reshaped by volcanism, plate tectonics, erosion
Read your book for
more specific
examples of plate
tectonic features
on Earth
(from Bennett et al.)
Earthquakes
●
What about plates sliding sideways?
●
Sideways movement → fault
–
Stresses build, produce earthquakes
–
Movement can be dramatic
LA and SF
together in 20
million years
(from Bennett et al.)
Hot Spots
●
Volcanic activity outside of plate boundary
–
Mantle plume rises at hot spot
(Wikipedia/Demis)
The Future
●
●
Plates move ~ 2 cm per year
We can project to the future
(from Bennett et al.)
How Did We Get Here?
●
Small planets = cool quickly
●
Plate tectonics not only due to size—Venus?
●
Size, rotation,
distance from Sun
seems to predict
outcome
(from Bennett et al.)
Summary
●
Terrestrial planets all looked the same when young
●
Geological differences arouse due to
●
–
Size
–
Rotation
–
Distance from Sun
This understanding allows us to predict what new
planets will look like, based on knowing a few physical
properties