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Geology of the Terrestrial
Planets
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Department
Terrestrial
1
Midterm!
Part I (Take home exam, including 10 points
from Mastering Astronomy, 50 pts) is
available, due October 26th, noon
This week, Part II (in class exam, 50 pts.)
– Taken in 3rd hour (week of 10/22 to 10/25)
– Bring SCANTRON (882 form) and #2 pencil
– Based on “Review Questions” handout, available
now!
Also: 10 of the 25 extra credit points are due
by October 26th, noon.
Lecture 9: Terrestrial Geology Basics
The Moon and Mercury
Moon
Mercury
The Moon’s geology
The Moon’s surface can be divided into
two main landforms: lunar maria and
highlands (mountainous and cratered)
regions.
Maria (plural of mare) are any of the
lowlands of the Moon (some circled by
mountains) that resemble a sea when
viewed from Earth.
Moon
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Moon
features
3
Nearly Full Moon
The Far
side of
the Moon
Lecture 9: Terrestrial Geology Basics
The Moon’s Surface
Moon
Mare
forms
The Maria were caused (3 to 4 billion years ago, just after the
Moon was formed) by large impacts cracking through the crust
and the consequent magma flow from the Moon’s mantle.
Asymmetry of maria between the two sides of Moon is caused
by differences in crust thickness (which ranges in depth from
60-100 km and is thinner on Earth-facing side).
This asymmetry also lead to the “locking” of one face of the
Moon always towards the Earth (since the maria are made of
denser materials).
The interior of the Moon has cooled too much for this to occur
again
Micrometeorites, sand sized particles from space, remain as
the only major erosion process
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interior
6
Walking
on the
Moon
Apollo 17
surface
December 1972
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Department
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Lecture 9: Terrestrial Geology Basics
The Moon and Mercury
Moon
Mercury
Mercury’s geology – extreme conditions
Radar observations show that Mercury rotates once
very 58.65 Earth days, which is precisely 2/3 of its
orbital period.
Mercury’s solar day is quite different from its sidereal
day. The solar day is 176 Earth days long (two
Mercurian years.)
Only 2 longitudes on Mercury experience noon while
the planet is at perihelion
High temperatures on Mercury can reach 425°C
(790°F), well above the melting point of lead (330°C
or 626°F).
On the night-side of Mercury, temperatures can fall to
-150°C (-250°F).
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Lecture 9: Terrestrial Geology Basics
Mercury and the Moon
Mercury’s geology - Moon Comparison
S1
Mariner 10 flew by Mercury in 1974 (and
subsequently twice more), returning a total of 4,000
photographs for the three fly-bys.
Mercury appears similar to our Moon; both are
covered with many impact craters.
Mercury’s craters are less prominent; the planet’s
surface gravity is twice that of the Moon so loose
material will not stack as steeply.
Ray patterns are also less extensive on Mercury
because of the higher gravity.
Mosaic
Map
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Department
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Lecture 9: Terrestrial Geology Basics
Mercury’s Surface
Mercury’s surface history is thought be:
Shrink
Merc
– Mercury was hot and melted due to radioactive Intercrater
decay and expanded in size
plain
– This fractured the crust and allowed lava to reach
the surface to form the intercrater plains
Smooth
– Lava eruptions in impact basins formed the
plains
smooth plains
– Then the interior cooled and the planet shrunk
scarps
cracking the surface forming the scarps
– This probably happened in the first 700 million
years after Mercury formed
S2
Scarp
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6_a2MercScarp
10
Mercury
Composite by
Mark Robinson
(Northwestern)
New
Merc
From
Sky &
Telescope
(September
2004)
Lecture 9: Terrestrial Geology Basics
Mercury and the Moon
A large “bulls-eye” impact crater called
S3
Caloris Basin is visible.
Caloris
The Moon has a similar impact region
This impact was so intense that there is Caloris
schematic
broken terrain in the region opposite of
the Caloris basin
S4Moon
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Caloris
Basin
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Lecture 9: The Terrestrial Planets
Mars by Lowell
Mars
Historical Mars
William and Caroline Herschel made first extensive
observations of Mars
Lowell v Photo
– In 1784, W. Herschel spoke with confidence about “inhabitants” of Mars
In 1879 Schiaparelli’s drawing of channels or canali on Mars
was misinterpreted by the public to mean canals dug by a race
of intelligent beings.
– Schiaparelli may have had an eye defect which made some details appear
as channels
Lowell, who opened his observatory in Flagstaff, AZ, in 1894,
reported he saw many canals. Other astronomers could not
confirm his findings.
Changes in the dark areas on Mars led to speculation that
there is vegetation on the planet that changes color in
response to seasonal growth.
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Mars’s Canals?
Lecture 9: Terrestrial Geology Basics
Mars
Mars’s Basics
Mars orbits the Sun at an average of 1.524 AU
(about 228 million km).
Mars’ orbit is more eccentric than Earth’s, so
Mars’ distance from the Sun varies from 210
million km to 250 million km.
Mars takes 1.88 Earth years to complete its
orbit around the Sun.
Polar caps of water-ice and carbon dioxide can
be seen
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Lecture 9: Terrestrial Geology Basics
Mars
Mars Rotation
Mars’ sidereal period is 24h37m; its solar
day is 24h40m long, very similar to that of
Earth.
Mars’ equator is tilted 25.2° with respect to
its orbital plane, close to Earth’s 23.4°.
We see seasons on Mars as we do on
Earth.
– The polar caps grow and shrink accordingly
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Mars Seasons
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Lecture 9: The Terrestrial Planets
Mars
Mars as Seen from Earth
Mars is best seen at opposition, once every
every 2.2 years (= 780 days = synodic period)
– All oppositions are not equal due to the significantly
elliptical orbit of Mars, so every 15 to 17 years
Mars has a much closer than average opposition
S1
Other special points on Mars’s (or any other
outer planet’s) orbit:
– Conjunction, eastern and western quadrature,
opposition
Configs
Mars can exhibit a significant gibbous phase
Mars at
near quadrature
Quadrature
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Oppositions of Mars (1997 – 2010)
2007
2005
P
2010
0.37
A
1997
A
1999
2001
2003
P
Mars during its opposition in 2003
MarsObs
Lecture 9: Terrestrial Geology Basics
Mars
Geology of Mars
Besides the polar caps, Mars has other
remarkable features
The southern hemisphere has most of the
higher elevation and the great impact region
called Hellas Basin and most of the impact
craters
The northern hemisphere has the lower
elevation, few impact craters and most of the
volcanoes
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Altitude Maps of Mars from the
Mars Global Surveyor (MGS)
Lecture 9: Terrestrial Geology Basics
Mars
Olympus
Mons
The largest volcano is Olympus Mons, who
height of 24 km (15 mi) is twice that of Earth’s
largest mountain.
– Several other large volcanoes can be found in the
surrounding Tharsis Region
S3,4
One reason Mars can “grow” larger volcanoes
than Earth is because they lack Earth-like
tectonic plates. Formed over a hot spot of lava
that wells up from within a planet, a volcano
can grow to enormous size if it does not move
off the hot spot.
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Lecture 9: Terrestrial Geology Basics
Mars
S5
There were some tectonic activities in Mars’ past: Valles
Marineris is an enormous canyon on Mars that stretches
nearly 4,800 km (3,000 mi).
– However, it was not carved out by a river nor a result of Earthlike plate tectonics
– Instead it is a split in the crust which caused the Tharsis
Region to bulge outward
– There do appear to be runoff channels on the edges of the
canyon which may have been formed by the outpouring of
subsurface water
There may be current geologic actively, though Mars will
“die” in the next few billion years
S9,10
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Department
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Valles Marineris
Outflow
channels
Lecture 9: Terrestrial Geology Basics
Mars
blueberries
Ancient Water on Mars
blueberries2
Could Mars have been water filled in its past?
– Outflow channels seem to imply that water flowed 2-3
billion years ago (based on crater counts)
Rovers Spirit and Opportunity (Mars Exploration
Missions: MER-A and MER-B; Rovers) landed on Mars
in 2004 looking for evidence of ancient water
– Opportunity found rocks that must have been soaking in
water at some time: Jarosite and the “blueberries”
containing hematite
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Department
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Lecture 9: Terrestrial Geology Basics
Mars
Channels
1.5km
Present Water on Mars
Under the current conditions, free flowing water
is unlikely to exist on Mars since the pressure
and temperature are too low.
– Water will only exist as a gas or solid on Mars
– However, there is evidence of “gullies” which seemed
to have running water in the recent past
However, water or water-ice may exist just
underneath the surface of the planet.
– Odyssey and Mars Express orbiter both saw evidence
for subsurface water
© Sierra College Astronomy
Department
Odyssey subH20
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Midterm!
Part I (Take home exam, including 10 points
from Mastering Astronomy, 50 pts) is
available, due October 26th, noon
This week, Part II (in class exam, 50 pts.)
– Taken in 3rd hour (week of 10/22 to 10/25)
– Bring SCANTRON (882 form) and #2 pencil
– Based on “Review Questions” handout, available
now!
Also: 10 of the 25 extra credit points are due
by October 26th, noon.
Lecture 9: Terrestrial Geology Basics
Current and Upcoming Mars Missions
Currently there:
– Mars Global Surveyor and Odyssey
(Orbiters;Relays)
– Spirit and Opportunity (Mars Exploration Missions:
MER-A and MER-B; Rovers)
– Mars Express
Beagle 2 rover crashed on surface, but orbiter is
working fine and it is taking some of the highest
resolution pictures of the Martian surface ever from orbit
– Mars Reconnaissance Orbiter (Launched: 12 Aug
2005)
Even higher resolution of surface, subsurface,
atmosphere (Inserted in Martian orbit on 10 March
2006, aerobraking has put into proper low orbit)
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Department
MRO
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Lecture 9: Terrestrial Geology Basics
Current and Upcoming Mars Missions
Up next:
– Phoenix lander (2007)
Digger arms, oven and portable laboratory
– Mars Science Laboratory (2009)
Bigger and better rover
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Department
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Lecture 9: Terrestrial Geology Basics
Venus
Venus’s Motions
Venus is easily seen in the sky with a maximum
elongation of 47 degrees
– (Ancient Greek names: Hesperus (evening) and
Phosphorus (morning))
Special points on Venus’s (or Mercury’s orbit):
– Inferior and superior conjunction
– Greatest western (morning) and eastern (evening)
elongation Each of these is repeated every 584 days
(Synodic period)
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Orbit
diagram
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Venus at Crescent
Lecture 8a: The Terrestrial Planets
Venus
Venus’s Motions
Starry
Venus can be seen high in the sky around
maximum elongation setting up to 3-4 hours
after sunset (or rising 3-4 hours before sunrise)
Venus can sparkle so brilliantly that it is often
mistaken for an airplane (or UFO) and in a dark
site can even cast a shadow (!)
Venus can be seen in the daytime under clear
sky conditions, if you know where to look
Like Mercury, Venus can transit the Sun, but is
far rarer
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Lecture 9: Terrestrial Geology Basics
Venus
Venus
What are the Major Geological Features of Venus?
Since Venus is only 5% smaller than the Earth, we
expect it to be geologically active
Orbiting probes Pioneer Venus 1 (1978), Venera 15
and 16 (1983-84), and Magellan (1990-93) have
produced detailed radar maps of Venus’s surface.
About two-thirds of Venus’s surface is covered with
rolling hills. Highlands occupy <10% of the surface,
with lower-lying areas making up the rest.
Venus has about 1,000 craters that are larger than a
few kilometers in diameter
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Venus
Radar
Globe
Venus
Radar
Map
S5-S9
Skip?
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Venus as seen in the UV
Pictures taken 5 hours apart
Clouds take about 4-5 days to circle planet
Lecture 9: Terrestrial Geology Basics
Venus
Venusian
features
What are the Major Geological Features of Venus?
While it has volcanoes and a lithosphere contorted by
tectonics, Venus has some unique features, such as
coronae, probably made of hot rising plumes of mantle
rock.
Volcanoes are still active (erupting in the last 100 million
years) since the atmosphere contains sulfuric acid
There is the lack of erosion on Venus: the winds are
very weak.
Venus
surface
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Lecture 9: Terrestrial Geology Basics
Venus
What are the Major Geological Features of Venus?
Venus has a lack of Earth-like plate tectonics: no super high
mountain ranges
Crater counts are uniform across the planet, suggesting an
uniform age for the planet’s surface which is estimated to be
750 million years old. The uniformity of this age suggest that
the entire planet “repaved” itself at that time.
Since Venus should be a warm underneath the lithosphere
as the Earth, the lithosphere of Venus must be thicker than
that of the Earth and resists fracturing into pieces
– No direct proof of this
– May have come about from higher temperature surface
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Lecture 9: Terrestrial Geology Basics
The Unique Geology of Earth
The Earth is the most active of the
terrestrial worlds
– The Earth’s size explains the abundance of
internal heat
– The erosion from wind and water is
explained by the Earth’s distance from the
Sun and the rotation rate
– The Earth’s plate tectonics is unique among
the terrestrial worlds.
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Lecture 9: Terrestrial Geology Basics
The Earth’s surface in motion
Plate tectonics is the movement of
fractured pieces of the lithosphere or
plates.
The plates of the Earth “float” on the
mantle as convection moves the plate
about the surface.
The plates move at a rate of a few
centimeters per year – about the rate of
fingernails on a human hand
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Drift
Plates
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Drift
Continental Motion
Plates2
Alfred Wegener is credited with first
developing the idea of continental drift the gradual motion of the continents relative
to one another.
He noticed that the coasts of South America
and Africa seem to fit together and that the
continents shared similar fossils
Not initially accepted because a mechanism SA
Afr
to move continents was not known.
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Plate Tectonics
In the mid-1950s began to observe
evidence for continental motion: midocean ridges
Drift
Plates2
Mantle material erupts onto the
Rift
ocean floor, pushing apart the
Subduc
existing seafloor on the either side.
This is referred to as seafloor
tectonics
spreading
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Department
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Plates2
The Earth’s surface has two very different types
of crust: seafloor and continental
– The seafloor crust is thinner, denser, and younger
It’s typically 5-10 km think
These plates get renewed in a process called subduction 2 crusts
and so the seafloor crust is never more the 200 million years
old.
As a result, the continents have been spreading away from
each other for 200 million years.
– The continental crust is thicker, less dense, and older
Typically between 20 and 70 km in thickness though its
weight pushes it down so that it only sticks out slight higher
than the seafloor.
Can be as old as 4 billion years.
subduction
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Plates2
subduction
Building up continents
Unlike seafloor crust, which get recycled, continental
crusts are gradually growing with time
Volcanism shapes west North America as volcanic
islands have merged into the rest of North America
Erosion played a big role the Great Plains and Midwest
Some mountains were formed when one plate
Himalayas
subducted under another
When two continent-bearing plates collided with each
other they also produced mountains. The Appalachians
formed from several collisions: two from South America
and one from western Africa.
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Department
Major geol.
features
42
Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Rift valley
Rifts, Faults, and Earthquakes
When continental plate are pulling apart, we
can get a rift valley like the one in East Africa
Plates that slide sideways are called faults
–
–
San Andreas is famous example and will eventually
bring LA and SF together in 20 million years
When a fault moves it can move at several meters in
a few seconds
Earthquake
Faults
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Department
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Hot Spots
Some volcanoes are created away from plate
boundaries under places known as hot spots.
The heat pushes up mantle (forming an island
if in the middle of the ocean)
The Hawaiian islands are the great example of
this
–
–
Main island of Hawaii is currently under a hot spot
which is moving southeast
Other islands “behind” the hot spot include Oahu (3
million years ago), Kauai (5 Mya), Midway (27 Mya)
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Department
Hawaii
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Lecture 9: Terrestrial Geology Basics
Plate Tectonics
Plate Tectonics Through Time
Since we know how the continents are drifting at the
present we can predict where they have been and
where they are going
About 200 million years ago all continents are together
as one called Pangea
Before that the continents have moved all around: a
billion years ago Africa was located at the South Pole drift
and Antarctica was near the Equator
Earth’s activity is certainly a function of its size and distance
from the Sun and its rotation rate
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Department
History
Terrestrial45
The End
© Sierra College Astronomy
Department
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