Transcript TerrestrialPlanetsRev2

The Terrestrial Planets
Andrew Rivers,
Northwestern University
Planetary regions in the Solar System
Jupiter lies just outside the “frost
line”, where ice can form.
Ices vaporized
Ices form and
can accrete
Sun melts ices inside the orbit of Jupiter. Inner planets form from rare
silicates and iron compounds, outer gas giants accrete from plentiful ices.
Formation of the Inner Planets
No ice = can’t easily
make planets big
Accretion of dust grains builds up to planet
Habitable Zone: distance from a star where
temperature would allow liquid surface
A Brief Visit to Venus
Venus goes through phases due to the relative position of the
Earth, Sun and Venus as Venus orbits interior to the Earth
Venus
• Basic Facts
– Radius =0.95 Radius of Earth
– Distance from sun= 0.7 AU
– Temp=750 K
• Young surface, Craters are rare
• Retrograde rotation
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–
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–
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orbit radius ~ 0.7 AU
orbit period: ~ 0.6 years
rotation period: ~0.66 years, retrograde .
almost everything else in the solar system rotates prograde.
Sun would rise in West, and set in East
Venus exhibits “retrograde rotation”
http://zebu.uoregon.edu/~soper/Venus/rotation.html
Mariner 10 mosaic and Magellan radar views of Venus
Active Geology: Volcanoes on Venus
• High amounts of Sulfur and sulfur compounds
seen in atmosphere
– Reactive compounds that quickly recombine with
other molecules
– Must be constantly replenished
• Possible source
– Volcanic activity
– Source of sulfur compounds on Earth
• Radar view of surface shows lava flows
– Reflectivity shows relative age
Lava flows on the Venus shield
volcano Sif Mons. The lighter ones
are more recent.
Image Credit: Ted Stryk
Venus surface from Venus lander Venera 14
Observation: Recent
mission Venus Express
(ESO) is studying its
atmosphere. Here a vortex
is seen near South Pole
The Runaway Greenhouse Effect Led
to a Superheated Venus
Why is Venus so Hot?
Recall the glass experiment
with Infrared radiation.
From http://www.astronomynotes.com/solarsys/s9.htm
Image Credit: UCAR: Goldilocks Principle
Comparison between Greenhouse Effect on Venus, Earth and Mars
The Earth as a Planet
The Earth at Night, a composite image
Comparing the Interiors of
Terrestrial Planets
Size is Destiny?
Why does the Earth have the
thinnest crust?
Note the differences between the largest and smallest planets.
What could explain the differences?
The Early Molten Earth
• The Earth formed after an
intense collision period
– Largest produced the moon?
• Because of molten state,
denser materials (iron)
would sink, lighter silicates
would float.
• Earth formed an iron core
Lord Kelvin predicted the Earth would
cool to current temperatures in 20-100
milliion years. Earth actually 4.5 Billion
years old. Why is it still so warm?
How we know
what we know?
Thin crust floating on plastic
layer makes the Earth possibly
the most geologically active
place in the solar system
The Structure of the Earth
Plate Boundaries
Plate regions. The theory of plate tectonics describes
continental motions and Earth surface changes over time.
The long chain of volcanoes of the Hawaiian chain are caused by the movement
of the Pacific plate over a stationary “hot spot” in the Earth’s Mantle
Tools of the Trade (1):
How we know Earth’s inner structure?
Earthquakes probe Earth’s interior
• Earthquakes generally caused by shifts among fault
lines.
• Earthquake waves travel through the Earth
– P-waves: Pressure waves, longitudinal
– S-waves: Shear waves, transverse
• Speed of wave depends on the medium
• Arrival times of earthquake P&S waves at three
seismic posts can be used to locate epicenter
P and S wave simulator: making waves
Displacement
Transverse, Shear (S) wave
Wavelength
Velocity
Amplitude
Distance
Waves/sec
eye
Longitudinal, Pressure (P) wave
Wave Basics
• Speed of wave depends only on medium
– Higher density=wave usually travels faster
• Transverse and longitudinal waves travel
at different speeds
– Class example: rod
• If there is a sudden change in medium
– Reflected wave
– Transmitted wave
• Wave enters medium at
angle=refraction
– Angle of refraction depends on speed of
waves in the two media
Refraction
depends on
densities
Shear waves not
supported, absorbed
Nick Strobel’s Astronomy Notes: http://www.jb.man.ac.uk/distance/strobel/solarsys/solsysb.htm
Structure of the Earth
derived from
earthquake studies
How do we know the
structure of the Earth?
Tools of the Trade (2):
Earth’s Magnetic Field
reveals the magnet within
Magnetic field of the Earth similar in
structure to a current loop
Conclusion: circulating electric currents in the
Earth's molten metallic core cause the
magnetic field.
Note: Venus has almost no
magnetic field. Why?
Aurora Borealis from space
Charged particles are
“guided” along magnetic
field lines, strongest at poles
Aurora Australis from Image
satellite
Oxygen emission spectrum
Earth’s Climate History
• Antarctic ice cores
– Yearly layers contain gas
bubbles (CO2
concentrations)
– Isotopic information can
be used to determine
temperature when layer
deposited
Recall: Earth
Recall the planetary
Greenhouse effect
comparison
Temperature trends on the surface of the earth. The rise is correlated
with the rise of greenhouse gasses in the atmosphere.
Antarctica
Ice Core
Tools of the Trade (3): Ice Cores
GISP2 ice core at 1837 meters depth with
clearly visible annual layers.
CO2 from
trapped
gasses
Isotope
concentration in the
ice indicates temp of
condensation
Properties of the Moon
• Mass=7.3x1022 kg (0.0123 Earths)
• Radius=1,737 km (0.272 Earths)
• Density=3340 kg/m3 (Earth=5430 kg/m3)
– Moon lacks iron core
• No significant atmosphere
• Rotational Period= 27.32166 days
• Orbital Period= 27.32166 days
Full Moon
Dark side of Moon
from Apollo 16
False colored moon as seen by Galileo spacecraft. Blue areas
Are Titanium rich, orange are metal poor regions
The Earth and Moon, as seen by NEAR spacecraft
Moon creates Earth Tides
Moon increasing in speed
due to tidal yank
Therefore moon is getting
farther away!
Friction between the ocean and the
seabed as Earth turns out from
underneath tidal bulges
Tidal bulge leads
Earth slows in rotation
by about 0.0023
seconds per century
From
http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/tides.html
Why do we always see same face of Moon?
• In the past, the moon rotated much faster
• Earth creates tides on moon 20 times larger
than moon creates on Earth
– Smaller mass of moon=greater squeeze
• Moon was rotating through its tidal bulge
– Friction force slowed down rotation
– Smaller mass of moon=faster slowdown.
– Drag stops when rotation period=revolution
period
Origin of the Moon
Observation: Earth only Terrestrial planet
with significant moon.
Observation: Density of
Moon less than the Earth.
Observation: Moon lacks
iron. Only small Fe core.
Observation: Oxygen isotopic ratios of moon
rocks match those of the Earth. Ratios in Mars
rocks and meteorites are different.
Giant Impact Theory
Moon formed from giant impact between
Earth and another “planetesimal”
30 minutes after impact
Lower density crust and mantle material of Earth
blasted out into orbit
Ejecta eventually coalesces by gravity
into moon.
5 hours after impact
Big Splash
How does impact theory explain
observations?
• Why are moon rocks similar to Earth rocks?
– Common origin, material for both came from
same part of solar system
• Why is moon lower density than Earth?
– Iron core of Earth was at center and was
untouched
– Lower density crust and mantle formed into moon
• Why is the moon “unique”?
– Luck. Other protoplanets did not have similar
catastrophic impacts after formation.
An Earthling’s Guide to Mars and
Martians
Mars through small telescope
(by Francisco Rodriguez Ramirez)
Dr. Andrew Rivers
Physics and Astronomy
Northwestern University
Mars Overview
• Basic Facts
– Diameter=0.53 diameter of Earth
– Average distance from sun= 1.5 AU
– Surface temp
• Maximum 20º C= 70º F
• Minimum -140º C=-220º F
– Two small moons, Phobos and Deimos
• Much smaller than Earth’s moon
• Orbit somewhat eccentric
– Primary victory for Kepler was to explain the motion of
Mars in the sky by assuming the orbit was elliptical
The Earth Moon system
as seen from Mars
Mars and small moon
Phobos and Deimos as
seen from small Earth
telescope.
Mosaic of the Valles Marineris hemisphere of Mars featuring the Valles
Marineris canyon system, and the three Tharsis shield volcanoes at the left
Mosaic of the Schiaparelli hemisphere of Mars featuring the impact
crater Schiaparelli, dark streaks causes by erosion and/or deposition by
wind and the Hellas impact basin at extreme lower right.
Missions to Mars:
A History
Size comparison between (left to right)
Spirt/Opportunity, Pathfinder and
Curiosity rovers
Mars Exploration
Rovers (Opportunity& Spirit)
Orbiter (Global Surveyor)
Lander (Phoenix)
The Mars Surface and Geology
• Mars has a shape like a half-peeled
orange
– Most of the lowland regions are in the
“top-half” surrounding the North Pole.
– Highlands dominate around South polar
regions
– Why
• No clear answer
• The result of a catastrophic impact early in
Mars’ history?
Mars Global Surveyor
Topographical map of Mars from the Mars Global Surveyor
featuring the Hellas impact crater and the South Polar regions.
Topographical map of Mars from the Mars Global Surveyor
featuring Valles Marineris,the three Tharsis volcanoes in the Tharsis
uplift region, and Olympus Mons, the mountain furthest to the left.
A Topographic Map of Mars
Why such large volcanoes?
• Mars does not have plate tectonics
– Solid crust
– Because Mars is smaller in size than the earth it cooled faster
• Thicker crust
• Thick single plate
• Shield volcanoes on the Earth
– Fixed hot spot underneath moving plate.
– We see a chain of successive small volcanoes
• On Mars, no plates
– Volcanoes can continue to build for much longer
Model of the
production of a shield
volcano through
mantle convection.
Recall: Earth has plate tectonics:
Shield volcanoes built in a chain
Olympus Mons seen from
the Mars Global Surveyor
and a comparison to Mauna
Kea, an Earth shield volcano,
and Mt. Everest
Water on Mars?
• Evidence for surface water on Mars in the past
– Some canyons show evidence of branching side valleys
(tributaries?) suggesting erosion by seepage of
groundwater
– Stacks of layered sediments similar to formations on earth
where they are deposited underwater.
– Mineralogical and other evidence in Martian meteorites.
– Martian hematite concretions (“blueberries”) show strong
similarities to concretions on the Earth
• Earth concretions form underground when minerals precipitated
from flowing groundwater
Opportunity rover on surface of Mars (Artist’s combination, not an actual image)
Observations: Water
Marble-shaped rocks known as concretions from Utah (left) formed
millions of years ago in groundwater-soaked rocks. Similar hematite-infused
“blueberries” from Mars discovered on Mars by NASA's Opportunity rover
are shown (right)
Observations: Water
Martian floodplain Ares Vallis
similarity to the Channeled
Scabland of Wash. State suggests
floods on Martian surface.
Layered sediments in Holden Crater
resulting from deposition of sediment in a
lake that could have occupied the crater.
Observations: Water
Mars Express (ESO mission)
Reull Vallis is believed to have formed when running water flowed in the
distant past, cutting a steep-sided channel through the Promethei Terra
Highlands before running on towards the floor of the vast Hellas basin.
The Evolution of the
Martian Atmosphere
• Evidence of past liquid water implies
different conditions in past
– Higher temperatures
• But the sun was about 20% dimmer in
the early history of the Solar System
• Why was Mars warmer in the past?
Comparing the Atmospheres: Venus, Earth, Mars
Near-Infrared Spectra of
Terrestrial planets
Note Ozone and water signatures
on Earth: Signs of life.
Greenhouse Warming Comparison
Image Credit: F. Begenal Lecture Notes, Class 14
Comparison between Greenhouse Effect on Venus, Earth and Mars
Comparing the Greenhouse
effect on the Atmospheres
of Venus, the Earth and Mars
Evolution of a Terrestrial Atmosphere: Earth
Evolution of a Terrestrial Atmosphere: Venus
Evolution of a Terrestrial Atmosphere: Mars