Transcript NATS1311_120408_bw

There will be a review session next Tuesday in the lab at 12:00 PM. I will be
available by appointment anytime between now and the final (December
16). Mr. Chang will also be available during his normal office hours and by
appointment. We are also available by phone or email.
Inside the Terrestrial Worlds
After they formed, the molten planets differentiated into three zones:
- core - made of metals
- mantle - made of dense rock
- crust - made of less dense rock
Most of Earth’s interior is rock - only narrow region of upper mantle is molten rock - where
lava comes from
Interior layers also categorized by strength of rock which depends on composition,
temperature, and surrounding pressure. Weaker rock can slowly deform and flow over
millions of years. Why asteroids are irregularly shaped - weak gravity unable to overcome
rigidity of rock. Gravity of larger world can overcome strength of solid rock, shaping it into a
sphere - will shape anything over about 500 km in diameter into a sphere in about 1 billion
years
Lithosphere - the rigid, outer layer of crust and part of the mantle which does not deform
easily - “floats” on softer rock beneath.
Inside the Terrestrial Worlds
Comparison of Terrestrial World Interiors
active geologyEarth and Venus(?) still
have molten cores
inactive geology - the
cores of Mercury, mars
and the Moon have long
since cooled and
solidified
Impact Cratering
objects hit planet at 10 – 70
km/s (30,000 - 250,000 km/hr)
- solid rock is vaporized
- a crater about 10 times the
size of the object and one to
two times as deep is
excavated
Impact Cratering
matter is ejected in all directions
- craters are circular
- large craters have a central peak like the way water rebounds in the
center when you drop a pebble in it
Production of a Crater Animation
Counting Craters to find Surface Age
Lunar highlands - crowded areas of cratering - rocks date to 4.4 billion years
Lunar maria - huge impact basins filled in by lava flow - relatively few craters - rocks date to 3 - 3.9
billion years
Heavy bombardment must have subsided very early in solar system history
Cratering rate decreased as Solar Systems aged.
The older the surface, the more craters are present.
History of Cratering Animation
Craters on Mars
Crater shapes reveal surface
conditions and history
Comet Shoemaker-Levy 9
Ripped apart by tidal forces to create “string of pearls”. Impacted
Jupiter in 1994. Created massive fireballs of hot gas rising thousands of
miles above impact sites. Scars larger than Earth lasted for months.
Comet Shoemaker-Levy 9
One by one, each fragment collided with Jupiter in July 1994.
- infrared cameras observed hot plumes ejected from the planet
- material from deep inside Jupiter was ejected, and fell… left dark spots
Such impacts probably occur on Jupiter once every 1,000 years.
This was a reminder to us that impacts still occur in the present!!
Comet Shoemaker-Levy 9
Something similar had happened to Callisto.
This crater chain is evidence that a string of nuclei once impacted it.
Impacts and Mass Extinctions on Earth
We know that larger objects have impacted
Earth
- Meteor Crater in northern Arizona
- caused by a 50-meter asteroid
- impact occurred 50,000 years ago
65 million years ago, many species,
including dinosaurs, disappeared
from earth
Sedimentary rock layer from that
time shows:
- Iridium, Osmium, Platinum
- grains of “shocked quartz”
- spherical rock droplets
- soot from forest fires
Impacts and Mass Extinctions on Earth
Elements like Iridium, rare on Earth, are found in meteorites.
Shocked quartz, found at Meteor Crater, forms in impacts.
Rock droplets would form from molten rock “rain.”
Forest fires would ensue from this hot rain.
All this evidence would imply that Earth was struck by an asteroid 65
million years ago.
In 1991, a 65 million year old impact
crater was found on the coast of
Mexico.
- 200 km in diameter
- implies an asteroid size of about
10 km across
- called the Chicxulub crater
Impacts and Mass Extinctions on Earth
We have a plausible scenario of how the impact led to mass extinction.
- debris in atmosphere blocks sunlight; plants die…animals starve
- poisonous gases form in atmosphere
Could it happen again?
This chart shows how frequently objects of various sizes will impact
Earth.
The odds of a large impact are small … but not zero!
The Tunguska Asteroid
In !908, an asteroid estimated to be about 40 m across exploded over
Tunguska, Russia - atmospheric friction caused it to explode before it hit the
ground
- Entire forests were
flattened and set of fire
- Knocked over people,
tents, furniture over 200 km
away
- Seismic disturbances
recorded 100 km away
- Atmospheric pressure
fluctuations detected 400
km away
- Energy equivalent of
several atomic bombs
released
Volcanism
Underground, molten rock, called magma, breaks through crack in the
lithosphere.
Trapped gases are released (outgassing):
- H2O, CO2, N2
Viscosity (thickness) of lava (typically basalt) determines type of volcano
Low viscosity - flat lava
plains (maria on the
moon)
Medium viscosity shallow-sloped shield
volcanoes - can be tall
but not very steep
High viscosity - tall,
steep stratovolcanoes
Tectonics
Tectonics - the action of internal forces and stresses on lithosphere
Convection cells in the mantle cause both:
- compression in lithosphere - mountains are produced
- extension in lithosphere - valleys are produced
Plate Tectonics
Continuing stress of mantle convection fractured Earth’s lithosphere into
more than a dozen plates
Plate tectonics - process in which these plates move over, under, and
around each other - unique to Earth
Plate Tectonics on Earth Animation
Erosion
movement of rock by ice, liquid, or gas
- valleys shaped by glaciers - Yosemite Valley
- canyons carved by rivers - Grand Canyon, Rio Grande
- sand blown by wind - Painted Desert of Arizona
erosion not only wears down features, it also builds them:
- sand dunes
- river deltas
- sedimentary rock - layers of piled sediments from erosion
Erosion
Erosion
Erosion
How Planetary Properties Affect each Process
impact cratering
- # of impacts same for all planets
- larger planets erase more craters
volcanism and tectonics
- requires interior heat
- retained longer by large planets
erosion
- requires an atmosphere
- large size for volcanic outgassing
- moderate distance from Sun
- fast rotation needed for wind
The Moon
highlands
older surface
more craters
mare
younger surface
3 – 4 billion yrs
fewer craters
dark basalt
heavily cratered, no atmosphere,
geologically inactive
Formation of the Maria
Several large impacts made huge crater basins.
- left cracks in lithosphere below
- mantle was not molten at this time - heat from differentiation and
acccretion had already leaked away
- at a later time, radioactivity heated up interior and molten basalt
leaked through the cracks (heat from radioactivity has been lost - the
Moon now has a cold interior)
This “runny” lava filled in the basins.
Tidal Heating On Jupiter’s Moon’s
Gravitational tidal heating keeps the interiors of the three inner moons of Jupiter hot
Moons have elliptical orbit and synchonous rotation
- as Ganymede completes one orbit, Europa completes exactly two orbits, and Io
completes exactly four orbits - moons periodically line up - causes orbital ellipticity.
- tidal bulges are constantly being flexed in different directions - generates friction
inside
Io
Jupiter’s tidal forces flex Io like a
ball of silly putty.
- friction generates heat
- interior of Io is molten
Volcanoes erupt frequently.
- sulfur in the lava accounts for
yellow color
- surface ice vaporizes and
jets away
Evidence of tectonics and impact
cratering is covered.
Volcanic Plumes
Lava fountain - active lava hot
enough to cause "bleeding" in
Galileo's camera - overloading of
camera by the brightness of the
target
Newly erupted hot lava flow. Dark,
"L"-shaped lava flow marks the
location of the November 1999
eruption.
Gas and Dust Plume
A broad plume of gas and dust about 80 km high above a lava flow
QuickTime™ and a
H.264 decompressor
are needed to see this picture.
“Movie” (two frames) of the Tvashtar volcano on Io taken by New Horizon’s
spacecraft on the way to Pluto
Europa
Metallic core, rocky mantle, and a crust
made of H2O ice
Its fractured surface tells a tale of
tectonics.
- few impact craters seen
- double-ridged cracks
- jumbled icebergs
These provide photographic evidence
of a subsurface ocean.
Europa has a magnetic field.
- implies liquid salt water beneath
the icy crust
Where liquid water exists, there could
be life!
Evidence of a Subsurface ocean
Jumbled crust with icebergs and surface cracks with double-ridged
pattern - caused by tidal flexing of thick layer of ice on top of liquid
ocean of water.
Europa Ice Rafts
Thin, disrupted, ice crust in the Conamara region of Europa
- white and blue colors outline areas blanketed by a fine dust of ice
particles ejected at the time of formation of the large (26 kilometer in
diameter) crater Pwyll 1000 kilometers to the south.
- a few small craters - less than 500 meters in diameter were probably
formed at the same time as the blanketing occurred by large, intact,
blocks of ice thrown up in the impact explosion that formed Pwyll.
Ganymede
Largest moon in the Solar System
Its surface has 2 types of terrain:
- heavily cratered, implies old
- long grooves, few craters, implies
young like Europa
It also has a magnetic field.
Could it have subsurface ocean?
- case not as strong as Europa’s
- tidal heating would be weaker
- would need additional heating
from radioactive decay
Geyser-like eruptions of ice particles and water vapor shoot from the south
pole of Saturn's moon, Enceladus