Download Lecture 5

Lecture 5
6/8/07
AST1001
Geology Basics
• Can probe the interior
of a planet with seismic
waves
• Core of the planet is
dense, made out of
metals
• Mantle is moderate
density
• Crust is very thin,
contains low density
rock
The Core
• The Earth’s core consists of
two parts: the molten outer
core and the solid inner
core
• The Earth is almost entirely
rock
• Other planets might be
similar, but no seismic data
• Differentiation causes the
density structure of the
planets
Other Planets
• The lithosphere is the outer, thin layer of
rocks
• Mars, Mercury, the Moon have very thick
lithospheres that extend almost to their
cores
• Earth, Venus have very large mantles
How Interiors Get Hot
• Accretion
– Planetesimals collide, bring energy to planet
• Differentiation
– Heavy stuff moves inward, reducing potential
energy
• Radioactive Decay
– Lots of radioactive elements in core, and
radioactive decays produce energy
How Interiors Cool Off
• Convection
– Hot things rise, cool things fall
– Examples: weather and boiling
water
• Conduction
– Contact allows heat transfer
• Radiation
– Light is radiated into space
Group Work
• The text does a example surface area to
volume calculation on page 255. Repeat the
calculation but now calculate the ratio using
Mars (radius = 3402 km). What does this
tell you about why Mars is no longer very
geologically active?
Magnetic Fields
• Magnetic fields occur
due moving charged
particles
• You need 3 things for a
planetary magnetic field
– An interior region of
electrically conduction
fluid
– Convection in that fluid
– Moderately rapid rotation
• Earth is the only
terrestrial planet to have
these things
Shaping Planetary Surfaces
• There are 4 ways of shaping planets
surfaces
–
–
–
–
Impact Cratering
Volcanism
Tectonics
Erosion
Cratering
• Impacting objects
typically travel at speeds
of 250,000 mi/hr
• Collisions convert that
kinetic energy into
moving rock to make
craters
• Craters can give us
information about the
planet’s past surface
Craters and Surface Aging
• Craters should be equally spaced and
equally dense on all planets if there is no
weathering
• Craters can be erased by erosion, lava, etc
• We can tell where activity happened by
looking at crater density
• Smaller planets are less geologically active
Volcanism
• Refers to molten lava coming
to the surface
• Magma rises because its less
dense and is squeezed to the
surface
• Lava creates volcanoes and
volcanic plains
• Also creates rocks like basalt
• Outgassing can bring different
gasses to the surface
Tectonics
• Refers to the stretching, compression or any
other force on the lithosphere
• Usually occurs due to mantle convection
• We have massive pieces of the crust here on
Earth, which leads to plate tectonics
Erosion
• Processes that break
down or transport
rock
• Can also create
things like sand
dunes, river deltas,
deposits,
sedimentary rock
Small Bodies
• Maria have formed on the
Moon due to lava outflows
• Today the surface of the
Moon is pretty much
unchanging
• Mercury is very similar, but
probably more volcanically
active
– Huge feature called Caloris
Basin
Mars
• Surface has many dramatic differences
– Southern hemisphere has high elevation with lots of
craters, including Hellas Basin
– Northern Hemisphere is low, has few craters
Volcanism on Mars
• Lots of volcanoes on northern plains
• Towering shield volcanoes
• The Tharsis Bulge is a continent sized
feature
• Valles Marineris
– Huge canyon
– Might have formed due to tectonic cracking
Water on Mars
• Lots of features that look like dry riverbeds
• No liquid water on the surface of Mars today
– Water is not stable on the surface
• Spirit and Opportunity have found lots of minerals
that would only form in water
Venus
• Magellan mapped the surfaces, down to
features 100 meters in size
• Lots of volcanoes on Venus
– Probably active
• Lots of contorted features, probably formed
by tectonics
• Virtually no erosion
Map of Venus
Tectonics on Earth
• Ample evidence to suggest that plates drift
around
• Results in a wide range of unique effects
– Earthquakes
– Hot spots
– Pangaea
Atmosphere Basics
• An atmosphere is just the gasses
surrounding the solid part of a planet
– Atmosphere always makes up a tiny fraction of
the planet’s mass
• Planets have a wide range of atmospheres
– Vary widely in composition, density
More Basics
• Atmospheres very thing
– 2/3 of air on Earth lies within 10 kilometers of
surface
• Air pressure occurs due to particles running
into things
• Air is held to the planet with gravity, so
there is more air near surface
• Bars are used to measure pressure
Atmospheric Effects
• Creates pressure
– Important for liquid water
•
•
•
•
Scatters and absorbs light
Creates wind and weather
Creates magnetosphere
Can cause greenhouse effect
Greenhouse Effect
• The greenhouse effect is a natural effect
• Earth absorbs some light from the Sun, reemits it as
infrared light
• Greenhouse gasses absorb the IR light, delay its
escape
• Too much of the greenhouse effect is a bad thing
Atmospheric Properties and
Height
• The atmosphere has
structure
– Troposphere is closest to
the surface
– Stratosphere
– Thermosphere
– Exosphere
Magnetosphere
• The Sun emits a low
density breeze of charged
particles called the solar
wind
• The Earth’s magnetic
sphere influences the
particles
• Particles can get trapped
along magnetic field lines
– For Van Allen Belts
– If they are in the
magnetosphere, then you
get aurora
Mercury and the Moon
• Very thin atmospheres
• Created by “surface ejection” of particles
• There might be water ice in polar craters
Mars Today
• Present day surface of Mars looks a lot like
a desert on Earth
• Atmosphere is very thin: 1% of pressure as
that on Earth
• Most UV radiation gets through the
atmosphere
Martian Seasons
• Mars has seasons similar to those on Earth
– Twice as long
• Eccentricity of orbit is important
– South has shorter, hotter summers and longer, colder winters
than North
• Can have strong winds
Mars in the Past
• For its first billion years,
Mars might have been
wet and warm
• Atmosphere was probably
lost once its magnetic
field went away
– Particles left Mars and
went into space
– Rocks could have also
taken oxygen
Group Work
• The escape velocity on Mars is about 5
km/s. The average daytime temperature is
230K. Using this information, compare the
peak thermal velocity of the atoms in
Mars’s atmosphere to the escape velocity.
How does this relate to potential
atmosphere loss by Mars?
Venus Today
• Surface is searing hot, very high
atmospheric pressure
• Almost no wind or weather
• Temperature is uniform and no seasons
• Sulfuric acid in the atmosphere
• Mysterious high speed upper winds
Venus and Its Clouds
• Earth gets rid of its carbon dioxide with
water
– Where did Venus's water go?
– Leading theory involves UV rays breaking
down the water
• Because Venus is slightly closer to the Sun,
water evaporates more, creates runaway
greenhouse effect