Download Lect09-2-8-17

Inner Solar System - Terrestrial Planets
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•Exterior (gas & droplets)
•Surface (solids & liquids)
•Interior (solid)
2) Exterior: Atmosphere
Huge
Effect
3) Surface
Little
Effect
1) Interior
Terrestrial Planets - Inside and Out
These typically do not change much after formation:
Conserved Quantity
• Mass & Size
Mass
• Distance from Sun
Orbital energy & angular momentum
• Composition
Atomic elements (except decay)
• Rotation
Angular momentum
Only affected by exchanges with space:
• Impacts (asteriods or comets)
• Atmospheric loss
Interior: Formation Properties
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On the previous slide, the properties of a planet that are listed on the left are set
at the time the planet is formed and do not change much afterward.
The reason the things don’t change much is that they are related to fundamental
laws of nature that say that certain things, like energy, are conserved.
Thus, for example, the rotation rate of a planet (listed in the left-hand column on
the slide) does not change much after the planet is formed, because the
angular momentum (listed at the right on the slide) is conserved.
At the bottom on the previous slide, it is noted that impacts by asteroids or
comets can make changes in a planet after it is formed.
Also, a planet can gradually lose some of its atmosphere to space.
• Mantle:
Molten:
melted rock
• Lithosphere:
Stiff: solid
“frozen”
rock
Thickness
depends on t
Interior: Internal Structure
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We have been able to figure out what the interior of the earth is like by
studying the behavior of seismic waves that are generated by earthquakes or
nuclear weapons tests. The waves refract as they propagate through the earth
as light does in going from air into water. From the arrival times at different
locations on the surface, we can get an idea of the earth’s interior composition.
This same technique is used to explore for oil.
• Heating:
Impacts
- Early on ( more than 3.8 billion years ago)
Differentiation
- Mostly shortly after formation
Radioactivity- Ongoing, decreasing, relatively small by now
Negligible heating of interior by Sun.
Differentiation is the process by which heavier elements, like iron,
sink toward the center of the planet’s interior, while lighter
elements rise.
• Cooling:
Conduction (molecular vibrations)
Convection (movement of rock in the mantle, see next slide)
Eruption (volcanoes)
All are ongoing at least until mantle freezes.
Interior: Heating and Cooling
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Convection cools the mantle material in much the same way that stirring your
coffee will make it tend to cool off faster.
When you stir your coffee, you cause hotter coffee from the bottom of your cup
to come up to the surface, where it can give up some of its heat easily to the
air.
Without the stirring, the coffee at the surface would cool off, but the coffee
below it would stay warm, since it could not contact the cool air directly.
Convection in the earth’s mantle is the motion of hotter mantle material up from
near the earth’s core toward the surface, where, like the coffee in contact with
the air, it can give up its heat more rapidly to the rocks of the earth’s crust.
After losing some of its heat, the cooled mantle material descends again toward
the earth’s core, where it picks up heat from the core that it can carry up to the
crust on its next convection cycle.
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Without convection, the earth’s interior could cool only through conduction.
Conduction is the process whereby heat flows from a hotter material to a cooler
one, with which it is in contact, without either material moving.
Heat is lost through the walls of your house in winter by conduction.
If coffee in your cup does not move, it loses heat through the walls of your cup
by conduction (which is why the cup feels hot).
If the mantle material inside a planet like the earth does not move, then it loses
heat through the surface of the planet by conduction.
Heat loss by conduction is generally much slower than heat loss by convection.
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2D example:
3D example:
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4 Surface 8
1 Interior 4
4
S/I 2
2
6 Surface 24
1 Interior 8
6
S/I
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• Rate of cooling increases with surface area
• Total heat increases with volume
• Close packed elements hide each other’s
surfaces, but have the same volume.
Interior: Surface Area vs. Volume
In the 2-D example on the previous slide,
a single square has a surface perimeter of length 4 meters
which encloses an interior area of 1 square meter.
For this square, we have 4 meters of surface to each square meter of interior
area.
If we pack 4 such squares together, we get a bigger square.
This larger square has a surface perimeter of length 8 meters
which encloses an interior area of 4 square meters.
For this larger square, we have 2 meters of surface to each square meter of
interior area.
Our square got bigger, but the ratio of its surface perimeter to its enclosed area
got smaller.
This happened because many of the surfaces of the component squares were
brought into contact, so that they are no longer surfaces.
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In the 3-D example on the previous slide,
a single cube has a surface area of 6 square meters
which encloses an interior volume of 1 cubic meter.
For this cube, we have 6 square meters of surface to each cubic meter of interior
area.
If we pack 8 such cubes together, we get a bigger cube.
This larger cube has a surface area of 24 square meters
which encloses an interior area of 8 cubic meters.
For this larger cube, we have 3 square meters of surface to each cubic meter of
interior area.
Our cube got bigger, but the ratio of its surface area to its enclosed volume got
smaller.
This happened because many of the surfaces of the component cubes were
brought into contact, so that they are no longer surfaces.
• Works for any given shape:
Surface area increases with square of size
Interior volume increases with cube of size
Sphere:
R
~ R*R
~ R*R*R
Area : 4 R 2
4 3
R
3
Volume
R

Surface Area
3
Volume :

A sphere of twice the radius has 8 times the volume, but it has only 4 times
the surface area. Since this larger sphere must cool its volume through its
surface, it will cool only half as effectively.
Interiors: Square-Cube Law
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Interior: Smaller Planets Cool Faster
• Rate of cooling increases with surface area
• Total heat increases with volume
• Time to cool:
~ R*R
~ R*R*R
Total Heat / Rate of Cooling ~ R
• Relative Thickness of lithosphere increases with decreasing size.
• Time to cool:
Total Heat / Rate of Cooling ~ R
• Relative Thickness of lithosphere increases with decreasing size.
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Of these, only Earth has a liquid outer core. We know this because of the
magnetic field that it generates. The magnetic field causes your compass
to point toward the north magnetic pole. On Venus or Mars, you would
not find your compass to be nearly so useful, because the magnetic field
is very much weaker.
The recent NASA Messenger satellite, now orbiting Mercury,
has led us to update the intelligence of the previous slide.
Mercury does have a molten outer core, and it does have a magnetic field
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The magnetic field of the earth resembles that of a bar magnet.
This is the latest information on the interior structure of Mercury.
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Interior: Effects on Larger Planets (Earth & Venus)
Effects on Surface:
• Thin lithosphere
• Convection in Molten mantle
• Motion of mantle against lithosphere
Faults
Volcanoes
Cracks & valleys
Mountain ranges
Effect on Atmosphere:
• Outgassing
Builds and replenishes atmosphere
Adds greenhouse gases
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The scale of the convective upwellings, which we call convection cells,
on the earth can be seen in this map. Only a handful of very large cells
cover the globe, as we see in the movie.
Venus has convective
cooling, but no Earthlike plate tectonics.
• No surface water.
• Very high surface
temperatures.
• Whole surface layer is
relatively buoyant, not
just “continents” in
limited regions of the
surface.
Lots of recent volcanism.
• Surface changes
relatively rapidly.
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Interior: Effects on Smaller Planets (Moon & Mercury)
Effects on Surface:
• Thick lithosphere
• No Convection near surface
• Dead geology
Surface dominated by cratering
Effect on Atmosphere:
• Outgassing stops
Bulk of atmosphere escapes into space
No greenhouse gases
Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Why so large a metal core?
• Composition of the material from
which the planet formed at this
small distance from the sun.
• Not result of very large impact.
• Built of much the same material
as the other terrestrial planets, but
with the qualification that there
was almost no water in the
environment.
Recent geologic activity?
• Well, it sure looks dead.
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Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
What is the surface made of?
• Analyze X-rays and gamma-rays
emitted from the surface.
• Caused by either radioactive
decay or by interaction with
energetic cosmic rays.
• Regions of the surface that were
sampled in this way are shown at
the right.
Fig. 1. Regions of Mercury (footprints)
sampled by XRS during analyzed
flares, numbered according
to Table 1. Outline colors reflect
derived Mg/Si ratios: white, Mg/Si ≈
0.6; yellow, Mg/Si ≈ 0.5;
blue, Mg/Si ≈ 0.4. Arrow indicates
spatially resolved measurement of a
portion of northern plains
material (16).
Mercury observed by the NASA Messenger Spacecraft
Schematic Illustration of
the Operation of
MESSENGER's GammaRay Spectrometer (GRS)
Galactic cosmic rays interact
with the surface of Mercury to a
depth of tens of centimeters,
producing high-energy (“fast”)
neutrons. These neutrons further
interact with surface material,
resulting in the emission of
gamma rays with energies
characteristic of the emitting
elements and low-energy
(“slow”) neutrons. Naturally
occurring radioactive elements
such as potassium (K), thorium
(Th), and uranium (U) also emit
gamma rays. Detection of the
gamma rays and neutrons by
GRS allows determination of
the chemical composition of the
surface
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Mercury observed by the NASA Messenger Spacecraft
Measurements of Mercury's surface by MESSENGER's Gamma-Ray Spectrometer (GRS)
reveal a higher abundance of the radioactive element potassium, a moderately volatile
element that vaporizes at a relatively low temperature, than previously predicted. Together
with MESSENGER's X-Ray Spectrometer (XRS), it also shows that Mercury has an average
surface composition different from those of the Moon and other terrestrial planets.
"Measurements of the ratio of potassium to thorium, another radioactive element, along with
the abundance of sulfur detected by XRS, indicate that Mercury has a volatile inventory
similar to Venus, Earth, and Mars, and much larger than that of the Moon," says APL Staff
Scientist Patrick Peplowski, lead author of one of the Science papers.
These new data rule out most existing models for Mercury's formation that had been
developed to explain the unusually high density of the innermost planet, which has a much
higher mass fraction of iron metal than Venus, Earth, or Mars, Peplowski pointed out.
Overall, Mercury's surface composition is similar to that expected if the planet's bulk
composition is broadly similar to that of highly reduced or metal-rich chondritic meteorites
(material that is left over from the formation of the solar system).
Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
What is the surface made of?
• Analyze X-rays and gamma-rays emitted from the surface.
• Not as exotic as everyone had hoped.
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Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Observations of
volatile elements in
Mercury’s crust rule
out multiple
scenarios for its
formation
• No super-large
collisions.
• Not made exclusively
from material bathed
for long times in very
high heat of the nearsun environment.
Recent new knowledge about Mercury
Messenger Spacecraft
These diagrams
illustrate the present
scientific consensus on
how the Moon was
formed. The idea is that
the Moon does not
contain enough very
dense material in its
core to have formed the
way Mars, Venus or
Earth did.
The recent Messenger
orbiter observations of
Mercury clearly show
that the material making
up Mercury’s surface
layers has a material
composition that is
inconsistent with its
having formed in this
fashion.
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Recent new knowledge about Mercury
Messenger Spacecraft
These diagrams
illustrate the present
scientific consensus on
how the Moon was
formed. The idea is that
the Moon does not
contain enough very
dense material in its
core to have formed the
way Mars, Venus or
Earth did.
The recent Messenger
orbiter observations of
Mercury clearly show
that the material making
up Mercury’s surface
layers has a material
composition that is
inconsistent with its
having formed in this
fashion.
Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Observations of
volatile elements
and also of
Mercury’s magnetic
field force us to
accept continued
heating in the
interior over long
times.
• Most likely source of
heating in the interior
is radioactive decay.
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Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Close-up views of
Mercury’s surface
prove that volcanic
flows occurred after
the end of the major
bombardment (after
3.8 billion years
ago).
Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Close-up views of
Mercury’s surface
prove that volcanic
flows occurred after
the end of the major
bombardment (after
3.8 billion years
ago).
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Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
Here we have a more
global view.
Credit: Courtesy of
Science/AAAS and
Brown University
A View Looking Down on the North Pole of Mercury (Center). Red circles show the locations of impact craters larger than
20 km in diameter. The area of contiguous northern high-latitude smooth plains mapped by MESSENGER from orbit (inside
the black line) covers 4.7 million square kilometers, over 6% of the surface of Mercury. Note that the number of craters
superposed on the plains is much less than in the surrounding areas, indicating the relative youth of the plains.
Recent new knowledge about Mercury from the NASA
Messenger Spacecraft
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