Transcript Lec11-022007 - Lunar and Planetary Laboratory

For the next few weeks:
Terrestrial Planets, their Moons,
and the Sun
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Announcements
• Reading Assignment
– Section 9-1 (pp 186-189) , 9-5 and 9-6 (pp 199-203)
• 3rd Homework is now posted on the course website (due Thursday
3/1)
• Term paper details are now posted on the website (due 4/17)
• Results from first exam will be briefly discussed today
– Solutions will be posted soon
• Public Lecture next Tuesday (2/27) at 7:30PM in this auditorium
– Prof. Bob Strom: “Global Warming”
– There will be a sign-up sheet for students in this class. Sign in and
get 5pts extra-credit added to your in-class activities score for each
one of these public lectures that you attend (including the first one,
if you attended it)
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
First Exam Results
•
•
•
•
Average: 69.1
Median: 70
High: 102-½ (5 scores over 99)
Total tests taken: 130
89 and above
78 – 88
67 – 77
56 – 66
55.5 and below
PTYS/ASTR 206
18
19
38
26
29
(13.8%)
(14.6%)
(29.2%)
(20.0%)
(22.3%)
Planetary Surfaces and Interiors
2/20/07
First mid-term exam grade distribution
25
20
15
Series1
10
5
0
95-100 90-95
85-90
PTYS/ASTR 206
80-85
75-80
70-75
65-70
60-65
55-60
50-55
45-50
Planetary Surfaces and Interiors
2/20/07
40-45
35-40
30-35
25-30
Approximate Curve
> 87.5
74 – 87
62.5 – 73.5
45 – 62
< 45
PTYS/ASTR 206
A
B
C
D
E
(15.4%)
(21.5%)
(30.8%)
(21.5%)
(10.8%)
Planetary Surfaces and Interiors
2/20/07
Today: Planetary Interiors and Surfaces
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Review: Formation of terrestrial planets
• Small dust particles accreted
to make “planetesimals”
• Planetesimals accreted (and
collided with other
planetesimals) to form
protoplanets
• The protoplanets were at least
partially molten
– denser iron-rich material fell
to the center, bringing
heavier metals with it,
making an iron-rich core
(differentiation)
–
A terrestrial
planet! Planetary Surfaces and Interiors
PTYS/ASTR
206
2/20/07
Internal structure of a terrestrial planet
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
All of the terrestrial planets, and Earth’s moon have a similar
internal structure, but the relative sizes are different
depending on how hot the interior got, how rapidly the object
cooled, and how much mass it has
The largest factor in determining this structure is the object’s
size
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Sources of heat
• Accretion
– Conversion of gravitational energy of
incoming material to kinetic energy, into
thermal energy
• Chemical Differentiation
– Conversion of gravitational energy of
falling denser materials within the
interior to thermal energy
• Radioactive decay of elements within
the body’s interior
– leads to a slight mass difference
between initial and final elements which
is converted to energy E=Δmc2
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Cooling processes in terrestrial planets
• Mantle convection in the
interior
– Hot material rises, cold
material falls (like boiling water
on a stove)
• Thermal conduction in the
lithosphere
– Like when a metal plate is
heated at one end – the other
end will soon get hot too
• Radiative loss through the
surface into space
• Volcanic eruptions
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Probing the interiors of planets
• The only reliable way to accurately probe
the interior of a planet is by analyzing
seismic activity – like Earthquakes
• Another reasonable approach is to
measure how the body rotates (by
measuring its libration)
• The existence of a planetary magnetic field
provides some basic information
• Theoretical and numerical models can be
constructed, but these are not as definitive
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Seismic Waves
• P and S waves (Primary
and Secondary)
– Move through the Earth’s
interior
– Provide information
about interior’s structure
• Surface Waves
– The rolling waves that
are felt on the surface
– Like water waves
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
P- and S-waves
P-waves, or compression waves
• P-waves
– Compressional (or
longitudinal) waves
– Can travel through
solids or liquids
• S-waves
– Shear (or transverse)
waves Cannot travel
through liquids
PTYS/ASTR 206
S-waves, or transverse waves
Planetary Surfaces and Interiors
2/20/07
Water waves
• S-waves do not
travel through the
Earth’s core
(creating a
“shadow zone” as
shown at below
right)
• This proves that
part of the core is
liquid
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Planetary Magnetic Fields
• Another important tool for “probing” the interior
of a planet
• Mercury – has a global magnetic field
– this is somewhat of a puzzle!
• Venus – does NOT have a global
magnetic field
– Slow rotation rate
• Earth – has a very strong global magnetic
field
• Mars – does not have a global magnetic
field, but there is evidence that it had one
in the past
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
• Permanent magnets lose their field if
raised to a temperature above about
500oC
– The Earth is hotter than this
nearly everywhere
• Earth’s field is also known to change
periodically
– Pole reversals
• It must be generating its own
internal magnetic field
• Need a circulating electric current
– Circulation and convection of
electrically conductive molten
iron in the Earth's outer core
PTYS/ASTR
206
Planetary
Surfaces and Interiors
produces
the magnetic
field
2/20/07
The Earth’s
Dynamo
Planetary Surfaces
• The appearance of a terrestrial planet’s
surface is determined by internal
geological activity, impacts with
asteroids and comets, and erosion
• Examples of internal activity include:
– Plate tectonics and volcanism
• Atmospheres can give rise to both
aeolian processes (wind erosion) and
water erosion through precipitation and
water flow, and other surface processes,
such as the flow of glaciers
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Evidence of Internal Activity:
Volcanoes
• Volcanoes – either active or extinct –
indicates whether the interior is currently
active, or was so in the past
– Earth – active volcanoes – active interior
– Venus – possibly active volcanoes, (most
are probably extinct) – active interior
– Mars – extinct volcanoes – no internal
activity
– Mercury – no volcanoes – probably no
internal activity
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Olympus Mons, Mars
Evidence of Internal Activity:
Plate Tectonics
• Plate tectonics is the shifting around
of large crustal “plates” on the
surface of a planet.
– It is the result of convection in the
interior
• Currently, only Earth has plate
tectonics
• One result of plate tectonics – huge
mountain ranges
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Dust devils on Mars
Aeolian processes
• Wind and water erosion have
effected the surfaces of Earth,
Mars, and Titan
– Sand Dunes exist on all
three bodies
– Dust devils exist on Mars
and Earth
• Mars rover Sprit is still working
because of a dust devil!
• Dust devils are much larger on
Mars
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Sand dunes on Titan
Impact Cratering
• The dominant geological
process in the solar system
• The number of craters on a
body is a good indicator of the
age of the surface
• Impacts have also affected the
atmospheres of the gas giants
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
History of Craters
• Less than 100 years ago
scientists felt that cratering due
to impacts was unrealistic and
highly improbable!
– Volcanism was the
explanation (can see
similar features associated
with terrestrial volcanoes)
• 1960’s – consensus that
impacts caused craters in
solar-system bodies
PTYS/ASTR 206
“Crater” Elegante (Pinacate balsatic
fields in Mexico, just below Arizona
border)
Planetary Surfaces and Interiors
2/20/07
• Shock effects in quartz (also seen in lunar
rock samples) due to shock waves produced
by the explosions associated with impacts
• Craters are nearly all circular in outline
– raised rims, concentric inner terraces,
and other features that are seldom found
in volcanic “craters”
– Circular also because of the shock
waves associated with the explosion
• not elongated!
• Volcanic “craters” are usually very small and
occur in a clearly volcanic environment.
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Evidence for
Craters due to
Impacts
Earth’s Impact Craters
• The Earth’s surface has probably
received more impact events than
the Moon
– Earth is larger (but its
atmosphere burns up smaller
ones).
• Only about 160 surviving craters
have been found thus far. So few
because …
– geological activity erases the
evidence (Earth’s surface is a
young one)
– Earth’s atmosphere provides a
shield against smaller asteroids
PTYS/ASTR 206
Manicougan crater, Quebec, Canada
(5th largest crater in the world)
Planetary Surfaces and Interiors
2/20/07
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07
Earth’s Impact Craters
PTYS/ASTR 206
Planetary Surfaces and Interiors
2/20/07