Transcript PPT

Asteroids and Meteorites
PTYS/ASTR 206
Asteroids/Meteorites
4/17/07
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Asteroids/Meteorites
4/17/07
• Asteroids
– Small rocky bodies in orbit
about the Sun
• Comets
– Small bodies that orbit the
Sun and (at least
occasionally) exhibits a coma
(or atmosphere) and/or a tail
• Meteoroids
– Small asteroids
• Meteorites
– the debris collected on Earth
• Meteors
– A brief flash of light (i.e. a
shooting star)
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Semantics
Asteroids (starlike)
1.
2.
3.
4.
5.
How were asteroids
discovered?
What is their origin?
What do they look like?
What are near-Earth
asteroids?
What happens when an
asteroid intersects Earth?
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Asteroids/Meteorites
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Ceres, the largest
asteroid in the
asteroid belt
Recall Bode’s Law
• As we discussed
previously, Bode’s
law is mathematical
representation of the
semi-major axis of
each planet
•
Start with the simple sequence
0 3 6 12 24 48 96 192 384
•
Then Add 4
4 7 10 16 28 52 100 196
•
Then divide by 10
0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.2 38.8
• Not a scientific “law”
in the usual sense
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Bodes Law prediction vs. actual
(semi-major axis in AU)
OBJECT
PREDICTED
ACTUAL
Mercury
0.4
0.39
Venus
0.7
0.72
Earth
1.0
1.0
Mars
1.6
1.52
???
2.8
???
Jupiter
5.2
5.2
Saturn
10
9.5
Uranus
19.6
19.2
Neptune
38.8
30.1
78
39.6
Pluto
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Discovery of Ceres
• Giuseppe Piazzi searched for
the missing planet
• Found Ceres
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Discovery of Ceres
• Ceres disappeared behind the
Sun and then reappeared – just
like a planet
• Ceres is about 2.8 AU from the
Sun – just as predicted by Bode’s
law
• However, Ceres looked like a star
must be a very small planet !
– It turns out that Ceres is
smaller than Pluto, but, by far,
the largest of the asteroids in
the asteroid belt
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Asteroids/Meteorites
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• Soon after Ceres was
discovered, other small
“planets” were found
– All about 2.8 AU from
the Sun
• New technique (ca.
1800’s):
– Invented by Max
Wolfe
– long-exposure
photography
– Look for asteroid trails
on photos
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Asteroids/Meteorites
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• Soon after Ceres was
discovered, other small
“planets” were found
– All about 2.8 AU from
the Sun
• New technique (ca.
1800’s):
– Invented by Max
Wolfe
– long-exposure
photography
– Look for asteroid trails
on photos
Did the Missing Planet Even Exist ?
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Discovery of the Asteroid Belt
• Astronomers were finding
many asteroids with the new
technique
– Wolfe alone discovered
over 200
Year
# known
1800
0
1844
4
• Modern Technique
– CCD images at various
times, have a computer
look for differences
1890
287
1900
452
1979
2125
• Have most likely found all
objects greater than 1km
across
1988
4044
2000
>10,000
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Searching for Asteroids and Comets
IMAGE 1
IMAGE 2
Difference between 2
images highlights
differences that may be
asteroids/comets !
IMAGE 1 – IMAGE 2
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Asteroid Names
• Many asteroids are discovered by amateurs
• The process can take decades
– After discovery, it is reported to the Smithsonian
Astrophysical Observatory Minor Planet Center
– Given a temporary name (1980 JE)
– If the object is found at the same location at 4
successive oppositions, the discover gives it a name
and it is assigned a number
• 1 Ceres
• 2 Pallas
• 3834 Zappafrank
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Asteroids
1.
2.
3.
4.
5.
How were asteroids
discovered?
What is their origin?
What do they look like?
What are near-Earth
asteroids?
What happens when an
asteroid intersects Earth?
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Asteroids/Meteorites
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Ceres, the largest
asteroid in the
asteroid belt
Origin of Asteroids
• There is no “missing planet”
• Asteroids are not the remains of a
long-destroyed large planet (not much
combined mass)
– The combined mass of all asteroids
in the asteroid belt is less than the
mass of our Moon
• Asteroids are relics of planetesimals
that failed to accrete into a full-sized
planet
– effect of Jupiter
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Jupiter’s effect on the Asteroid Belt
•
Numerical simulations indicate
that the existence of Jupiter
makes it unlikely that a planet
would have formed in the
asteroid belt
•
If Jupiter were not there, it is
likely that a fifth terrestrial planet
would have formed
•
Jupiter also depleted the
asteroid belt removing any that
got close to Jupiter (and were
flung out into the solar system)
•
Note: Asteroid belt doesn’t look
like movie asteroid belts, it is
mostly empty space
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Kirkwood Gaps
• Even today, gravitational
perturbations by Jupiter
deplete certain orbits
within the asteroid belt
• The resulting gaps, called
Kirkwood gaps, occur at
simple fractions of
Jupiter’s orbital period
• Similar to gaps in
planetary rings (but keep
in mind eccentricity!)
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• Located at the Lagrange
points of Jupiter
– Stable orbits, same
period as Jupiter
– Named for Trojan war
characters
– About 50 Trojan’s are
known, many more are
suspected
Trojan Asteroids
• Earth has no Trojans
– However we have
spacecraft at Earth’s
Lagrange points
(ACE/SOHO/Genesis
and others)
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Asteroids
1.
2.
3.
4.
5.
How were asteroids
discovered?
What is their origin?
What do they look like?
What are near-Earth
asteroids?
What happens when an
asteroid intersects Earth?
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Ceres, the largest
asteroid in the
asteroid belt
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Asteroids Physical Structure Size and Shape
• All are smaller than 1000 km
– Ceres is the largest with a
diameter of 934 km (more
than 2 times smaller than
Pluto)
• Only the largest are round
– Uniform brightness
Gaspra
• Most are irregular shaped
– Elongated asteroids vary in
brightness as seen from
Earth as they rotate and
present varying amount of
cross-sectional area
• Some have moons
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Physical Structure: Composition
• Some are differentiated
– Iron cores ?
• Density of some are consistent
with them being made of rocky
silicates
• Dark (low albedos)
– Carbon rich ?
• Others have very low densities
indicating that they are
probably rubble piles
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Ida and its moon Dactyl
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Some Asteroids are “Rubble Piles”
• A survey of asteroid rotation
rates show that many rotate
quite slowly
– If an asteroid is a collection
of smaller chunks (i.e. a
rubble pile), it would not be
able to rotate very quickly
without flying apart
• Spherical (or really small!)
asteroids rotate more rapidly
– They are solid
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Asteroid Collisions
• Some asteroids
have huge craters,
like 253 Mathilde
shown here
• Being a rubble
pile, it more-easily
absorbs energy
from the collision
and remains intact
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Asteroids
1.
2.
3.
4.
5.
How were asteroids
discovered?
What is their origin?
What do they look like?
What are near-Earth
asteroids?
What happens when an
asteroid intersects Earth?
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Ceres, the largest
asteroid in the
asteroid belt
Near-Earth Asteroids
• Asteroids that are within the orbit of
Mars are called Near-Earth Objects
(NEOs)
– 2500 are currently known
• Fairly recently, it was thought that
Asteroid 2004 MN4 had a 1 in 60
chance of hitting Earth in 2029
– An impact has now been ruled
out
– It is about a 400-meter sized
asteroid (would cause a lot of
damage)
• The larger asteroid 1950 DA (1-km)
has a 1:300 chance of striking
Earth in 2880
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Image by Jim Scotti of LPL’s
Spacewatch program
Possibility of an Impact
• Calculating asteroid trajectories,
precisely, can be tricky
– Need a detailed mapping of the
Sun’s gravitational field
– Need a better understanding of
the characteristics of the asteroid
(rotation, orbit, shape, etc.)
– Yarkovsky effect
• Diurnal and seasonal variations
in the thermal radiation emitted
by the asteroid can act as a
miniature propulsion system
Radar image of 1950 DA
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NEO’s and Impacts with Earth
• 30-meter sized asteroids come
close to Earth about every 2
years
– They strike Earth every
6000 years or so
• A recent close call
– There was an explosion
over the Mediterranean on
June 6, 2002 (a 10-meter
sized asteroid)
• Have probably found all
objects larger than 1km, so
probably no global effects
without advance warning
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Asteroids
1.
2.
3.
4.
5.
How were asteroids
discovered?
What is their origin?
What do they look like?
What are near-Earth
asteroids?
What happens when an
asteroid intersects Earth?
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Asteroids/Meteorites
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Ceres, the largest
asteroid in the
asteroid belt
Collision Course
•
“Burns” up in atmosphere (outer
layers vaporize, very little heating
of rock occurs)
•
If large enough it slows down to
terminal velocity, then falls to
Earth as a meteorite
•
If larger still, will strike the
surface with a tremendous
amount of kinetic energy creating
an explosion
– This is what created
Berringer Meteor Crater, and
flung ejecta out to a distance
of 1-2 km. Events like this
occur ~6000 yrs
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Meteor
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Collision Course
•
“Burns” up in atmosphere (outer
layers vaporize, very little heating
of rock occurs)
•
If large enough it slows down to
terminal velocity, then falls to
Earth as a meteorite
•
If larger still, will strike the
surface with a tremendous
amount of kinetic energy creating
an explosion
– This is what created
Berringer Meteor Crater, and
flung ejecta out to a distance
of 1-2 km. Events like this
occur ~6000 yrs
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“Impact” with surface
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Ejecta Blanket
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Ejecta Blanket
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Ejecta Blanket
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Ejecta Blanket
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Collision Course
•
“Burns” up in atmosphere (outer
layers vaporize, very little heating
of rock occurs)
•
If large enough it slows down to
terminal velocity, then falls to
Earth as a meteorite
•
If larger still, will strike the
surface with a tremendous
amount of kinetic energy creating
an explosion
– This is what created
Berringer Meteor Crater, and
flung ejecta out to a distance
of 1-2 km. Events like this
occur ~6000 yrs
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Meteorites
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Meteorites
•
All meteorites were broken off from larger objects and have
fallen to Earth from space, but not all are from asteroids.
Some are from the Moon and Mars (but not many). No Earth
meteorites have been recognized.
•
Falls are observed to land, finds are just found lying on
ground
•
3 main categories
–
Stony
•
•
–
–
•
Chondrites (contain chondrules)
Achondrite
Stony-Iron
Iron
Many other sub-classes of meteorites
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Stony
• Most abundant meteorite falls,
but only about 10% of
collection
– Hard to distinguish from Earth
rocks, erode quickly in Earth’s
atmosphere
• Fusion Crust can make
identification easier
• Some of them are the oldest
rocks ever found (4.56 Ga)
– This is one way we get the
age of the solar system
• One class has almost an
identical composition to that of
the Sun (outside of H and He)
– Building blocks of the planets
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Stony-Iron and Iron
• Stony-Iron – rarest in both fall
and in collections
• Iron – not very abundant fall,
most abundant in collection
– Easy to find
– Made of metal (uncommon
on Earth’s surface)
• Only source of metal to
humans before 2000BC
– Take longer to erode
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Meteorite/Asteroid Connection
• Undifferentiated asteroids
– Chondrites
• Differentiated asteroids
– Achondrites (mantle)
– Stony-Iron (mantlecore boundary)
– Iron (core)
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Widmanstätten pattern in Iron Meteorites
• Provides a conclusive test
that the material is indeed
meteoritic
• The pattern is produced as
the iron-nickel material is
cooled slowly (millions of
years) – like the core of an
asteroid parent body that
remains molten for a long
time after its formation
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• most abundant type of stony
meteorite
• very primitive chemistry
– not been modified by melting
or differentiation of a parent
body
– Part of bodies that accreted
right at the time the solar
system formed
– Most contain chondrules
(small spherical-shaped
glassy-like objects
embedded within the
meteorite)
• They can also contain some of
the material that existed prior to
the formation of the Sun
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Chondrites
Carbonaceous chondrites
• A small class of chondritic
meteorites that contain high
levels of water and organic
compounds
• The presence of volatile
elements and water indicate
that the object was not
heated significantly
– Material straight from the
original solar-nebula
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