Download Asteroids The Asteroid Belt Composition and Classification

Asteroids
The Asteroid Belt
• The orbits of most of the asteroids lie between those of
• The asteroids all orbit the
Mars and Jupiter
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Asteroid belt
• More than 10,000 asteroids have well-determined orbits
• Asteroids 2410 and 4859 are named for the two of the
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authors of our book, Morrison and Fraknoi
There are about a million asteroids with a diameter
greater than 1 km
The largest asteroid is Ceres and was the first to be
discovered in 1801
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Diameter just under 1000 km
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Composition and Classification
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Reflectivity = 3%
C class, most numerous
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Carbonaceous or carbon-rich
• Some reflect like the Moon
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Reflectivity = 20%
S class, second most populous
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Stony composition
• Some are very bright
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Reflectivity = 60%
M class, much less numerous
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Year
D (AU)
Dia. (km)
Class
Ceres
1801
2.77
940
C
Pallaa
1802
2.77
540
C
Vesta
1807
2.36
510
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Hygeia
1849
3.14
410
C
Interamnia
1910
3.06
310
C
Davida
1903
3.18
310
C
Cybele
1861
3.43
280
C
Europa
1868
3.1
280
C
Sylvia
1866
3.48
275
C
Juno
1804
3.67
265
S
Psyche
1852
2.92
265
M
Patientia
1899
3.07
260
C
Euphrosyne
1854
3.15
250
C
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together at different distances from the Sun
• Apparently the asteroids are still located near their
birthplaces
The Largest Asteroids
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• The different classes of asteroids are grouped
Name
Metal
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Where Different Asteroids are Found
• Asteroids are not all alike
• Some are very dark like a lump of coal
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Jupiter
Typical spacing is millions
of km
physical characteristics and probably resulted from
collisions between asteroid
Many are probably missing from the original distribution
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Mars
• The asteroids seem to group into families that have similar
the Moon
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Sun in the same direction
as the planets
The asteroid belt contains
orbits with semimajor
axes between 2.2 and 3.3
AU
The asteroids are not
particularly close together
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• The total mass of the asteroids sums to about the mass of
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Trojans
Orbit of Mars
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Orbit of Jupiter
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Vesta, A Volcanic Asteroid
Asteroids Up Close
• Vesta is a very unusual asteroid
• Much brighter than other main belt objects
• Surface is covered with basalt
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• To get to Jupiter, Galileo had to traverse the
asteroid belt
• Galileo has close encounters with two main-belt
asteroids, Gaspra and Ida
• Gaspra is and S-type asteroid 16 km long
Indicates volcanism in spite of its small size
• Some meteorites have been found with compositions
similar to Vesta
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4.4 to 4.5 billion years
old
Whatever process
created Vesta was
early and short lived
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• Ida is a larger S-type asteroid 56 km in length
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• Hubble found a crater
on Vesta deep enough
to expose the mantle
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Cratering suggests it is 200 million years old
Cratering shows it is 1 billion years old
Ida has a satellite, Dactyl, whose orbit was used to
calculate the mass and hence the density of Ida
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Portraits of Asteroids
Gaspra
Ida
Galileo images of the small main-belt
asteroid, Gaspra. The dimensions of
Gaspra are 16 x 11 x 10 km.
Asteroid Ida from Galileo images.
Ida is 56 km in length.
2.5 g/cm3, similar to primitive rocks
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As Close as it Gets
• One February 12, 2001 the NEAR (Near Earth
Asteroid Rendezvous) Shoemaker spacecraft
landed on the surface of the asteroid Eros
Picture taken by NEAR Shoemaker at 120 m. The
vertical lines at the bottom resulted when the spacecraft
struck the surface while transmitting the picture
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Picture taken by NEAR Shoemaker at 170 m.
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The Moons of Mars
The Trojans
• The moons of Mars, Deimos and Phobos, are
• The Trojan asteroids are located far beyond the
thought to be captured asteroids
Deimos
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main belt at about the same distance as Jupiter
• The Trojan asteroids are dark and sizable
• There is one group ahead and one group behind
Jupiter
Phobos
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Asteroid in the Outer Solar System
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Earth Approaching Asteroids
• Asteroids exist with orbits that carry them far
• In 1989, a 200 m object passed with 800,000 km
outside the orbit of Jupiter
• Chiron is one such asteroid
of Earth and in 1994 a 10 m object passed with
105,000 km of Earth
• About 500 NEOs (near earth objects) are known
• The orbits of NEO are unstable
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Diameter of 200 km
During closest approach to the Sun, brightened by a
factor of 2
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• Pholus is another such asteroid
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Ventures out past the orbit of Neptune
Is the reddest object ever observed
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Will collide with terrestrial planet
Will be ejected from the solar system
• We are naturally interested in NEOs since an
encounter with one would be unpleasant
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Comets
The Orbits of Comets
• A comet is a relatively small chuck of icy material
• Newton recognized that the orbits of comets were highly eccentric
• Edmund Halley published calculations in 1705 for the orbits of 24
that develops an atmosphere as it approaches the
Sun
• Comets can develop tails
• Comets move with respect to the background stars
but are much more unpredictable than planets
• Comets are the best preserved, most primitive
material available in the solar system
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comets and predicted that a particular comet would return in 1758
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It did and was named Halley’s Comet
• Halley’s Comet last appeared in 1986 and was studied by several
spacecraft
May provide unique access to the material that formed
the planets 4.5 billion years ago
• Comets spend most of their lives very far away
from the Sun where it is very cold
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The Comet’s Structure
• Most comets exist in the Oort cloud
atmosphere of gas and dust
• This material comes from the nucleus of the
comet
• The comet has
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Huge spherical cloud surrounding the solar system
Extends out to 50,000 AU
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Nucleus (1-10 km)
Coma (105 km)
Hydrogen envelope (107 km)
Ion tail (directly away from the Sun)
Dust tail (away from comet’s motion)
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Location and Origin of Comets
• When we see a comet, we see its temporary
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Orbits are stable
Occasionally a comet will be perturbed and enter the
solar system
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The gravitational sphere of influence of the Sun
Only then is a comet visble
About 1013 comets may exist, 1000 times the mass of
the Earth
• Comets also are found in the Kuiper cloud
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Flattened disk just outside the orbit of Pluto
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The Fate of Comets
Comet Shoemaker-Levy 9
• Comets spend nearly their entire life in the Oort
Comet Shoemaker-Levy 9
broken up into 21 pieces
photographed by Hubble
cloud at a temperature near absolute zero
• It a comet, enters the inner solar system then
several things can happen
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May survive passing the Sun and return to the Oort
cloud
May hit the Sun or come so close that it is vaporized
May interact with a planet
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Hubble photo showing the impact
of fragment G
Impact the planet
Get speeded up and ejected from the solar system
Perturbed into an orbit with a shorter period
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Comet will rather quickly end its life
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Meteors
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atmosphere from space
These particles vaporize in the atmosphere at heights of 80 to 130
km
The typical bright meteor is produced by a particle with a mass less
than 1 gram
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No larger than a pea
• If a particle the size of a golf hits the atmpshere, a much brighter
trail is created
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Fireball
• Most of the meteors that strike the Earth can be
associated with specific comets
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Some visible some not visible
• A meteor shower consists of passing through the
debris of a comet
• These meteor showers seem to come from one
spot in the sky
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• If a bowling ball size object hits the atmosphere, it has a good
Radiant
• Meteor showers are often
chance of reaching the ground
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Meteor Showers
• Meteors are the result of solid particles entering the Earth’s
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designated by the constellation
they seem to come from
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Major Annual Meteor Showers
Shower Name
Date of Maximum
Associated Comet
Comet’s Period
(Years)
Quandrantid
January 3
Unknown
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Lyrid
April 21
Thatcher
415
Eta Aquarid
May 4
Halley
76
Delta Aquarid
July 30
Unknown
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Perseid
August 22
Swift-Tuttle
105
Draconid
October 9
Giacobini-Zinner
7
Orionid
October 20
Halley
76
Taurid
October 31
Encke
3
Leonid
November 16
Tempel-Tutlle
33
Nature of Meteor Showers
• No shower meteor has ever reached the surface
• From the flight paths, one can deduce that the
particles are very light or porous
• Comet dust is apparently fluffy, inconsequential
stuff
• The most reliable meteor shower is the Perseid
shower (comes from the Perseus constellation on
August 11)
One can estimate that total mass of of the particles in
the Perseid swarm is nearly a billion tons from the
Swift-Tuttle comet
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Geminid
December 13
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Phaethon
1.4
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Comet Swift-Tuttle was last seen in 1992 and is predicted to
return in 2126 and will have a close pass with Earth
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Meteorites
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• Traditionally meteorites have been placed into three broad classes
atmosphere is called a meteorite
Hundreds fall on the Earth every year
Meteorites do not come from comets
First documented case in modern times was
recorded in 1803
Meteorites are discovered in two ways
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Stones
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Nearly pure nickel-iron
Silicate or rocky
Stony-irons
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Ice cap collects over a large area and preserves the meteorites
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Irons
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About 25 per year are found
Antarctica is a fertile ground for finding meteorites
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Observed meteorite falls
Meteorite finds
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Meteorite Classification
• A meteor that survives its fall through the
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Mixture of stone and metallic iron
Class
Falls
Finds
Antarctic
Primitive stones
88%
51%
85%
Differentiated
stones
8%
1%
12%
Irons
3%
42%
2%
Stony-irons
1%
5%
1%
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Ages and Compositions of Meteorites
• Meteorites include the oldest and most primitive materials
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available for direct study
Using radioactive dating, the average age of meteorites is between
4.54 ± 0.1 billion years
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Usually taken as the age of the solar system (4.5 billions years)
• Meteorites almost certainly originate from asteroids
• Two famous meteorites (both fell in 1969)
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Murchison (Australia)
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Allende (Mexico)
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Carbonaceous. Contained complex organic molecules, amino acids
Contained material older than the solar system
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Material formed by previous generations of stars
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