Download Chapter 12 Asteroids Comets and D arf Asteroids, Comets, and

Chapter 12
Asteroids Comets,
Asteroids,
Comets and Dwarf
D arf
Planets
Their Nature, Orbits, and Impacts
12.1 Asteroids and Meteorites
Our goals for learning:
• What are asteroids like?
• Why is there an asteroid belt?
• Where do meteorites come from?
Can Asteroids destroy the Earth ?
Asteroid is messing up my picture!
Vermon of the sky!
y
Galileo at launch from shuttle
Great asteroid images on the way to Jupiter!
Gaspra close up via Galileo
HOW DID GALILEO
GET SAFELY ACROSS
THE ASTEROID
BELT!
Ida & moon Dactyl
Galileo photo album
Mathilde
G
Gaspra
Id
Ida
NEAR –Shoemaker
Spacecraft
MISSION
Fall 2000spring 2001
N.E.A.R. =
NEAR
EARTH
ASTEROID
RANDEVOUS
Eros
Eros close-up
Asteroid
Facts
• Asteroids are rocky leftovers of planet formation.
g is Ceres,, diameter ~1,000
,
km
• Largest
• 150,000 in catalogs, and probably over a million with
diameter >1 km.
• Small asteroids are more common than large asteroids.
• All the asteroids in the solar system wouldn’t add up to
even a small
ll terrestrial
t
t i l planet.
l t
Gaspra
Matilde
eros
id
ida
Asteroids are cratered and not round
Asteroids with
ith Moons
• Some large asteroids
have their own
moon
• Asteroid Ida has a
tiny moon named
Dactyl
Densit of Asteroids
Density
• Measuring orbit of
asteroid’s moon tells
us asteroid’s mass
• Mass and size tell us
asteroid’s density
• Some asteroids are
solid rock; others
just piles of rubble
Asteroid Orbits
• Most asteroids orbit
in a belt between
Mars and Jupiter
• Trojan asteroids
follow Jupiter’s
Jupiter s
orbit
• Orbits of near-Earth
asteroids cross
Earth’s orbit
Thought Question
Whyy are there very
y few asteroids beyond
y
Jupiter’s
p
orbit?
A. There was no rocky material beyond Jupiter’s
orbit.
orbit
B. The heaviest rocks sank towards the center of
the solar system.
C. Ice could form in the outer solar system.
D. A passing star probably stripped away all of
those asteroids
asteroids, even if they were there at one
time.
Thought Question
Why are there very few asteroids beyond Jupiter’s
orbit?
A There
A.
Th was no rocky
k material
t i l beyond
b
d Jupiter’s
J it ’
orbit.
B The heaviest rocks sank towards the center of the
B.
solar system.
C. Ice could form in the outer solar system.
D. A passing star probably stripped away all of
those asteroids, even if they were there at one
time.
ti
Why is there an asteroid belt?
Thought Question
Which explanation for the belt seems the most
plausible?
A. The belt is where all the asteroids happened to
form.
B. The belt is the remnant of a large terrestrial
planet that used to be between Mars and Jupiter.
C. The belt is where all the asteroids happened to
survive.
Thought Question
Which explanation for the belt seems the most
plausible?
A The belt is where all the asteroids happened to
A.
form.
B. The belt is the remnant of a large terrestrial
planet that used to be between Mars and Jupiter.
C. The belt is where all the asteroids happened
pp
to survive.
But WHY didn’t they
f
form
a little
littl planet?
l t?
Origin of Asteroid Belt
• Rockyy planetesimals
p
between Mars and
Jupiter did not
accrete into a planet.
planet
p
gravity,
g
y,
• Jupiter’s
through influence of
orbital resonances,
stirred up asteroid
orbits and prevented
their accretion into a
planet.
Orbits of known Asteroids
Kirkwood’s Gaps
• The asteroid orbits are not evenly distributed
throughout the asteroid belt between Mars and Jupiter.
• There
Th
are severall gaps where
h
no asteroids
t id are ffound:
d
Kirkwood’s gaps (purple bars below)
These correspond
to resonances of
the orbits with the
orbit of Jupiter.
p
Example:
2:3 resonance
Orbital Resonances
• Asteroids in orbital
resonance with
Jupiter experience
periodic nudges
n dges
• Eventually those
nudges move
asteroids out of
resonant orbits,
leaving gaps in belt
Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt.
Apollo Amor
Apollo-Amor
Objects:
Asteroids
with elliptical
orbits,
reaching into
the inner
solar system.
Some
potentially
colliding
g
with Mars
or Earth.
Trojans:
Sharing
stable
t bl orbits
bit
along the
orbit of
Jupiter:
Trapped in
the
Lagrangian
points of
Jupiter.
The Search for
N B lt Asteroids
Non-Belt
A t id
Dedicated telescopes like
th LINEAR ttelescope
the
l
monitor the sky for non-belt
asteroids that could
potentially collide with Earth.
Survey covers mostly
regions away from the
Galactic plane: faint
asteroids are difficult to
detect against the
background of the
Milky Way.
Where do meteorites come from?
Meteor Terminology
• Meteorite: A rock from space that falls
through
g Earth’s atmosphere
p
• Meteor: The bright trail left by a meteorite
Meteors burn in atmosphere
SPORADIC METEOR
It’s a METEORITE—IF IT HITS THE GROUND
CAN I GET HIT?
A dog in Cairo
A lady’s hip in Alabama
M t it Impact
Meteorite
I
t
Chicago, March 26, 2003
Barringer Meteorite Crater
One of Earth’s youngest IMPACT craters
Earth hit by a 62 Meter Nickel Iron Meteorite
50 000 years ago 1 mile
50,000
il across
How do we know that this is not a volcano?
Analysis of Meteorites
3b
broad
d categories:
t
i
• Iron
meteorites
• Stony
meteorites
• Stony
Stony--Iron
meteorites
Nickel-Iron Meteorite 6% of Falls
66% of finds! Why?
Polished and etched with acid we see the Widmanstatten patterns
Cooled very slowly so Nickel-iron crystals grew. Few deg/ 106 yrs
FROM A PLANETESMAL CORE
Stoney Meteorites
92% of falls, 26% finds
This is a CHONDRITE
Veryy old
CHONDRULES
NOTE: CHONDRULES ARE SMALL GLASSY ROCK BITS
PRESENCE IMPLIES CHONDRITES NEVER MELTED!
From a region of molten rock and iron
Planetesimal core / mantle boundary!
STONY-IRON METEORITE—pallasite great FOR Jewelry
Typical Meteorite Jewelry and knives for outer of this world gifts!
The Origins of Meteorites (2)
• Planetesimals cool and differentiate
• Collisions eject material from
different depths with different
compositions and temperatures.
• Meteorites can not have been
broken up from planetesimals very
long ago
 so
remains of planetesimals
should still exist
exist.
 Asteroids
Stones
St
Stony-Irons
I
Irons
Finding Meteorites
M t meteorites
Most
t it are smallll and
dd
do nott produce
d
significant
i ifi
t craters.
t
Good place to find meteorites: Antarctica!
Distinguish
between:
• Falls = meteorites which have been
observed to fall (fall time known).
• Finds = meteorites with unknown fall time.
Meteorites from Moon and Mars
• A few meteorites arrive from the Moon and
Mars
• Composition differs from the asteroid
fragments.
fragments
• A cheap (but slow) way to acquire moon rocks
and Mars rocks.
rocks
12.2 Comets
Our goals for learning:
• What are comets like?
• Where do comets come from?
What are comets like?
Comet move slowly over several weeks across the sky!
Can they Kill me? Throughout history, comets have been considered as portents of
doom, even very recently: Appearances of comet Kohoutek (1973),
doom
(1973) Halley (1986),
(1986) and
Hale-Bopp (1997) caused great concern among superstitious.
Comet Facts
• Formed beyond the frost line, comets are
icy counterparts to asteroids.
• Nucleus of comet a “dirty snowball”
• Most comets do not have tails.
• Most comets remain perpetually frozen in
y
the outer solar system.
• Only comets that enter the inner solar
y
ggrow tails.
system
S
Sun-grazing
i Comet
C
t
The first signs of an approaching comet
COMA -vast cloud of gas &dust up to 106 Km (80 earth diameters!)
Comet Ikeya-Seki 12km diameter 1965 dawn sky
WWII Battleship Binoculars
great for comet hunting
Discover a Comet
You Name it!
Hyakutaki
You discover it you Name it!
Hyakutaki core from Astrophysical Observatory at CSI
METEOR SHOWER
LEONIDS 1833
LEONIDS 1966
Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year.
Why are there Meteor Showers?
RADIENT—SHOWER METEORS ALL FROM SAME SPOT
Meteors in a shower appear to emanate from the same area of sky
because of Earth’s
Earth s motion through space and they are all traveling
parallel together
RADIENT=
ALL SHOWER
METEORS
ARE IN SAME
ORBIT
MOST
SHOWERS
HAVE BEEN
LINKED TO
COMETS
METEORS
ARE COMET
PIECES
EARTH
CROSSES
COMETS
ORBIT!
WHAT HAPPENS IF THE
PARENT COMET IS
AT THE INTERSECTION
POINT
Comets eject small particles that follow the comet around in its
orbit and cause meteor showers when Earth crosses the comet’s
orbit.
orbit
Growth of Tail
Anatom of a Comet
Anatomy
• Coma is atmosphere
p
that comes from
heated nucleus
• Plasma tail is gas
escaping
p g from coma,,
pushed by solar
wind
• Dust tail is pushed
by photons
Gas Tail
Spectrum
Emission
Lines of ionized
Gases excited
By UV of SUN
CO+,, CO2+
CO
CH+,CN+,N2+
C+,Ca+
COMA spectrum
UV Breakup parts
Of H2O,CO
O CO2,CH
CH4
NH3 &Na, Fe, K,
Ca, etc
Comet
Dust
Fills Solar
system
COMET PARTS
TWIN TAILS
TYPE I
Gas pushed by
Solar Wind
Gas Tails have
Wisps & twists
Because!
Ions effected by
M
Magnetic
ti field
fi ld
In the Solar wind
TYPE II
Dust pushed
Gently by
Light
pressure
Dust tails obey gravity
And gently curve
COMA
Or HEAD
ICY NUCLEUS few KM diameter
Gas and Dust Tails of
Comet Mrkos in 1957
Comet Hale Bopp (1997)
N cle s of Comet: Tempel 1
Nucleus
• A “dirty snowball”
• Source of material
for comet’s tail
Deep Impact Mission
• Mission to studyy
nucleus of Comet
Tempel 1
• Projectile hit surface
on Julyy 4,, 2005
• Many telescopes
studied aftermath of
impact
The Geology of Comet Nuclei
Comet nuclei contain ices of water, carbon dioxide, methane,
ammonia, etc.:
Materials that should have condensed from the outer solar nebula.
Those
compounds
sublime
(transition from
solid directly to
gas p
g
phase)) as
comets approach
the sun.
Densities of comet
nuclei: ~ 0.1 – 0.25 g/cm3
Not solid ice balls, but
fluffy material with
significant amounts of
empty space.
Where do comets come from?
Only a tiny number of
comets enter the inner
solar system - most
stayy far from the Sun
Oort cloud:
On random orbits
extending to about
50 000 AU
50,000
Kuiper belt:
On orderly orbits
from 30-100 AU in
disk of solar system
100,000AU
Oort Cloud
Origin of
Long Period Comets
10,000AU
Note: 1 ly=63,000AU
Passing star sends a Cometary Shower
into the Solar system?
How did they get there?
• Kuiper belt comets formed in the Kuiper belt: flat
plane, aligned with the plane of planetary orbits,
orbiting in the same direction as the planets.
• Oort cloud comets were once closer to the Sun,
but they were kicked out there by gravitational
i t
interactions
ti
with
ith jovian
j i planets:
l t spherical
h i l
distribution, orbits in any direction.
12.3 Pluto: Lone Dog No More
Our goals for learning:
• How big
g can a comet be?
• What are the large objects of the Kuiper belt
like?
• Are Pluto and Eris planets?
How big can a comet be?
Pluto’ss Orbit
Pluto
• Pl
Pluto
t will
ill never hit Neptune,
N t
even though
th
h their
th i orbits
bit
cross, because of 3:2 orbital resonance
• Neptune orbits three times during the time Pluto orbits
twice
Is Pluto a Planet?
•
•
•
•
•
By far the smallest planet.
Not a gas giant like other outer planets.
planets
Has an icy composition like a comet.
H a very elliptical,
Has
lli i l inclined
i li d orbit.
bi
Pluto has more in common with comets than
with the eight major planets
What is Pluto like?
• Its moon Charon is nearly as large as Pluto
itself (p
(probablyy made byy a major
j impact)
p )
• Pluto is very cold (40 K)
• Pluto has a thin nitrogen atmosphere that
will refreeze onto the surface as Pluto’s
orbit takes it farther from the Sun.
Sun
Other Icy
Ic Bodies
• There are manyy icyy
objects like Pluto on
elliptical, inclined orbits
beyond Neptune.
Neptune
• The largest of these,
Eris, was discovered in
summer 2005, and is
even larger
g than Pluto.
• Note it has a Moon
• Eris is no longer called
Xena or 2003 UB313
Pluto and Eris
• Pluto’s size was overestimated after its discovery
in 1930, and nothing of similar size was
discovered for several decades
• Now other large objects have been discovered in
Kuiper Belt, including Eris
• The International Astronomical Union (IAU) now
classifies Pluto and Eris as dwarf planets
• The gravity of a dwarf planet is not strong enough
to make it round
K iper Belt Objects
Kuiper
• These large,
g , icyy
objects have orbits
similar to the
smaller objects in
the Kuiper Belt that
become short period
p
comets
• So are they very
large comets or very
small planets?
Other Kuiper Belt Objects
• Most have been discovered very recently so
little is known about them.
• NASA’s New Horizons mission will study
Pluto and a few other Kuiper Belt object in
a planned flyby.
12 4 Cosmic Collisions: Small Bodies
12.4
Versus the Planets
Our goals for learning:
• Have we ever witnessed a major
j impact?
p
• Did an impact kill the dinosaurs?
• Is the impact threat a real danger or media
hype?
• How do the jovian planets affect impact
rates and life on Earth?
Have we ever witnessed a major
impact?
Comet SL9 caused a string of
violent impacts on Jupiter in
Summer of 1994, reminding us
that catastrophic collisions still
happen.
Tidal forces tore it apart during a
previous encounter with Jupiter
Summer of 1994 SL9 on direct collision course with Jupiter!
Impact plume
from a fragment
of comet SL9
rises high above
Jupiter’ss surface
Jupiter
Dusty debris at an impact site
Jupiter’s
Moon
Callisto
Why do we
Think we are
Seeing
FRESH
CRATERS?
What’s that
Bode for us?
Did an impact kill the dinosaurs?
ICE
Hungarian Stamp
Mass Extinctions
• Fossil record shows occasional large dips in
the diversity
y of species:
p
mass extinctions.
• Most recent was 65 million years ago,
ending the reign of the dinosaurs
dinosaurs.
Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks
but often found in meteorites.
• Luis and Walter Alvarez found a worldwide
layer containing iridium
iridium, laid down 65
million years ago, probably by a meteorite
impact.
impact
• Dinosaur fossils all lie below this layer
Iridium Layer
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
l
lower
rockk layers
l
Consequences of an Impact
• Meteorite 10 km in size would send large
amounts of debris into atmosphere.
p
• Debris would reduce sunlight reaching
Earth’ss surface
Earth
surface.
• Resulting climate change may have caused
mass extinction.
extinction
Red hot rock fell all over the earth. All forests were on fire!
Dust plunged Earth into a winter that lasted years
Likel Impact Site
Likely
• Geologists
g
have
found a large
subsurface crater
abo t 65 million
about
years old in Mexico
Comet or
asteroid
about 10
km in
diameter
approaches
Earth
Is the impact threat a real danger
or media hype?
Facts about Impacts
• Asteroids and comets have hit the Earth.
j impact
p is onlyy a matter of time: not IF but
• A major
WHEN.
• Major impacts are very rare.
• Extinction level events ~ millions of years.
j damage
g ~ tens to hundreds of years.
y
• Major
Impact features from space.
Bosumtwi impact feature
Clearwater impact craters
Deep Bay impact crater
Manicougan Impact feature (Canada) from shuttle
Wolf impact crater
Tunguska, Siberia: June 30, 1908
j disintegrated
g
and exploded
p
in
A ~40 meter object
the atmosphere
The Tunguska Event
• The Tunguska event
in Siberia in 1908
d t
destroyed
d an area th
the
size of a large city!
• Explosion of a large
object, probably an
Apollo asteroid of 90 –
190 m in diameter, a
few km above the
g
ground.
Area of destruction from the Tunguska
event superimposed
p
p
on a map
p of
Washington, D.C. and surrounding
beltway.
•Energy release
comparable to a 12megaton nuclear
weapon!
Meteor Crater, Arizona: 50,000 years ago (50 meter object)
The asteroid with our name on it
• We haven’t seen it yet.
• Deflection is more p
probable with years
y
of
advance warning.
• Control is critical: breaking
g a bigg asteroid
into a bunch of little asteroids is unlikely to
help.
• We get less advance warning of a killer
comet…
NEO’s
NEO’S
Long period comet
The worst problem
How do the jovian planets affect
impact rates and life on Earth?
Influence of Jovian Planets
Gravity
G
i off a jjovian
i planet
l
(especially
(
i ll Jupiter)
J i ) can redirect
di
a
comet
Jupiter has directed some comets toward Earth but has ejected many
more into the Oort cloud.
Was Jupiter necessary for life
on Earth?
Impacts can extinguish life
life.
But were they
y necessary
y for
“life as we know it”?
If the Dinosaurs were here would we be?
Freq enc of Impacts
Frequency
• Small impact
p
happen almost daily.
• Impacts large
enough to cause
mass extinctions are
many millions of
years apart
Impact videos
• Video 1 KT event simulation
Video 2 Apophis
p p p
possibilities
FOR MORE INFORMATION
NASA Impact Hazard Website:
What are we doing about it?
http:// impact.arc.nasa.gov
NASA NEO Program Website
http:// neo.jpl.nasa.gov
So you want to hunt Asteroids
IAU:MINOR PLANET CENTER
http://CFA-WWW.HARVARD.EDU