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Astronomy 110: SURVEY OF ASTRONOMY
7. Cosmic Debris
1. The Asteroid Belt
2. Comets & Dwarf Planets
3. History of the Solar System
“Look here brother / Who you
jiving with that Cosmik Debris”
— Frank Zappa
Overview: Structure
100 AU
S
20,000 AU
5 AU
Inner system:
terrestrial planets,
asteroids.
Outer system:
giant planets and
moons, “KBOs”.
Oort Cloud:
comets.
In addition to the major planets, our solar system
contains a variety of asteroids, “dwarf planets”, and
other rubble. These objects continue to play an
important role in the evolution of major planets; they
also provide a window on conditions in the early solar
system.
1. THE ASTEROID BELT
a. Asteroid Properties
b. Belt Structure
c. Asteroid Impacts
Asteroid Properties
Comet Nuclei
Asteroids and comets to scale
Mission to Eros
Eros: The Final Approach
The Last Global Rotation Movie
Most asteroids may be rubble piles: loose collections
of fragmented rock held together by self-gravity.
Giant Asteroids
Ceres
975 km
Large asteroids
are complex
objects which
appear to have
differentiated.
Largest Asteroid May Be 'Mini Planet'
Vesta
Computer Model
530 km
Hubble Reveals Huge Crater on the Surface of the Asteroid Vesta
Meteors and Meteorites
A meteor enters Earth’s
atmosphere; a meteorite
survives the fall.
Fireball Meteor Over Groningen
Samples From the Asteroid Belt
Primitive meteorites
are old (4.6 Gyr); they
are relics of the solar
system’s formation.
Processed meteorites
come from differentiated
asteroids which were
fragmented by collisions.
Origin of
Processed
Meteorites
Key Stages in the Evolution of the Asteroid Vesta
Family Members
Crust
Magnesium-Sliicate
Mantle
(Olivine)
Surface
Lavas
Iron-Nickle
Core
Stony
Irons?
As smaller bodies in the early Solar System
fall together, the asteroid agglomerates.
Heavier elements sink to the
center as the asteroid heats.
This forms a separate core,
mantle, and outer crust. Lava
from the interior flows to the surface.
PR95-20 • ST ScI OPO • April 19, 1995 • B. Zellner (GA Southern Univ.), NASA
A fragment of Vesta
Hubble Maps the Ancient Surface of Vesta
Occasional impacts with other bodies
break off pieces of the crust, exposing
the mantle.
Hubble Maps the Ancient Surface of Vesta
Origin of
Processed
Meteorites
Key Stages in the Evolution of the Asteroid Vesta
Family Members
Crust
Magnesium-Sliicate
Mantle
(Olivine)
Surface
Lavas
Iron-Nickle
Core
Stony
Irons?
As smaller bodies in the early Solar System
fall together, the asteroid agglomerates.
Heavier elements sink to the
center as the asteroid heats.
This forms a separate core,
mantle, and outer crust. Lava
from the interior flows to the surface.
PR95-20 • ST ScI OPO • April 19, 1995 • B. Zellner (GA Southern Univ.), NASA
A fragment of Vesta
Hubble Maps the Ancient Surface of Vesta
Occasional impacts with other bodies
break off pieces of the crust, exposing
the mantle.
Hubble Maps the Ancient Surface of Vesta
Belt Structure
14 August 2006
Hildas
Trojans
Mars
Jupiter
Trojans
Wikipedia: Asteroid belt
Belt Structure
Hildas
Trojans
Mars
Inner Belt: a < 2.5 AU
Mid Belt: 2.5 AU < a < 2.8 AU
Outer Belt: a > 2.8 AU
Jupiter
Trojans
Wikipedia: Asteroid belt
Trojans
Hildas
Outer
Middle
Inner
Resonances With Jupiter
Resonances with Jupiter sort asteroids by orbital period;
period determines semi-major axis (Kepler III: P2 = a3).
Asteroid Families
Many asteroids are members of families; they have
similar orbits and compositions (indicated by colors).
Asteroid Belt Populations
Inner belt asteroids (left) and families (right).
Origin of Families
Hildas
Trojans
PO
W
Large Asteroid Breakup — Don Davis
!
Mars
Fragments are scattered
on similar orbits.
Jupiter
Trojans
Wikipedia: Asteroid belt
A Suspected Asteroid Collision
Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris
Origin of Near-Earth Objects (NEOs)
Mars
Some fragments wind
up on orbits which are
resonant with Jupiter.
Their orbits grow more
elliptical, finally entering
the inner solar system.
Wikipedia: Asteroid belt
Asteroid Impacts
Asteroid Impacts
Age: 49,000 yr
1.2 km
Barringer Meteor Crater, Arizona
Age: 212 Myr
70 km
Manicouagan, Quebec, Canada
Historical Impacts
Tunguska (1908): impactor exploded in
air with H-bomb force.
Jupiter (1994): string of comets
hit planet; visible from Earth
Chicxulub (pronounced tʃikʃu'lub)
Cretaceous–Tertiary extinction event
Impact crater map
Iridium-rich layer (65 Myr old)
Chicxulub Impactor:
A Possible Timeline
1. Baptistina parent body (170 km
diameter) smashed ~160 Myr ago.
Large Asteroid Breakup — Don Davis
2. Fragment hits Moon, forming
Tycho crater (110 Myr ago).
3. Fragment hits Earth, forming
Chixulub (65 Myr ago).
K-T event
Impact Threat
Small impacts are more
common than big ones.
Millions of years ago
Wikipedia: K-T extinction event
The fossil record shows
many mass extinctions
over Earth’s history.
Only K-T is associated
with a definite crater.
2. COMETS AND DWARF PLANETS
a. The Kuiper Belt
b. Comets
Largest Known Kuiper Belt Objects (and Satellites)
Wikipedia: Kuiper Belt
Pluto and Charon
Double planet with 2 small moons;
possibly formed by giant impact
(similar to Earth-Moon system).
• orbit mass: MPluto = 0.002 M⊕♁
3
density:
~
2
g/cm
•
rd rock, 2/3rd ice
composition:
1/3
•
• thin atmosphere: N2, CH4, CO
Pluto has probably differentiated;
Charon is too small to melt itself.
Hubble Maps Pluto
Pluto’s Orbit
Pluto’s orbit is highly tilted
(inclination i = 17°) to the
rest of the solar system.
Wikipedia: Pluto
Pluto is in a 3:2 resonance
with Neptune. This is a
stable resonance — no
real change can occur.
Pluto’s Orbit
KBO Orbits
Classical: outside
Neptune’s orbit
Resonant: like
Pluto’s orbit
Scattered: highly
elliptical
Plan View of the Solar System
KBOs and Comets
Classical and resonant
KBOs are safe from
Neptune’s influence.
KBOs above
this line cross
Neptune’s orbit
3:2
Scattered KBOs which
cross Neptune’s orbit
are easily perturbed.
The Resonant KBOs
These scattered KBOs may become comets.
Comets
Comets are icy
objects which fall
into the inner
solar system.
Warmed by the
Sun, they may
develop long tails.
un
To
S
Comets
Com
Mot et’s
ion
Comet Halley
At other times, a comet is an inert lump of ice & dust.
Changes in a Comet
Comet Nuclei
Asteroids and comets to scale
Deep Impact: the Nucleus of Tempel 1
Evidence of Cometary Ice
Analysis of collision
debris suggests Tempel
formed ~ 30 AU from
the Sun.
Tempel Alive With Light
Origins of Comets
After 1000 passages (or
less) a comet nucleus
disintegrates. Where do
new comets come from?
Short-period comets (P<200 yr):
• stay close to plane of ecliptic
• originate in Kuiper belt
• scattered by Neptune (et al. ???)
Long-period comets (P>200 yr):
• arrive from all directions
• originate in Oort cloud
• scattered by passing stars
Comets and Meteor Showers
Comets shed dust, sand and
gravel which slowly spread
out as they move along the
comet’s orbit.
If the Earth encounters one
of these trails, we get a
meteor shower.
Meteor Showers
Comet Encke
Perseid Meteor Shower
Raining Perseids
Major Meteor Showers
Forty Thousand Meteor Origins Across the Sky
3. HISTORY OF THE SOLAR SYSTEM
a. Four Facets of Formation
Four Facets of Formation
1. Cloud collapse
ordered motion of Solar System
Orbits & spins are “fossils” of motion in early Solar System.
2. Frost line
two types of planets
Terrestrial planets form near Sun, jovian planets further away.
3. Impacts & encounters
exceptions to the rules
E.g., Earth’s big Moon, and some objects with unusual motions.
4. Planet migration
rearrange outer Solar System
Form Oort cloud & Kuiper belt; cause late heavy bombardment.
Cloud Collapse
1. A gas cloud starts to collapse due to its own gravity.
2. It spins faster and heats up as it collapses.
3. Vertical motions die out, leaving a spinning disk.
4. The solar system still spins in the same direction.
1. What would happen if the gas cloud had no rotation
whatsoever to begin with?
A. The cloud would collapse more before forming a disk.
B. The cloud would collapse less before forming a disk.
C. The cloud would fly apart instead of collapsing.
D. The cloud would fall straight in and not form a disk.
2. Which of the following is not explained by the idea of
cloud collapse?
A. All planets orbit in nearly the same plane.
B. All planets orbit in nearly the same direction.
C. Most planets spin in roughly the same direction.
D. The square of a planet’s orbital period is
proportional to the cube of its semi-major axis.
The Frost Line
The disk was hot at the center, and cool further out.
Inside the frost line, only
rocks & metals can condense.
Outside, hydrogen compounds
can also condense.
The frost line was between the present orbits of Mars
and Jupiter — roughly 4 AU from the Sun.
The Frost Line: Jovian Planets
1. Outside the frost line,
icy planetesimals were
very common, forming
planets about 10 times
the mass of Earth.
2. These planets attracted nearby gas, building up giant
planets composed mostly of H and He.
3. The disks around these planets produced moons.
3. What would have happened if our solar system
formed with no oxygen (hence, no H2O)?
A. Only terrestrial planets (small, rocky) would form.
B. Only jovian planets (giant, gassy) would form.
C. Jovian planets might form beyond the CH4 and NH3
frost lines.
D. No planets of any kind would form.
4. Which of the following is not explained by the frost
line idea?
A. Terrestrial planets are much smaller than jovian
planets.
B. Jupiter and Saturn are composed of the same mix
of elements as the Sun itself.
C. Jovian planets have large satellites which orbit in the
same direction as the planet spins.
D. All jovian planets have rings.
Impacts & Encounters
1. Giant impacts in early solar system:
— explain rotation of Uranus,Venus
— form Moon from collision debris
2. Satellite capture after near-miss:
— moons of Mars captured from asteroid belt
— Triton captured from Kuiper belt
Impacts & Rotation
If proto-planets side-swipe and
merge, the angular momentum of
their initial orbit is transformed
into rotation of the merged planet.
Stellar Collisions
If the initial orbit is tilted with respect to the solar
system, the merged planet’s spin will also be tilted.
5. Venus spins backward and slower than any other
planet, taking 243 Earth days to rotate once. How
might this have come about?
A. Venus was side-swiped by a large asteroid, which
reversed its rotation.
B. Venus formed in a head-on collision, which left it
with almost no angular momentum.
C. Venus suffered a glancing collision with another
planet, without actually merging.
Formation of the Moon
Moon-forming impact
Mars-sized planet (Thea) hits
Earth about 4.5 Gyr ago.
Moon forms from debris:
This explains why Moon is poor in metals and volatiles.
6. Which of the following facts does the giant impact
hypothesis explain?
A. The Moon’s surface composition is similar to
Earth’s outer layers.
B. The Moon has a very small iron core for its size.
C. The Moon is poor in easily vaporized substances.
D. The Moon orbits the Earth in the same direction as
the Earth spins.
E. All of the above.
Planet Migration
A planet embedded in a disk around a star can excite
spiral waves — this process robs the planet of angular
momentum, causing it to spiral inward.
Planet Migration: The Nice Model
Migration is expected whenever planets interact with
disks; did this happen in our Solar System?
Wikipedia: Nice Model
1. Giant planets
born closer to Sun;
icy planetesimals
orbit in outer disk.
2. Jupiter & Saturn
migrate into 2:1
resonance; Uranus
& Neptune switch.
3. Planetesimals are
scattered outward,
populating Kuiper
belt & Oort cloud.
Outcome of the Nice Model
1. Kuiper belt drastically thinned and moved outward
to present position.
— many objects in resonances with Neptune
2. Majority of icy planetesimals scattered by Jupiter
into extremely elliptical orbits, forming Oort cloud.
— can’t form in place; density much too low
3. Some planetesimals scattered inward, explaining the
Late Heavy Bombardment.
— can match history of impacts on Moon’s surface