Download The Origin of the Solar System

H205
Cosmic Origins
APOD
Today: Forming the Solar
System
Finish EP6
The Origin of
the Solar
System
 All objects in the Solar System seem to have formed at nearly the
same time, out of the same original cloud of gas and dust
 Radioactive dating of rocks from the Earth, Moon, and some
asteroids suggests an age of about 4.5 billion yrs
 A similar age is found for the Sun based on current observations
and nuclear reaction rates
A theory of the Solar
System’s formation
must account for what
we observe:
 Planets orbit in the same direction and in the same plane
 Two families of planets: terrestrial & Jovian
 Compositions of planets
 Ages 4.5 billion years (or less)
 Other details – structure of asteroids, cratering of
planetary surfaces, detailed chemical composition of
surface rocks and atmospheres, etc.
The Solar
Nebula
Hypothesis
 Derived from 18th century ideas of Laplace and Kant
 Proposes that Solar System evolved from a rotating,
flattened disk of gas and dust (an interstellar cloud),
the outer part of the disk becoming the planets and
the inner part becoming the Sun
The Solar
Nebula
Hypothesis
 Explains the Solar
System’s flatness and
the common direction of
motion of the planets
around the Sun
 Interstellar clouds are common between the stars in
our galaxy and this suggests that most stars may have
planets around them
Interstellar Clouds
The cloud that formed
Solar System was
probably a few light
years in diameter and 2
solar masses
Typical clouds are 71%
hydrogen, 27% helium,
and traces of the other
elements
Interstellar Dust
Clouds also contain
tiny dust particles
called interstellar
grains
Grain size from large
molecules to a few
micrometers
They are a mixture of
silicates, iron and
carbon compounds,
and water ice
Dust grains are very small about 4000 could fit across a
sucker stick
In the Beginning…
Triggered by a
collision with
another cloud or a
nearby exploding
star, rotation
forces clouds to
gravitationally
collapse into a
rotating disk
The Solar
Nebula
 Over a few million years the cloud collapses into a rotating disk
with a bulge in the center
 This disk, about 200 AU across and 10 AU thick, is called the solar
nebula
 The bulge becomes the Sun and the disk condenses into planets
Temperatures in
the Solar Nebula
 Before the planets formed,
the inner part of the disk
was hot, heated by gas
falling onto the disk and a
young Sun – the outer disk
was colder than the
freezing point of water
Astronomers have observed
many gas/dust disks where
planets may be forming
Condensation
Condensation occurs when gas
cools below a critical
temperature at a given gas
pressure
 Gas molecules bind together to
form liquid or solid particles
 On Earth, water vapor condenses
to form clouds
 Steam condenses on the
bathroom mirror
Condensation in
the Solar Nebula
 Iron vapor condenses at 1300 K, silicates condense at 1200 K, and
water vapor condenses at room temperature
 In a mixture of gases, materials with the highest vaporization
temperature condense first
 The Sun kept the inner solar nebula (out to almost Jupiter’s orbit)
too hot for anything but iron and silicate materials to condense
 The outer solar nebula cold enough for ice to condense
The Formation of the Planets
Grains stick
together
 Next step is for the tiny
particles to stick
together, perhaps by
electrical forces, into
bigger pieces in a process
called accretion
 As long as collisions are
not too violent, accretion
leads to objects, called
planetesimals, ranging in
size from millimeters to
kilometers
Planetesimals
 Planetesimals in the inner
solar nebula were rockyiron composites, while
planetesimals in the outer
solar nebula were icyrocky-iron composites
 Planets formed from
“gentle” collisions of the
planetesimals, which
dominated over more
violent shattering
collisions
Formation of the Planets
Simulations show that
planetesimal collisions
gradually lead to
approximately circular
planetary orbits
As planetesimals grew
in size and mass their
increased gravitational
attraction helped
them grow faster into
clumps and rings
surrounding the Sun
Formation of the Planets
Planet growth was
especially fast in the
outer solar nebula due
to:
Larger volume of
material to draw upon
Larger objects (bigger
than Earth) could start
gravitationally capturing
gases like H and He
Craters
Everywhere!
Continued
planetesimal
bombardment and
internal
radioactivity melted
the planets and led
to the density
differentiation of
planetary interiors
Formation
of Moons
 Moons of the outer
planets were probably
formed from
planetesimals orbiting
the growing planets
 Not large enough to
capture H or He, the
outer moons are mainly
rock and ice giving them
solid surfaces
Where did OUR
Moon come from???
Lunar Formation Before Apollo
Three hypotheses for the Moon’s origin:
Co-Accretion Theory
Capture Theory
Fission Theory
The Spongmonkies: We Like The Moon
It’s the
little dot
inside!
The Earth-Moon System
The Moon’s diameter is ¼ of the Earth’s
The Moon’s surface is very dark, like a
charcoal briquet
The Moon’s mass is 1/80th of the Earth’s
The Moon has no atmosphere
Something’s Different…
 Unlike most of the other moons in the solar system, the
Moon is very large relative to its central planet
 These oddities indicate that the Moon formed
differently from the other solar system moons!
Earth and
Moon by
Richard
Swarts
Co-Accretion
Theory
Earth and Moon
were twins,
forming side by
side from a
common cloud of
gas and dust
Capture
Theory
Moon was
originally a small
planet orbiting
the Sun and was
subsequently
captured by
Earth’s gravity
during a close
approach
Fission
Theory
The Moon spun
out of a very
fast rotating
Earth in the
early days of
the Solar
System
One small
step for man,
one giant leap
for mankind
Moon
Rocks!
Hypotheses Failed!
Each of these hypotheses gave different
predictions about Moon’s composition:
In capture theory, the Moon and Earth would be very
different in composition, while twin theory would
require they have the same composition
In fission theory, the Moon’s composition should be
close to the Earth’s crust
Moon rock samples proved surprising
For some elements, the composition was the same,
but for others, it was very different
None of the three hypotheses could explain these
observations
So Where Did the Moon Come From?
Did the Earth and Moon form together as a
binary system?
NO…
Did the Earth capture the Moon after both
were already formed?
NO…
Did the early Earth break apart into the
Earth-Moon system?
NO…
What do the Moon rocks tell us?
The Moon was formed from the Earth’s crust
What Is the Earth Made of?
The Earth is
made of many
chemical elements
Iron, Silicon,
Magnesium,
Oxygen, Carbon
And water!
Earth rocks
contain water!
What’s Inside the Earth?
The chemical elements
on Earth are
separated (Earth is
differentiated!)
Iron and nickel in the
core
Lighter elements in
the mantle and crust
What about the MOON?
What is it made
of?
Where did it
come from?
It’s not
green
cheese!
What’s Inside the Moon?
The Moon has a
TINY iron core
The Moon does
not appear to be
as differentiated
as the Earth
Maybe there is a
tiny iron core inside
What Have We Learned from Moon Rocks?
The composition of moon rocks is
very similar to the composition of the
Earth’s crust
But, unlike the
Earth, the Moon
has very little
iron
And Moon
rocks are dry!
Giant Impact
Theory
Does it work?
It sounds like
science fiction!
The Large
Impact
Hypothesis
 Moon formed from
debris blasted out
of the Earth by the
impact of a Marssized body
 Age of lunar rocks
and lack of impact
site on Earth
suggests collision
occurred at least
4.5 billion years ago
Impact
Theory
 A large
body, about
the size of
Mars
impacts the
early Earth
Told through the art of William Hartmann
The
collision
blasts
material
from the
Earth’s
crust into
space
The material forms a disk around the Earth
 Material in the disk coalesces to form the Moon
 The early Moon is hot from the heat of accretion
The Large Impact Solution
This “large impact” idea explains:
The impact would vaporize low-melting-point
materials (e.g., water) and disperse them explaining
their lack in the Moon
Only surface rock blasted out of Earth leaving
Earth’s core intact and little iron in the Moon
Easily explains composition similarities and
differences with Earth
The splashed-out rocks that would make the Moon
would more naturally lie near the ecliptic than the
Earth’s equatorial plane – explains why the plane of
the Moon’s orbit is not the Earth’s equatorial plane
Explains Earth’s tilted rotation axis
What Happened to the Moon’s Iron?
The Earth
gobbled it!
Forming the Moon We See Today
As Moon’s surface solidified, stray
fragments from original collision created
craters that blanket highlands
A few of the larger fragments created
the large basins for the maria to form
By the time the maria filled with molten
material and solidified, little material was
left for further lunar bombardment –
thus the smooth nature of the maria
Highlands and Mare
The Early Moon
When the Moon formed it was much
closer to the Earth
The Moon is moving away from the
Earth at about 2 inches per year
The day and the month were originally
much shorter than they are today
Tides were originally much greater
Leftovers of the Solar System
 Asteroids and comets are
remnants of the
formation of the Solar
System
 Some may be planetesimals
 Best source of information
about the Solar System’s
early years
 Asteroids and comets play
a central role in planetary
impact and in particular
can have a large influence
on Earth’s biological life
Asteroids
Asteroid Gaspara, image
from Galileo spacecraft
 Asteroids are small, generally rocky bodies that orbit Sun
 Most asteroids (thousands) lie in the asteroid belt, a region
between the orbits of Mars and Jupiter
 The combined mass of all the asteroids is probably less than
1/1000 the mass of the Earth
The Asteroid Belt
Size and Shape
of Asteroids
 Asteroids are small, so their
sizes are best determined
from infrared
measurements: bigger bodies
emit more IR than smaller
ones at the same
temperature
 Asteroids range in size from
1000 km across (Ceres) down
to kilometer-sized objects
and even smaller
Asteroid Ceres from Hubble images
Asteroid Itokawa was visited by a Japanese spacecraft
in 2005. The spacecraft will return to Earth in 2010.
Asteroid
Composition
 Reflection spectra show that asteroids belong to three
main compositional groups: carbonaceous bodies, silicate
bodies, and metallic iron-nickel bodies
 Inner-belt asteroids tend to be silicate-rich and outerbelt asteroids tend to be carbon-rich
 Some asteroids are loose lumps of material held
together by gravity
Origin of the Asteroids
Asteroid
Belt
Structure
 Regions of the asteroid
belt seemingly empty of
asteroids are called
Kirkwood Gaps
 The gaps are caused by the
same resonance process
that causes the gaps in
Saturn’s rings
 Trojan asteroids are two
loose swarms located
along Jupiter’s orbit, 60°
ahead and 60° behind
Composition of
Comets
 Spectra of coma and tail show comets are rich in water, CO2, CO
 Solar UV radiation dissociates H2O, creating a large hydrogen
cloud around the comet
 After many close solar passes, comets’ gas is exhausted
 Density ~ 0.2 g/cm3 - comets are “fluffy,” not compacted ices
Origin of Comets
Most comets come
from the Oort
Cloud, the
spherical shell of
trillions of icy
bodies believed to
lie far beyond
Pluto’s orbit to a
distance of about
150,000 AU
The Oort Cloud &
the Kuiper Belt
 Comets originally orbited among the giant planets as planetesimals, then
were tossed into the Oort cloud by those planets
 The shape of the Oort cloud is determined from observations of comet
orbits
 Some comet orbits seem to come from a flatter, less remote region – the
Kuiper belt, which extends from Neptune’s orbit out to some unknown
distance – may contain much more mass than the asteroid belt
 Comets in the Oort cloud are a frigid 3 K and only warm up enough to
emit gas when they enter the inner Solar System
Late Bombardment, the Kuiper
Belt, and the Oort Cloud
 Evidence suggests that
the Solar System was
bombarded with impacts
about 3.8 million years
ago
 Probably resulted from
dynamical rearrangement
of the Solar System due
to the interaction of the
giant planets and the
comet swarm
This
model
explains:





giant planet orbits
the Trojan Asteroids (both for Jupiter and
Neptune)
the Kuiper belt
the giant planet irregular satellites
the “Late Heavy Bombardment”
http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html
Asteroids around Other Stars
Dust comes from
collisions of asteroids
We can detect dust with
Spitzer
Observed 24 micron IR
excess due to dust
Dust implies asteroids
and planets
For Next Week
Extrasolar Planets
Origin of Elements