Download Dating the Universe

Dating the Universe
But first...
Graded Paper 1 returned today...
Lecture 6:
Formation of the Solar System
Astro 202
Prof. Jim Bell ([email protected])
Spring 2008
How did the Solar System Form?
!Observational Constraints
– Planetary motions
– Planet, asteroid, comet compositions
– Meteorite and cosmic dust ages
– Other solar systems?
!The Solar Nebula Model
– Formation of the Terrestrial Planets
– Formation of the Giant Planets
• Chemical Condensation ("Lewis") Model
Paper 2 is due at beginning of class on Feb. 14
Don’t wait until the last minute!
Paper 3 to be posted online next Tuesday 2/12
Observational Constraints
! For a theory of the formation of our solar
system to be viable, it must be consistent with
our presently-available data
! Theories must fit 3 classes of constraints:
(1) Motions: The observed spin and orbital motions
of the planets, asteroids, and comets
(2) Composition: The measured or inferred
compositions of planets, asteroids (meteorites),
and comets
!Planetary Evolution
Astro 202
4
"Motion" Constraints
Top view
! All of the planets orbit the Sun in roughly the same plane (the
ecliptic), which is very close to the Sun's equatorial plane
! The orbits of the major planets are nearly circular
! Planets, asteroids, and most comets circle the Sun counter-
clockwise as viewed from above (exceptions: some comets)
! The Sun and most of the planets generally spin in this same
counterclockwise direction and have very small tilts relative to the
plane of the solar system (exceptions: Venus, Uranus, Pluto)
! Nearly all the planets and asteroids spin with the same period,
Edge on
roughly 20±10 hours per "day" (exceptions: Venus, Pluto)
! The Sun contains 99.9% of the mass of the solar system, but the
planets contain 99.7% of the conserved angular momentum
Astro 202
5
Astro 202
6
7
Astro 202
8
Compositional Constraints
! The compositions of the planets vary with distance
from the Sun
–
–
–
–
The inner planets are dense and rocky/metal-rich
The outer planets are primarily gas (H, He) and ices
Asteroids in the main belt are transitional
Comets and KBOs are primarily ices
! Most of Jupiter and Saturn, and the atmospheres of
Uranus and Neptune, have approximately the same
composition as the Sun
! Many meteorites appear to be chemically different
(more "pristine") than planets and moons
Astro 202
Major Characteristics of our
Solar System: Summary
Age Constraints
! The primitive (unaltered) carbonaceous and
(1) Large bodies have orderly motions
(2) Planets fall into two main categories
ordinary chondrite meteorites ALL have
ages of approximately 4.5 billion years
! Tiny fractions of some very primitive
meteorites and cosmic dust particles have
ages even slightly older than this
! The oldest rocks from the Moon are about
4.4 billion years old
! More details in next 4 lectures...
• Small, rocky inner planets
• Giant, gas- and ice-rich outer planets
(3) Swarms of asteroids and comets abound!
(4) There are notable exceptions to the
trends
• Tilt of Uranus, tilt of Pluto's orbit, spin of Venus, orbit of
Triton, ...
Astro 202
9
Astro 202
Historical Models for the Formation
of the Solar System
The Solar Nebula Theory
! Catastrophic Models
! The leading model today is called the solar nebula theory
– A large comet collided with the Sun and ejected material that
cooled to form the planets [Buffon, 1745]
– Another star made a close pass by the Sun, tidally pulling
material out that cooled to form the planets [Bickerton, 1880]
! Nebular Models
– Sun and planets formed from a vast nebula of gas
[Descartes, 1644] that swirled around and condensed into
planets [Kant, 1755]
– Nebula contracted into a fast spinning disk, shedding "rings"
of material that formed the planets [Laplace, 1796]
– Previous generations of stars lived and died before our solar
system was formed ("cosmic recycling")
– The violent death of a previous star or stars formed an enormous
cloud of hydrogen, helium, tiny amounts of heavier elements, and
interstellar dust particles in this part of our galaxy
– This cloud collapsed under its own gravity and spun into a disk
– Most of the mass fell into the center of the disk and formed the Sun
– A tiny fraction of the leftovers formed the planets, moons, etc.
! All have problems with the basic physics or matching
some of the constraints, but nebular models "best"
Astro 202
10
11
– Some day (maybe 5 billion years from now) the Sun will die and
redistribute most of this matter back into an interstellar cloud...
Astro 202
12
Cosmic Recycling...
The Solar Nebula
Theory
Ours may not
be the first
solar system to
occupy this
part of the
galaxy...
Astro 202
13
Astro 202
14
The Violent Young Sun
"accretion"
The "Standard Model":
From
gas clouds to gas giants...
Astro 202
15
Original nebula may have had ~2 solar
masses of material; half of it lost by the
young Sun's strong "bipolar outflow"
Astro 202
16
Putting the age of the solar
system into perspective...
How long did this take?
! FAST according to computer models
– Contraction, spin-up of original cloud of gas takes
only a few million years
– Condensation of dust and settling of the dust into a
rotating disk may have taken only ~ 100 years!
– Clumping of dust by self-gravity into 1-5 km
planetesimals may have taken only ~ 1000 years!
– Accretion of planetesimals into ~1000 km objects
may have occurred over only ~1 million years
– Growth of a few objects to even larger Moon, Mars,
or Earth sizes in only about 100 million years
Astro 202
Our galaxy forms
17
The Cosmic Calendar
(from Carl Sagan’s Cosmos)
Astro 202
Implications...
Testing the Solar Nebula Theory
!The original catastrophic theories
! How do we decide if this is right? (we weren't there!)
assumed that the Sun formed first and
the planets formed later, by accident
– Do the models rely on the laws of physics and chemistry or
do they require miracles or ad hoc assumptions?
– Do the models accurately predict the present nature and
distribution of the planets?
– Implies that planets are rare
!Nebular theories assume that the Sun
– Are there clues in the compositions of the planets or comets
or meteorites that could help to support this model?
and planets form together
– Implies that planetary systems are common
Astro 202
18
19
– Can we look elsewhere in the galaxy for evidence for this
kind of process of solar system formation?
Astro 202
20
Compositional Evidence
Compositions of Meteorites
• A few special "primitive"
carbonaceous chondrite meteorites
contain grains of dust with ages
greater than 4.5 billion years
• Samples of the Sun's "parent body"?
• Confirms that meteorites (and by
inference, asteroids and comets)
provide the best way to study the
"building blocks" of the solar system
• Some of these ancient stones also
have complex organic molecules
and even amino acids
• The solar nebula theory
predicts a specific
chemical condensation
sequence as the hot
gaseous nebula cooled...
• In general, this sequence
is consistent with the
observed changes in
planetary composition
with solar distance
• Often called the "Lewis
Model"
Astro 202
Orion
21
Observational Evidence:
Nebulae...
Star-forming Regions...
Dusty disks around nearby stars!
"Brown Dwarfs" and
Planets around other stars!
Astro 202
23
Astro 202
Allende
22
Chemical Condensation Model
(the "Lewis" Model)
• Only rocks and metals could condense out near the Sun
• Farther away, lower temperatures allow lighter condensates
• The solar nebula theory
predicts a specific
chemical condensation
sequence as the hot
gaseous nebula
cooled...
• Model worked out by
astronomer John Lewis
and others (for example,
in the March 1974
Scientific American)
Astro 202
25
Astro 202
26
Assumptions...
Predictions of the Lewis Model
! Pressure, Temperature of the nebula
– From a different model...
! Composition of the nebula
– Solar: 75% H, 24% He, 1% everything else
! Phase diagrams of nebula materials
– When do solids condense from the gas?
! Equilibrium chemistry
– Slow solid/gas reactions, predictable products
! Rocky inner planets, icy outer ones
– Yes
! Mercury will have a large metallic core
– Yes
! H2O increases from Venus to Earth to Mars
– Maybe, but interiors not well understood...
! Volatile-rich materials in the asteroid belt
– Consistent with some asteroid spectra & meteorites
! "Unprocessed" outer satellites half rock, half ice
– Consistent with Ganymede and Callisto data
Astro 202
27
Astro 202
28
But there are problems...
Formation of the
Terrestrial Planets
! Pluto & Triton should be almost all ices
– But densities near “rockier” 2 g/cm3 ???
! Condensed grains merge into larger and larger
aggregates, forming 1-10 km size planetesimals
! Planetesimals grow by gravitational accretion
! The Moon
– Few volatiles, depletion of Fe and Ni
– Implies formation inside Mercury's orbit!
– But Moon rocks not primarily metal oxides!
! Small number of protoplanets 100-1000 km
(Moon, Mars masses) form
! Continued accretion of planetesimals adds
energy and heat, melting the growing
protoplanets
! Oddballs due to exceptional circumstances?
– Moon, Venus, Uranus, Pluto, ...
Astro 202
29
Self-Gravity... makes a planet?
! Melted planets differentiate and lose most of
their volatiles (gases and ices)
! Terrestrial planets not massive enough to be
able to hold on to light Hydrogen & Helium gas
Astro 202
30
Formation of the Giant Planets
! Additional supply of ices and other volatiles in the outer
R < 400 km
solar system allowed the protoplanets to grow much
larger, perhaps 5-10 times the mass of the Earth
! Masses large enough to keep Hydrogen and Helium
gravitationally bound to the protoplanet
! Giant protoplanets heated as their cores collapse under
more and more mass: same way stars start to form!
! But the giant planets in our solar system were not
massive enough to initiate fusion of Hydrogen into
Helium, so the cores cooled to their present state
Astro 202
R ! 400 km +
31
Astro 202
32
Solar Systems within Solar Systems
Giant Planets vs.
Terrestrial Planets
!Why did proto-Jupiter &
proto-Saturn grow so big
but proto-Earth didn’t?
!Possible solution:
Putative Jovian Nebula
– At lower Jupiter/Saturn temperatures, icy
materials could condense out of the nebula
– So: more solid material could condense
near Jupiter/Saturn orbit than near Earth
Solar Nebula
Astro 202
33
Jupiter/Saturn vs.
Uranus/Neptune
34
Jupiter Rules!
There appear to have been two possible paths for the
formation of the giant planets in our solar system:
! If Jupiter formed early enough, it
• If the protoplanet grew big enough (5-10 Earth masses)
very early when there was still lots of nebular gas:
- accretes a lot of gas, has a solar-like composition
- Jupiter and Saturn!
• If the protoplanet grew big enough (5-10 Earth masses)
but not until later, when there was less nebular gas:
- accretes only a little gas, less Sun-like composition
- Uranus and Neptune!
Astro 202
Astro 202
35
could have prevented the formation of a planet in
the asteroid belt.
! Why?
! Jupiter's gravity influences nearby planetesimals
! Nearby planetesimal orbits made more eccentric
– Higher eccentricity means higher impact velocities
– Higher impact velocities mean less "sticking"
– Planetesimals cannot accrete and grow very well
Astro 202
36
Planetary Evolution
Caveats
! After only 100 to 500 million years,
! The Chemical Condensation Model is a
simplistic view of a complex process!
– Was chemical equilibrium reached
everywhere?
– Many of the "leftovers" that we see today from
solar system formation have undergone
complex evolutionary histories
! Thus it is important to find and study the most
primitive solar system bodies (asteroids, comets)
Astro 202
37
most of the "debris" in the solar system was
either swept up by the planets or ejected
! The planets started to cool and assume their
present appearances
! Each planet or moon has followed it own
evolutionary course, governed by its size,
composition, solar distance, and the
occasional random catastrophic impact...
Astro 202
38
Astro 202
Initial "inventory" of
internal heat governs a
planet's evolution 40
How Do Interiors
Get Hot?
Heat In
Heat Out
Astro 202
39
Summary
Summary
! The solar system formed about 4.5 billion years ago
– More soon on where that number comes from...
! The leading theory is that it formed rapidly from a
! Observations of the Sun, planets, asteroids, and
comets can be used to constrain models for the
formation of the solar system
! The currently-favored model is the solar nebula
theory, which postulates that the Sun and planets
were formed from a spinning cloud of gas & dust
! This theory explains many of the observations
spinning cloud (nebula) of gas and dust
! Condensation and accretion of solids occurred quickly:
much of the "action" over in ! 108 years
! The outer planets were massive enough (especially
Jupiter and Saturn) to accrete gases from the nebula
! But the inner planets were not!
! Jupiter may have prevented the formation of another
terrestrial planet in the asteroid belt
– Motions of the planets
– Compositions of the Sun, planets, and meteorites
– Ages of meteorites and cosmic dust particles
Astro 202
41
Next Time...
How do we know how old space
rocks are?
Guest: Dr. Matt Hedman
Reading:
Read PBD 7
Check out
Matt’s book:
Astro 202
42