Transcript Lecture 5. Origin of the Solar System, Formation of the Earth

Lecture 5. Origin of the Solar System, Formation of the
Earth
reading: Chapter 4
Early Observations of Saturn
Galileo’s 1616 sketch:
Galileo first observed Saturn’s rings in 1610 with his new telescope.
But they were fuzzy - couldn’t identify them - “ears”.
“Ears” changed shape.
1655 Christiaan Huygens, used a better telescope.
Discovered rings.
1675 Giovanni Domenico Cassini saw there were multiple
rings with gaps between them. (Cassini Division)
Many rings?
Rings solid?
Nebular Hypothesis
French mathematician & astronomer
Pierre Simon Laplace 1785
Used math to study Saturn’s rings.
Realized if solid, gravity would disrupt it.
Calculated Saturn must be a
rotating sphere of gas.
If mass in the center,
periphery rotates rapidly,
outer part distends
outward to form disk
If spinning faster, would form a ring.
If gravitational interactions in the ring, get several rings.
Then reasoned that if the center part is not a planet but a star, the
disk or ring could form planets - Nebular Hypothesis.
Idea also described by Immanual Kant in 1775.
Modern Nebular Hypothesis
New stars in Milky Way:
98% H and He
2% heavy elements
Start out with an interplanetary cloud of gas and dust The Solar Nebula.
Only way to collapse the cloud: gravitational perturbation
- start cloud spinning
- gravity will pull matter together
- most mass will concentrate in center
- cloud will spin faster
- cloud forms a disk, 1000 AU
- protoplanetary disk
Process of accretion
- gas and dust denser
- particles collide (bounce, stick, or break up)
- if stick, can stick to more particles
Accretion
Process of accretion not very well understood.
Disk made of same stuff as the interplanetary cloud:
- mostly H and He
- ices H2O, CH4 (methane), NH3 (ammonia)
- rock silicates
- evenly distributed in the disk
Particles for planetesimals = little planets
- when 1 km across, gravity attracts more particles
- few 100 km across, is planetesimal
- get thousands of planetesimals
Sun gets more massive, enters T Tauri phase.
Planetesimals impact each other and grow into planets.
Thermonuclear reactions begin.
T- Tauri Phase
Early Sun:
- lasts 100 million years
- H burning not yet begun
- heated as they contract and grow
- intense X-rays, radio waves, intense solar wind
- loses ~50% of its mass early on
- solar wind blows away residual gases & volatiles
- large radii
- about half have disks
Heating the Solar Nebula
Sun heats up the disk:
- inner part hot
- outer part cold
What happens when you heat:
- H and He (gets warmer)
- ices (melt or vaporize into gas)
- rock (gets warmer)
M V
E M
J
S
U
Mercury
Inner Solar System
H and He get heated, pushed out.
Ices vaporize, pushed out.
Rock left.
Get Terrestrial Planets.
Venus
Venera 14 lander images of Venus
Gas Giants
Cassini spacecraft image
during Jupiter flyby, 2003,
Not rocky, not icy.
Started to grow as large rocky/icy bodies.
Gravity started to suck in H and He from the disk.
Runaway gravitational attraction of gas.
Occurred before T Tauri phase.
Gas giants formed early.
Composition of solar nebula
& early Sun.
Jupiter is 5.2 AU
Saturn is 9.5 AU
Uranus, taken by Keck telescope, 2004
More Gas Giants
Uranus & Neptune have both
abundant gas and ices.
More enriched in C and N
than Jupiter and Saturn.
Uranus:
19.2 AU
83% H
15% He
2% CH4
Neptune:
30.1 AU
85% H
13% He
2% CH4
Neptune, taken by Voyager 2
Outer Solar System
Bodies formed more slowly, far
apart.
Pluto:
40 AU, Surface T -235 to -210˚C
70% rock and 30% water ice
other ices: methane, ethane, carbon monoxide
Kuiper Belt:
disk shaped region
30-50 AU many small icy bodies
source of short period comets
Oort Cloud:
much further out; spherical cloud
significant fraction of mass of solar system
(Jupiter mass)
extends out 3 light years!
source of long period comets
Pluto and Charon (its moon) taken
by Hubble Space Telescope
Evidence
See halos of dust and gas around other stars.
Similar dimensions and our solar system.
Also finding abundant gas giant
planets around other stars.
Young stars (T Tauri) have jets of matter
from intense solar wind.
Disks and planet formation may be
common in the universe!
Primitive meteorites:
- age of the solar system.
- early accreting bodies
composed of.
How Long Did it Take?
To form disk: 50,000 - 100,000 years
Initial accretion: 10 million years.
Disk destroyed by T Tauri star: by 25 myrs.
Lots of left over planetesimals and dust.
Slowly will impact other surfaces,
until most are gone.
Accretion is still occurring!
But most was finished by 3.8 Ga.
Evidence: craters on the Moon
-know age of Moon surface from Moon
rocks returned by astronauts
- observe craters sizes and abundance.
The Very Young Earth
Molten at the surface - very hot.
Sources of heat:
1. Impact heating melts rock
violent surface
2. Radioactive decay
- unstable isotopes decay into more stable daughter elements
- heat released
- lots of radioactive elements early on
- most long decayed
3. Core formation
Earth is very large - takes a long time for it to cool.
Differentiation of the Earth
We know interior of the Earth has a different composition from the
surface.
Crust - basalt and granite
Mantle - Mg and Si rich rock (plastic - 67% of Earth’s mass)
Outer core - molten S, Fe, Ni
32% of Earth’s mass
Inner core - solid S, Fe, Ni
}
Differentiation of the Earth, cont.
Process not well understood.
Heated planetesimals (silicate rock, volatiles, C).
Melting occurs.
Dense elements sink (Fe, Ni), others float (Si, Al, O).
Releases additional heat (gravitational potential energy).
Titius-Bode Law
1772 J. E. Bode
1776 J. D. Titus
Noted a progression of sizes of the orbits of the planets.
Distance derived by adding 4 to the series of numbers:
0, 3, 6, 12, 24, 48, 96.
Only something was missing!
1801 Giuseppe Piazzi of Sicily
- creating a star catalogue
Series - T-B
Actual AU
Law
- found a star-like body
Mercury
0+4=4
3.9
0.39
- had retrograde motion
Venus
3+4=7
7.2
0.71
Earth
6 + 4 = 10
10
1.0
Mars
12 + 4 = 16
15.2
1.52
???
24 + 4 = 28
-
-
Jupiter
48 + 4 = 52
52.0
5.20
Saturn
96 + 4 = 100
95.4
9.54
Mathematician calculated orbit
at 2.77 AU
1802 others found star-like points
there “aster-oid”
total mass < 10% of the moon.
Lecture 6. Formation of the Moon, Absolute Ages,
Radiometric Dating
reading: Chapter 4