reading: Chapter 4

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

Early Observations of Saturn

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 multiplerings with gaps between them. (Cassini Division)

Many rings?Rings solid?

Galileo’s 1616 sketch:

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 distendsoutward 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, thedisk 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 He2% 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)

J S UMEVM

H and He get heated, pushed out.Ices vaporize, pushed out.Rock left.Get Terrestrial Planets.

Inner Solar System

Venera 14 lander images of Venus

Mercury

Venus

Gas Giants

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.

Cassini spacecraft imageduring Jupiter flyby, 2003,

Gas giants formed early.Composition of solar nebula

& early Sun.Jupiter is 5.2 AUSaturn is 9.5 AU

Uranus & Neptune have both abundant gas and ices.More enriched in C and N than Jupiter and Saturn.

Uranus:19.2 AU83% H15% He2% CH4

Neptune:30.1 AU85% H13% He2% CH4

More Gas Giants

Uranus, taken by Keck telescope, 2004

Neptune, taken by Voyager 2

Outer Solar System

Pluto and Charon (its moon) takenby Hubble Space Telescope

Bodies formed more slowly, farapart.

Pluto:40 AU, Surface T -235 to -210˚C70% rock and 30% water iceother ices: methane, ethane, carbon monoxide

Kuiper Belt:disk shaped region30-50 AU many small icy bodiessource of short period comets

Oort Cloud:much further out; spherical cloudsignificant fraction of mass of solar system

(Jupiter mass)extends out 3 light years!source of long period comets

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.

To form disk: 50,000 - 100,000 yearsInitial 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.

How Long Did it Take?

The Very Young Earth

Molten at the surface - very hot.

Sources of heat:1. Impact heating melts rock

violent surface2. 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 thesurface.

Crust - basalt and graniteMantle - Mg and Si rich rock (plastic - 67% of Earth’s mass)Outer core - molten S, Fe, NiInner core - solid S, Fe, Ni } 32% of Earth’s mass

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. Bode1776 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!

Series - T-B Law

Actual AU

Mercury 0 + 4 = 4 3.9 0.39

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

1801 Giuseppe Piazzi of Sicily- creating a star catalogue- found a star-like body- had retrograde motion

Mathematician calculated orbitat 2.77 AU

1802 others found star-like pointsthere “aster-oid”

total mass < 10% of the moon.

reading: Chapter 4

Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating