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

1
Lecture 5. Origin of the Solar System, Formation
of the Earth
reading Chapter 4
2
Early Observations of Saturn
Galileos 1616 sketch
Galileo first observed Saturns rings in 1610
with his new telescope. But they were fuzzy -
couldnt 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?
3
Nebular Hypothesis
French mathematician astronomer Pierre Simon
Laplace 1785 Used math to study Saturns
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.
4
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
5
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.

6
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

7
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)
8
Inner Solar System
Mercury
H and He get heated, pushed out. Ices vaporize,
pushed out. Rock left. Get Terrestrial Planets.
Venus
Venera 14 lander images of Venus
9
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
10
More Gas Giants
Uranus, taken by Keck telescope, 2004
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
11
Outer Solar System
Bodies formed more slowly, far apart. Pluto 40
AU, Surface T -235 to -210C 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
12
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.

13
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.

14
The Very Young Earth

• Molten at the surface - very hot.
• Sources of heat
• Impact heating melts rock
• violent surface
• Radioactive decay
• - unstable isotopes decay into more stable
  daughter elements
• - heat released
• - lots of radioactive elements early on
• - most long decayed
• Core formation
• Earth is very large - takes a long time for it to
  cool.

15
Differentiation of the Earth
16
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).
17
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 - found a star-like body - had
retrograde motion Mathematician calculated
orbit at 2.77 AU 1802 others found star-like
points there aster-oid total mass lt 10 of the
moon.
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
18
Lecture 6. Formation of the Moon, Absolute Ages,
Radiometric Dating
reading Chapter 4