Download Formation of the Solar System/Chapter 7

Homework #4
 Due Wednesday, February 24, 11:59PM
 Covers Chapters 6 and 7
 Estimated time to complete: 1 hour
 Read chapters, review notes before starting
Where did asteroids and
comets come from?
Asteroids and Comets
•
•
•
Unused leftovers from the accretion process
Rocky/metal asteroids inside frost line
Icy comets (with some rock/metal) outside frost line
Captured Moons
•
Unusual moons of some planets may
be captured planetesimals (such as
Mars’ moons).
Summary of Solar System Formation
4) Due to temperature gradient, rock/metal condenses
everywhere in Solar System, while hydrogen compounds
only condense to solids (ices) beyond frost line.
5) Planets grow via accretion from dust  planetesimals 
planets
6) Outer large planetesimals large enough to accrete a lot of
hydrogen/helium  form mini-Solar Systems with moons.
7) Solar wind (when Sun turns ‘on’) clears out remaining
hydrogen/helium gas to stop further planet growth.
8) Asteroids/comets leftover unused bits and pieces that didn’t
accrete onto larger planets.
How do we explain
“exceptions to the rules”?
Nebular theory has to be amended to account
for these exceptions.
Exceptions to the Rule
1. Venus rotates backwards on its axis – why?
1. Uranus rotates on its side – why?
1. There is water (a hydrogen compound) on Earth
and Mars, a place where hydrogen compounds
should not be – why?
2. Earth has a very large moon for a terrestrial planet
– why?
Our theory will need to account for these Solar System
anomalies.
Period of Heavy Bombardment
 Leftover
planetesimals
bombarded other
objects shortly after
Solar System
formation (few
hundred million years
after formation) 
many big collisions
led to the exceptions
to the rule
 Jupiter responsible
for a lot of chaos
Odd Rotation of Venus and
Uranus
 Giant impacts
might explain the
different rotation
axes of Venus
and Uranus –
each were
”smacked” at
some point
during the Period
of Heavy
Bombardment
Origin of Earth’s Water
Water may have
come to Earth by
way of icy
planetesimals
(large comets)
formed beyond
the frost line
colliding with
Earth during
Period of Heavy
Bombardment.
How do we explain the
existence of our large Moon?
Giant Impact
Mars-sized object collided with Earth during the Period of
Heavy Bombardment.
When did the planets form?
We cannot find the age of a planet, but we can
find the ages of the rocks that make it up.
We can determine the age of a rock through
careful analysis of the proportions of various
atoms and isotopes within it.
(isotopes: atoms with same number of protons,
but different number of neutrons)
Radioactive Decay
Some isotopes
decay into other
nuclei.
A half-life is
the time for half
the nuclei in a
substance to
decay.
Potassium (K)
spontaneously
decays into
Argon (Ar).
Age Estimation Via Radioactive Decay
40K
has a half-life of 1.25 billion years  decays to 40Ar
New rock has 100% 40K and 0% 40Ar.
Rock that is 1.25 billion years old has 50% 40K and 50% 40Ar
Measure ratio of 40K-to-40Ar  tells age of rock
(high ratio  young rock, low ratio  old rock)
(Carbon-14 dating) has a half life of only ~5700 years 
not suitable for dating objects millions orbillions of years old
(it’s a common myth that carbon-14 dating is used to
determine how old the Earth is or how old dinosaur bones
are)
14C
decays to 206Pb with a half-life of 4.5 billion years  get
consistent ages with 40K-40Ar studies.
238U
When did the planets form?
•
•
•
•
Radiometric dating gives us time since rock
crystallized (so the melting and re-forming
of rock, such as inside a volcano, “resets”
the clock for radiometric dating  rock is
“young” again.)
Planets, including Earth, probably formed
4.5 billion years ago.
Oldest meteorites are 4.55 billion years old.
Oldest moon rocks are 4.4 billion years old.
Chapter 6 Study Guide
1) Solar System (SS) – Sun, 8 planets (4 terrestrial, 4 Jovian),
dwarf planets, asteroids, comets
1) Sun – >99.9% of total mass of SS, 98-99%
hydrogen/helium
3) Terrestrial planets – small, near Sun, rock/metal, high
density, no/few moons, no rings
4) Jovian planets – large, far from Sun, gaseous (mostly
H/He/hydrogen compounds with small rock/metal cores),
low density, many moons, ring system
5) Asteroids – small, rocky/metal objects mostly in asteroid
belt between Mars and Jupiter (not remains of shattered
planet!)
Chapter 6 Study Guide
6) Comets – icy bodies beyond Neptune in Kuiper belt (40100 AU) or Oort cloud (~50,000 AU)
7) Rules of Solar System I: all planets orbit Sun in same
direction in same plane (most planets rotate in same
orientation too)
8) Rules of Solar System II: planets divided into inner
terrestrial and outer Jovian planets
9) Rules of Solar System III: asteroids, comets exist
10) Exceptions: Venus and Uranus’s strange rotation, Earth’s
large Moon, water on Earth
11) Nebular theory best describes formation of Solar System
Chapter 6 Study Guide
11) See Summary of Solar System Formation earlier in this
lecture  conservation of angular momentum, energy
play an important role, dust  planetesimal  planet
12) Inside frost line only rock and metal could condense
(terrestrial planets + asteroids), outside frost line
rock/metal/hydrogen compounds (ices) could also
condense (Jovians + comets)
13) As Jovians grew via accretion, they attracted large
amounts of H/He and grew very large
14) Jovians acted like mini-Solar Systems  moon systems
15) Planet growth ended when young Sun turned “on” and
generated a solar wind that blew away remaining gas
Chapter 6 Study Guide
16) “Exceptions” believed to be caused by an early “Period of
Heavy Bombardment” – large bodies hit Venus, Uranus
(changing rotation), and Earth (stripped matter formed
Moon, water brought to Earth by comets)
17) Age of Earth/Moon determined from radiometric
dating(for example, Potassium-40 turns slowly into Argon40), NOT carbon-14 dating (half-life for decay is way too
short)
Chapter 7
Earth and the Terrestrial
Worlds
Mercury
craters
smooth plains
cliffs
no atmosphere
“a geologically
dead” world
Venus
Volcanoes
Few craters
very thick atmosphere
extremely hot surface
Radar view of a
twin-peaked
volcano
Earth
volcanoes
few craters
mountains
riverbeds
moderate atmosphere
liquid water
Moon
craters
smooth plains
no atmosphere
“a geologically
dead” world
Mars
some craters
volcanoes
very thin atmosphere
(dried) riverbeds?
Insert ECP6 Figure 7.26
Why have the terrestrial planets (plus
Earth’s Moon) turned out so differently,
even though they formed at the same
time from the same materials?
Geological activity (or lack thereof) is the key
Earth’s Interior
Core: Highest
density; nickel
and iron
Mantle:
Moderate
density; silicon,
oxygen, etc.
Crust: Lowest
density; granite,
basalt, etc.
Why do water and oil separate?
A)
B)
C)
Water molecules repel oil molecules electrically.
Water is denser than oil, so oil floats on water.
Oil is more slippery than water, so it slides to the
surface of the water.
D) Oil molecules are bigger than the spaces between
water molecules.
Full credit for all answers, even if you are wrong.
Why do water and oil separate?
A) Water molecules repel oil molecules electrically.
B) Water is denser than oil, so oil floats on
water.
C) Oil is more slippery than water, so it slides to the
surface of the water.
D) Oil molecules are bigger than the spaces between
water molecules.
Full credit for all answers, even if you are wrong.
Differentiation
Important concept!
Gravity pulls highdensity material to
center.
Lower-density
material rises to
surface.
Material ends up
separated by
density.
Differentiation
happened when
planet was still hot
and liquid/molten.
Terrestrial Planet Interiors
Applying what we have learned about Earth’s
interior to other planets tells us what their
interiors are probably like, mainly from their
average densities.
What causes geological activity?
A planet’s internal heat determines the amount of
geologic activity  key point of understanding!
Can Rock Flow?
 Rock stretches when pulled
slowly (especially when
very warm) but breaks
when pulled rapidly.
 The gravity of a large world
pulls slowly on its rocky
content, shaping the world
into a sphere.
 Bodies over 500 km in
diameter will become
spherical in ~1 billion years
by slow, slow deformation
of rock by gravity.
Heating of Planetary Interiors
When
Earth
was
young
Now
Accretion and
differentiation when
planets were young
Potential energy 
kinetic energy  heat
Radioactive decay in
core is most important
heat source today
(Uranium, Potassium,
Thorium)