Lecture Notes
Arny Chapter 7
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We will not cover section 7.3.
Chapter 7 - Survey of the Solar System
Due to lack of time, we will spend
only one day covering the solar system.
That's the bad news, the good news is chapter 7 is a single chapter summarizing
the solar system, perfect for us.
Some of the material we've already learned or will learn in lab.
Planets all orbit the Sun in the same direction,
as do most of the asteroids and comets.
The solar system formed from a giant cloud of gas and dust, gravity and
spin shaped that cloud into a disk; that explains the shape and orbits of
the solar system.
More about this later.
The planets (ignoring Pluto) form two distinct
groups or families:
Inner rock and metal planets.
Outer gas giant planets.
The Sun and planets all appear to have
been formed about 4.5 billion years ago.
Section 7.1 - Components of the Solar System
The Sun:
More than 700 times more mass than the rest of the solar system combined!
71% H, 27% He, + heavier elements
(Note: H (hydrogen) and He (helium) are the two lightest elements.)
The Sun is about 10 times bigger than Jupiter which is roughly 10 times
bigger than the Earth (a million times larger than the Earth in terms of
volume).
[See figure 7.1 on page 217.]
Inner or Terrestrial Planets:
[See figures 7.2, 7.3, and 7.8.]
Earth, Venus, Mars, Mercury, and the Moon (from largest to smallest) are
all similar.
All are in the inner part of the solar system.
They are all small (compared to other planets and the Sun), from 6378 km
in radius (Earth) to 1738 km (Moon).
All are made of rock and metal.
Spectroscopy (chapter 3) can be used to determine the composition of atmospheres
and surfaces of planets.
But what about the interiors?
Density is a convenient and easy way of estimating interior compositions.
Density (often written as the Greek letter rho) is mass divided by volume.
Half-way between 3 and 8 is 5.5, nearly
the density of Venus.
You could come up with other possibilities but this simplest guess is believed
correct.
Venus is composed of about half rock and half metal.
| Planet | Density | Composition |
| Mercury | 5.43 g/cm^3 | half rock, half metal (60% met) |
| Venus | 5.25 | half rock, half metal |
| Earth | 5.52 | half rock, half metal |
| Moon | 3.34 | mostly rock, a little metal |
| Mars | 3.93 | 80% rock, 20% metal |
| Jupiter | 1.33 | rock, metal core H liquid, gas |
| Saturn | 0.69 | Similar to Jupiter (more gas) |
| Uranus | 1.32 | Similar to Jupiter |
| Neptune | 1.64 | Similar to Jupiter |
Asteroids:
Most are made of rock-type material, some are pure metal.
Small, maybe 10 km across on average.
Most orbit in the asteroid belt, located between the orbits of Mars
and Jupiter.
Mars has two small moons that are probably captured asteroids.
Comets:
Mostly made of ice.
The most common liquids and ices in the solar system are
water (H20)
carbon dioxide (CO2)
ammonia (NH3) and
methane (CH4).
Comets are small, typically 10 km across.
The Kuiper Belt is like the asteroid
belt, but of comet-like bodies.
The Kuiper Belt extends beyond the
orbit of Neptune.
[No good picture of this in the text,
picture a swarm of dots in a ring
shape stretching beyond Neptune.]
Pluto is actually the largest of these
Kuiper Belt Objects (KBOs), if we
had known about the Kuiper Belt
before discovering Pluto, it would
not have been called a planet.
Occasionally a KBO will be deflected
(by some near collision?) and fall into
a highly elliptical orbit.
When these objects pass close to the Sun, the ices evaporate away and create
the long tails we associate with comets.
Kuiper Belt comets orbit in the ecliptic
plane, in normal direction around Sun.
These become "short-period comets".
There appears to be a second reservoir
of potential comets in the solar system.
The Oort Cloud is believed to be more than a trillion potential comets
occupying a vast sphere (not a flat disk like the Kuiper Belt) far beyond
the planets.
[See figure 7.6 on page 221.]
Some of these objects fall into the solar system and appear as comets as
well.
Oort Cloud comets usually have orbits highly
inclined, often retrograde (opposite to normal orbiting direction).
These are "long-period comets" (usually they come in and go out,
not to return for maybe thousands of years).
Halley's comet came from the Oort Cloud but had a close encounter with a
planet like Jupiter or Earth and now has a short-period (76 year) orbit.
Moons:
The inner planets have few or no moons.
The outer planets have many moons, like miniature solar systems.
Some moons appear to have formed in place around their planet, others appear
to have been captured from elsewhere.
Section 7.2 Origin of the Solar System
We have discovered many patterns in the solar system.
Now we try to answer the questions posed by those patterns, namely
Why are inner planets all made of rock
and metal?
Why are outer planets larger with lots of liquids and gases?
Why is there an asteroid belt?
Why is there an Oort Cloud?
and more.
The nebula probably had a slight rotation
to begin with, maybe because it is part of the larger rotating Milky Way
galaxy.
As the nebula contracted in size, its spin increased.
Material in the disk is moving correctly
to orbit the center.
Material above or below the disk is pulled inward.
The cloud, which began spherical, is shaped into a disk (pancake shape)
by the rotation and gravity.
See figure 7.10 on page 226.
Most of the cloud's material fell into the center forming a single large
object that became our Sun.
Things are starting to get explained!
That most of the material fell into the center explains why the Sun is so
much larger than any planet.
The disk shape of the cloud explains why all the planets ended up orbiting
in the same plane.
The original cloud's rotation explains why the planets orbit the Sun all
in the same direction that the Sun rotates.
So we have a central Sun
surrounded by hot, circula-
ting gases.
Gas will be much hotter
closer to the Sun (material
which fell in further and is
now being warmed by Sun).
See figure 7.13 on page 229.
Hot close to Sun, cooler further out.
The contracting gases got very hot.
But the orbiting gases will now
start to cool off.
As the gas cools, particles can condense.
The particles slow down and stick together to form solids.
Like water condensing to form dew when moist air is cooled or steam condensing
into drops of water on surfaces.
Metals condense when the temperature drops
below about 1300K.
Rocks condense when the temperature drops below about 1000K.
Ices (H20, NH3, CH4, CO2, ...) condense at temperatures below 300K.
Gases (H, He) only condense if the temperature goes below a frigid 50K.
Near the Sun it was hot; only rock
and metal condensed.
The other elements remained as gases.
Far from the Sun it was cooler;
rocks, metals, and ices all condensed.
But not cold enough for H and He gases to condense.
The condensed particles (similar to snowflakes)
start to stick together to form boulders, this is called accretion.
Further accretion builds up mountain-size objects, typically 10 km across.
These are called planetesimals ("little planets").
Gravity pulls planetesimals together to form planets.
Violent collisions cause the newly-formed planets to be very hot.
The hot planets undergo differentiation, heavier elements sinking
to centers of the planets.
This is why the Earth and other planets tend to have the heavier metals
in their core and lighter rocks and ices towards the surface.
We've explained so much more...
The inner solar system was full of rock and metal planetesimals.
So rock and metal planets formed in the inner solar system.
The outer solar system was filled with
rock, metal, and ice planetesimals.
More ice than rock and metal because ice made of more common elements.
The outer planets and moons are made more of ice than rock and metal.
Because there were more planetesimals,
larger planets formed in the outer solar system.
The planets got so large that they could suck in and hold onto the H and
He gas around them.
This explains how outer planets got so large and it explains their composition.
The Jovian planets controlled the gas around
them like the Sun controlled the gas around it.
Thus we can understand also how the giant planets ended up with many moons
looking like miniature solar systems.
All of this occurs in maybe just a few
million years.
Then the Sun got going, the light and particles from the Sun (the "solar
wind") pushes all the remaining gas and dust out of the solar system.
No more planetesimals can form.
Not all the planetesimals were used up
in forming planets (and moons).
What happened to the leftover planetesimals?
They would continue to orbit the Sun, perturbed occasionally by planets,
until
* crash into the Sun
* crash into a planet or moon causing a crater (the craters covering
the planets and moons are a testament to this, see figure 7.14 on page
231).
* were captured as a satellite (small moon)
* found a safe orbit
Asteroids are inner solar system planetesimals which have survived until
today, prevented from pulling together into a planet due to interference
from nearby Jupiter's gravity.
Asteroids are very interesting, they're the leftover raw material from which
planets like the Earth were formed.
The Kuiper Belt is a haven for outer solar system planetesimals.
The Asteroid Belt and Kuiper Belt are locations where planetesimals could
orbit safe from planetary perturbations.
* were deflected out of the solar system
This is largely the origin of the Oort Cloud, comets (planetesimals) thrown
out of the solar system by gravitational encounters with large planets.
There may have been more planets early
on then there are now.
There may have been titanic collisions between planets and moons.
But even that can explain features found in the solar system:
It is believed that our Moon was once a
Mars-size planet that collided with the Earth!
Part of that object rebounded in the collision and settled into orbit as
our Moon.
[Discussed in the text in section 6.4, see figure 6.15 on page 200.]
Venus rotates backwards from the direction
the other planets rotate (the other planets all rotate in the same direction
they orbit the Sun).
Venus seems to be "upside-down".
This may have occurred because of some huge collision.
Other mysterious discoveries in the solar system can also be explained as being the result of such collisions.
< ** Lecture Usually Ends Around Here ** >
Planetary Atmospheres
The giant planets have so much gravity that they can hold onto anything
that comes their way.
They have the same "atmosphere" they've always had, an outer layer
of mostly hydrogen gas.
Mercury (and the Earth's Moon) have no
atmosphere, both for the same reason.
They are fairly small and have low escape velocities.
Any atmospheric gases are heated enough by the Sun that they can escape
those planets.
For the Earth, Venus, and Mars, lighter
gases (which tend to move faster) like H and He can escape into space.
The actual composition of the atmosphere can be due to a combination of
effects:
Original gases when formed.
Gases from the planet's interior, volcanic vents and the like.
Gases delivered from outside, like colliding comets.
And in the case of the Earth, there is a further processing of the gases
by the life on Earth.
We are skipping section 7.3 although you may find it interesting reading.
End chapter 7.
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