Monday April 12th 1999
Announcements:

Lecture notes:

Class 38    "A Tour of the Solar System"
In this section of the course we will move back much closer tohome to study our solar system.  Our ultimate goals will be to
answer questions like:
-why are the planets of the solar system arranged the way theyare?
-how and when did our solar system form?
However before we can answer grand questions like these we will need a basic working understanding of the planets in the solar system.

First a scale model to give you an idea of the size of the solar system.
Earth = grain of salt 1/3 of mm in diameter
Moon = small speck of pepper 1 cm away
Sun small plum 4 meters from Earth
Mercury Venus Mars = grains of salt
Jupiter = apple seed 20 m away
Saturn = smaller seed 36 m away
Uranus and Neptune = large salt grains
Pluto = speck of pepper over 150 m away.

Solar system is mostly empty space.

Basic Revolution and Rotation Facts.
The solar system basically has a disk shape.  Leads to the ecliptic that we talked about in earlier classes.  The planets
revolve about the Sun in orbits that lie close to a common plane. Mercury is tipped by 7 degrees to Earth's orbit.
Pluto is tipped by 17.2 degrees.  All the rest are tipped by less than 3.4 degrees.

Rotation of Sun and planets is related to disk shape.  Sun's equator inclined only 7.25 degrees to Earth's orbit.  Most of the
other planets are tipped less than 30 degrees.  Except Venus rotates backwards and Uranus and Pluto are highly tipped.

All the planets revolve about the Sun in the same sense -- counterclockwise as seen from north.

Two basic kinds of planets.
Another powerful and unifying concept for solar system studies is that the planets can be grouped into 2 basic types.
type 1 = terrestrial planets = small sense rocky worlds.
Mercury Venus Earth Mars.
type 2 = Jovian planets = gas giants = large low density worlds.
Jupiter Saturn Uranus Neptune.

Pluto is an oddball - some people think it's not really a planet.

The use of these 2 general groupings is known as comparative planetology.  A helpful way to see the "big picture" about the
solar system without getting lost in the many details and interesting facts about each individual planet.
 
 

Terrestrial Planets	Jovian Planets
smaller size	larger size
closer to sun	farther from sun
higher temperature	lower temperature
rocky solid surface	gaseous surface
few or no moons	many moons/rings

Slides shown.

• Mercury - small rocky worlds only 40% larger than Earth's moon.  Thousands of overlapping craters from meteorite impacts.
• Mercury and Caloris - Caloris basin - immense bull's eye crater from asteroid impact.  1400 km across.
• Venus - similar size to Earth.  Cloud layers are clearly visible.  runaway greenhouse   Atmosphere is 100x as dense as Earth.
• Surface temp is 750K.  Rotates backward.
• Venus by Magellan - 1990's satellite.  Magellan radar map of full planet (cuts through clouds).  Fairly smooth surface but some mountains.
• Venus Mountain as mapped by Magellan - Venus has some volcanoes - may be currently active but no good proof for this.
• Earth from Space - hospitable climate and other conditions for life.
• Mars features - most of the planet disk is visible.  About half Earth's diameter and 1/10 the mass.  Polar caps have frozen CO2. Low density atmosphere <1% Earth's
• Mars - Olympus Mons - largest known volcano in the solar system - about the size of Texas at its base (700 km diameter 25 km high)  currently inactive.
• Mars - Viking 2 site - 1976 touchdown - red color due to iron oxide - rust.
• Mars - Mars pathfinder - July 4 1997 landing.  USA has active Mars exploration program going on now.
• Jovian planets - "Gas giants"
• Jupiter with its moons Io Europa - atmosphere bands and great red spot due to cloud layers.  Small set of rings and many moons.
• Jupiter - comparison of great red spot and Earth.  Earth sized hurricane.  Going on for at least 300 years.
• Saturn and its Rings - rings made of trillions of icy particles - dust grains to boulders.  Total mass is that of a small moon.
• All particles orbiting.  From tidal destruction of satellite?
• Close up of Saturn rings - rings made of tens of thousands of ringlets.  Also large ring gaps.
• Uranus - relatively featureless atmosphere.  Spin axis is tipped over to orbital plane - most extreme seasons in solar system.  Has rings and many moons.
• Neptune - has features - great dark spot and "scooter" clouds.  Blue color due to methane gas.  Has rings and many moons.
• Pluto and Charon - found only in 1930.  no good spacecraft pictures yet - only HST.  Has a moon Charon that orbits each 6.4 days.  Pluto diameter = 1/5 of Earth's Charon = 1/10 of Earth's.  We suspect it is mostly water ice.
• Comet SL9- from HST - comet was captured by Jupiter's gravity and torn apart into little bits - 21 fragments
• Comet SL9- summer 1994 impact.  Made scars as big as Earth.  Impacts were millions of megatons of TNT.  Such impacts probably happen each 100 years or so.

The age of the solar system.  People think that the Sun and the planets all formed at about the same time in the distant past.
We will discuss the details of this process in a later class but today we will focus on how people figured out when the planets
and sun formed.

The key to this is radioactive dating - radioactive dating lets people determine the ages of rocks with reasonable precision.

Rocks are ultimately made up of atoms.  So far in this class we have only discussed atoms with stable nuclei - that is if you
leave the atom alone it will remain unchanged for an arbitrarily long time.

However there are also atoms with unstable nuclei.  If you leave these alone their nuclei will slowly breakdown into lighter
"daughter" nuclei - this happens spontaneously.

This phenomenon of unstable nuclei is known as radioactivity - since radiation is also emitted as part of decay.

Examples are uranium 235 - (92 protons 143 neutrons)
                        uranium 238 -(92 protons 146 neutrons)
                        plutonium - 241 (94 protons 147 neutrons)
                        thorium - 232 ( 90 protons 142 neutrons)
Each type of radioactive element takes a characteristic time to decay into its "daughter" element - "half life" drops by 1/2
Not per atom but statistically.  So if we start with 1 billion atoms after 1 half life only 1/2 billion.  After 2 half lives
only 1/4 billion and so on.

The half life for U 235 is 713 million years.  For U238 it is 4.5 billion years.

Half lives have been precisely measured for many radioactive elements.

So to measure the age of a rock measure the amount of lead 206 and uranium 238.  Since we know the rate of decay we can work out the rock age.  If equal amounts of parent/daughter then 1 half life.

If 1/4 parent/3/4 daughter then 2 half lives.... etc.

In reality scientists use somewhat more complex methods that are more robust and reliable.

Scientists have gathered and dated rocks from:
Earth - oldest are 4 billion years old
Moon - oldest are 4.5 billion years old
Mars meteorites - oldest is 4.5 billion years
other meteorites - all about 4.5 billion years old.
Suggests that solar s system formed about 4.5 billion years ago.