index:
A Survey of the Solar System:

Origin of the Solar System:
     Solar Nebula Theory
     Stages in Planet Building

TwoKindsofPlanets:
     Terrestrial (Earthlike) Planets
     Jovian (Jupiter-like) Planets

The Earth as a Planet:

The Moon: Our Nearest Neighbor:
       The Geometry of the Sky:
     Earth, Moon, and Sun
        (phases, eclipses, tides)

Lessons from 
    Comparative Planetology

"Gravityís Dance"

Individual Planets

Major moons:

Meteorites, Comets and
    Asteroids:
 

A Survey of the Solar System

• The solar system is composed almost entirely of empty space.

 -- 99% of the mass of the solar system is in the Sun.
 -- the sizes of the objects in the solar system are comparable to a tennis-ball-sized Sun and grains of sand and "bb"s strewn across an area far larger that a football field.
 
• All the planets revolve around the Sun in nearly the same plane.

 -- Pluto's orbit tipped 17.2°, but all the others are nearly coplanar (meaning that visually they all take nearly the same path around the sky - through the zodiac and near the ecliptic plane).
 
• The rotation of the Sun on its axis and all the planets are in the same (counterclockwise) direction.

 -- exceptions (Venus, Uranus, Pluto rotation axes) explained by catastrophic events.
 -- revolutions of most satellites are in the same (counterclockwise) direction and in nearly the same plane. Exceptions (retrograde orbits) like Saturn's Pheobe are probably all captured moons.
 
• Each planet is a little more than twice the distance from the Sun as the nearest inward neighbor [Titius-Bode rule].

 -- exceptions are only for "missing planet" at the radius of the asteroids and imperfect agreement for the position of Pluto, which is thought to be an ejected moon.
 
• Planetary material:

 -- ~75% hydrogen   ~25% helium   ~2% heavier elements
 -- this is nearly identical to the composition ratios of the Sun itself.
 
• Common age of all dated materials - 4.6 billion years.

 -- Moon rocks, meteorites, Sun's evolutionary age, etc.

Any description of the origin of the solar system (or any other planetary system) must be able to explain the origin of these properties, both similarities and differences.

• Solar Nebula Theory:

 -- all stars form because of the gravitational contraction of the denser parts of very large clouds of gas and dust.
 -- as the star is being born (contracting and heating as a protostar, prior to the turn-on of nuclear fusion), it remains surrounded by the cocoon of gas and dust.
 -- rotation of the cloud flattens much of the material into a spinning disk around the protostar's equator.
 -- planets form from this rotating disk of gas and dust around the young star. 
 * first condensation of small clumps of material from gas and dust. 
 * then accretion into larger clumps called planetesimals. (this is the same as is found in asteroids, meteorites, and cometary material)
 * the largest clumps attract the most additional material (because they have the strongest gravity), sweeping up the smaller chunks, and more gas and dust to form protoplanets and finally the planets.
 -- the condensation sequence places large quantities of dense solid material such as metals and rocky material (which can condense at higher temperature) near the Sun, and large quantities of lighter, less-dense material such as hydrogen, helium, water, and hydrocarbons (which will not condense unless it is much colder) far from the Sun.
 -- as the nuclear fusion of the star turns on, the gas and dust cocoon is blown away by the increasing stellar wind.

This is a very general process!! It should be common to the  formation of all stars. 

* Evidence of protoplanetary disks of warm dust found around such nearby stars as b-Pictoris,t-Tauri and Vega.
* Using Doppler and other techniques, many planets have been found orbiting other stars, including complex planetary systems with up to four planets (these searches find large  planets in small orbits near the star most easily.)

Stages in Planet Building: (can also be called "the History of Terrestrial Planets" because evidences of these stages are seen most easily on the solid surfaces of these planets.)

  -- Condensation and accretion: the process that brought the brought the matter of the planet together into one place also left the early planetary surface scarred with craters.

  -- Differentiation: internal heat caused the molten interior of the planet and separation of the dense material to the core and lighter, less dense material to the surface.

  -- Volcanism and flooding: the hot interior of the planet and the cooler solid crust floating on top is breached by volcanoes and the shifts of plate tectonics. This process left the low-lying areas of the surfaces covered with nearly flat basaltic sheets of lava (mare, or maria(plural)), and on Earth with a covering of water (the oceans). 

  -- Weathering, surface erosion, and sedimentation: these processes remake the planetary surfaces by the interactions of the planetary lithosphere (atmosphere, crust, and upper mantle.) 

  -- Freezing of the planetary interior: Planetary interiors cool at different rates, with large planets cooling the slowest and tiny worlds cooling rapidly.

The  same process is seen throughout the solar system; the only difference is the condensation temperature and distance from the Sun
 -- Some planets and natural satellite seem to have stopped and frozen part-way through the process.
 -- meteoroids, asteroids, and the Oort cloud (the origin of comets) are all thought to be debris left over as remnants of the creative process that built the planets.
(Condensation accretion, differentiation, and weathering take place on the Jovian planets, but without solid surfaces the cycle is clearly incomplete; and in the smallest planets which have already cooled internally to a completely solid state, the process is essentially complete, with no residual activity.)

Terrestrial (Earthlike) Planets:

 -- small, dense, rocky: densities from 3.5 - 5.5 gm/cm³
 -- thin tenuous atmospheres; contain very little H₂ or He
 -- lie in the inner solar system close to the Sun; small, fast orbits take less few years.
 -- have few moons and no rings.
 -- generally rotate on their axes rather slowly
 -- all went at least part way through the "history of terrestrial planets" sequence (also observable on the solid moons of the outer solar system.) Mercury's and our Moon's surfaces appear frozen with heavy cratering (old surface); while the Earth, which is much larger and thus cooled slower, still has volcanism, plate tectonics, and weathering ongoing.
 

Jovian (Jupiter-like) Planets:

 -- large, gaseous, low density: average densities all less than 1.5 gm/cm³
 -- dense atmospheres; nearly 85% H₂, 15% He, traces of others including methane, ammonia, and other trace gases (which give these planets their colors); these planets are almost 'starlike' in their composition; also fantastic weather patterns and strongest (fastest) winds in the solar system.
 -- lie in the outer solar system far beyond the asteroid belt; long, slow orbits which take many years.
 -- each have complex ring systems.
 -- each have many moons;  the counts for each of the observed moons for each jovian planet are now up around thirty, with more small moons being found each year. Some of the moons are as large as the terrestrial worlds.
 -- interiors - liquid molecular and metallic hydrogen, with small innermost core of ice and rock.
 -- motion of liquid metallic hydrogen in the planetary interiors cause dynamo effect and large magnetic fields.

Earth's Interior:

  -- studied by plotting seismic waves from earthquakes.
  -- thin crust (20-70 km thick) and 

*  the light upper part of the earth's interior;
*  a thin lower-density rocky layer, very active, floating
  on the denser mantle. 
*  plate tectonics caused by convection in the mantle, moves the parts of the surface around continually.
*  edges of the plates are areas of mountain ranges and trenches caused by pushing of one plate over the top of the other, and of earthquake activity and volcanoes.

  -- thick mantle (~3000 km thick) made up mostly of silicate (or rocky) materials.

*  thick layer of metal oxides, 
*  not molten but 'plastic', and undergoing continuous convection.

  -- core (~3500 km thick) made of liquid (outer) and solid (inner) metallic iron and nickel.

*  high density; primarily molten, except the inner core that is under too high pressure.
*  flow of liquid metal in Earth's core leads to Earth's magnetic field by dynamo effect.

 The Age of the Earth:

  -- ~4.5 billion years, although the oldest rocks on the Earth's surface are less than 3.6 billion years (most of the oldest rocks are destroyed by the continual weathering processes.)
 

 Earth's Atmosphere:

  -- origin is primarily from gases given off from volcanoes and from comet impacts [read text carefully.]

*primeval atmosphere  -- hydrogen, helium, methane, ammonia, from the solar nebula. No O₂ or O₃.
*  present atmosphere -- planet accretes planetesimals with volatile materials such as carbon dioxide, water vapor, and nitrogen;  UV broke up methane, etc.; life consumed carbon dioxide, which freed O₂ into the atmosphere.

  -- atmosphere has gone through many stages of evolution, continually eroding the crust.
  -- ozone layer and UV: O₃ blocks most ultraviolet radiation, which can mutate cells and cause cancer, from reaching the earth's surface (but O₃ can be destroyed by reaction with chloroflourocarbon compounds such as freon®).
  -- greenhouse effect: caused by trapping of infrared radiation deep in the atmosphere by carbon dioxide and water vapor.
  -- weather caused by interaction of Sun's heating and convective flow, with the water brought in from over the oceans to over the land by prevailing wind patterns influenced by the Earth's rotation [eastward flow at temperate latitudes ('Canadian fronts')  and westward flow at tropical latitudes (hurricanes).]
  -- cycle of evaporation, clouds, precipitation, and erosion causes weathering.
 

 Earth's Magnetosphere:

  -- dynamo effect from convection and churning of Earth's liquid metal core causes Earth's magnetic field.
  -- interaction of Earth's magnetic field and charged particles from the solar wind cause the aurora ('northern lights') in annular regions near the Earth's north and south magnetic poles.
 

Earth's Seasons:

  -- caused by 23.5° tilt of the Earth's axis. [More information elsewhere.]

the History and appearance of the Moon:

  -- the Moon is less dense than the Earth, and is made up of many of the same materials that make up the Earth's mantle and crust, and has the same age: 4.3 billion years -- giant collision theory.
  -- the surface is heavily cratered, and has large basaltic (dark colored rock) planes called maria -- lava flow sheets. Highlands and mountains are result of old tectonic activity, or ancient huge impacts. The lunar interior is probably cold enough now to be fully solid throughout.
  -- the same side always faces the Earth; the back side is far more heavily cratered.
  -- the Moon is the only celestial body visited so far by manned space probes from Earth. -- July 20, 1969; Neil Armstrong and Edwin "Buzz" Aldrin, Apollo 11, and five later mission. 
  -- recently water found on floors of polar craters.

the Phases of the Moon:

 -- [More information elsewhere.]

Eclipses:

 -- [More information elsewhere.]

the Tides:

 -- [More information elsewhere.]

• Planetary Atmospheres:

   -- Ozone:
  * in the earth's upper atmosphere; losses due to chloroflourocarbon compounds like Freon® (manmade).
  * absorbs UV radiation - protects against skin cancer and mutations; Mars has none.
  -- carbon dioxide: 
   * absorbs infrared radiation and increases the Greenhouse effect; rising temperatures could flood large coastal areas.
   * increasingly denser atmosphere could trap more heat.
   * CO₂ levels strongly affected by mankind's burning of fossil fuels.
   * worlds with liquid water oceans absorb CO₂, and reduce the greenhouse effect (difference between Venus and Earth.) [NOTE: that there are actually opposing actions here from water, because gaseous water in the atmosphere as clouds or vapor act as greenhouse gases just like CO₂, and increase atmospheric heating.]
  -- liquid water:
   * liberated from rocks in interior during volcanism/ flooding stage.
   * a large amount of water found in rocky meteorites.
  -- free oxygen (O₂):
   * only liberated in large quantities by biological activity in early life.
   * Paradox that life needs oxygen, but that the best way to increase oxygen is life.
-- wind systems:
   * long lasting cyclonic storms and high winds (up to 1000 mph) are found on the jovian planets, and are an interesting and instructive parallel to earthly hurricanes (although energy source is internal planetary heating, rather than Sun, which causes Earth's weather).
-- Titan:
   * this Saturn's moon is the only moon to have a thick (dense) atmosphere, in fact one of the thickest in the solar system.
   * contains most of the complex hydrocarbons of life, but no water.
• Satellites and Rings:

 -- satellites (moons):
   * all the large outer worlds have large enough gravitational fields to pull in many moons.
   * Generally the moons closest to the planet orbit in or near the planet's equatorial plane in nearly circular orbits, and were probably formed when the planet was formed. The moons farthest from the planet are often smaller, in tilted, retrograde, or elliptical orbits, and can be captured satellites with composition similar to comet nuclei.
   * the small terrestrial worlds have few or no moons.
   * Earth and Pluto, with moons nearly as large as the planet, are oddballs.
-- ring systems:
   * very thin disk (a few kilometers thick); rocks and dust. 
   * seen for all four gas giants.
   * due to tidal forces very near massive bodies - "Roche limit" caused when the tidal forces from the planet are stronger than the tensile strength of rocks - tearing apart any moon that strays too close to the planet.
• Planetary Interiors:

 -- density:
   * because of the condensation sequence, the innermost planet (Mercury) has one of the largest and densest cores (primarily because the highest density materials, such as iron and nickel metals, have the highest melting points and thus stay solid in the hottest region near the Sun). 

     density = mass/volume

      = M_/4_/3pr3
 -- stages of planet building:
   * condensation, accretion, planetesimals (asteroids), protoplanets, planets, clearing of the solar nebula. 
   * history of terrestrial planets: cratering, differentiation, volcanism/flooding, weathering/rebuilding of the surface
   * the same process is seen throughout the solar system; the only difference is the condensation sequence and distance from the Sun (which leads to different composition for different worlds).
   * some planets and satellites seem to have stopped and frozen part way through the process, or have cooled all the way to inertness.
 -- volcanism:
   * several worlds have evidence for dormant volcanoes:
Venus and Mars. 
   * Only on four worlds have we seen active (currently erupting) volcanoes: Earth (rock); Io (sulfur), Triton (liquid nitrogen geysers), and Europa (waterspouts; only identified after a late reanalysis of Voyager data).
 -- magnetic fields:
   * due to the flow of liquid conductors (metals) in the planetary interiors - "dynamo effect". 
   * molten iron or iron alloys in terrestrial interiors (best example is Earth).
   * liquid metallic hydrogen in gas giant interiors (best example is Jupiter).
 -- size and SIZE:
   * the larger the world, the better it traps heat. Jupiter (and all the gas giants) still give off more heat trapped from when they formed than they receive from the Sun. 
   * the largest planets have a strong enough gravitational attraction that when they were forming they swept out a huge volume of space - large enough to create tiny planetary systems of their own.

• Every particle in the universe pulls on every other particle in the universe.

 
• Orbital mechanics is the 'bathroom scales' that we use to weigh the planets (and also stars, and galaxies, etc.).

-- Use Kepler's Third Law, a separation of the planet to its's moon and the moon's orbital period find the planet's mass (remebering that the moons mass is much s than the planet).
P²(yrs) = A³(AU) _{/(m1+m2)(multiples of the Sun's mass)}
        or
P²(sec) = 4p² x A³(meters)/_{G x (m1+m2)(kilograms)}
where G = 6.67x10^-11m³/_kg._sec2

Tidal forces: the differential forces when large objects pull on each other, cause planetary distortion, internal churning, heating and even the tearing apartt of the rocks that make up the world, leading to:

   --  tidal locking: Earth's Moon, Mercury, Pluto/Charon; one side always faces the larger body (Mercury odd 2:3 orbit/rotation locking)
   --  tidal heating: Io, Europa, others (causes active vulcanism, sub-surface planetary oceans, etc.).
   --  formation of planetary rings: all of the Jovian planets; the Roche limit (~2.5 times the planets diameter) is as close as a large moon can get before the tidal forces rip the moon apart.
 

• Detection of 'new' worlds: 

   --  Both Neptune and Pluto were mathematically predicted before discovery because of the gravitational influence they had that influenced and perturbed the orbits of the other outer planets.
   --  the same is true of the detection of extrasolar planets (they cause a wobble of their parent star's position, which is detectable using the Doppler effect).
 
• etc. . . . (We will see gravityís effects far beyond our solar system in coming weeks.)

Individual Planets:
      know the properties of each of the planets, including:

• Earth's Moon:

  -- only object visited by manned missions; maria, highlands, cratering, origin, locked rotation from tidal drag; eclipses; tides.
 
• Mercury:

  -- heavy cratering, scarps; densest planet, odd (locked) rotation; why no atmosphere; finished evolutionary sequence.
 
• Venus: 

  -- dense atmosphere (carbon dioxide, sulfuric acid clouds, very strong greenhouse effect); impact cratering; clear evidence of significant  volcanic activity sometime in its history; Magellan radar mapping, Venera landers (Russia).
 
• Mars:

  -- small, thin atmosphere (escape velocity of gas molecules near their average kinetic energy); seasons (variations in polar caps, water trapped in permafrost, dry ice); evidence of large dormant volcanoes; evidence of past thick atmosphere and liquid surface water; visit of Viking lander (70's), Mars pathfinder ('97), Mars Global Surveyor (present); Life?
 
• Jupiter:

  -- more mass than all other planets combined; Great Red Spot; thin ring; Shoemaker-Levy 9 comet impacts; huge (strong) magnetosphere; liquid metallic hydrogen.
 
• Saturn:

  -- huge complex ring system of dust and boulders; average density less than water (0.7gm/cm³)
 
• Uranus:

  -- tipped over (very odd seasons); evidence of many cometary impacts.
  -- discovered by amateur astronomer William Herschel with his homemade backyard telescope.
 
• Neptune:

  -- Great Dark Spot; 1000mph winds; very cold; 
  -- first planet discovered because of a mathematical prediction - tested the orbit of Uranus, and found a discrepancy which had to be caused by the presence of another massive object out there.
 
• Pluto:

  -- never visited by space probes yet; its density, size (diameter), atmosphere, and surface albiedo (brightness or darkness) variations found by mutual eclipses with its moon Charon and occultations of stars; eccentric, highly tipped (17°) orbit; 
  -- discover in 1930 by Clyde Tombaugh in response to orbital variation in Uranus' orbit unexplained by the presence of Neptune; later found that Pluto's masscould not account for that variation either; it was far too small!! There must be some other massive object(s) out there!!

know the names and some of the distinguishing properties of each of the four Galilean moons of Jupiter: Io, Europa, Ganymede, and Callisto; and of the distinguishing properties of the following moons of the other outer planets: Titan and Mimas (Saturn), Miranda (Uranus), and Triton (Neptune).

• Asteroids:

-- composition:
   * iron or rocky materials. 
   * some of the rocky asteroids have some carbon compounds - carbonaceous chondrites.
-- origin: 
   * asteroid belt, between Mars and Jupiter (rubble debris from "failed planet", gravitational resonances with Jupiter).
• Comets:

-- very elongated elliptical orbits.
-- composition:
   * "dirty snowballs" - water and carbon ices and dust.
-- origin:
   * Oort Cloud, Kuiper belt (the remnant of the protoplanetary disk), both beyond the outer planets.
-- visibility: 
   * comet nucleus (~10 km across)heats and sublimates away as it approaches the Sun, creating a 100,000km coma, and ion and dust tails (can be over 2AU long, ion tail directed away from Sun, dragged by solar wind).
• Meteorites:

-- A meteor is the streak of light caused when a meteoroid (an piece of rock or metal or ice) streaks through the Earth's atmosphere. A particularly bright meteor is called a bolide.
--  Any material that survives the fiery passage through the atmosphere is a meteorite.
-- composition:
   * have the same composition as asteroids.
-- visibility:
   * most meteors visible at night are from the frictional destruction of objects the size of a grain of sand or a pea burning up in the Earth's upper atmosphere (~100 miles up)
   * each meteor is visible for a fraction of a second to several seconds, based on its size and brightness.
   * ~5-10/hr fall into the atmosphere that are bright enough to see from any one point on Earth's surface, with that number almost doubling between the hours of midnight and dawn (that being the 'ram' direction of Earth's orbital motion).
-- meteor showers: 
   * caused when Earth passes in its orbit through a comet path (sweeping up the debris emitted as the comet sublimates away and left all along the comet orbit).
   * all meteors in a single shower appear to come from a single point in the sky (the radiant), which is the direction toward the comet's orbit.
-- impact cratering:
   * Earth-crossing paths of asteroids and comets can lead to catastrophic impacts; and are even believed by many scientists to have caused the death of the dinosaurs.
   * kinetic energy of orbiting objects:
E = ¹/₂mv²
   * the energy of this orbital motion has to be dissipated as friction (heat) or explosively in cratering and destruction.