Lecture 3: Overview of the Planets

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Lecture 3 Overview of the Planets
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Notes

• Homeworks due right now!
• Anything handed after class, or tomorrow loses 5
  pts (out of 50)
• Anything handed Thursday before class loses 10
  pts.
• Nothing accepted after Thursdays class (beginning
  of class)
• Still need yellow sheets from some people
• Alcide, Benkowski, Distler, Gonzalez,
  Kannanaikkel, Meyerson

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The planets
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A Scale Model Solar System
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Densities

• Density of an object is Mass / Volume
• The Sun is 1.4 grams/cm3 or 1.4 gcc
• Water is 1.0 gcc Very convenient.
• Aerogel is .003 gcc ? lowest density material.
• Ice is 0.917 gcc ? Ice floats
• Granite is 2.2 gcc
• Diamond is 3.5 gcc
• Pure Lead is 11.34 gcc
• Gold is 19.3 gcc

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Measuring Density

• Easy. Divide Mass by Volume. We need
• 1. The size (to get the volume)
• 2. The mass.
• How to get the size?
• 1. Measure the Distance, and
• 2. The angular size of the planet.
• How to get the Mass? (using Keplers third law)
• 1. Measure the Distance,
• 2. Measure the Angular size of a planets orbit,
• 3. The orbital period of the planet.

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Measuring size
Credit astronomynotes.com
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Determining Planetary Mass
Why doesnt this work well for planets and the
Sun?
Credit astronomynotes.com
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Example, Mars

• Our observations of Mars at its closest approach
  to Earth (parallax, and angular extent)
• d 7.8x1010 m away.
• A 0.005 degrees
• Using D2pdA/360
• D (2p7.8x1010m0.005deg)/360deg
• D 6.8x106 m or 6800 km
• To determine its Mass, we will use its moon
  Phobos and mM(5.916x1011)a3/T2
• a 9.37x106 m
• T 27,576 sec
• M 6.4x1023 kg
• Finally, the Volume is V4pr3/3 and Density ?M/V
  of Mars
• ?M/V 3885 kg/m3 3.885 gcc

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Planetary Densities

• In small groups
• Which object is more dense?

• Earth or Venus
• Earth or Mars
• Jupiter or Saturn
• Uranus or Neptune
• Pluto or Neptune
• Platinum or Gold

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Planetary Densities

• Earth or Venus
• Earth or Mars
• Jupiter or Saturn
• Uranus or Neptune
• Pluto or Neptune
• Platinum or Gold

• Mercury 5.4 gcc
• Venus 5.3 gcc
• Earth 5.5 gcc
• Mars 4.0 gcc
• Jupiter 1.3 gcc
• Saturn 0.7 gcc
• Uranus 1.3 gcc
• Neptune 1.6 gcc
• Pluto 2.1 gcc
• Gold 19.3 gcc
• Platinum 21.5 gcc

Figure Adler Planetarium
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Categories of planets

• Using just size and density, we can separate the
  planets into some simple categories
• Terrestrial Planets Mercury, Venus, Earth/Moon
  and Mars
• Small size
• High Density
• Inner Solar System
• Giant Planets Jupiter, Saturn, Uranus and
  Neptune
• Large
• Low Density
• Outer Solar System
• Dwarf Planets Pluto, Ceres, Eris possibly
  many KBOs
• Large enough to be round (diameter 900 km for
  asteroids, diameter 400 km for KBOs)
• Not the dominant body in their region of the
  Solar Sytem

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Terrestrial Planets
CreditAstronomynotes.com
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The Planets-Compression vs. Composition

• Densities all greater than 4 gcc
• Must be composed of rock and metal in their cores
• We could expect a correlation between size and
  density, if they are all made of the same
  material
• More mass, higher density
• Less mass, lower density
• As the mass increases, the body compresses more
  and densities increase???
• This works for Earth and Venus, similar size,
  mass and density
• Mars and the Moon are both smaller and less dense
  than the Earth and Venus
• Mercury, is the outlier, much less massive, but
  as dense as Earth? Mercury must have a
  different composition!

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Giant Planets
CreditAstronomynotes.com
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Giant Planets Compression vs. Composition

• The Giant planets are much more massive than the
  Terrestrial planets.
• Well, we know they are less dense than the
  terrestrial planets, so what does this suggest
  about possible compositions?
• What sorts of things could we build a Giant
  planet out of, that would allow such low
  densities?

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Giant Planets Compositions and Composition

• The Giant planets contain lots of Hydrogren and
  Helium
• Thus, have compositions more similar to the Sun
  than the terrestrial planets.

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Pluto

• How does Pluto fit into our scheme of densities?
• It is small compared to all other planets.
• Pluto has a moderate density around 2.0 gcc.
• What could it be made of?

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Other Solar System densities, trends and other
bodies..

• The four large moons of Jupiter
• Moon Semi-major axis Density
• Io 0.42 million km 3.3 gcc
• Europa 0.76 million km 3.0 gcc
• Ganymede 1.07 million km 1.9 gcc
• Callisto 1.88 million km 1.8 gcc
• What is going on here????
• Asteroids? Thought to be mainly rocky, but
  density measurements are challenging. How could
  we measure it?
• Comets? Mostly icy, also difficult to measure.

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Chemistry

• Hydrogen and Oxygen are both very important, both
  very reactive, and abundant.
• Each can form compounds with each other, and
  other elements (H20 water being an obvious one)
• Hydrogen with Carbon CH4 is methane, very
  common
• Oxygen with Carbon CO2 is Carbon dioxide, also
  very common
• In some places, Hydrogen is more abundant than
  Oxygen, this is called a reducing environment.
• Where Oxygen dominates, this is called an
  oxidizing environment.
• Outer Solar System Hydrogen dominates
• Inner Solar System Oxygen dominates

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Different Matter

• Elements and compounds can be found in four
  different physical states, depending on
  temperature and pressure
• Solid, liquid, gas and plasma.
• We will frequently refer to matter in the Solar
  System as
• Gas -- atmospheres
• Ice -- well, ice, but not always water ice.
• Rock planetary cores
• Metal planetary interiors..

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Gas

• Atmospheres are all gas
• Earths atmosphere is special, consisting of
  nitrogen (N2 78) and oxygen (O2 20).
• Mars and Venus have atmospheres made mostly of
  carbon dioxide (C02). Without life on Earth, our
  atmosphere would have much more CO2.
• Jupiter and Saturn have atmospheres consisting
  mostly of Hydrogen (H2) and Helium (He). In this
  way they look similar to the Sun.
• Uranus and Neptune also have atmosphere with lots
  of Hydrogen and Helium, but also have higher
  density, suggesting more heavy elements.

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Ice

• Volatiles are molecules that are liquid or gas at
  moderate temperatures, but freeze into ices at
  low temperatures.
• If a substance sublimes, it vaporizes from an ice
  to a gas. (think dry ice)
• On Mars, CO2 is the main volatile. However, water
  ice has been found there recently.
• Other volatiles Carbon Monoxide (CO), ammonia
  (NH3), and methane (CH4).
• Volatiles are the main constituent in comets, and
  many planetary satellites.

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Polar Caps
What can Martian polar caps tell us about seasons
on Mars, or even its axis tilt?
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Rock

• At higher temperatures, the volatiles evaporate
  completely, leaving behind rock.
• Earth and Moon heavily composed of rock.
• Most common rocks are silicates oxides of
  silicon, aluminum and magnesium.

Creditrst.gsfc.nasa.gov/
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Metal

• At even higher temps, rock is transformed.
• Metallic elements in rock may separate out
  typically iron, magnesium and nickel.
• The core of Earth and (much of) Mercury are
  metallic.
• Some asteroids appear to be pure nickel or iron.

Creditgibeon meterorite
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Chemical Trends

• Lets review the trends we have seen so far.
• From the inner to outer Solar System.

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Rocks vs. Minerals

• A Rock is assembled by various compounds or
  elements, called minerals.
• A mineral is a single substance (homogenous)
• A rock is a mixture of different minerals
  (inhomogenous)
• An elemental mineral contains only one element
• Gold (AU)
• Graphite or Diamond (C)
• A compound mineral is comprised of miltiple
  elements bonded together
• Quartz (SiO2) or Hematite (Fe2O3)
• Pure elemental minerals are rare in nature, we
  usually find compound minerals.

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Types of Rocks

• Rocks are divided according to their origin
• Igneous rocks that formed directly by cooling
  from a molten state. These make up 2/3 of the
  Earths crust.
• Sedimentary composed of fragments of other
  rocks that are cemented together. Limestone,
  chalk, shale andsandstone are examples.
• Metamorphic these are produced by burying
  either of the other two types, processing them
  with high temperatures and pressures. Burying and
  returning rocks to the surface is one result of
  Earths movement of continental plates. Marble is
  the prime example, and if formed from limestone.

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Rock formation summary

• Our three processes.

Creditmineraltown.com
Cresit msnucleus.org
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Rock formation

• Igneous rock formation, at a lava flow.

Sedimentary rocks exposed
Metamorphic rocks
Creditthe rock cycle website
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The Giants Causeway??
Led Zeppelin Houses of the Holy, and
Some dudes at the Giants
causeway in N. Ireland.
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Primitive rocks

• We have covered the three types of rocks which
  have been created on Earth, from molten rock, or
  other rocks.
• However, could there be rocks from before this.
  From the beginning of the Solar System?
• Well, yes, just not on Earth. Any primitive rocks
  that accreted to Earth when it was forming have
  since been altered from various mechanisms which
  generated lots of heat (impacts, radioactive
  decay).
• The core of the Earth got hot enough to melt and
  become liquid. The densest materials could then
  sink to the center. This is called
  differentiation.
• Is there any place were we could fine primitive
  rocks?

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Differentiation

• One asteroid, Vesta, is known to be
  differentiated. It is not as large as Ceres
  though, and not round enough to qualify as a
  dwarf planet.

• The Dawn mission is going to travel to both Vesta
  and Ceres, in 2011 and 2015 (using ion
  thrusters).
• Scheduled to launch June 2007.
• Already been canceled and revived once.

Creditdawn.jpl.nasa.gov
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Atmospheres

• There is a wide spectrum of atmospheres in the
  Solar System.
• Both Earth and Jupiter have atmospheres, despite
  huge difference in composition, size, mass and
  location in the Solar System.

Credit nineplanets.org
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Atmospheres

• Meanwhile, two similar sized moons of Jupiter and
  Saturn, Ganymede and Titan, are completely
  different
• Titan has an atmosphere
• Ganymede does not.

Credit nineplanets.org
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Atmosphereshow to get one

• How does a planet get an atmosphere?
• It can form with one (primordial or captured), or
• It can create its own (outgassing or secondary
  material).
• Impacts of comets etc.
• The gas in an atmosphere must remain bound to the
  planet over the age of the Solar System.
• Each Molecule in the gas can escape the planet if
  it achieves escape velocity in the upward
  direction.
• What properties of a gas determine the velocity
  the gas particles?

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Atmospheres how to lose one

• Keeping at atmosphere can be non-trivial.
• Impacts asteroids and comets with enough mass
  could remove large portions of an atmosphere
• Thermal escape If it is hot enough, or grows
  hotter, then the random motions of the gas could
  exceed escape velocity, allowing gas to escape
  (perhaps the ungrateful residents of the planet
  could change the chemistry of the atmosphere
  enough to significantly heat the planet up)
• Charged particles The solar wind could scour the
  outer atmosphere stripping particles away.
• Thermal escape happens in the exosphere, where
  the atmosphere is very thin, and molecules rarely
  collide.

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Atmospheres

• Examples
• Giant planets
• Massive, large escape velocities tens of km/s
• Can even keep light gases, like hydrogen and
  helium
• Terrestrial planets
• Less massive, but dense, but hotter than giant
  planets.
• Can retain thinner atmospheres
• Cant keep lightest gases
• Titan, Saturns moon
• Not massive enough to keep light gases
• Just right to keep heavy gases, like methane and
  nitrogen
• Similar to Ganymede, which has not
  atmosphere?????

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Differential Escape

• For a given temperature, lighter gas molecules
  will have higher velocities than heavier ones
• Escape velocity depends on gravity, or the mass
  of the body
• Hence, smaller and warmer bodies lose more gases
  than large cool ones.
• So the outer planets can keep the light gases,
  like hydrogen and helium.
• And planets which and smaller and warmer can only
  keep the heavy gases, like nitrogen and oxygen.

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Earth vs. Titan

• Troposphere is where weather occurs.
• The Stratosphere is very stable, where some jets
  fly.
• Ozone layer, absorbs lots of radiation from the
  Sun.
• Meteors often burn up in the mesosphere.
• Thermosphere, or ionsphere is where auroras
  happen and space shuttles orbit.

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Earth vs. Titan
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Earth and Titans atmosphere
Image Credit JPL/Space Science Institute
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Summary of atmosphere diversity

• The main reasons for the differences between the
  atmospheres of various planets and moons
• Initial capture from the Solar Nebula
• Large outer planets could capture and hold all
  gases from the initial solar nebula, which is
  reflected in compositions similar to the Sun.
• Small inner planets could not hold onto lighter
  gases.
• Later out-gassing
• Both outer and inner planets out-gassed part of
  their atmosphere, but this is more dominant for
  the inner planets.
• Biological agents
• Only Earth has changed it atmosphere
  significantly due to biological effects.
  Otherwise our atmosphere would look more like
  that of Mars and Venus.

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Done and Done

• Questions?
• Next homework will be online Thursday, due on Feb
  20th..

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