Astronomy 330

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Astronomy 330

• Lecture 20

http//solarsystem.nasa.gov/index.cfm
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Astronomy 330 The outer planets (Uranus,
Neptune, Pluto)

• Uranus and Neptune compared to Jupiter and Saturn
• Smaller
• Higher densities
• Higher proportion of methane
• Not giants, but have intermediate masses between
  Jupiter, Saturn and Earth

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Astronomy 330 The planets size comparison
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID178
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Astronomy 330 Overview

• The distance from Saturn to Uranus is almost the
  same as the distance from Earth to Saturn
• The distance from Uranus to Neptune is slightly
  larger.
• Neptune seems to be more active than Uranus with
  an internal source of heat
• Pluto is completely different, smaller than Moon
  and resembles the large, icy satellites of the
  gas giants and is also similar to Kuiper belt
  objects. May be similar to building blocks of
  giant planets cores.

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Astronomy 330 Planet Orbits
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID175
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Astronomy 330 Discovery

• These planets are either invisible or barely
  visible to the naked eye.
• These planets (Uranus, Neptune and Pluto) have
  only been discovered in the recent past.
• Uranus and Neptune have only been directly
  explored by Voyager 2 (remember Voyager 1 only
  visited Saturn and then flew out of the
  ecliptic plane).
• Pluto has only been observed telescopically from
  Earth.

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Astronomy 330 Uranus

• 1781, William Herschel, an amateur at the time
• Was making star charts and noticed a disk-like
  object in one field of view.
• He thought it was a comet.
• Subsequent observations showed that it moved more
  slowly than a comet.
• Even more observations allowed computation of its
  orbit and showed it to be a planet orbiting at 19
  AU.
• Titius-Bode law predicted a planet at this
  distance.

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Astronomy 330 Sir William Herschel (1738-1822),
Caroline Herschel (1750-1848)
http//star.arm.ac.uk/history/herschel.html
http//www.klima-luft.de/steinicke/ngcic/persons/h
erschel_c.htm
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Astronomy 330

• Titius-Bode law says that planets should occur at
  particular distances from the Sun and only at
  those distances. This has been shown to be false
  and is just a coincidence (e.g. Neptune and Pluto
  dont fit).
• Herschel became famous and switched careers to
  become an astronomer.
• Uranus father of Saturn who is father of Jupiter.

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Astronomy 330 Titius-Bode Law

• Start with the number sequence
• 0 3 6 12 24 48 96 192 384 768
• Add 4 to each number
• 4 7 10 16 28 52 100 196 388 772
• Then divide by 10
• 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 77.2
• This is close to the actual distances of the
  planets in AU
• 0.39 0.72 1.00 1.52 5.20 9.54 19.19 30.07 39.48
• Titius-Bode law is NOT a law of nature and is
  just a coincidence (but it does work for moons
  around some of the planets too)

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Astronomy 330 Discovery of Neptune

• Uranus can be seen by the naked eye
• Uranuss orbit could also be accurately
  determined from observations of its position in
  the sky.
• The computed orbit of Uranus which took into
  account the gravitational effects of the Sun and
  the known planets, did not match with
  observations.
• Some other body must be perturbing its orbit,
  this is how Neptune was discovered.

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Astronomy 330

• The discrepancies where small (15 seconds of
  arc), but real.
• Adams and Leverrier (1845-1846) independently
  calculated where to look (they made assumptions
  using Titius-Bodes law).
• Galle discovered Neptune telescopically in 1846
  1-2 degrees from where it was supposed to be.
• Neptunes distance from the Sun is inconsistent
  with Titius-Bode law.
• Galileo may have observed Neptune, but thought it
  was just a star.

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Astronomy 330 Discovery of Pluto

• Even more accurate tracking of the orbits of both
  Uranus and Neptune suggested that there was yet
  another body perturbing their orbits.
• Percival Lowell (remember him?) calculated where
  to look for this planet and also undertook a
  systematic search.
• Planet was not discovered until after his death
  by Clyde Tombaugh in 1930 using a newly designed
  telescope for such a search.
• Tombaugh discovered Pluto BEFORE going to college.

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Astronomy 330

• It turns out that the supposed perturbations of
  Uranus and Neptune were caused by observational
  inaccuracies, not by Pluto. Pluto is much to
  small to cause such perturbations!

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Astronomy 330 Clyde Tombaugh, 1906-1977
http//starchild.gsfc.nasa.gov/docs/StarChild/whos
_who_level2/tombaugh.html
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Astronomy 330 Density and Composition

• Uranus and Neptune are smaller than Jupiter and
  Saturn and have masses about 15 times that of
  Earths
• Saturn has a smaller mass than Jupiter and its
  density is lower. This means that its internal
  composition is probably the same as Jupiters
  since it undergoes less internal compression.

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Astronomy 330

• Uranus and Neptune have higher densities than
  Saturn (about the same as Jupiter). Therefore,
  they must have different compositions than
  Jupiter and Saturn, otherwise they would be even
  less dense than Saturn (smaller mass, less
  gravity, less compression, lower density).

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Astronomy 330 Relative Sizes of gas planets
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID180
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Astronomy 330

• Uranus and Neptune have a higher proportion of
  elements heavier than H and He to account for
  their densities.
• Ice and Rock are logical choices.
• Oxygen is the most abundant reactive element
  after Hydrogenit combines with H easily to form
  water ice.
• Rock is material made of Si and O.
• Models of the interiors of these planets indicate
  that they should have rocky, icy, inner cores.

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Astronomy 330

• Surrounding these cores, models predict
  relatively thin envelopes of liquid and gaseous
  hydrogen plus other gases.
• Uranus and Neptune are too small to form metallic
  hydrogen like Jupiter and Saturn.
• Maybe these planets even have thick water clouds,
  but there is no direct evidence for thiswater
  vapor has not been detected on either planetbut
  ammonia is deficient and could indicate that is
  dissolved in water.
• Knowledge of interiors of planets is very sketchy
  at this time!!!

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Astronomy 330 The different interiors of the gas
giants
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID166
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Astronomy 330 Differences between Uranus and
Neptune

• Density of Uranus is 1.3 g/cm3 and for Neptune is
  is 1.6 g/cm3this indicates either structural
  differences or compositional differences.
• Uranus lacks an internal source of heat, Neptune
  has a significant heat source.
• This was discovered using IR observations at 25
  micronssuggested that the planets had similar
  temperatures even though Neptune is so much
  farther from the Sun.

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Astronomy 330

• Models suggest that Neptune could still be
  radiating its primordial heat.
• Rock and ice retain their heat very well and, as
  we see from Neptunes density, it probably has a
  higher proportion of these materials.
• Uranus has its rotation axis inclined by 98o
  which puts it almost in the plane of the orbit of
  the planet. Also, the obits of Uranuss rings
  and satellites are perp. to its rotational axis
  as well. Maybe caused by a collision with
  another massive body? Dont know!

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Astronomy 330 Uranuss titlted axis (Keck
telescope, IR, July 2004)
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID3163
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Astronomy 330 Pluto

• A completely different sort of planet.
• Smaller than our Moon
• Density of 2.1 g/cm3.
• Suggested that it is composed partly of water ice
  and resembles Triton, the large moon of Neptune.
• It definitely isnt a gas planet and it also is
  not a displaced terrestrial planet (why do you
  think that is?).

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Astronomy 330 Pluto and Charon observed by HST
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2100
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Astronomy 330

• Pluto probably is similar to the icy objects
  which collided to form the cores of the gas
  giants.
• It is the smallest planet and the largest of the
  Kuiper belt objects (also called ice dwarfs or
  trans-Neptunian objects).

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Astronomy 330 Atmospheres of U. and N.

• Through a telescope U. and N. appear greenish and
  are free of cloud structures.
• Voyager saw almost no cloud structure on Uranus,
  Neptune shows some white clouds (this is at
  visible wavelengths!!!).
• This is caused by the fact that sunlight
  penetrates deeply and is scattered by methane
  (CH4) as it travels back to the surfacegives
  these planets their blue-green color.

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Astronomy 330 Appearance
Neptune
Uranus
http//solarsystem.nasa.gov/planets/index.cfm
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Astronomy 330 Atmospheric compositions

• Methane absorption bands are seen in spectra as
  on Jupiter and Saturn
• However, these absorption bands are much stronger
  and other gases are also seen.
• Indicates that the proportion of methane is
  higher on Neptune and Uranus and is consistent
  with the higher proportion of ices in the total
  masses we have already discussed.

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Astronomy 330

• Uranus and Neptune are deficient in H and He
  relative to Jupiter and Saturn.
• Where the pressure is about 1 bar (i.e. like sea
  level) the temperature is about 73 K (-200 C).
  This is the temperature of liquid N and is much
  colder than on either Jupiter or Saturn.
• Ammonia is frozen solid at this temperature and
  pressure.
• Water may be present at even greater depths.

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Astronomy 330

• Voyager showed that Uranus has a thin haze on its
  sunlit pole indicating that some sort of
  photochemistry is going on.
• IR spectra showed that the ratio of H/He is even
  more like the Solar value since He does not
  precipitate out into metallic H as on Jupiter and
  Saturn.

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Astronomy 330

• These measured compositions are consistent with a
  two step formation process.
• Two step formation for gas giants
• Core forms from colliding planetisimals composed
  of rock and ice (things like comets and Pluto)
• Then these cores gravitationally accumulate H/He
  gas from solar nebula.
• Other atmospheric constituents contributed by
  out-gassing from rocky-icy cores.
• Jupiter and Saturn acquired more H/He than Uranus
  and Neptune and this explains their different
  relative compositions.

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Astronomy 330 Atmosphere Temperatures

• As we have seen Jupiter and Saturn show IR
  emissions from methane and methane photochemical
  byproducts.
• Uranus showed none of this.
• Neptune shows emissions from methane and ethane.
• This difference is caused by a strong temperature
  inversion (where the temperature increases with
  altitude) in Neptunes atmosphere and only a weak
  one on Uranus.

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Astronomy 330

• The higher temperature in the inversion region
  can be detected as emission against the backdrop
  of the colder lower layers of the atmosphere.
• As on Jupiter and Saturn, we expect the
  compositions here to be affected by
  photochemistry and by charged particles from the
  Sun which have been trapped in magnetospheres of
  the planets.

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Astronomy 330 Photochemical smog on Neptune
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2069
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Astronomy 330

• If our atmosphere was at the temperature of that
  of Uranus or Neptune in their tropopauses (-218o
  C) it would condense into a mixture of ices.
• At this temp. H and He are gaseous (and Neon too)
  and substances such as N, methane and CO can have
  significant vapor pressures.
• Neptune shows all of these gases except Neon.
  Also, HCN detected indicating a source for N in
  Neptunes upper atmosphere.

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Astronomy 330 Radio observations

• At short radio wavelengths we detect thermal
  radiation from the atmospheres.
• As one goes to longer wavelengths one can see
  deeper into the atmosphere, and presumably to
  higher temperature regions.
• This is what is seen on Jupiter, Saturn and
  Neptune (increasing emissions as the radio
  wavelength is increased).
• This is NOT seen on Uranusthe temperature does
  not increase with depth into the atmosphere.

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Astronomy 330

• Uranus does not have a source of internal heat as
  the other gas planets.
• There is not enough energy to maintain convection
  currents in the lower atmosphere and convection
  operates only at high altitudes where the Sun
  heats it. This is similar to the Earths oceans
  where the top layers are warm, but at depth a
  uniform, global temperature is reached.

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Astronomy 330

• The lack of a strong source of internal heat also
  helps explain the absence of N2 and CO in the
  upper atmosphere of Uranus since strong vertical
  convection is required to dredge up these
  materials from the interior.
• Radio brightness observations of Uranus and
  Neptune are also inconsistent with calculations
  for the expected emission from these planets.

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Astronomy 330

• In the 3-10 cm region of the spectrum U. and N.
  radiate more energy than expected and this
  indicates that the atmospheres of U. and N. are
  transparent at these wavelengths. This could be
  due to a low ammonia content of the atmospheres.

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Astronomy 330 Summary of Atmospheric Properties
of Uranus
http//www.windows.ucar.edu/
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Astronomy 330 Weather

• Difficult to observe since we require clouds and
  the atmospheres of Uranus and Neptune are very
  clear.
• This is especially true for Uranusalso the
  inclination of its axis makes study difficult.
• Neptune has seasons like Earths since its axis
  is tilted at 27o but the seasons are 165 times
  longer !!!
• Since Uranus is tipped to 98o, for 42 years one
  pole received all the solar radiation. Equinoxes
  occur when the equator faces the Sun.

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Astronomy 330 Seasons on Uranus
http//www.windows.ucar.edu/tour/link/uranus/atmo
sphere/U_seasons.htmleduhigh
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Astronomy 330

• Under these conditions we would expect Uranus to
  have one big Hadley cell, but Voyager found that
  the rotation of Uranus dominates and its banded
  like the other gas giants. This is also strange
  since Uranus has not been measured to have an
  internal source of heat to drive smaller hadley
  circulations to create bands.
• Neptune does have well defined clouds and it is
  easier to study is atmospheric flow patterns and
  its weather.

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Astronomy 330 Uranus and Neptune both have
banded wind patterns like Jupiter and Saturn !
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID3163
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Astronomy 330 Neptunes bands
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2424
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Astronomy 330

• Neptunes clouds are detectable from Earth but
  Voyager discovered dark clouds and spots as well.
• Neptune has a feature known as the great dark
  spot (GDS) which is very similar to the GRS on
  Jupiter.
• The GDS is a giant eddy about the size of the
  Earth and its winds rotate counterclockwise
  around and area of high pressure. It also occurs
  in the sourthern hemisphere like the GRS.

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Astronomy 330

• Smaller cyclones are also visible at higher
  latitudes.
• The GDS seems to have disappeared since Voyagers
  visit (based on telescopic observations).
• The composition of the clouds of Neptune is
  unknownprobably methane ice.

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Astronomy 330 Neptunes GDS (Its gone now!)
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID138
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Astronomy 330 But its still stormy (HST and
IRTF)!
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2117
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Astronomy 330 Rotation and wind speeds

• Uranuss rotation was unknown until Voyager
  (1986). Clouds showed a period of about 16
  hours. This is the speed of the winds plus
  rotation.
• The period of rotation of Uranus in its deep
  interior by measuring is magnetic field is 17.2
  hours.
• The wind speed on Uranus varies with latitude.

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Astronomy 330

• Easier to measure winds and rotation on Neptune
• Period of rotation from magnetic field is
  measured to be 16.8 hours.
• The changes in wind speed with latitude on
  Neptune are almost identical with Uranuss.
• Uranus is extremely curious since it is heated
  near one pole only, the winds should increase as
  you go to the equator. They dont, they decrease
  and the fastest winds are at high latitudes.
  Also, the measured temperatures at both poles are
  the same!

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Astronomy 330

• Some process is redistributing the heat from the
  sunlit pole to the rest of the planetprobably
  something to do with its internal structure. We
  just dont know!
• Uranus is just plane weird!

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Astronomy 330 Reading

• Read Chapter 14 of Morrison and Owen.