Astronomy 330

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Astronomy 330

• Lecture 20
• Part 2

http//solarsystem.nasa.gov/index.cfm
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Astronomy 330 Magnetospheres

• Uranus and Neptune have significant magnetic
  fields.
• However, at Uranus it is tipped 60o to the axis
  of rotation AND it is displaced relative to the
  center of the planet.
• On the surface of Uranus the magnetic field
  strength varies by 0.1 to 1.1 gauss (on Earth it
  is constant at about 0.3 gauss).

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Astronomy 330 Magnetosphere Scales
http//dosxx.colorado.edu/magneto/scales.html
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Astronomy 330

• Uranus has a magnetosphere similar in size to
  Saturns.
• The compositions of the atomic particles in
  Uranuss magnetosphere are simpler, and come
  mostly from Hydrogen escaping from Uranuss
  atmosphere.
• The magnetosphere is also inclined like the
  dipole field of the planet and because of this
  the magneto-tail rotates like a corkscrew as the
  planet rotates.

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Astronomy 330 Charged Particle (Plasma) Sources
http//dosxx.colorado.edu/magneto/plasma.html
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Astronomy 330

• The sunlit side also experiences UV radiation
  emitted by the escaping H atoms from it
  atmosphere.
• This is called electroglow and is also seen on
  Jupiter, Saturn and Titan.
• Uranus also shows auroras like the the other gas
  giant planets.
• Auroras are seen as UV and IR emission from H3.
• Auroras occur near the equator on Uranus.

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Astronomy 330 Uranuss Magnetosphere
http//dosxx.colorado.edu/magneto/uranus.html
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Astronomy 330 Neptunes Magnetosphere

• Neptunes inclination of its rotational axis is
  27o.
• Neptunes magnetic dipole is offset from the
  planets center by about 1/2 of its radius (For
  Uranus it is 1/3)
• The dipole it further tilted by 47o from the
  rotational axis on Neptune
• The field strength is 1/2 that of Uranus and it
  varies from place to place due to the tilt and
  the fact that it is offset.
• So Neptune is just as weird as Uranus!

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Astronomy 330 Neptunes Magnetosphere
http//dosxx.colorado.edu/magneto/neptune.html
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Astronomy 330

• Why are the magnetic field of these planets the
  way they are?
• The best theory so far is that generated in the
  mantles of these planets, not in their cores as
  in other planets.
• The mantles are composed of pressure-ionized ices
  which surround dense, rocky cores.
• Here, compounds of H, C, O and N become ionized
  due to the high pressures and become conducting
  to electricity.

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Astronomy 330

• Note this is extremely speculative since we do
  not have a very clear picture of the internal
  structure of these planets, but it is consistent
  with other ideas about how the planets formed and
  how they are constructed.

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Astronomy 330 Pluto and its Moon

• Is Pluto really a planet?
• Plutos orbit is the most eccentric of all the
  planets.
• Plutos orbit is also highly inclined.
• So, its orbit is in some ways like that of a
  comet.
• Pluto is also very small, about 2/3 the size of
  the Moon and its rotation period is 6.4 days
  (only Mercury and Venus have longer rotation
  periods).

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Astronomy 330 Relative Size of Pluto
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Astronomy 330 The orbit of Pluto
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Astronomy 330

• Plutos orbit crosses the orbit of Neptune.
  Maybe its an escaped satellite of Neptune?
• However, Pluto and Neptune obit this Sun in a
  resonance configuration which prevents Pluto from
  coming within 17 AU of Neptune which make this
  unlikely.

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Astronomy 330 Plutos Moon

• Discovered in 1978 and its name of Charon.
• Its discovery also makes it more likely that
  Pluto formed independently of Neptune.
• In 1985, Charon occulted Pluto (passed in front
  of). Observation then showed the relative sizes
  and brightnesses of Pluto and Charon. Modern IR
  measurements are also important for studying
  these objects.
• Charon orbits Pluto at a distance of 20,000 km
  and this is the closest orbit of any satellite
  and its planet in the S.S. except for Phobos.

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Astronomy 330

• The period of the orbit of Charon is exactly
  equal to the rotation rate of Pluto.
• This is another example of tidal resonance.
  Pluto is tidally locked by Charoneventually the
  Earth will rotate with the same period as the
  moons revolution.
• The mass ratio of the Charon to Pluto is 1/10 and
  this is the largest in the S.S. (Moon/Earth is
  1/80).

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

• Estimates of Plutos mass first came from
  measuring it effects on Neptune and Uranusvery
  uncertain and ranged from .2 to 7 Earth masses.
• This uncertainty also made estimates of its
  composition uncertain.
• With the discovery of Charon, we can use Newtons
  laws to measure Plutos massit is 1/400 that of
  the Earth! Much, much smaller than previous
  estimates.

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Astronomy 330

• Combining this mass with occultation measured
  diameter we can compute the density of Plutoit
  is 2.1 g/cm3.
• This density if identical to the density of
  Triton (Neptunes moon).
• So, Triton and Pluto a probably made of the same
  stuff.
• The orbit of Charon also defines the rotation
  axis of Pluto since they are tidally locked. The
  inclination of its orbit is 112o to the ecliptic.
  In other words the rotation axis is nearly in
  the plane of the S.S. (like Uranus).

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Astronomy 330

• So three planets rotate backwards
  (retrograde)can you name them?

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Astronomy 330

• So three planets rotate backwards
  (retrograde)can you name them?
• Venus, Uranus, and Pluto!

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Astronomy 330

• Pluto exhibits surface brightness changes as it
  rotates.
• Albedo ranges from 0.3 to 0.5 (Moon is about 0.1
  and Triton is about 0.8).
• Spectral measurements showed methane ice is
  present.
• Also, ices of CO and N are present too.
• Again, this similar to Triton except that Pluto
  most have more dark material and it seems to lack
  CO2. Also its methane to nitrogen ratio is
  higher.

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Astronomy 330 Pluto surface map
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID569
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Astronomy 330 Out best guess for the composition
of Pluto
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID264
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Astronomy 330

• Because of the similarities between the surface
  compositions of Triton and Pluto, it is also
  reasonable that they have similar atmospheric
  compositions.
• Since Nitrogen is present on Plutos surface, it
  must dominate in its atmosphere since Nitrogen
  will have the highest vapor pressure.
• A stellar occultation in 1988 showed that a thin
  atmosphere to be present on Pluto.
• If Nitrogen is the primary constituent of this
  atmosphere it will have a temp. of 35-40 K and a
  pressure of 1-20 microbars.

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Astronomy 330

• Observations of Pluto and Charon together and
  those when Charon is occulted indicate that the
  surface of Charon is covered in water ice.
• Water is also present on Triton and presumably on
  Pluto (but it is hard to detect).
• These is no evidence for methane on Charon and it
  may have been driven off by a large impact and
  the heating it caused.

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Astronomy 330

• The eccentricity of Plutos orbit also affects
  the atmosphere of Pluto since sometimes it is
  closer to the Sun and at other times very far
  away. This varies from 29.7 AU to 49.7 AU and
  the intensity of the sun varies by a factor of
  about 3.
• The atmosphere of Pluto is vapor which is in
  equilibrium with its solid (icy) form (i.e. N),
  so as Pluto moves to aphelion its atmosphere will
  freeze out and essentially disappear.
• Perihelion occurred in 1989.

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Astronomy 330 Think Pluto is the last planet?
Think again!

• Quaoar-discovered 2002, Sedna-discovered 2004,
  probably really just large Kuiper or Oort objects

http//www.gps.caltech.edu/mbrown/sedna/