Lecture 19

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Lecture 19The giant planets

• Meteo 466

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Different planetary types

• Relative masses
• Jupiter MJ ? 300 ME
• Sun MSun ? 1000 MJ
• Smallest star
• M ? 0.08 MS
• ? 80 MJ
• Deuterium burning limit 13 MJ
• (Objects bigger than this are classified as
  brown dwarfs)

318 ME
95 ME
14.5 ME
17.2 ME
1 ME
Beatty et al., (1999), Fig. 14-1.
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What is a brown dwarf?

• Two competing definitions
• Objects between 13 and 80 MJ, which can burn
  deuterium but not hydrogen
• Substellar objects (lt80 MJ) which form by
  gravitational collapse (like a star) rather than
  by core accretion (like a planet)

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Time evolution of giant planets and brown dwarfs

• Both giant planets and brown dwarfs cool and
  shrink as
• they age
• Note that, for an evolved giant planet (or brown
  dwarf), the
• radius is nearly independent of the mass

Beatty et al., (1999), Fig. 14-1.
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Planetary radius vs. mass

• Curves represent theoretical predictions for
  planets with
• different compositions
• Dots represent actual planets

Beatty et al. (1999), Fig. 14-3
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Giant planet radii
Beatty et al. (1999), Table 14-1

• Easy facts to remember
• Jupiter is about 10 times Earths diameter
• The Sun is about 10 times Jupiters diameter

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Solar nebula composition
Ref. J. K. Beatty et al., The New Solar System
(1999), Fig. 14-2.

• Jupiter and Saturn are close to solar
  composition, although
• each is depleted in H and He (Saturn more so
  than Jupiter)
• Uranus and Neptune are mostly composed of
  various ices

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Giant planet interior structure
Beatty et al., (1999), Fig. 14-7
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Rotation and planetary shape
Ref. J. K. Beatty et al., The New Solar System
(1999), Ch. 14.

• How a planet deforms as it rotates depends on
  its internal
• structure
• -- A spherical mass distribution acts like a
  point source
• A planets gravitational field depends on its
  mass distribution
• Accurate measurements of a planets
  gravitational field by
• an orbiting spacecraft can therefore yield
  insights into the
• planets internal structure

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Beatty et al. (1999), Table 15-1

• Saturn is depleted in He relative to the other
  giant planets
• He rains out of metallic hydrogen at low
  temperatures
• Jupiters interior is too hot for this to have
  had much effect
• Uranus and Neptune are too small to have metallic
  hydrogen
• Uranus and Neptune are highly enriched in CH4
  (and presumably in NH3 and H2S) relative to
  Jupiter and Saturn

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Beatty et al. (1999), Table 15-1

• Jupiters H2O abundance is probably much higher
  than measured
• The Galileo probe descended through a downwelling
  region
• The air had presumably been dried out by being
  lofted to higher altitudes and cooled

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Giant planet radiation balance

• Jupiter, Saturn, and Neptune all emit more energy
  than they receive from the Sun
• What could be going on here?

Beatty et al. (1999), Fig. 15-9
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Specific luminosities of the planets

• Earth (like meteorites) gets heated internally by
  radioactive decay
• Jupiter, Saturn, and Neptune all have significant
  internal heat sources in addition to radioactive
  decay
• The extra heat is thought to come from continued
  gravitational contraction (the Kelvin-Helmholtz
  mechanism)

Beatty et al. (1999), Fig. 14-9
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• An aside
• Lord Kelvin thought that the Sun produced its
  energy by gravitational contraction
• From this, he deduced that the Earth could not be
  more than 30 million years old
• This conflicted with Charles Darwins estimate
  for the minimum age of the Earth, 300 million
  years, published 3 years earlier in On the Origin
  of Species
• On the Age of the Suns Heat
• By Sir William Thomson (Lord Kelvin)
• Macmillan's Magazine, vol. 5 (March 5, 1862), pp.
  288-293.From reprint in Popular Lectures and
  Addresses, vol. 1, 2nd edition, pp. 356-375.

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Kelvins big faux pas

• It seems, therefore, on the whole most probable
  that the sun has not illuminated the earth for
  100,000,000 years, and almost certain that he has
  not done so for 500,000,000 years.
• This analysis was wrong, of course, because Lord
  Kelvin was unaware of nuclear fusion
• Darwin, however, was so taken back by Kelvins
  authority as a physicist that he deleted all
  references to the age of the Earth in later
  editions of his book

Image from Wikkipedia
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Heat flow in the Jovian planets

• Why does Uranus appear to lack an internal heat
  source?

Beatty et al. (1999), Fig. 14-9
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Uranus (from Voyager 2)

• Perhaps the problem stems from Uranus high
  (98o)
• obliquity
• Equilibration of emitted energy probably takes a
  significant
• fraction of a Uranian year ( 84 Earth years)

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• Lets take a visual tour through the outer Solar
  System
• We can do this because of many pictures returned
  by Voyager 1 and 2 in the late 1970s/early
  1980s, the Galileo mission to Jupiter in the
  1990s, and the Cassini mission, which is in
  orbit around Saturn at present
• The Hubble Space Telescope also has taken many
  beautiful pictures

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Jupiter from Voyager 1

• Many obvious and persistent features
• Belts (dark) and zones (light)
• Lots of clouds and eddies
• The Great Red Spot (probably first observed by
  Giovanni Cassini in 1665)

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Jupiter fronted by Io and Europa
Photo from Voyager 1
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Jupiters belts and zones
Beatty et al. (1999), Fig. 15-15
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Zonal winds on the giant planets

• Belts and zones are regions of alternating
  easterly and
• westerly winds (on Jupiter, at least)
• Equatorial jets are prograde (westerly) on
  Jupiter and Saturn,
• retrograde on Neptune

Beatty et al. (1999), Fig. 15-7
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Clouds on the giant planets

• Jupiter and Saturn have at least 3 different
  types of visible
• clouds NH3 (ammonia), NH4SH (ammonium
  sulfide), and H2O
• Uranus and Neptune have only CH4 clouds

Beatty et al. (1999), Fig. 15-8
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Deeper cloud layers on Jupiter

• Theoretical calculations by Jonathan Lunine
  (Univ. of Arizona)
• On hot Jupiters around other stars, these more
  refractory compounds condense out at higher
  altitudes and can be seen in planetary spectra

Beatty et al. (1999), Fig. 14-13
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Jupiters Great Red Spot

• The Red Spot was only really red during the
  1970s
• Rotation is counterclockwise, which is
  anti-cyclonic,
• as the spot is in Jupiters southern hemisphere

Photo from Voyager 1
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Great Red Spot (infrared picture)

• Picture taken by Galileo orbiter (June, 1996)
• Pink areas represent colder, higher clouds

Beatty et al. (1999), Fig. 15-8
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Saturn (from Voyager 1)

• Much like Jupiter, but it has
• a spectacular ring system
• Rings are thought to decay
• with time (from collisions)
• and hence must be
• replenished, also by
• collisions
• Rings are within Saturns
• Roche limit (the distance
• within which tidal forces
• overwhelm gravity)

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Saturn (from Cassini)

• The rings have gaps, which are created by mean
  motion resonances with different moons
• Biggest gap is called the Cassini division

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Uranus rings and moons

• Uranus also has a system of rings and moons
• Image has been enhanced to make them appear
  brighter relative to the planet
• The discovery of these rings should have ended
  any debate about how to pronounce Uranus ?

Beatty et al. (1999), Fig. 16-13
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Uranus (from Voyager 2)

• Almost featureless in the visible
• Why are Uranus and Neptune blue?

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Uranus reflection spectrum
CH4 absorbs in the red ?
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Neptune (from Voyager 2)

• Neptune is more interesting to look at
• At the time of the Voyager 2 observations,
  Neptune had a Great Blue Spot
• The Blue Spot has since disappeared

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Pluto and Charon (from HST)

• This is the clearest view yet of the distant
  planet Pluto and its moon, Charon, as revealed by
  NASA's Hubble Space Telescope (HST).
• Picture taken in 1994
• Hubble's corrected optics show the two objects as
  clearly separate and sharp disks. This now allows
  astronomers to measure directly (to within about
  1 percent) Pluto's diameter of 1440 miles (2320
  kilometers) and Charon's diameter of 790 miles
  (1270 kilometers).

NASA Planetary Photojournal
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Computer enhanced images of Pluto

• The New Horizons spacecraft is on its way to
  Pluto and
• will arrive there in 2015
• The PI, Alan Stern, just stepped down last week
  as NASA
• Associate Administrator for Space Science

NASA Planetary Photojournal