April 10, 2006 Astronomy 2010

Giant Planets

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Jovian Worlds

Uranus Neptune Jupiter

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Saturn

Astronomy 2010 2

Exploration

   First spacecrafts: pioneer 10 (1972) & 11 (1973).

  Can we navigate through the asteroid belt? What are the radiation hazards near the planets?

Pioneer 10 flew by Jupiter 1973 and flew out the solar system.

Pioneer 11 flew by Jupiter 1974 and was deflected towards Saturn which it reached in 1979.

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Exploration

   Voyager 1 & 2 (launched 1977)   Highly productive Missions Carried 11 scientific instruments including cameras and spectrometers, devices to measure the magnetospheres Voyager 1   Reached Jupiter 1979 and Saturn 1980 Used gravity assist towards Saturn Voyager 2   Reached Jupiter four month later than Voyager 1 Reached Saturn 1981, Uranus 1986, Neptune 1989  Multiple Flybys possible thanks to approximate alignment of the planets - occurs once in 175 years April 10, 2006 Astronomy 2010 Voyager 2 4

Voyager 2

Exploration

 Galileo space probe  Launched 1989    Reached Jupiter December 1995 Deployed a small entry probe for a direct study of Jupiter’s atmosphere Sept. 2003, probe sent into Jupiter’s atmosphere to end its mission.

 Cassini   Launched 1997 Reached Saturn in 2004, now in orbit.

 Will deploy entry probe for April 10, 2006 Titan in Jan 2005 Astronomy 2010 6

Galileo Space Probe

Galileo – Jupiter Entry Probe

        Mass 339 kg Plunged at shallow angle into Jupiter at speed of 50 km/s.

Slow down by friction against the Jovian atmosphere, temperature of shield to 15000 o C Speed dropped to 2500 km/h Deployed parachute for actual entry in the atmosphere Transmission of data to orbiter – retransmission to Earth 57 minute descent/recording 200 km downward/500 km sideward April 10, 2006 Astronomy 2010 8

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Some Scientific results of the Galileo mission

The discovery of a satellite (Dactyl) of an asteroid (Ida). Confirmation of the existence of a huge ancient impact basin in the southern part of the Moon's far side (inferred from Apollo data but never before mapped). Evidence of more extensive lunar volcanism than previously thought. Discovery of an intense interplanetary dust storm (the most intense ever observed). Discovery of an intense new radiation belt approximately 50,000 km (31,000 miles) above Jupiter's cloud tops. Jovian wind speeds in excess of 600 kilometers per hour (> 400 mph) were detected. Far less water was detected in Jupiter's atmosphere than estimated from earlier Voyager observations and from models of the Comet Shoemaker-Levy 9 impact. Far less lightning activity (about 10% of that found in an equal area on Earth) than anticipated. The individual lightning events, however, are about ten times stronger on Jupiter than the Earth. Helium abundance in Jupiter is very nearly the same as its abundance in the Sun (24% compared to 25%). Extensive resurfacing on Io due to continuing volcanic activity since the 1979 Voyagers fly-bys. Preliminary data support the evidence of magnetic fields for both Io and Ganymede. Evidence for liquid water ocean under Europa's ice.

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Jovian Planets - Basics

    Large distance from the sun.

Long period.

Jupiter/Saturn similar in composition + internal structure.

Uranus/Neptune smaller, differ in composition and structure.

Basics Properties of the Jovian Planets

Planet Distance Jupiter (AU)

5.2

Period (years)

11.9

Diameter (km)

142800

Mass (Earth=1)

318

Saturn Uranus Neptune

9.5

19.2

30.1

29.5

84.1

164.8

120540 51200 49500 95 14 17

Density (g/cm3)

1.3

0.7

1.2

1.6

Rotation (hours)

9.9

10.7

17.2

16.1

Appearance

 Only the upper atmosphere of the giant planets is visible to us  Composed primarily of hydrogen and helium gas    Jupiter/Saturn  Uppermost clouds made of ammonia crystals (NH 3 ) Neptune  Methane (CH 4 ) Uranus  no clouds, featureless haze April 10, 2006 Astronomy 2010 11

Rotation

 How does one determine the rotation rate of the giants?

 On Jupiter:  1 st option: Use dynamic surface features (storms).

  However the cloud rotation may have nothing to do with the rotation of the mantle and core… 2 nd option: look at periodic variations of radar waves associated with the magnetic field produced deep inside the planet.

 Rotation 9h 56 min.

 Same technique used to measure the rotation of other giant planets:  Saturn : 10h40 m.

 Uranus/Neptune : ~ 17 hours.

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Seasons on the Giants

Jupiter tilted by 3 o No seasons to speak of.

Saturn Tilted by 27 o Long seasons Neptune Tilted by 27 o Long seasons.

Uranus Tilted by 98 o Practically orbiting on its side Rings + satellites follow same pattern 21 year seasons!!!

Why this odd tilt? Giant impact!

Giant Planets – Giant Pressure

 Giant planets composed mainly of H, He, but…  Because of its enormous size, hydrogen and helium in the center of Jupiter is compressed enormously –.

 Estimated pressure: 100 million bars.

 Central density of 31 g/cm 3 .

 Earth by contrast has 4 million bars and 17 g/cm 3 in its center.

 Giant Planet implies Giant Pressure!!

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Consequences of the Pressure

 Few 1000 km below the surface, hydrogen is in a liquid state  Still deeper, the liquid is further compressed and begins to act like a metal.

  On Jupiter part of the interior is metallic hydrogen!

Saturn is less massive – most of its interior is liquid – but not metallic.

 Neptune/Uranus are probably too small to liquefy hydrogen April 10, 2006 Astronomy 2010 16

More about Composition

 The planets also have a core composed of heavier materials  Possibly the original rock/ice bodies that formed before gas were abundantly captured by the planets.

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Internal Heat Source (1)

 Because of their large sizes, all giant planets were strongly heated during their formation - Jupiter was the hottest  Some of the primordial heat still remains  Giant planets may also generate energy internally by slowly contracting  Even a small amount of shrinkage can generate significant heat  Raises the temperature of the core and atmosphere above the temperature due to the Sun energy supply.  Jupiter has the largest internal source of energy  4x10 17 Watts  A cross between a planet (like earth) and a star  Internal heat is primordial heat April 10, 2006 Astronomy 2010 18

Magnetic Fields

   All four giant planets have strong magnetic fields and associated magnetospheres The magnetospheres are large – extend for millions of km in space Jupiter’s field was discovered in the late 1950’s  Radio waves detected from Jupiter  Electrons circulating in the magnetosphere produce the radio waves by a process called synchrotron emission.

 Magnetic fields of Saturn, Uranus, and Neptune discovered by flyby spacecraft.

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Magnetospheres

 Jupiter’s magnetic field not aligned with its axis of rotation. – tipped by 10 o  Uranus/Neptune have tilts of 60 o and 55 o .

 Saturn’s field is perfectly aligned with its axis of rotation.

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Atmospheres

 The part of the planets accessible to direction observation.

 Dramatic examples of weather patterns.

 Storms on these planets can be larger than Earth!

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Atmospheric Composition

 Methane (CH 4 ) and ammonia (NH 3 ) were first believed to be the primary constituents of the atmospheres.

 We know today that hydrogen and helium are actually the dominant gases.

  First based on far-infrared measurements by Voyager Less helium in Saturn’s atmosphere  Precipitaion of Helium? Energy Source of Saturn?

 Best measurements of composition by Galileo spacecraft (1995) upon atmosphere entry.

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Clouds and Atmospheric Structure

 Jupiter’s clouds are spectacular in color and size.

 Color: orange, red, brown   Fast motion Saturn is more “subdued”: clouds have nearly uniform butterscotch hue.

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Winds and Weather

 Many regions of high/low pressure  Air flow between these regions  Sets up wind patterns distorted by the fast rotation of the planet.

 Wind speeds measured by tracking cloud patterns.

 Differences with Earth  Giant planets spin much faster than Earth   Rapid rotation smears out air circulation into horizontal (east-west) patterns parallel to the equator.

No solid surface  No friction or loss of energy – why tropical storms on Earth eventually die out…  Internal Heat contributes as much energy to the atmosphere as sunlight (except for Uranus).

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What causes the winds?

 Coriolis Effect!

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Jupiter/Saturn Winds

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Uranus/Neptune Winds

 Rather similar to the winds on Jupiter/Satrun  True on Uranus in spite of the 98 O tilt.

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Storms

 Always present on the giant planets   Superimposed on the regular circulation patterns Large oval shape high-pressure regions on both Jupiter and Neptune  Most famous: Jupiter Great Red Spot  In the Southern hemisphere  30000 km long (when Voyager flew by)   Present since first seen 300 years ago Changes in size – but never disappears Astronomy 2010 From Voyager 1, 1979 31 April 10, 2006

Great

Red

Spot

 Counterclockwise rotation  Period : 6 days  Similar disturbances formed in 1930s on Jupiter  Smaller circles near the red spot  Cause unknown  Long lived because of  absence of ground, and  their size  Expected life - centuries April 10, 2006 Astronomy 2010 32

Neptune Great Dark Spot

 First seen in 1989 by Voyager  10000 km long  17 day period  Had disappeared (faded?) in June 1994  New dark spot seen in Nov 1994  Faded by 1995  Do storms form and disappear faster on Neptune?

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Saturn Spots

 Large storms rare on Saturn  Appear to be connected with the seasons  Spot outbreaks every 30 years or so near the equator  Most recently in 1990 April 10, 2006 Astronomy 2010 34