Download 1 Lecture #28: Uranus

Lecture #28: Uranus
The Main Point
• Uranus:
– General properties.
Uranus, the first planet discovered by
telescopes, is a mid-sized and highly tilted
Jovian class planet with a bland atmosphere
and a very dark ring system.
– Atmosphere.
– Interior.
– Origin and evolution.
• Reading: 7.1 (Uranus) and 11.1
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The Jovian Planets
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Basic Properties of Uranus
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Discovery
Average Distance from Sun: 2.87 billion km (a=19.2 AU).
Orbital period: 84.1 years; eccentricity: 0.046.
Period of Spin around axis: ~17 hours and retrograde.
Tilt of Uranus' spin axis: 98° (extreme seasons).
Mass: 8.7x1025 kg = 15 ME ; Radius: 25,600 km = 4.0 RE.
Density = 1.3 g/cm3 (data indicate mostly "rocky/icy").
No "surface" like the terrestrial planets: clouds/haze visible.
Gravity = 9.0 m/sec2 (0.92 times Earth's).
Cloud-top temperature: 60 K (-213°C).
Atmosphere: Mostly H, He, CH4--but only ~15% of planet.
Moons: 27 presently known (12 discovered since late 1990s).
Uranus has a small set of dark, dusty rings.
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First planet discovered by telescope:
– Uranus had been observed as early as 1690 but was
catalogued as a star (very slow sky motion).
– Recognized as a planet in 1781 by the musician and
amateur astronomer William Herschel (1738-1822), after
a systematic search of the sky with a 7 foot refractor.
– Herschel wanted to name the planet Georgium Sidus
(George's planet) after his patron, King George III.
– But Uranus (pronounced YUR uh nus), the Greek god of
the heavens, was proposed by others and the name stuck.
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Space Missions
Observations
• Uranus' disk spans
only about 2 to 4
arcseconds.
• Only one encounter: Voyager 2.
Mission
Date
Voyager 2 1986
Goals and Results
Flyby; high resolution imaging and other studies
– It can be barely visible to the naked eye, but you must
have very dark skies and know exactly where to look.
• Uranus appears slightly blue and relatively bland
through most ground based telescopes.
• More detail can be seen in higher resolution HST
images, or using telescopes in the infrared.
• Features in Uranus' atmosphere are very small and
subtle compared to those on Jupiter or Saturn.
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No plans have been made for any new space missions to Uranus.
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Atmospheric Composition
Features in Uranus' Atmosphere
• Determined by spectroscopy from
telescopes and spacecraft.
• Observed (upper) composition very
“solar”.
• Zones and belts weak or absent.
• Smaller clouds, storms do occur.
Name
Hydrogen
Helium
Methane
Water
Carbon Monoxide
Hydrogen Disulfide
Ammonia
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Atmospheric Structure
Altitude (km)
Clouds/hazes of
methane gas give
Uranus its distinctive
blue/green color
Symbol
% Volume
H2
83
He
15
2
CH4
H2O
???
CO
~1 x 10-5?
1 x 10-6
H2S
NH3
2 x 10-7
C2H4
7 x 10-7
CH3NH2 3 x 10-7
• The ratio of H2/He in
Uranus' atmosphere is
almost identical to that
in the Sun.
• Much more He and
CH4 than detected on
Jupiter or Saturn.
• Implies that Uranus
atmosphere still "well
mixed" (heavier He
did not differentiate?)
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Uranus' Interior
• Slightly higher density, higher Helium abundance, and lower internal
pressures & temperatures argue that Uranus and Neptune have very
different interior structures than Jupiter and Saturn.
• “ice” and “rock” are much larger fractions of Uranus and Neptune, where
“ice” here means volatiles like CH4, NH3, H2O, and “rock” means silicate
minerals like the terrestrial planets.
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100
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Strange Seasons!
Internal vs. External Heat Sources
~1986
• Again, examine the energy balance.
• For Uranus: Outgoing = Incoming.
• Unlike Jupiter & Saturn (& Neptune), Uranus
doesn't generate its own internal heat.
• Why not???
• Lack of internal heating may explain the rarity of
clouds & other "weather" on Uranus.
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• Uranus is tilted on its
side, essentially "rolling"
around the solar system!
• This is another factor that
may influence differences
between Uranus and the
other gas giant planets.
• What caused this strange
tilt? UNKNOWN
– Primordial?
– Giant Impact?
– Tides?
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21 year time steps
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Satellites
Uranus has a strong magnetic field
• Uranus has 5 "regular" large satellites, discovered by
telescopic observations between 1787-1948.
• But Uranus has no
metallic H, so how is
the field generated?
• Hypothesis: Field is
generated in the core by
high pressure ionization
of ices/rock (making
them conductive).
• Uranus' field is tilted
and offset relative to its
spin axis: supports the
idea that the field is
generated in the
rocky/ice mantle?
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~2028
– All are relatively dark and icy (based on spectroscopy).
• 10 smaller moons discovered by Voyager 2.
• 12 more tiny outer moons discovered in 1997-2005.
• Naming scheme based on Shakespeare and Pope.
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More details: Lecture 30
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Rings!
Summary
• Uranus is a gas giant planet with a thick H, He, and
CH4 atmosphere.
• Uranus is tilted at 98° to the ecliptic: odd seasons.
• Uranus' atmosphere has many fewer clouds and other
features than Jupiter's or Saturn's.
• Uranus has a
system of 9 dark
"main" rings.
• These rings are also
surrounded by belts of
fine-grained dust.
• Discovered in 1977 by
watching a star "blink
out" behind them.
• Made of organics+ices?
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– No internal heat of its own.
• Uranus' interior consists mostly of "icy" and "rocky"
materials at high pressure and temperature.
– Strong & offset magnetic field generated in the “mantle”.
More details in Lecture 31...
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• Astro
Uranus
has 27 known satellites and a ring system.
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Next Lecture...
• Neptune:
– General properties.
– Atmosphere.
– Interior.
– Origin and evolution.
• Reading: 7.1 (Neptune) and 11.1
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