GEOL 3045: Planetary Geology
Lysa Chizmadia
Uranus
Introduction
 3rd largest planet
– Seventh from Sun (or eighth)
 1st discovered in modern times
– 1781 by William Herschel
 Eccentricity = 0.046
Image from: http://www.solarviews.com/eng/uranus.htm
– 18 - 20 AU
 Axial Tilt = 97.9º
– Earth = 23.5º
 Orbital period = 84.1 years
– Each pole 42 years continuous Sun, then dark
 Rotational period = 17.9 hours
 Mass = 8.69 x 1025 kg
– ~14.5 Earths
 Average  = 1.29 g/cm3
–  = 5.52 g/cm3 (2nd lowest  in solar system)
 At least 27 moons
Image from: http://en.wikipedia.org/wiki/Uranus
Internal Structure
 Atmosphere:
– Pressure: 1.2 bars
 Earth: ~ 1 bar
– Outer 20% of radius
 ~0.5 Earth masses
– ~83% H, ~15% He & 2% CH4
 Blue color due to absorption of red light by methane
 Icy mantle
–
–
–
–
~13.4 x Earth masses
Hot dense fluid
Composition: H2O, NH3, etc.
High electrical conductivity
 Rocky Core:
–
–
–
–
~0.55 x Earth masses
 ~ 9 g/cm3
P ~ 8 Mbars
T ~ 5000 K
Image from: http://www.solarviews.com/eng/uranus.htm
Atmosphere
 Structure:
– Troposphere
 Highly complex cloud structure
–
–
–
–
False color showing banding
H2O clouds
NH4HS clouds
NH3 or H2S clouds
CH4
 Responsible for Far IR emissions
– T ~ 59 K
– Stratosphere
 Contains H-Cs formed from UV
radiation of CH4
 May be responsible for bland appearance
– Thermosphere/Corona
 H2 and H
 Extends 2 Uranus radii (~8 Earth radii)
Images from: http://en.wikipedia.org/wiki/Uranus
Dark Spot
Climate
 At equator, winds retrograde
White Storm
– 50-100 m/s
 Wind speeds  to 60 lat
– 150-200 m/s
 Towards poles, winds prograde
 At poles, winds fall to 0 m/s
 Dark spots common on
Neptune never observed on
Uranus until 2006
Images from: http://en.wikipedia.org/wiki/Uranus
Inner Rings
Rings
Dark ring system
2nd discovered
At least 13 distinct rings
Opacity & gaps indicate recent
formation
 Probably from shattered satellite
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
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Fine Structure
False color to show structure
Shepherd Satellites
Images from: http://www.solarviews.com/eng/uranus.htm
Images from: http://en.wikipedia.org/wiki/Uranus
Magnetic Field
 ~50 x Earth’s magnetic field
 Tilted 59 from rotation axis
 Not centered with planet
– ~1/3 towards south pole
 Therefore, highly asymmetric
– Strength in S ~0.1 G
– Strength in N ~1.1 G
 ~ Neptune
 May be generated in CH4H2O ocean instead of in core
Image from: http://en.wikipedia.org/wiki/Uranus
Heat Output
 Coldest of giant planets
– 49 K (~ same as Pluto)
 Neptune radiates 2.6 x more heat than receives
from Sun
– Uranus radiates almost none
 Heat flux < Earth
 Hypotheses:
– When knocked over, lost primordial heat
– Barrier in atmosphere prevents heat from reaching
space
Image from: http://en.wikipedia.org/wiki/Uranus
Summary
 1st discovered in modern times
 2nd least dense planet
 Internal structure
– Atmosphere
– Icy mantle (hot, dense liquid)
– Rocky core
 Rotates on side
– Probably due to massive collision
 Complex atmospheric structure
– Troposphere
– Stratosphere
– Thermosphere/Corona
 Asymmetrical magnetic field
 Coldest of giant planets
Image from: http://en.wikipedia.org/wiki/Uranus