Uranus and Neptune
Outer Worlds of the Solar System

May 5, 2011, jeb

OUTLINE Discovery and Exploration Basic Properties of Uranus and Neptune Rotation of Uranus Atmospheres of Uranus and Neptune Magnetospheres and Internal Structure of Uranus and Neptune Moons of Uranus and Neptune Rings of Uranus and Neptune Golden Globe Awards for Uranus and Neptune Other Sources on Uranus Other Sources on Neptune

Discovery and Exploration

Uranus (1781)
• discovered by William Herschel, British Astronomer
  • discovered by chance while viewing sky with telescope
  • named for the mythical father of Saturn
• Uranus seen from Earth with large telescope

Neptune (1846)
• Deviations in orbit of Uranus led to prediction of Neptune's orbit by two astronomers:
  • John Adams (British) and Urbain Leverrier (French)
  • first observed (where predicted) by German astronomer Johann Galle
• Neptune seen from Earth with large telescope

Voyager 2
• with the flybys of Uranus (1986) and Neptune (1989), our knowledge of these two outer planets expanded greatly

Names

• Uranus named after the ancient Greek deity of the sky. The equivalent Roman god Caelus, was the father of Saturn, grandfather of Jupiter, and great grandfather of Mars.
• Neptune named after god of the sea in Roman mythology

Basic Properties of Uranus and Neptune

Rotation of Uranus

Tilt of Uranus
• Rotation axis tilted relative to orbital plane
  • tilt angle 98^o
  • consequence:
    • north pole points directly at the Sun at one point in orbit
      • south total darkness
    • south pole points directly at the Sun 1/2 year later
      • north total darkness
• Very strange seasons:
  • poles have summers with Sun directly overhead
    winters with Sun never appearing
  • equator
• Moons also orbit in the skewed equatorial plane
• No other planet has such a large tilt

Origin of Tilt
• Catastrophic event?
• Grazing collision with another planet sized body?

Atmospheres of Uranus and Neptune

Elements
• 84% molecular hydrogen (H₂)
• 14% helium
• methane (CH₄) - 2% on Uranus and 3% on Neptune
• very little ammonia (NH₃)
  • frozen out (much colder than Jupiter and Saturn)



Comparison of the Jovian Atmospheres




	Jupiter	Saturn	Uranus	Neptune
molecular hydrogen (H2)	86%	92%	84%	84%
helium	14%	7%	14%	14%
methane (CH4)	trace%	0.2%	2%	3%
ammonia (NH3)	trace%	0.02%	0%	0%



Surface Temperatures
• Uranus has a surface temperature of 58 K
  • no significant internal heat source
• Neptune has a surface temperature of 59 K
  • radiates 2.7 times more heat than it receives from Sun
Color of Atmospheres
• Uranus and Neptune are a bluish color (Uranus blue-green)
  • due to methane in the atmosphere
    • methane absorbs out the red
  • Neptune has more methane, and is distinctly blue
Weather
• Uranus
  • Featureless appearance
    • no internal heat source
  • Flows below surface
    • very efficient energy transfer
    • south pole (currently in total darkness) only a few degrees colder than north pole
• Neptune
  • Cloud and band structure more easily seen
    • internal heat source -> convection
  • Cloud streaks
  • Upper atmosphere slightly warmer than Uranus
  • Great Dark Spot discovered by Voyager 2 (1989)
    • similar to Great Red Spot on Jupiter
    • but has not survived
      • gone in 1994, although other features have appeared (Hubble Space Telescope)

Magnetospheres and Internal Structure of Uranus and Neptune

Strength of Magnetic Fields

• internal magnetic fields 100 times Earth
  • 1/10 of Saturn
• at cloud tops (4 times Earth's radius) comparable to field strength at surface of Earth

Tilt and offsets of magnetic fields

• Uranus and Neptune magnetic fields are tilted to large angles relative to the rotation axis
  • 60^o on Uranus
  • 46^o on Neptune
• Uranus and Neptune magnetic fields are not centered on center of planet
  • large offsets

Internal Structure

• Rocky cores
  • about the size of the Earth, but about 10 times as massive
• no metallic hydrogen (found in Jupiter and Saturn)
• source of magnetic field differs from Jupiter and Saturn (where metallic hydrogen is responsible)
  • frozen out ammonia -> ionic layer?

Moons of Uranus and Neptune

Major moons of Uranus and Neptune

Uranus

27 known moons
• three distinct classes:
  • 11 small very dark inner moons mostly discovered by Voyager 2
    • one only recently discovered
  • 5 medium sized moons
    • image shows five largest moons of Uranus and two midsize moons of Neptune(Proteus and Nereid)
  • 11 newly discovered (since 1997), much more distant, small moons
• most have nearly circular orbits in the plane of Uranus' equator
  • the outer 11 have much more elliptical orbits
• dirty surfaces
  • radiation darkening (chemical reactions)
• Strangest - Miranda -jagged and fractured

Neptune

• 13 known moons
• not a very regular moon system
  • no moons on roughly circular, equatorial, prograde orbits
• Outer moon - Nereid
  • Elongated orbit
  • 1.4 million km -> 9.7 million km
    • perspective, the semi-major axis of Moon's orbit is 0.384 million km
• Triton

Triton (moon of Neptune)

• 7th largest moon in Solar System
  • next to Jupiter's Galilean moons, Saturn's Titan, and Earth's Moon
• Retrograde orbit (going around the "wrong" way)
  • circular
  • inclined at 20^o to equator of Neptune
• 37K surface temperature
• One of three objects in Solar System known to have a nitrogen dominated atmosphere
  • The other two are Earth, and Titan (Saturn)
• Geysers
  • vaporized nitrogen erupting from below the surface
• Surface lacks much cratering
  • Numerous frozen "lakes" of water ice may be of recent volcanic origin
• Origin of Triton?
  • Must explain
    • Surface features of Triton
    • Retrograde orbit of Triton
    • related to elongated orbit of Nereid?
  • Candidate theories:
    • 1.) captured by Neptune (not so long ago, astronomically speaking)
    • 2.) formed with Neptune -> then a catastrophic event disrupted orbit
  • what role did Pluto play in this origin?
    • orbits of Neptune and Pluto cross
• Future of Triton
  • Spiraling toward Neptune
    • caused by retrograde orbit which generates tidal bulge on Neptune
  • Tidal gravitation of Neptune will rip Triton apart
    • Perhaps in 100,000,000 years, which Triton reaches Roche Limit
    • will this produce spectacular Rings?

Rings of Uranus and Neptune

Uranus

• 11 Rings are known
  • Nine rings discovered from Earth (1977)
    • stellar occultation
• dark, narrow, widely spaced
  • widest ring (epsilon ring) is 100 km wide, and shows much internal structure
• shepherd satellites
  • Cordelia and Ophelia shepherd the epsilon ring
• undiscovered shepherd satellites?
• all within the Roche Limit

Neptune

• 5 known rings
  • 3 quite narrow
  • 2 broad and diffuse
• Outer ring (Adams) clumps into arcs
• all within the Roche Limit
  • Adams ring is just outside the approximate Roche Limit of 2.4 radii, but actually lies within the true Roche Limit

General Theory of Rings

• All jovian worlds have rings
• Rings are believed to be relatively short-lived (astronomically speaking)
• Therefore: formation is a common event
  • otherwise we would not see rings on all 4 planets
• Local environment important:
  • Moons, catastrophic events, etc.
• No comprehensive theory yet exists

Other Sources on Uranus

StarDate Guide to Uranus
Welcome to the Planets - Uranus
Views of the Solar System - Uranus
The Nine Planets - Uranus

Other Sources on Neptune

StarDate Guide to Neptune
Welcome to the Planets - Neptune
Views of the Solar System - Neptune
The Nine Planets - Neptune