Transcript Uranus and Neptune

Discovery of Uranus, Neptune, Pluto
•
•
•
•
•
The last 3 planets were discovered with
telescopes, in the last 150 years or so (too faint to
see with eyes)
Studied (glimpsed) by Voyager 2, powered by
“gravity slingshot” ; explored all Jovian planets
Uranus was discovered accidentally by William
Herschel
Neptune was discovered ‘mathematically’!
Following observations of perturbations in the orbit
of Uranus by an unseen object, Adams and Levellier
independently predicted its existence
Pluto is not a (Jovian) planet, more like a moon,
planetesimal, or asteroid; was discovered in 1930 by
Clyde Tombaugh
Uranus
View of Uranus from Earth
(something strange ?)
Orbit of Uranus around the Sun:
Rotation axis tilted at 98 degrees !
Uranus
• Uranus and Neptune are almost twins in size ~
25,000 Kms radius
• Uranus is tilted on its side by 98 degrees; each pole
faces the sun for half the orbital period (84 years)
• Cause of tilt? Where on Uranus would you go to
spend summer vacation ?
• Uranus rotates somewhat backwards like Venus;
rotation period = 17hr 14m
• Weak magnetic field ~ 0.74 x Earth’s, could be a very
small metallic mantle
• Methane atmosphere, hence greenish hue
• H2O (ice) mantle
Neptune
Neptune and the Great Black Spot
Methane ice clouds on Neptune
Atmosphere of Neptune (c.f. Jupiter)
A pale imitation of Jupiter!
Neptune
• Neptune’s atmosphere very active, with convection
currents
• ‘Great Black Spot’ ~ 10,000 Kms long, cyclonic
storm like the GRS on Jupiter
• Internal energy source (gravity) like Jupiter; emits
2.6 times the energy it receives from the sun
• Tilted at 30 degrees
• Strange moon Triton, which rotates backwards,
probably a captured Kuiper Belt Object
• Magnetic field tilted ~ 50 degrees to rotation axis;
mag poles not aligned with geo poles (Earth and
Jupiter are about 10 degrees)
Magnetic and geographical axes
Neptune’s Magnetic axis is NOT centered
Structures of Uranus and Neptune
Relatively thin atmospheres compared to Jupiter and Saturn,
but still quite extensive compared to terrestrial planets
Atmospheres of Uranus and Neptune
• More He than Jupiter and Saturn
• Insufficient gravity  loss of H
• H is locked up in methane CH4, 10 x
Jovian, (also more carbon)  absorbs red,
reflects blue
• Hydrocarbon ‘smog’ – mainly frozen
methane haze and clouds with ethane,
acetylene, etc.
• Discovery of Jupiter’s ring(s) by Voyager 2
Discovery of Ring Systems in
Jovian Planets
• Jupiter --------• Saturn --------• Uranus --------• Neptune ---------
Voyager 2
Galileo
Occultation
Occultation
- Occultation  Obscuration of starlight by rings
- Jupiter’s ring is very thin and dusty material
Occultation of starlight by rings of
Uranus
Dusty rings of Uranus not seen from the Earth; occultation method used
following the discovery of Jupiter’s ring by Voyager 2
‘Occultation Spectrum’ due to rings of
Uranus – dimming of starlight blocked
by the rings as seen from the Earth
Uranus’ rings and moons
Uraniun system with rings
Note the rotation axis and position of rings
Voyager 2 view of Rings of Uranus
(dusty, extended atmosphere makes
rings unstable)
Relatively more extended atmospheres due to lower gravity than
Jupiter and Saturn
Neptune’s rings (similar to Uranus
– dusty ambient atmosphere)
The slit in the middle blocks out light from the main body of the planet,
Thereby enabling the faint rings to be seen
Rings of Uranus and Neptune
• Both Uranus and Neptune have ring
systems
• Made mainly of rocks, not ice like Saturn,
therefore nearly invisible
• Owing to low gravity, the ring structures
are diffuse and unstable: extend into the
moons (hence heavily cratered) or spiral
into the planet
• Probably last no more than 10-100 million
years
Moons of Uranus and Neptune
• Uranus has 5 large moons, and 10 small
ones, all heavily cratered
• Miranda shows large oval structures; may
have shattered several times and reassembled
• Neptune has 5 small moons (< 100 Kms)
bunched near the rings; followed by
Proteus at ~ 200 Kms, and then Triton
• Uranus and Neptune do not have large
Galilean satellites; did not form moons on
their own as in Jupiter and Saturn
Moons of Uranus – heavily cratered
due to interaction with loose ring
particles and dust
Miranda – Broken and re-assembled ?
Triton – captured by Neptune ?
Orbits opposite to direction of rotation of Neptune
Triton
• Triton is more dense, ~ 2 g/cc, indicating
rocky composition, than the moons nearer
to Neptune which are mostly ice
• Reverse of the trend for Galilean satellites
• Triton might have been a Kuiper Belt
Object, captured by Neptune
• Has a very thin atmosphere, 0.00002 bars
• T = 38 K  N and CH4 freeze and “snow”
• Polar caps extend to 25 degree latitude