Uranus
and Neptune Rings, Moons, Pluto/Charon
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version
April
27, 2000
Announcements
Uranus
and Neptune Rings and Moons, Pluto/Charon Preview
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Jupiter,
Saturn, Uranus, Neptune, Pluto
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Uranus's
rings
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Neptune's
rings
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Uranus's
satellites
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Neptune's
main moon Triton
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Triton's
retrograde orbit
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Pluto
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moon
Charon nearly as large
Uranus's
Rings
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Rings
discovered by accident with stellar occultation in 1977
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occultation
observed to measure diameter of Uranus
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Voyager
2 flyby allowed for measurement of composition
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Rings
composed of low reflectivity particles about 1 m in diameter
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as
reflective as coal
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What
are Saturn's rings composed of?
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Does
Jupiter have rings?
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Why
are Uranus's (and Neptune's) rings so dark?
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not
sure
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rings
are probably coated with methane ice
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methane
is CH4
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methane
ice is shiny
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but
methane ice bombarded by electrons forms carbon compounds
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carbon
compounds often black
Uranus's
rings backlit
Neptune's
Rings
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First
detected using stellar occultations
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Studied
in more detail with Voyager 2
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Ring
particles probably similar to Uranus's
Neptune's
Rings
Uranus's
Satellites
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15
found so far
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Densities
measured for 5 largest satellites
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density
~ 1500 kg/m3
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what
are these satellites made from?
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ice/rock
mix
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5 largest
satellites:
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Miranda,
Ariel, Umbriel, Titania, Oberon
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Umbriel
and Oberon heavily cratered
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dead
or alive?
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Umbriel
is particularly dark
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Evidence
of fluid flow on Ariel
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There
are a few craters in the channel, at least one crater covered over by fluid
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how
old is the lava flow?
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probably
billions of years old
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Miranda
looks traumatized
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possibly
result of collision or chucks during formation moved around
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then
surface cooled and solidified before completely smooth
Infrared
image of Uranus and 8 of its moons
Channel
on Ariel
The
contorted surface of Miranda
Cratering
rate on our Moon. Is this valid to apply to Uranus's moons?
Neptune's
Main Moon Triton
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7 other
known moons probably like Uranus's smaller satellites
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Triton:
last one of the "major moons"
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Lots
of unusual features
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Not
many craters
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Plumes
observed by Voyager 2
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Either
from warm interior
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heated
tidally by previously eccentric orbit (because of gravitational capture--see
next slide)
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Plumes
form dark smudges on surface
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smudges
appear wind-blown
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Triton
indeed has a thin atmosphere with winds
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mostly
nitrogen
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what
other solar system bodies have mostly Retrograde (backwards) orbit
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clockwise
or counterclockwise?
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Triton
probably formed somewhere else and was captured by Neptune
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4 outer
moons of Jupiter 1 of saturn also retrograde
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Triton
much bigger than these
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Triton's
orbit is decaying
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eventually
Triton will spiral inside the Roche limit
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what
happens to gravitationally bound objects inside the Roche limit?
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atmospheres?
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Some
terrain on Triton resembles Europa and Ganymede's ice cracks
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Unique
"Cantaloupe" terrain
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Surface
temperature 37 K, cold enough for nitrogen ice
Triton
from Voyager 2
Europa's
ice crack surface
Triton's
Retrograde Orbit
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Retrograde
(backwards) orbit
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clockwise
or counterclockwise?
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Triton
probably formed somewhere else and was captured by Neptune
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Probably
had a wild eccentric orbit for a while
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Source
of tidal heating
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Tidal
forces worked to circularize orbit (see below)
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4 outer
moons of Jupiter 1 of saturn also retrograde
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Triton
much bigger than these
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Triton's
orbit is decaying
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Recall
tides on Earth
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How
many high tides per day?
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TWO!
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Lump
raised by our Moon in oceans not right under moon
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leads
Moon because of rotation of Earth
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pulls
Moon into larger orbit
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Lump
raised on Neptune by Triton falls behind Triton because of retrograde orbit
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eventually
Triton will spiral inside the Roche limit
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what
happens to gravitationally bound objects inside the Roche limit?
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They
break up
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Why
doesn't this happen to us?
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we
are chemically, not gravitationally bound
Pluto
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Success
of Adams and Le Verrier at predicting Neptune's existence led people to
examine Neptune's orbit for problems
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problems
with the orbit of what planet led to the discovery of Neptune?
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Uranus
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William
Pickering and Percival Lowell predicted position of "planet X"
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They
were wrong
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Pluto
too small to detect using their data
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What
imagined geographic feature on Mars did Percival Lowell help "observe"
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However,
Clyde Tombaugh did find Pluto at Lowell observatory in 1930
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Pluto's
orbit eccentric compared to other planets
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sometimes
even inside of Neptune
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Retrograde
rotation
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what
other planets have retrograde rotation
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Venus,
Uranus
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Pluto
has a moon that is almost as big as it is
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Charon
rotates synchronously
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Pluto
rotates once per Charon's orbit
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Staring
at each other
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What
causes synchronous rotation?
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Our
view of Pluto with Hubble Space telescope is like Lowell's view of Mars
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Pluto
probably has Nitrogen polar ice caps
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Similar
in size, density to Triton
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recall
Triton was probably captured by Neptune
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Pluto
got away
Two
hemispheres of Pluto
Uranus
and Neptune Rings and Moons, Pluto/Charon Summary
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Jupiter,
Saturn, Uranus, Neptune, Pluto
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Uranus's
rings
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Neptune's
rings
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Uranus's
satellites
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Neptune's
main moon Triton
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Triton's
retrograde orbit
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Pluto
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moon
Charon nearly as large
