Download THE FRIGID REALM

The Frigid Realm
The Outer Solar System
Uranus: Planet #7
Properties:
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Discovered accidentally in 1781 by William Herschel.
Just barely visible to the unaided eye, but was not recorded in antiquity.
Gas giant planet, mass ~14.5 M, radius ~ 4 R.
Presents an almost featureless blue-green appearance to our view.
Orbit & Rotation:
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Average distance from the Sun ~19 AU, orbital period = 84 years.
Oddly, its rotation axis is tilted 98 from its orbital plane; Uranus is effectively
“lying on its side.”
Like the other gas giants, Uranus rotates rapidly, in about 17 hours.
Uranus experience very weird seasons, since its rotation axis alternatively
points towards then away from the Sun.
Atmosphere & Interior:
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Its atmosphere is composed of hydrogen (84%), helium (14%), & methane
(2%). Atmospheric color is due to methane, which absorbs more red light
than blue light. The reflected light is deficient in red photons, and appears
blue to our view.
No ammonia is observed, unlike the situation for Jupiter & Saturn.
The few clouds or spots observed are located deep in the atmosphere.
Voyager 2 discovered that the Urainian magnetic field (~100X Earth’s in
strength) is offset from the center of the planet and misaligned with the
rotation axis. No explanation for this.
Uranus lacks an internal heat source, unlike the other three outer planets.
The interior pressures are too low for metallic hydrogen to form, so hydrogen
remains molecular down to the rocky core.
A slushy layer of water ice and dissolved ammonia may form the “mantle”
within Uranus.
Moons & Rings:
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5 mid-sized ice moons, all orbit in the equatorial plane
Ring system discovered accidentally in 1977, when a star passing behind
Uranus “blinked out” early, indicating that material was in orbit around the
planet.
The rings appear dark (low reflectivity), narrow, and widely spaced, very
different than Saturn’s rings. Shepherd satellites work to confine these rings,
as with Saturn’s F ring.
Visited by Voyager 2, which provided close-up images of the moons & rings.
Neptune: Planet #8
Properties:
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Existence was predicted by the laws of celestial mechanics, based on
Neptune’s gravitational effect on the orbit of Uranus.
Adams and Leverrier independently made this prediction in the mid-1840’s.
The German astronomer Galle discovered Neptune in 1846. A prototypical
example of a theory making a prediction of new natural phenomena, which is
then shown to be accurate.
Requires a telescope to see; Neptune is never visible to the unaided eye.
Gas/Ice giant planet, mass ~17 M, radius ~ 4 R.
Appears blue-colored with distinct atmospheric features.
Orbit & Rotation:
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Average distance from the Sun ~30 AU, orbital period = 164 years.
Axis tilt is 29.6, very similar to Saturn’s.
The planet rotates in about 16 hours, the outer atmosphere actually takes an
hour longer to complete one rotation.
Atmosphere & Interior:
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Atmospheric coloration is due to methane, which is more abundant on
Neptune than Uranus.
Also possess an odd magnetic field, offset and misaligned from the planet’s
center.
The interior contains an outer gas layer (H, He, & methane), and an inner
rocky core. A mantle of compressed water ice and ammonia is theorized to
exist between these two zones.
Moons & Rings:
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One large moon, Triton, and several small medium-to-small ice moons.
Triton orbits in a retrograde sense, an oddity, different than all other large
moons. Its surface has been reprocessed, and appears to contain active
nitrogen geysers. The amount of surface activity is surprising, given its
temperature.
Nereid possesses one of the most elongated orbits of any known moon.
Pluto: Planet (?) #9
Properties:
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Discovered by Clyde Tombaugh in 1930, at Lowell Observatory, who was
looking for “Planet X”. The predicted position, based on erroneous
discrepancies in the orbits of Uranus & Neptune, was only a few degrees from
Pluto’s actual position.
Pluto is never larger than 0.1 arc second as seen from Earth; therefore we
know little about its overall properties.
A small world, smaller than the Moon. Mass = 2  10-3 M, radius ~ 0.18 R.
Like Earth, Pluto possesses a moon, discovered in 1978, named Charon.
The only planet not visited by spacecraft. The New Horizons mission, to
flyby Pluto & enter the Kuiper Belt, is scheduled for launch in Feb 2006.
Orbit & Rotation:
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Very unusual orbit compared to the other planets: inclined 17 to the ecliptic
plane, and an orbital eccentricity = 0.25.
Average distance from the Sun ~39.5 AU, orbital period = 248 years. Due to
its elliptical orbit, Pluto sometimes is closer to the Sun than Neptune.
Pluto reached perihelion in 1989, and is now moving slowly away from the
Sun.
Rotates in 6.4 Earth days, in a retrograde sense, like Venus.
Atmosphere, Surface & Interior:
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The inferred composition (based on Pluto’s mean density) is most similar to a
medium-sized ice moon. Pluto is made up largely of water ice, and appears
to be a twin of Neptune’s moon Triton.
Methane exists in solid form on the surface; this gas may also contribute to
an extremely thin atmosphere.
Highly processed images from HST suggest some surface detail on Pluto, a
detailed map will have to wait for the New Horizons spacecraft, which will
reach Pluto in 2015.
Moons:
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Charon is more than half the size of Pluto, and about 1/6 the mass. The
Pluto/Charon system is effectively a “double planet”.
Charon’s orbit is inclined 118 to the plane of Pluto’s orbit around the Sun; a
very unusual orientation.
Pluto & Charon are tidally locked, each presenting the same face to the other
at all times.