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Astronomy
A. Dayle Hancock
[email protected]
Small 239
Office hours:
MTWR 10-11am
Uranus Neptune & Remote
Worlds
Uranus and Neptune
Orbits and Atmospheres
Internal Structure
Magnetic Fields
Rings Uranus's Large Moon
Neptune's Triton
Pluto
http:// physics.wm.edu/~hancock/171/
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Uranus
Uranus is the 7th planet from the Sun. It was discovered in
1781 by William Herschel.
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Neptune
Neptune is the 8th planet from the Sun. It was discovered in
1846 by noting the small discrepancy in Uranus's orbit and
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calculating its position using Newton's law of gravity.
Uranus
When Voyage 2 pasted by
Uranus in 1986, the Jovian
gas planet appeared nearly
featureless. Clouds were not
visible even in the close range
images. The atmosphere is
82.5% H, 15.2% He which is
similar to Jupiter and Saturn.
Unlike Jupiter and Saturn,
Uranus's atmosphere has
2.3% methane (CH4). T = 55K!
Clouds, storms and bands are visible in the IR. Uranus
orbits 19.2 AU from the Sun. The most unusual thing about
Uranus is its axis of rotation is tilted 97.9o to its orbital plane.
Since Uranus's orbital period is 84 years, it is summer at the
south pole for 42 years and winter for 42 years
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Neptune
Voyage 2 past by Neptune in
1989. While it has about the
same diameter as Uranus, it is
18% more massive. The
atmosphere is 79% H and 18%
He, It has 3% CH4 and almost
no H2O. It is cold at 55K. There
are clouds (like the white Cirrus
clouds of CH4 ice.) There are
also bands and storms visible
in enhanced images. The Great
Dark spot has disappeared.
Neptune orbits 30 AU from the Sun. Neptune's orbital
period is 165 years. Uranus and Neptune receive little
solar energy. Unlike Uranus, Neptune emits more energy
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than it receives probably due to slow contraction.
Densities of Uranus and Neptune
The densities of Uranus (1320 kg/m 3) and Neptune (1640
kg/m3) are as large or larger than Jupiter (1330 kg/m 3) and
Saturn (690 kg/m3). Both planets have a higher concentration
of heavy elements that Jupiter or Saturn which is unexpected.
Their great distances from the sun is also surprising. That far
out there should have not been enough material to form these
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relatively large planets.
Magnetic Fields of Uranus and Neptune
Magnetometer on Voyage 2 reveled some surprises about
the magnet fields of Uranus and Neptune. The axis of the
Magnetic field on Uranus is tilted at 59o to its rotation axis.
On Neptune it is 47o. Both fields are offset from the center
of the planet. The fields are created by the dynamo effect.
The conducting rotating liquid is high compressed H 20 and
NH3 which is conducting.
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Rings of Uranus
Uranus has three rings. The rings are narrow ( <10 km)
and dark. They were discovered in 1977 by observing the
occultation of a star with ground based telescopes. They
are thought to be composed of carbon compounds which
resulted from radiation darkening of CH4 ice. The two faint
outer rings were only discovered in 2005.
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Rings of Neptune
The rings of Neptune
are dark and thin like
Uranus. They are
also thought to be
radiation darkened
CH4. The big surprise
about Neptune's
rings is that they are
disappearing. They
could be gone in a
century if the decay
continues.
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Moons of Uranus
Uranus has 5 large moons and
22 small moons. Of the 5 large
moons Umbriel and Oberon are
heavily cratered dead worlds.
Ariel and Titania surface
appears to have cracked and
the surface flooded with ice
lava. Miranda (bottom image)
is cratered but has the unique
'chevron' feature with cliffs
twice as high as Mt. Everest.
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Moons of Neptune
Neptune has 13 known
moons. Most are small icy
bodies similar to the small
moons of Uranus. Triton is
the one large moon. Triton is
in a retrograde orbit around
Neptune. The orbit is
inclined by 23o. It probably
formed else where in the
solar system. It surface is
mainly water ice and is young
from tectonic activity due to
tidal forces. There are signs of old ice volcanoes. Triton has
a thin tenuous atmosphere. Because of the retrograde orbit,
Triton is moving closer to Neptune. It will eventually reach
the Roche limit and break up forming a major ring system.11
Pluto
Pluto was discovered in 1930. It is no longer considered a
planet because of it small size (2300 km). Its orbit is highly
eccentric (e= .25) and the plane of its orbit is steeply inclined
(17.2o). This is more than any planet. Its large eccentricity
cause it to be closer to the Sun than Neptune during part of
its orbit. It axis of rotation is tilted by 120 o (causing extremely
long seasons) and its rotation is retrograde.
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Pluto
Before July 2015, the best images of Pluto were from the
Hubble space telescope. Because Pluto is so far way
(semimajor axis = 39.5 AU), the Hubble images do not
show any distinct surface features.
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Pluto
In July of 2015, the New Horizons space probe flew past
Pluto and its moons. This image from the flyby shows a
more complex object than expected. Pluto's surface is
composed of more than 98% nitrogen ice, with traces of CH 4
and CO. Pluto's density is 1.860 kg/m 3. The interior is
probably differentiated with a rocky core and an ice mantle.
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Charon
Charon is the largest moon of
Pluto. In fact, the diameter of
Charon (1190 km) is about half
the size of Pluto (2270 km),
This is a recently downloaded
image of Charon from the New
Horizon flyby. Note the riff near
the equator and the area flat
dark area near the north pole.
Charon orbital period is 6.4
days which is the same as the
rotation period of Pluto and the rotation period of Charon.
Charon and Pluto always keep the same face toward each
other.
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Trans-Neptunian Objects
Eris like Pluto is a 'dwarf planet'. Eris is the largest of the
trans-Neptunian objects. Eris is highly reflective. Its
simimajor axis is 68 AU. Like Pluto and the other trans
Neptunian objects its orbit has a large eccentricity and the
orbit plane is tilted to the ecliptic. Many have satellites of their
own (Dysnomia in the case of Eris). Over 35,000 transNeptunian object are thought to exist.
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Trans-Neptunian Objects
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Kuiper Belt
Most of the TransNeptunian object lie within
the Kuiper belt which
extends from 30-50 AU.
Eris and Sedna are
actually even further out.
The Kuiper belt consists
of objects left over from
the formation of the solar
system. It does not
appear to be unique to
our solar system. The
image shows the edge on
'Kuiper' belt of a star
which is 57 light years
away.
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