Uranus, named after the the father of the Roman god Saturn, is the
seventh planet in the Solar System and third of the gas giants. It is
the third largest planet by diameter, yet fourth most massive.
Uranus has the distinction of being the first planet discovered
in modern history. Actually, its discovery as a planet almost did not
happen. In 1781, the astronomer William Herschel was charting the stars
found in the Gemini constellation when he observed a disk-like object.
His intial conclusion was that he had discovered a comet and reported
his findings as such to the Royal Society of England. However, Herschel
was puzzled when he calculated the object's orbit. Instead of the more
elliptical path occuring with comets, he found that it was much more
circular. This observation, which was confirmed by other astronomers at
the time, led Herschel to conclude that he had, in fact, discovered a
new planet. Shortly thereafter, it was widely accepted that Herschel had
discovered an unknown planet.
As a result of his discovery, Herschel was given the privilege of
naming the new planet. The name he chose was Georgium Sidus, which is
latin for Georgian Planet. He opted for this name to honor then king of
England, George III. This name, however, was not widely accepted, and as
a result others began to suggest names. The name Uranus was put forth
in the tradition of naming planets after deities in Roman mythology.
Over time the scientific community accepted this as the planet's name.
At present, the only planetary mission to visit Uranus is Voyager
2. This lone encounter, which occurred in 1986, provided a large amount
of data and discoveries. The spacecraft took thousands of pictures of
Uranus and its moons and rings. Although the images of the planet showed
little other than the uniform blue-green color seen from Earth-based
telescopes, other images revealed the presence of ten previously unknown
moons and two new rings. At this time, no future missions are scheduled
for Uranus.
Atmosphere
Due to its stark blue appearance, the Uranian atmospheric
patterns have been much more difficult to observe than, say, those of
Jupiter or even Saturn. Fortunately, the Hubble Space Telescope has
provided much more insight into the structurial nature of Uranus'
atmosphere. Through more advanced imaging techonologies than Earth-based
telescopes or Voyager 2, Hubble has shown that there are latittudinal
bands much like those found on the other gas giants. Additionally, the
winds associated with these bands can blow in excess of
576 km/hr.
The reason behind the monotonous atmospheric appearance is the
composition of the top-most layer of the atmosphere. The visible cloud
layers are composed primarily of methane, which absorbs those visible
wavelengths corresponding to the color red. Thus, the reflected
wavelengths are those of blue and green.
Beneath this outer methane layer, the atmosphere is composed of roughly 83% hydrogen (H
2)
and 15% helium with trace amounts of methane and acetylene. This
composition is similar to that of the other gas giants. Uranus'
atmosphere is drastically different in another regard, though. Whereas
Jupiter and Saturn's atmospheres are primarily gaseous, Uranus' conatins
much more ice. This indicates that the Uranian atmosphere is extremely
cold. In fact, at approxiamtely
-224° C,
its atmsophere is the coldest found in the Solar System. What is even
more interesting is data indicates that this extreme temperature is
constant globally, occuring even on the side that is not sunlit.
Interior
Uranus' interior is thought to consist of two layers: a core and
mantle. Current models suggest that the core is primarily composed of
rock and ice and is approximately .55 times the mass of the Earth. The
planet's mantle is believed to be
8.01 x 1024 kg,
or about 13.4 times the mass of the Earth.
Furthermore, the mantle is
composed of water, ammonia and other volatile elements. What
distinguishes Uranus' mantle from those of Jupiter and Saturn is that it
is icy, though not in the traditional sense. Instead, the ice is very
hot and thick. The mantle is
5,111 km thick.
What is most surprising about Uranus' interior and one of the
most distniguishing features with respect to the other gas giants is
that it does not emit more energy than it receives from the Sun.
Considering that even Neptune, which is very similar in size to Uranus,
produces approximately 2.6 times the amount of heat that it receives
from the Sun, scientists are very intrigued by the low heat that Uranus
generates. There are two popular theories for this phenomenon. The first
says that Uranus was struck by a large body, dispersing into space most
of the heat that planets normally retain from their formations. The
second theory claims that there is some barrier preventing the internal
heat from making its way to the planet's surface.
Orbit & Rotation
When Uranus was discovered it expanded the radius of the known
Solar System by almost a factor of two. What this means is that, on
average, Uranus' orbit is about
2.87 x 109 km.
The consequence of such an enormous distance is that it takes sunlight
around two hours and forty minutes to reach Uranus—that is almost twenty
times as long as it takes sunlight to reach the Earth! This huge
distance also means that a year on Uranus lasts almost 84 Earth years!
At 0.0473, Uranus' orbital eccentricity is just slighty less than
that of Jupiter's .0484, making it the fourth most circular orbit of
all the planets. The result of Uranus' fairly small orbital eccentricity
is that the difference between its perihelion of
2.74 x 109 km and aphelion of
3.01 x 109 km is just
2.71 x 108 km.
Perhaps the most interesting thing about Uranus is how odd its
rotation is compared to all of the other planets'. The axis of rotation
for every planet other than Uranus is roughly perpendicular with their
orbital plane. However, Uranus' axis is tilted almost 98°, which
effectivley means that Uranus rotates on its side. The result of this is
that Uranus' north pole points at the Sun for half of its year, while
the south pole points at the Sun the other half of its year! In other
words, it is daytime on one Uranian hemispheres, while it is night time
on the other for 42 Earth years at a time. Furthermore, due to this
extreme rotation, Uranus does not have days like on other planets—that
is, the Sun doesn't rise and set like on other planets.
The cause for this highly unusal axial tilt is theorized to be
the effect of a large body striking Uranus with such force that it
essentially knocked the planet over on its side.
Rings
Although Saturn's rings have been well known for some time, it
wasn't until 1977 that the planetary rings surrounding Uranus were
discovered. The reason behind this is twofold: their distance from the
Earth and their low reflectivity of light. Nonetheless, the Voyager 2
spacecraft identitfied two more on its fly-by mission in 1986, followed
by the Hubble Space Telescope discovery of two additional rings in 2005.
The total number of known rings currently sits at thirteen, the largest
and brightest of which is the epsilon ring.
Uranus' rings differ from those found around Saturn in both the
size of its particles and the particles' composition. First, the
particles making up Saturn's rings are small, with few larger than
several meters in diameter, whereas Uranus' rings contain many bodies up
to twenty meters in diamter. Second, the particles of Saturn's rings
are largely composed of ice. Uranus', however, are composed of both ice
and considerable dust and debris.
Uranus Facts
Discovery Date: 1781
Discoverer:
William Herschel
Number of Moons: 27
Rings: Yes
Time it takes light to reach Uranus from the Sun:
2 hours 40 minutes
Mass:
8.68 x 1025 kg
Mean Diameter:
50,724 km
Mean Circumference:
159,354 km
Volume:
6.83 x 1013 km3
Density:
1,270 kg/m3
Surface Gravity:
8.87 m/s2
Length of Day:
17 hours 14 minutes
Length of Year:
84 Earth Years
Orbital Eccentricity:
0.0473
Mean Orbital Distance from the Sun:
2.87 x 109 km
Perihelion:
2.74 x 109 km
Aphelion:
3.01 x 109 km
Axial Tilt: 97.8°
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