Uranus planet Facts
Uranus is named for the personifi cation of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 bil-lion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant plan-ets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour.
Uranus has been visited by a space-
craft only once—by the U.S. Voyager 2
probe in 1986. Before then, astronomers
had known little about the planet, since its
distance from Earth makes the study of its
visible surface difficult even with the most
powerful telescopes available. Earth-
based attempts to measure a property as
basic as the planetary rotation period had
produced widely diff ering values, ranging
from 24 to 13 hours, until Voyager 2 fi nally
established a 17.24-hour rotation period
for the Uranian interior. Since Voyager’s
encounter, advances in Earth-based obser-
vational technology have added to
knowledge of the Uranian system.
Uranus and its neighbour Neptune,
the next planet outward from the Sun,
are nearly twins in size. Measured at the
level of the atmosphere at which the
pressure is one bar (equivalent to Earth’s
sea-level pressure), Uranus’s equatorial
radius of 25,559 km (15,882 miles) is 3.2
percent greater than that of Neptune.
But Uranus has only 85 percent the mass
of Neptune and thus is significantly less
dense. The difference in their bulk den-
sities—1.285 and 1.64 grams per cubic
cm, respectively—reveals a fundamental
difference in composition and internal
structure. Although Uranus and Neptune
are significantly larger than the terres-
trial planets, their radii are less than
half those of the largest planets, Jupiter
and Saturn.
Because Uranus’s spin axis is not per-
fectly parallel to the ecliptic, one of its
poles is directed above the ecliptic and
the other below it. (The terms above and
below refer to the same sides of the eclip-
tic as Earth’s North and South poles,
respectively.) According to international
convention, the north pole of a planet is
defined as the pole that is above the eclip-
tic regardless of the direction in which
the planet is spinning. In terms of this
definition, Uranus spins clockwise, or in a
retrograde fashion, about its north pole,
which is opposite to the prograde spin of
Earth and most of the other planets.
When Voyager 2 flew by Uranus in 1986,
the north pole was in darkness, and the
Sun was almost directly overhead at the
south pole. In 42 years, or one-half the
Uranian year, the Sun will have moved to
a position nearly overhead at the north
pole. The prevailing theory is that the
severe tilt arose during the final stages of
planetary accretion when bodies compa-
rable in size to the present planets
collided in a series of violent events that
knocked Uranus on its side. An alternate
theory is that a Mars-sized moon, orbit-
ing Uranus in a direction opposite to the
planet’s spin, eventually crashed into the
planet and knocked it on its side.
Uranus’s rotation period of 17.24
hours was inferred when Voyager 2
detected radio wave emissions with that
period coming from charged particles
trapped in the planet’s magnetic field.
Subsequent direct measurements of the
field showed that it is tilted at an angle of 58.6° relative to the rotation axis and
that it turns with the same 17.24-hour
period. Because the field is thought to
be generated in the electrically conduct-
ing interior of the planet, the 17.24-hour
period is assumed to be that of the inte-
rior. The relatively fast rotation causes an
oblateness, or flattening of the planet’s
poles, such that the polar radius is about
2.3 percent smaller than the equatorial
radius. Winds in the atmosphere cause
cloud markings on the visible surface to
rotate around the planet with periods
ranging from 18 hours near the equator
to slightly more than 14 hours at higher
latitudes.
The atmosphere
Molecular hydrogen and atomic helium
are the two main constituents of the
Uranian atmosphere. Hydrogen is detect-
able from Earth in the spectrum of
sunlight scattered by the planet’s clouds.
The ratio of helium to hydrogen was
determined from the refraction (bend-
ing) of Voyager 2’s radio signal by the
atmosphere as the spacecraft passed
behind the planet. Helium was found to
make up 15 percent of the total number of
hydrogen molecules and helium atoms, a
proportion that corresponds to 26 per-
cent by mass of the total amount of
hydrogen and helium. These values are
consistent with the values inferred for
the Sun and are greater than those
inferred for the atmospheres of Jupiter
and Saturn. It is assumed that all four
giant planets received the same propor-
tions of hydrogen and helium as the Sun
during their formation but that, in the
cases of Jupiter and Saturn, some of the
helium has settled toward their centres.
The processes that cause this settling
have been shown in theoretical studies
not to operate on less-massive planets
like Uranus and Neptune.
Methane absorbs strongly at near-
infrared wavelengths, and it dominates
that part of the spectrum of reflected light
even though the number of methane mol-
ecules is only 2.3 percent of the total.
Astronomers determined this estimate of
methane abundance using Voyager 2’s
radio signals that probed to atmospheric
depths at which the methane-to-hydro-
gen ratio is likely to be constant. If this
constancy is characteristic of the planet
as a whole, the carbon-to-hydrogen ratio
of Uranus is 24 times that of the Sun.
(Methane [CH4
] comprises one atom of
carbon and four of hydrogen.) The large
value of the carbon-to-hydrogen ratio
suggests that the elements oxygen, nitro-
gen, and sulfur also are enriched relative
to solar values. These elements, however,
are tied up in molecules of water, ammo-
nia, and hydrogen sulfide, which are
thought to condense into clouds at levels
below the part of the atmosphere that can
be seen. Earth-based radio observations
reveal a curious depletion of ammonia
molecules in the atmosphere, perhaps
because hydrogen sulfide is more abun-
dant and combines with all the ammonia
to form cloud particles of ammonium
hydrosulfide. Voyager’s ultraviolet spec-
trometer detected traces of acetylene and
ethane in very low abundances. These
gases are by-products of methane, which
dissociates when ultraviolet light from
the Sun strikes the upper atmosphere.
On average, Uranus radiates the same
amount of energy as an ideal, perfectly
absorbing surface at a temperature of
59.1 kelvins (K; −353 °F, −214 °C). This
radiation temperature is equal to the
physical temperature of the atmosphere
at a pressure of about 0.4 bar. Temperature
decreases with decreasing pressure—i.e.,
with increasing altitude—throughout this
portion of the atmosphere to the 70-mil-
libar level, where it is about 52 K (−366 °F,−221 °C), the coldest temperature in
Uranus’s atmosphere. From this point
upward the temperature rises again until
it reaches 750 K (890 °F, 480 °C) in the
exosphere—the top of the atmosphere at
a distance of 1.1 Uranian radii from the
planetary centre—where pressures are on
the order of a trillionth of a bar. The cause
of the high exospheric temperatures
remains to be determined, but it may
involve a combination of ultraviolet
absorption, electron bombardment, and
inability of the gas to radiate at infrared
wavelengths.
Voyager 2 measured the horizontal
variation of atmospheric temperature in
two broad altitude ranges, at 60–200 mil-
libars and 500–1,000 millibars. In both
ranges the pole-to-pole variation was
found to be small—less than 1 K (1.8 °F, 1
°C)—despite the fact that one pole was
facing the Sun at the time of the flyby.
This lack of global variation is thought to
be related to the efficient horizontal heat
transfer and the large heat-storage capac-
ity of the deep atmosphere.
Although Uranus appears nearly
featureless to the eye, extreme-contrast-
enhanced images from Voyager 2 and
more-recent observations from Earth
reveal faint cloud bands oriented paral-
lel to the equator. The same kind of zonal
flow dominates the atmospheric circula-
tion of Jupiter and Saturn, whose
rotational axes are much less tilted than
Uranus’s axis and thus whose seasonal
changes in solar illumination are much
different. Apparently, rotation of the
planet itself and not the distribution of absorbed sunlight controls the cloud
patterns. Rotation manifests itself
through the Coriolis force, an effect that
causes material moving on a rotating
planet to appear to be deflected to either
the right or the left depending on the
hemisphere—northern or southern—
being considered. In terms of cloud
patterns, therefore, Uranus looks like a
tipped-over version of Jupiter or Saturn.
The wind is the motion of the atmo-
sphere relative to the rotating planet. At
high latitudes on Uranus, this relative
motion is in the direction of the planet’s
rotation. At equatorial latitudes the rela-
tive motion is in the opposite direction.
Uranus is like Earth in this regard. On
Earth these directions are called east and
west, respectively, but the more general
terms are prograde and retrograde. The
winds that exist on Uranus are several
times stronger than on Earth. The wind is
720 km (450 miles) per hour prograde at a
latitude of 55° S and 400 km (250 miles)
per hour retrograde at the equator.
Neptune’s equatorial winds are also ret-
rograde, but those of Jupiter and Saturn
are prograde. No satisfactory theory
exists to explain these differences.
Uranus has no large spots like
Jupiter’s long-lived Great Red Spot or
the Great Dark Spot observed on
Neptune by Voyager 2 in 1989. Voyager’s
measurements of the wind profile on
Uranus came from just four small spots
whose visual contrast was no more than
2 or 3 percent relative to the surround-
ing atmosphere. Because the giant
planets have no solid surfaces, the spots
must represent atmospheric storms. For
reasons that are not clear, Uranus seems
to have the smallest number of storms of
any of the giant planets.
Uranus’s
moons and rings
Uranus’s 27 known moons are accompa-
nied by at least 10 narrow rings. Each of
the countless particles that make up the
rings can be considered a tiny moon in
its own orbit. In general, the rings are
located closest to the planet, some small
moons orbit just outside the rings, the
largest moons orbit beyond them, and
other small moons orbit much farther out.
The orbits of the outermost group of
moons are eccentric (elongated) and
highly inclined to Uranus’s equatorial
plane. The other moons and the rings are
essentially coplanar with the equator.
Moons
Uranus’s five largest moons range from
about 240 to 800 km (150 to 500 miles) in
radius. All were discovered telescopically
from Earth, four of them before the 20th
century. Ten small inner moons were
found by Voyager 2 in 1985–86. They are
estimated to be between about 10 and 80
km (6 and 50 miles) in radius, and they
orbit the planet at distances between
49,800 and 86,000 km (31,000 and 53,500
miles). The innermost moon, Cordelia,
orbits just inside the outermost rings,
Lambda and Epsilon. An 11th tiny inner
moon, Perdita, photographed by Voyager
near the orbit of Belinda remained unno-
ticed in the images until 1999 and was
not confi rmed until 2003. Two additional
inner moons, one near Belinda’s orbit,
Cupid, and the other near Puck’s, Mab,
were discovered in observations from
Earth in 2003. All 18 of the above are reg-
ular, having prograde, low-inclination,
and low-eccentricity orbits with respect
to the planet.
Nine small outer moons in roughly
the same size range as the Voyager fi nds
were discovered from Earth beginning in
1997. These are irregular satellites, hav-
ing highly elliptical orbits that are
inclined at large angles to the planet’s
equator; all but one also orbit in the retro-
grade direction. Their mean distances
from the planet lie between 4 million and
21 million km (2.5 million and 13 million
miles), which is 7–36 times the distance of
the outermost known regular moon,
Oberon. The irregular moons likely were
captured into orbits around Uranus after
the planet formed. The regular moons
probably formed in their equatorial orbits
at the same time that the planet formed.
The four largest moons— Titania ,
Oberon , Umbriel , and Ariel , in order of
decreasing size—have densities of 1.4–1.7
grams per cubic cm. This range is only
slightly greater than the density of a
hypothetical object that would be
obtained by cooling a mixture of solar
composition and removing all the gas-
eous components. The object that
remained would be 60 percent ice and 40
percent rock. In contrast to these four
is Miranda, the fifth largest Uranian
moon, but only half the size of Ariel or
Umbriel. Like the smaller moons of
Saturn, Miranda has a density (1.2 g/cm3
[0.7 oz/in3 ]) that is slightly below the
solar composition value, which indicates
a higher ice-to-rock ratio.
The discovery of Uranus
Uranus was discovered by the English
astronomer William Herschel, who had
undertaken a survey of all stars down to
eighth magnitude—i.e., those about five
times fainter than stars visible to the naked
eye. On March 13, 1781, he found “a curious
either nebulous star or perhaps a comet,”
distinguished from the stars by its clearly
visible disk. Its lack of any trace of a tail
and its slow motion led within months to
the conclusion that the object was a planet,
rather than a comet or an asteroid, moving
in a nearly circular orbit well beyond
Saturn. Observations of the new planet
during the next 65 years revealed discrep-
ancies in its orbital motion—evidence of
gravitational forces on Uranus that were
not due to any other known planet, which
ultimately led to the discovery of yet more
distant Neptune in 1846.
Herschel suggested naming his
new discovery Georgium Sidus (Latin:
“Georgian Planet”) after his patron, King
George III of England, while the French
favoured the name Herschel. The planet
was eventually named according to the
tradition of naming planets for the gods
of Greek and Roman mythology; Uranus
is the father of Saturn, who is in turn the
father of Jupiter.
Umbriel
Umbriel is the third nearest of the fi ve
major moons of Uranus and the one hav-
ing the darkest and oldest surface of the
group. Its discovery is attributed to the
English astronomer William Lassell in
1851, although the English astronomer
William Herschel, who discovered Uranus
and its two largest moons, may have
glimpsed it more than a half century ear-
lier. Umbriel was named by Herschel’s
son, John , for a character in Alexander
Pope’s poem The Rape of the Lock .
It orbits Uranus once every 4.144 days
at a mean distance of 265,970 km (165,270
miles). Umbriel has a diameter of 1,170
km (727 miles) and a density of about 1.4
g/cm3 (0.81 oz/in3 ).
The only images of Umbriel’s surface
have come from the U.S. Voyager 2 space-
craft’s fl yby encounter with the Uranian
system in 1986. These show that Umbriel
is distinct from the other major moons of
Uranus in having no evidence of past tec-
tonic activity, as shown by the presence
of many large impact craters. The most
notable feature of the hemisphere imaged
by Voyager is a bright ring, dubbed
Wunda, that appears to line the fl oor of a
crater 40 km (25 miles) across.
Titania
Titania is the largest of the moons of
Uranus . It was fi rst detected telescopi-
cally in 1787 by William Herschel , who
had discovered Uranus itself six years earlier. Titania was named by William’s
son, John Herschel, for a character in
William Shakespeare’s play A Midsummer
Night’s Dream.
Titania orbits at a mean distance of
435,840 km (270,820 miles) from the cen-
tre of Uranus, which makes it the second
outermost of the planet’s major moons.
Its orbital period is 8.706 days, as is its
rotational period. It is thus in synchro-
nous rotation, keeping the same face
toward the planet and the same face for-
ward in its orbit. Its diameter is 1,578 km
(980 miles), and it has a density of about
1.71 g/cm3
(1 oz/in3
).
Titania was observed close up on only
one occasion, when the U.S. Voyager 2
spacecraft swiftly flew through the
Uranian system in 1986. Spacecraft images
show its surface to have many bright
impact craters up to 50 km (30 miles) in
diameter, but few large ones, along with
trenches and a deep fault line extending
roughly 1,600 km (1,000 miles). These and
other related features strongly suggest
the occurrence of internal geologic pro-
cesses in the moon’s ancient past.
Oberon
The outermost of the five major moons of
Uranus and the second largest is Oberon,
which was discovered in 1787 by William
Herschel. As with Titania, it was also
named by William’s son for a character in
William Shakespeare’s play A Midsummer
Night’s Dream.
The mean distance of Oberon from
the centre of Uranus is about 582,600 km
(362,000 miles), and its orbital period is
13.46 days. Like all of Uranus’s large moons,
Oberon rotates synchronously with its
orbital period, keeping the same hemi-
sphere toward the planet and the same
hemisphere forward in its orbit. The moon
has a diameter of 1,522 km (946 miles) and
a density of 1.63 g/cm3 (0.94 oz/in3 ) .
Photographic images transmitted by
the U.S. Voyager 2 spacecraft when it flew
past the Uranian system in 1986 revealed
that Oberon’s surface is old and that a few
of the numerous bright craters appear to
have been flooded by some kind of dark
material that upwelled from the moon’s
interior.
Spacecraft exploration
Although the missions of the twin Voyager
1 and 2 spacecraft originally called for fly-
bys of only Jupiter and Saturn, the timing
of Voyager 2’s launch allowed for a change
to its trajectory so that it could be retar-
geted to Uranus and Neptune for an
extended mission, which ultimately was
carried out. After more than eight years
in space, Voyager 2 sped through the
Uranian system on Jan. 24, 1986. Its
instruments provided an accurate deter-
mination of the masses and radii of the
planet and its major moons, detected
Uranus’s magnetic field and determined
its strength and orientation, and mea-
sured the planet’s interior rotation rate.
Images of the Uranian system, which
totaled more than 8,000, revealed for the
first time the weather patterns in the
planet’s atmosphere and the surface char-
acteristics of the moons. In addition to
Voyager’s discoveries of new moons, a
ring, and dust bands between the rings, it
provided details of ring structure at scales
not achievable from Earth.
Yet, despite these achievements,
Voyager left many unanswered questions
that only another spacecraft mission or a
major advance in Earth-based observa-
tional technology would be able to
address. As of 2009, no missions to
Uranus are planned.
The interior
Although Uranus has a somewhat lower
density than Jupiter, it has a higher pro-
portion of elements heavier than hydrogen
and helium. Jupiter’s greater mass (by a
factor of 22) leads to a greater gravitational
force and thus greater self-compression
than for Uranus. This additional compres-
sion adds to Jupiter’s bulk density. If
Uranus were made of the same propor-
tions of material as Jupiter, it would be
considerably less dense than it is.
Different models proposed for the
Uranian interior assume different ratios of
rock (silicates and metals), ices (water,
methane, and ammonia), and gases (essen-
tially hydrogen and helium). At the high
temperatures and pressures within the
giant planets, the “ices” will in fact be liq-
uids. To be consistent with the bulk density
data, the mass of rock plus ice must consti-
tute roughly 80 percent of the total mass
of Uranus, compared with 10 percent for
Jupiter and 2 percent for a mixture of the
Sun’s composition. In all models Uranus is a fluid planet, with the gaseous higher
atmosphere gradually merging with the
liquid interior. Pressure at the centre of the
planet is about five megabars.
Scientists have obtained more infor-
mation about the interior by comparing a
given model’s response to centrifugal
forces, which arise from the planet’s rota-
tion, with the response of the actual
planet measured by Voyager 2. This
response is expressed in terms of the
planet’s oblateness. By measuring the
degree of flattening at the poles and relat-
ing it to the speed of rotation, scientists
can infer the density distribution inside
the planet. For two planets with the same
mass and bulk density, the planet with
more of its mass concentrated close to
the centre would be less flattened by rota-
tion. Before the Voyager mission, it was
difficult to choose between models in
which the three components—rock, ice,
and gas—were separated into distinct lay-
ers and those in which the ice and gas
were well mixed. From the combination
of large oblateness and comparatively
slow rotation for Uranus measured by
Voyager, it appears that the ice and gas
are well mixed and a rocky core is small
or nonexistent.
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