The Uranus
(Uranus: History,Discovery,Internal structure,Atmosphere,Planetary rings)
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest
planetary mass in the Solar System. Uranus is similar in composition to
Neptune, and both have different bulk chemical composition from that of
the larger gas giants Jupiter and Saturn. For this reason, scientists
often classify Uranus and Neptune as "ice giants" to distinguish them
from the gas giants. Uranus's atmosphere is similar to Jupiter's and
Saturn's in its primary composition of hydrogen and helium, but it
contains more "ices" such as water, ammonia, and methane, along with
traces of other hydrocarbons. It is the coldest planetary atmosphere in
the Solar System, with a minimum temperature of 49 K (−224 °C; −371 °F),
and has a complex, layered cloud structure with water thought to make
up the lowest clouds and methane the uppermost layer of clouds. The
interior of Uranus is mainly composed of ices and rock.
Uranus
is the only planet whose name is derived from a figure from Greek
mythology, from the Latinised version of the Greek god of the sky
Ouranos. Like the other giant planets, Uranus has a ring system, a
magnetosphere, and numerous moons. The Uranian system has a unique
configuration among those of the planets because its axis of rotation is
tilted sideways, nearly into the plane of its solar orbit. Its north
and south poles, therefore, lie where most other planets have their
equators. In 1986, images from Voyager 2 showed Uranus as an almost
featureless planet in visible light, without the cloud bands or storms
associated with the other giant planets. Observations from Earth have
shown seasonal change and increased weather activity as Uranus
approached its equinox in 2007. Wind speeds can reach 250 metres per
second (900 km/h; 560 mph).
History
Like
the classical planets, Uranus is visible to the naked eye, but it was
never recognised as a planet by ancient observers because of its dimness
and slow orbit. Sir William Herschel announced its discovery on March
13, 1781, expanding the known boundaries of the Solar System for the
first time in history and making Uranus the first planet discovered with
a telescope.
Discovery
Uranus
had been observed on many occasions before its recognition as a planet,
but it was generally mistaken for a star. Possibly the earliest known
observation was by Hipparchos, who in 128 BCE might have recorded it as a
star for his star catalogue that was later incorporated into Ptolemy's
Almagest. The earliest definite sighting was in 1690, when John
Flamsteed observed it at least six times, cataloguing it as 34 Tauri.
The French astronomer Pierre Charles Le Monnier observed Uranus at least
twelve times between 1750 and 1769, including on four consecutive
nights.
Sir
William Herschel observed Uranus on March 13, 1781 from the garden of
his house at 19 New King Street in Bath, Somerset, England (now the
Herschel Museum of Astronomy), and initially reported it (on April 26,
1781) as a comet. Herschel "engaged in a series of observations on the
parallax of the fixed stars", using a telescope of his own design.
Herschel recorded in his journal: "In the quartile near ζ Tauri ... either Nebulous
star or perhaps a comet." On March 17 he noted: "I looked for the Comet
or Nebulous Star and found that it is a Comet, for it has changed its
place." When he presented his discovery to the Royal Society, he
continued to assert that he had found a comet, but also implicitly
compared it to a planet:
The
power I had on when I first saw the comet was 227. From experience I
know that the diameters of the fixed stars are not proportionally
magnified with higher powers, as planets are; therefore I now put the
powers at 460 and 932, and found that the diameter of the comet
increased in proportion to the power, as it ought to be, on the
supposition of its not being a fixed star, while the diameters of the
stars to which I compared it were not increased in the same ratio.
Moreover, the comet being magnified much beyond what its light would
admit of, appeared hazy and ill-defined with these great powers, while
the stars preserved that lustre and distinctness which from many
thousand observations I knew they would retain. The sequel has shown
that my surmises were well-founded, this proving to be the Comet we have
lately observed.
Herschel
notified the Astronomer Royal, Nevil Maskelyne, of his discovery and
received this flummoxed reply from him on April 23, 1781: "I don't know
what to call it. It is as likely to be a regular planet moving in an
orbit nearly circular to the sun as a Comet moving in a very eccentric
ellipsis. I have not yet seen any coma or tail to it."
Although
Herschel continued to describe his new object as a comet, other
astronomers had already begun to suspect otherwise. Finnish-Swedish
astronomer Anders Johan Lexell, working in Russia, was the first to
compute the orbit of the new object. Its nearly circular orbit led him
to a conclusion that it was a planet rather than a comet. Berlin
astronomer Johann Elert Bode described Herschel's discovery as "a moving
star that can be deemed a hitherto unknown planet-like object
circulating beyond the orbit of Saturn". Bode concluded that its
near-circular orbit was more like a planet than a comet.
The
object was soon universally accepted as a new planet. By 1783, Herschel
acknowledged this to Royal Society president Joseph Banks: "By the
observation of the most eminent Astronomers in Europe it appears that
the new star, which I had the honour of pointing out to them in March
1781, is a Primary Planet of our Solar System." In recognition of his
achievement, King George III gave Herschel an annual stipend of £200 on
condition that he move to Windsor so that the Royal Family could look
through his telescopes.
Orbit and rotation
Uranus orbits the Sun once every 84 years. Its average distance from the Sun is roughly 20 AU
(3 billion km; 2 billion mi). The difference between its minimum and
maximum distance from the Sun is 1.8 AU, larger than that of any other
planet, though not as large as that of dwarf planet Pluto. The intensity
of sunlight varies inversely with the square of distance, and so on
Uranus (at about 20 times the distance from the Sun compared to Earth)
it is about 1/400 the intensity of light on Earth. Its orbital elements
were first calculated in 1783 by Pierre-Simon Laplace. With time,
discrepancies began to appear between the predicted and observed orbits,
and in 1841, John Couch Adams first proposed that the differences might
be due to the gravitational tug of an unseen planet. In 1845, Urbain Le
Verrier began his own independent research into Uranus's orbit. On
September 23, 1846, Johann Gottfried Galle located a new planet, later
named Neptune, at nearly the position predicted by Le Verrier.
The
rotational period of the interior of Uranus is 17 hours, 14 minutes. As
on all the giant planets, its upper atmosphere experiences strong winds
in the direction of rotation. At some latitudes, such as about 60
degrees south, visible features of the atmosphere move much faster,
making a full rotation in as little as 14 hours.
Internal structure
Uranus's
mass is roughly 14.5 times that of Earth, making it the least massive
of the giant planets. Its diameter is slightly larger than Neptune's at
roughly four times that of Earth. A resulting density of 1.27 g/cm3
makes Uranus the second least dense planet, after Saturn. This
value indicates that it is made primarily of various ices, such as
water, ammonia, and methane. The total mass of ice in Uranus's interior
is not precisely known, because different figures emerge depending on
the model chosen; it must be between 9.3 and 13.5 Earth masses. Hydrogen
and helium constitute only a small part of the total, with between 0.5
and 1.5 Earth masses. The remainder of the non-ice mass (0.5 to 3.7
Earth masses) is accounted for by rocky material.
The
standard model of Uranus's structure is that it consists of three
layers: a rocky (silicate/iron–nickel) core in the centre, an icy mantle
in the middle and an outer gaseous hydrogen/helium envelope. The core
is relatively small, with a mass of only 0.55 Earth masses and a radius
less than 20% of Uranus's; the mantle comprises its bulk, with around
13.4 Earth masses, and the upper atmosphere is relatively insubstantial,
weighing about 0.5 Earth masses and extending for the last 20% of
Uranus's radius. Uranus's core density is around 9 g/cm3, with a
pressure in the centre of 8 million bars (800 GPa) and a temperature of
about 5000 K. The ice mantle is not in fact composed of ice in the
conventional sense, but of a hot and dense fluid consisting of water,
ammonia and other volatiles. This fluid, which has a high electrical
conductivity, is sometimes called a water–ammonia ocean.
The
extreme pressure and temperature deep within Uranus may break up the
methane molecules, with the carbon atoms condensing into crystals of
diamond that rain down through the mantle like hailstones.
Very-high-pressure experiments at the Lawrence Livermore National
Laboratory suggest that the base of the mantle may comprise an ocean of
liquid diamond, with floating solid 'diamond-bergs'.
The
bulk compositions of Uranus and Neptune are different from those of
Jupiter and Saturn, with ice dominating over gases, hence justifying
their separate classification as ice giants. There may be a layer of
ionic water where the water molecules break down into a soup of hydrogen
and oxygen ions, and deeper down superionic water in which the oxygen
crystallises but the hydrogen ions move freely within the oxygen
lattice.
Although
the model considered above is reasonably standard, it is not unique;
other models also satisfy observations. For instance, if substantial
amounts of hydrogen and rocky material are mixed in the ice mantle, the
total mass of ices in the interior will be lower, and, correspondingly,
the total mass of rocks and hydrogen will be higher. Presently available
data does not allow a scientific determination which model is correct.
The fluid interior structure of Uranus means that it has no solid
surface. The gaseous atmosphere gradually transitions into the internal
liquid layers. For the sake of convenience, a revolving oblate spheroid
set at the point at which atmospheric pressure equals 1 bar (100 kPa) is
conditionally designated as a "surface". It has equatorial and polar
radii of 25,559 ± 4 km (15,881.6 ± 2.5 mi) and 24,973 ± 20 km (15,518 ±
12 mi), respectively. This surface is used throughout this article as a
zero point for altitudes.
Atmosphere
Although
there is no well-defined solid surface within Uranus's interior, the
outermost part of Uranus's gaseous envelope that is accessible to remote
sensing is called its atmosphere. Remote-sensing capability extends
down to roughly 300 km below the 1 bar (100 kPa) level, with
a corresponding pressure around 100 bar (10 MPa) and temperature of 320
K (47 °C; 116 °F). The tenuous thermosphere extends over two planetary
radii from the nominal surface, which is defined to lie at a pressure of
1 bar. The Uranian atmosphere can be divided into three layers: the
troposphere, between altitudes of −300 and 50 km (−186 and 31 mi) and
pressures from 100 to 0.1 bar (10 MPa to 10 kPa); the stratosphere,
spanning altitudes between 50 and 4,000 km (31 and 2,485 mi) and
pressures of between 0.1 and 10−10 bar (10 kPa to 10 µPa); and the
thermosphere extending from 4,000 km to as high as 50,000 km from the
surface. There is no mesosphere.Moons
Uranus
has 27 known natural satellites. The names of these satellites are
chosen from characters in the works of Shakespeare and Alexander Pope.
The five main satellites are Miranda, Ariel, Umbriel, Titania, and
Oberon. The Uranian satellite system is the least massive among those of
the giant planets; the combined mass of the five major satellites would
be less than half that of Triton (largest moon of Neptune) alone. The
largest of Uranus's satellites, Titania, has a radius of only 788.9 km
(490.2 mi), or less than half that of
the Moon, but slightly more than Rhea, the second-largest satellite of
Saturn, making Titania the eighth-largest moon in the Solar System.
Uranus's satellites have relatively low albedos; ranging from 0.20 for
Umbriel to 0.35 for Ariel (in green light). They are ice–rock
conglomerates composed of roughly 50% ice and 50% rock. The ice may
include ammonia and carbon dioxide.
Among
the Uranian satellites, Ariel appears to have the youngest surface with
the fewest impact craters and Umbriel's the oldest. Miranda has fault
canyons 20 km (12 mi) deep, terraced layers, and a chaotic variation in
surface ages and features. Miranda's past geologic activity is thought
to have been driven by tidal heating at a time when its orbit was more
eccentric than currently, probably as a result of a former 3:1 orbital
resonance with Umbriel. Extensional processes associated with upwelling
diapirs are the likely origin of Miranda's 'racetrack'-like coronae.
Ariel is thought to have once been held in a 4:1 resonance with Titania.
Uranus
has at least one horseshoe orbiter occupying the Sun–Uranus L3
Lagrangian point—a gravitationally unstable region at 180° in its orbit,
83982 Crantor. Crantor moves inside Uranus's co-orbital region on a
complex, temporary horseshoe orbit. 2010 EU65 is also a promising Uranus
horseshoe librator candidate.
Planetary rings
The
Uranian rings are composed of extremely dark particles, which vary in
size from micrometres to a fraction of a metre. Thirteen distinct rings
are presently known, the brightest being the ε ring. All except two
rings of Uranus are extremely narrow – they are usually a few kilometres
wide. The rings are probably quite young; the dynamics considerations
indicate that they did not form with Uranus. The matter in the rings may
once have been part of a moon (or moons) that was shattered by
high-speed impacts. From numerous pieces of debris that formed as a
result of those impacts, only a few particles survived, in stable zones
corresponding to the locations of the present rings.
William
Herschel described a possible ring around Uranus in 1789. This sighting
is generally considered doubtful, because the rings are quite faint,
and in the two following centuries none were noted by other observers.
Still, Herschel made an accurate description of the epsilon ring's size,
its angle relative to Earth, its red colour, and its apparent changes
as Uranus traveled around the Sun. The ring system was definitively
discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and
Jessica Mink using the Kuiper Airborne Observatory. The discovery was
serendipitous; they planned to use the occultation of the star SAO
158687 (also known as HD 128598) by Uranus to study its atmosphere. When
their observations were analysed, they found that the star had
disappeared briefly from view five times both before and after it
disappeared behind Uranus. They concluded that there must be a ring
system around Uranus. Later they detected four additional rings. The
rings were directly imaged when Voyager 2 passed Uranus in 1986. Voyager
2 also discovered two additional faint rings, bringing the total number
to eleven.
In
December 2005, the Hubble Space Telescope detected a pair of previously
unknown rings. The largest is located twice as far from Uranus as the
previously known rings. These new rings are so far from Uranus that they
are called the "outer" ring system. Hubble also spotted two small
satellites, one of which, Mab, shares its orbit with the outermost newly
discovered ring. The new rings bring the total number of Uranian rings
to 13. In April 2006, images of the new rings from the Keck Observatory
yielded the colours of the outer rings: the outermost is blue and the
other one red. One hypothesis concerning the outer ring's blue colour is
that it is composed of minute particles of water ice from the surface
of Mab that are small enough to scatter blue light. In contrast,
Uranus's inner rings appear grey.
Exploration
In
1986, NASA's Voyager 2 interplanetary probe encountered Uranus. This
flyby remains the only investigation of Uranus carried out from a short
distance and no other visits are planned. Launched in 1977, Voyager 2
made its closest approach to Uranus on January 24, 1986, coming within
81,500 km (50,600 mi) of the cloudtops, before continuing its journey to
Neptune. The spacecraft studied the structure and chemical composition
of Uranus's atmosphere, including its unique weather, caused by its
axial tilt of 97.77°. It made the first detailed investigations of its
five largest moons and discovered 10 new ones. It examined all nine of
the system's known rings and discovered two more. It also studied the
magnetic field, its irregular structure, its tilt and its unique
corkscrew magnetotail caused by Uranus's sideways orientation.
Voyager
1 was unable to visit Uranus because investigation of Saturn's moon
Titan was considered a priority. This trajectory took Voyager 1 out the
plane of the ecliptic, ending its planetary science mission.:118
The
possibility of sending the Cassini spacecraft from Saturn to Uranus was
evaluated during a mission extension planning phase in 2009, but was
ultimately rejected in favour of destroying it in the Saturnian
atmosphere. It would have taken about twenty years to get to the Uranian
system after departing Saturn. A Uranus orbiter and probe was
recommended by the 2013–2022 Planetary Science Decadal Survey published
in 2011; the proposal envisages launch during 2020–2023 and a 13-year
cruise to Uranus. A Uranus entry probe could use Pioneer Venus
Multiprobe heritage and descend to 1–5 atmospheres. The ESA evaluated a
"medium-class" mission called Uranus Pathfinder. A New Frontiers Uranus
Orbiter has been evaluated and recommended in the study, The Case for a
Uranus Orbiter. Such a mission is aided by the ease with which a
relatively big mass can be sent to the system—over 1500 kg with an Atlas
521 and 12-year journey. For more concepts see Proposed Uranus
missions.
