Uranus has a relatively featureless appearance at visible wavelengths. Even from Voyager 2 at a distance of 80,000 km there were few distinguishable features. This is believed to be due to Uranus being further from the Sun than Jupiter and Saturn, which means its temperature is lower (only 58 degrees Kelvin in the upper atmosphere). This decreases the liklihood of chemical reactions making the colorful compounds that give the surface features on Jupiter and Saturn. In addition, the upper atmosphere is thought to have a high- level petrochemical haze that obscures features lower in the atmosphere.
The blue color is because of methane gas in the atmosphere, which absorbs red and orange light strongly, leaving more blue light to be scattered to the observer. The clouds are thought to be mostly methane ice, with a temperature at the cloud tops of about -221 degrees Celsius.
Voyager 2 confirmed the suspicion that Uranus had a magnetic field. The field is about 50 times stronger than that of the Earth and is tilted about 60 degrees with respect to the rotation axis. As a result, the magnetic field moves like a corkscrew as Uranus rotates, as illustrated in the following movie (5 MB). One hypothesis for this behavior of the magnetic field is that it originates in a thin conducting shell outside the core of the planet rather than deep in the core as for the Earth or Jupiter. The pressure would not be high enough for the relevant conducting material to be metallic hydrogen. A mixture of water, methane, and ammonia under sufficient pressure could provide the requisite electrical conductor.
The magnetosphere contains belts of charged particles similar to those of the Earth. The rings and most of the moons orbit within the magnetososphere and thus are protected from the Solar wind.
The atmosphere of Uranus, like those of the larger gas giants Jupiter and Saturn, is composed primarily of hydrogen and helium. At depth it is significantly enriched in volatiles (dubbed "ices") such as water, ammonia and methane. The opposite is true for the upper atmosphere, which contains very few gases heavier than hydrogen and helium due to its low temperature. Uranuss atmosphere is the coldest of all the planets, with its temperature reaching as low as 49 K.
The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300[a] and 50 km and pressures from 100 to 0.1 bar; the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10−10 bar; and the hot thermosphere (or exosphere) extending from an altitude of 4,000 km to several Uranian radii from the nominal surface at 1 bar pressure. Unlike Earths, Uranuss atmosphere has no mesosphere.
The troposphere hosts four cloud layers: methane clouds at about 1.2 bar, hydrogen sulfide/ammonia clouds in at 3– 10 bar, ammonium hydrosulfide clouds at 20–40 bar, and finally water clouds below 50 bar. Only the upper two cloud layers have been observed directly—the deeper clouds remain speculative. Above the clouds lie several tenuous layers of photochemical haze. Discrete bright tropospheric clouds are rare on Uranus, probably due to sluggish convection in the planets interior. Nevertheless observations of such clouds were used to measure the planets zonal winds, which are remarkably fast with speeds up to 240 m/s.
Little is known about the Uranian atmosphere as to date only one spacecraft, Voyager 2, which passed by the planet in 1986, has studied it in detail. No other missions to Uranus are currently scheduled.
Although there is no well-defined solid surface within Uranuss interior, the outermost part of Uranuss gaseous envelope (the region accessible to remote sensing) is called its atmosphere. Remote sensing capability extends down to roughly 300 km below the 1 bar level, with a corresponding pressure around 100 bar and temperature of 320 K.
The observational history of the Uranian atmosphere is long and full of errors and frustrations. Uranus is a relatively faint object, and its visible angular diameter is smaller than 4″. The first spectra of Uranus were observed through a prism in 1869 and 1871 by Angelo Secchi and William Huggins, who found a number of broad dark bands, which they were unable to identify. They also failed to detect any solar Fraunhofer lines—the fact later interpreted by Norman Lockyer as indicating that Uranus emitted its own light as opposed to reflecting light from the Sun. In 1889 however, astronomers observed solar Fraunhofer lines in photographic ultraviolet spectra of the planet, proving once and for all that Uranus was shining by reflected light. The nature of the broad dark bands in its visible spectrum remained unknown until the fourth decade of the twentieth century.
The key to deciphering Uranuss spectrum was found in the 1930s by Rupert Wildt and Vesto Slipher, who found that the dark bands at 543, 619, 925, 865 and 890 nm belonged to gaseous methane. They had never been observed before because they were very weak and required a long path length to be detected. This meant that the atmosphere of Uranus was transparent to a much greater depth compared to those of other giant planets. In 1950, Gerard Kuiper noticed another diffuse dark band in the spectrum of Uranus at 827 nm, which he failed to identify. In 1952 Gerhard Herzberg, a future Nobel Prize winner, showed that this band was caused by the weak quadrapole absorption of molecular hydrogen, which thus became the second compound detected on Uranus. Until 1986 only two gases, methane and hydrogen, were known in the Uranian atmosphere. The far-infrared spectroscopic observation beginning from 1967 consistently showed the atmosphere of Uranus was in approximate thermal balance with incoming solar radiation (in other words, it radiated as much heat as it received from the Sun), and no internal heat source was required to explain observed temperatures. No discrete features had been observed on Uranus prior to the Voyager 2 visit in 1986.
InJanuary 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km providing the first close-up images and spectra of its atmosphere. They generally confirmed that the atmosphere was made of mainly hydrogen and helium with around 2% methane.The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. Only a limited number of discrete clouds were observed.
Inthe 1990s and 2000s, observations by the Hubble Space Telescope and by ground based telescopes equipped with adaptive optics systems (the Keck telescope and NASA Infrared Telescope Facility, for instance) made it possible for the first time to observe discrete cloud features from Earth. Tracking them has allowed astronomers to re-measure windspeeds on Uranus, known before only from the Voyager 2 observations, and to study the dynamics of the Uranian atmopshere.[1
The Hubble Space Telescope (HST)is a space telescope that was carriedinto orbit by a Space Shuttle in 1990and remains in operation. A 2.4meter (7.9 ft) aperture telescopein low Earth orbit, Hubbles fourmain instruments observe in the nearultraviolet, visible, and nearinfrared. The telescope is namedafter the astronomer Edwin Hubble.
Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely sharp images with almost no background light. Hubbles Ultra-Deep Field image, for instance, is the most detailed visible-light image ever made of the universes most distant objects. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.
Although not the first space telescope, Hubble is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. The HST was built by the United States space agency NASA, with contributions from the European Space Agency, and is operated by the Space Telescope Science Institute. The HST is one of NASAs Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.
Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, scientists found that the main mirror had been ground incorrectly, significantly compromising the telescopes capabilities. However, after a servicing mission in 1993, the telescope was restored to its intended quality.
Hubble is the only telescope designed to be serviced in space by astronauts. Between 1993 and 2002, four missions repaired, upgraded, and replaced systems on the telescope, but a fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved one final servicing mission, completed in 2009. The telescope is now expected to function until at least 2014. Its scientific successor, the James Webb Space Telescope (JWST), is to be launched in 2018 or possibly later.
OverviewIn 1985, Howard B. Keck of the W. M. KeckFoundation gave $70 million to fund the design andconstruction of the Keck I Telescope. The keyadvance that allowed the construction of the Keckslarge telescopes was the ability to operate smallermirror segments as a single, contiguous mirror. In thecase of the Keck each of the primary mirrors iscomposed of 36 hexagonal segments that worktogether as a single unit. The mirrors were madefrom Zerodur glass-ceramic by the Germancompany Schott AG . On the telescope, eachsegment is kept stable by a system of active optics,which uses extremely rigid support structures incombination with adjustable warping harnesses.
During observation, a computer-controlled system of sensorsand actuators adjusts the position of each segment, relative toits neighbors, to an accuracy of four nanometers. This twice-per-second adjustment counters the effect of gravity as thetelescope moves, in addition to other environmental effects thatcan affect the mirror shape.Each Keck telescope sits on an altazimuth mount. During thedesign stage, computer analysis determined that this mountingstyle provides the greatest strength and stiffness for the leastamount of steel, which totals about 270 tons per telescope. Theweight of each telescope is about 300 tons.
•The telescopes are equipped with asuite ofinstruments, both cameras and spectrometers that allow observations acrossmuch of the visible and near infraredspectrum
Thecomposition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of molecular hydrogen and helium. The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, was determined from the analysis of Voyager 2 far infrared and radio occultation observations.
Knowledge of the isotopic composition of Uranuss atmosphere is very limited. To date the only known isotope abundance ratio is that of deuterium to light hydrogen: 5.5+3.5 −1.5 × 10−5, which was measured by the Infrared Space Observatory (ISO) in the 1990s. It appears to be higher than the protosolar value of 2.25 ± 0.35×10−5 measured in Jupiter.The deuterium is found almost exclusively in hydrogen deuteride molecules which it forms with normal hydrogen atoms.
Structure The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; the stratosphere, spanning altitudes between 50 and 4000 km and pressures between 0.1 and 10−10 bar; and the thermosphere/exosphere extending from 4000 km to as high as a few Uranus radii from the surface. There is no mesosphere.
Temperature profile of the Uraniantroposphere and lower stratosphere.Cloud and haze layers are also indicated.
but keep in mind that the core of Jupiter is more like 24,000 K – much hotter. The core of Uranus has a density of about 9 g/cm3, which makes it about twice as dense as the average density of the Earth.
For astronomers, Uranus has an unusually low temperature; and that’s a mystery. One ideas is that the same impact that knocked Uranus off its rotational axis might have also caused it to expel much of its primordial heat. With the heat gone, Uranus was able to cool down significantly further than the other planets. Another idea is that there’s some kind of barrier in Uranus’ upper atmosphere that prevents heat from the core to reach the surface.
We have written manystories about Uranus onUniverse Today. Here’s anarticle about a dark spot inthe clouds on Uranus, andhere’s an article about thecomposition of Uranus.
Uranus has a mass of roughly 14.5 times that of Earth, which makes it the least massive of the giant planets. Astronomers know that it’s mostly made of various ices, like water, ammonia and methane. And they theorize that Uranus probably has a
Thecore of Uranus probably only accounts for 20% of the radius of Uranus, and only about 0.55 Earth masses. With gravity of all the outer mantle and atmosphere, regions in the core experience a pressure of about 8 million bars, and have a temperature of 5,000 Kelvin. That sounds hot, like as hot as the surface of the Sun.
Sweet Moon," William Shakespeare wrote in "A Midsummer Nights Dream," "I thank thee for thy sunny beams; I thank thee, Moon, for shining now so bright." Centuries later, the moons of Uranus pay homage to the famous playwright. The Hubble Space Telescope captured this false- color image of Uranus and its moons. While most of the satellites orbiting other planets take their names from Greek mythology, Uranus moons are unique in being named for Shakespearean characters, along with a couple of the moons being named for characters from the works of Alexander Pope.
Oberon and Titania are the largest Uranian moons, and were first to be discovered -- by William Herschel in 1787. William Lassell, who had been first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Nearly a century passed before Gerard Kuiper found Miranda in 1948. And that was it until a NASA robot made it to distant Uranus.
Oberon is the second largest moon of Uranus. Discovered in 1787, little was known about this moon until Voyager 2 passed it during its flyby of Uranus in January 1986. Oberon is heavily cratered -- similar to Umbriel -- especially when compared to three other moons of Uranus: Ariel, Titania and Miranda. Like all of Uranus large moons, Oberon is composed of roughly half ice and half rock. Oberon has at least one large mountain that rises about 6 km off the surface.
Discovery:Oberon was discoveredin January 1787 byWilliam Herschel.
How Oberon Got its Name: Oberon is named for the king of the fairies in Shakespeares "A Midsummer Nights Dream." Moons of Uranus are named for characters in William Shakespeares plays and from Alexander Popes "Rape of the Lock
Uranus is the only giant planet whose equator is nearly at right angles to its orbit. A collision with an Earth-sized object may explain Uranus unique tilt. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint.
Featured Mission: Voyager 2 Most of what we know about Uranus came from Voyager 2s flyby in 1986. The spacecraft discovered 10 additional moons and several rings before heading on to Neptune.
The largest ring is twice the diameter of the planets previously known rings. The rings are so far from the planet, they are being called Uranus "second ring system." One of the new moons shares its orbit with one of the rings. Analysis of the Hubble data also reveals the orbits of Uranus family of inner moons have changed significantly over the past decade.
sincedust orbiting Uranus is expected to be depleted by spiraling away, the planets rings must be continually replenished with fresh material. "The new discoveries demonstrate that Uranus has a youthful and dynamic system of rings and moons," said Mark Showalter of the SETI Institute, Mountainview, California.
Showalter and Jack Lissauer of NASAs Ames Research Center, Moffet Field, Calif., propose that the outermost ring is replenished by a 12-mile-wide newly discovered moon, named Mab, which they first observed using Hubble in 2003.
Hubble uncovered the rings in August 2004 during a series of 80, four-minute exposures of Uranus. The team later recognized the faint new rings in 24 similar images taken a year earlier. Images from September 2005 reveal the rings even more clearly.
The image is a color composite made from short exposures, showing the disk of Uranus with some cloud features. Just to the left and right of the color image of the disk are a combination of deeper, panchromatic images showing Uranuss inner rings; the brightest is the Epsilon Ring. The satellite Mab is visible as eight dots adjacent to the outer ring on the right side.