Keynote lectures/Uranus

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The aurorae of the planet Uranus are in line with its equatorial rings. Credit: NASA, ESA, and L. Lamy (Observatory of Paris, CNRS, CNES).

Uranus is a gaseous object in orbit around the Sun at a distance of less than one light-year. Jupiter and Saturn are systematically closer to the Sun, and Neptune is systematically further from the Sun.

Theory of Uranus[edit | edit source]

Here's a theoretical definition:

Def. the "seventh planet in our solar system,[1] is called Uranus.

A "huge rock, possibly the size of a small planet, [may have] collided with Uranus, causing it to tilt dramatically, affecting its spin, its magnetic field, and even its heat distribution."[2]

"Along with affecting the planet's tilt, the impact event may have also spurred on the development of Uranus's thick icy outer layer, which keeps the heat from planet's core locked inside. Uranus is the only planet in our solar system that doesn't leak heat from its core, reaching temperatures of -371 degrees Fahrenheit at some of its chilliest points."[3]

"The impact likely caused Uranus's larger moons and rings, which orbit in line with its rotation, to gain their unique path as well."[3]

Electromagnetics[edit | edit source]

The magnetic field of Uranus as observed by Voyager 2 in 1986. S and N are magnetic south and north poles. Credit: Ruslik0.

"The discovery of [Uranus]'s non-dipolar, non-axisymmetric magnetic [field at the right] destroyed the picture-established by Earth, Jupiter and Saturn-that planetary magnetic fields are dominated by axial dipoles."[4]

"Planetary magnetic fields are generated by complex fluid motions in electrically conducting regions of the planets (a process known as dynamo action), and so are intimately linked to the structure and evolution of planetary interiors."[4]

Three-dimensional "numerical dynamo simulations [...] model the dynamo source region as a convecting thin shell surrounding a stably stratified fluid interior."[4]

This "convective-region geometry produces magnetic fields similar in morphology to [that] of Uranus [The field is] non-dipolar and non-axisymmetric, and [results] from a combination of the stable fluid's response to electromagnetic stress and the small length scales imposed by the thin shell."[4]

The planet had "a strong planetary magnetic field of Uranus and an associated magnetosphere and fully developed bipolar magnetotail [and a] detached bow shock wave [which] was observed upstream at 23.7 Uranus radii (1 RU = 25,600 km) and the magnetopause boundary at 18.0 RU. [The] maximum magnetic field of 413 nanotesla was observed at 4.19 RU [The] planetary magnetic field is well represented by that of a dipole offset from the center of the planet by 0.3 RU. The angle between Uranus' angular momentum vector and the dipole moment vector has the surprisingly large value of 60 degrees. [The] field of Uranus may be described as that of an oblique rotator. The dipole moment of 0.23 gauss R3U, combined with the large spatial offset, leads to minimum and maximum magnetic fields on the surface of the planet of approximately 0.1-1.1 gauss. The rotation period of the magnetic field and [that] of the interior of the planet is estimated to be 17.29±0.10 [hr]."[5]

Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.[5][6] In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.[5] This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high as 1.1 gauss (110 µT).[5] The average field at the surface is 0.23 gauss (23 µT).[5] In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator.[6] The dipole moment of Uranus is 50 times that of Earth.[5][6] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.[6] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giants, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.[7][8]

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.[5][6][9] Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's.[5][6] Uranus's magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.[5][10]

Uranus's magnetosphere contains charged particles: protons and electrons with small amount of H2+ ions.[6][9] No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.[9] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.[9] The density of low energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm−3.[11] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.[9] The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.[9] This may be the cause of the uniformly dark colouration of the moons and rings.[12] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.[13] Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere.[14]

Meteors[edit | edit source]

Zonal wind speeds are plotted as detected on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data. Credit: .

Uranus has a complex, layered cloud structure, with methane thought to make up the uppermost layer of clouds.[15] With a large telescope of 25 cm or wider, cloud patterns may be visible.[16] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[17] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[17]

In the 1990s, the number of the observed bright cloud features grew considerably partly because new high resolution imaging techniques became available.[18] Most were found in the northern hemisphere as it started to become visible.[18] An early explanation - that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them - was shown to be incorrect: the actual number of features has indeed increased considerably.[19][20] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[20] They appear to lie at a higher altitude.[20] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud may have persisted since Voyager flyby.[18] Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.[18] For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed Uranus Dark Spot was imaged.[21] The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.[22]

For a short period from March to May 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[20][23]

On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus's surface, giving astronomers more insight into the planet's atmospheric activity.[21]

The bright collar at −45° latitude is also connected with methane clouds.[24] Other changes in the southern polar region can be explained by changes in the lower cloud layers.[24]

The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[18] The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[18] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s.[18][25] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[18][26] Closer to the poles, the winds shift to a prograde direction, flowing with the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.[18] Windspeeds at −40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.[18] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.[18][25][27] Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks".[28]

Lightnings[edit | edit source]

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.[29][30][31] 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'.[32][33] Scientists also believe that lightning storms make rainfalls of solid diamonds occur on Uranus, as well as on Jupiter, Saturn, and Neptune.[34][35]

Cosmic rays[edit | edit source]

"[F]or the regions of the giant planets, especially Uranus and Neptune, ... ionization is due mainly to cosmic rays."[36]

Ultraviolets[edit | edit source]

The images at the top of the article are in the ultraviolet showing the aurorae of Uranus.

"These are among the first clear images, taken from the distance of Earth, to show aurorae on the planet Uranus. Aurorae are produced when high-energy particles from the Sun cascade along magnetic field lines into a planet's upper atmosphere. This causes the planet's atmospheric gasses to fluoresce. The ultraviolet images were taken at the time of heightened solar activity in November 2011 that successively buffeted the Earth, Jupiter, and Uranus with a gusher of charged particles from the Sun. Because Uranus' magnetic field is inclined 59 degrees to its spin axis, the auroral spots appear far from the planet's north and south poles. This composite image combines 2011 Hubble observations of the aurorae in visible and ultraviolet light, 1986 Voyager 2 photos of the cyan disk of Uranus as seen in visible light, and 2011 Gemini Observatory observations of the faint ring system as seen in infrared light."[37]

Violets[edit | edit source]

Six 15-second narrow-angle images were used to extract color information from the extremely dark and faint rings. Two images each in the green, clear and violet filters were added together and averaged to find the proper color differences between the rings. Credit: NASA/JPL.

"This false-color view of the rings of Uranus was made from images taken by Voyager 2 on Jan. 21, 1986, from a distance of 4.17 million kilometers (2.59 million miles). All nine known rings are visible here; the somewhat fainter, pastel lines seen between them are contributed by the computer enhancement. Six 15-second narrow-angle images were used to extract color information from the extremely dark and faint rings. Two images each in the green, clear and violet filters were added together and averaged to find the proper color differences between the rings. The final image was made from these three color averages and represents an enhanced, false-color view. The image shows that the brightest, or epsilon, ring at top is neutral in color, with the fainter eight other rings showing color differences between them. Moving down, toward Uranus, we see the delta, gamma and eta rings in shades of blue and green; the beta and alpha rings in somewhat lighter tones; and then a final set of three, known simply as the 4, 5 and 6 rings, in faint off-white tones. Scientists will use this color information to try to understand the nature and origin of the ring material. The resolution of this image is approximately 40 km (25 mi). The Voyager project is managed for NASA by the Jet Propulsion Laboratory."[38]

Blues[edit | edit source]

This is Uranus in the blue. Credit: NASA.
The planet's atmospheric details are very difficult to see in visible light. Credit: NASA.

The images show Uranus through a blue filter.

Cyans[edit | edit source]

This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. Credit: NASA/JPL/Voyager mission.
Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by Voyager 2. Credit: NASA.
The first dark spot on Uranus ever observed is in an image obtained by ACS on HST in 2006. Credit: NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute).
Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).

In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening.

"Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color."[15]

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[17] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[17][25] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[24] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[17] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[25] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[25] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.[39]

On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus's surface, giving astronomers more insight into the planet's atmospheric activity.[21] Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus's extreme axial tilt results in extreme seasonal variations in its weather.[40][22] Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere have existed for less than 84 years, or one full Uranian year. A number of discoveries have been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.[41] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[42] Stratospheric temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.[43] The majority of this variability is believed to occur owing to changes in the viewing geometry.[19]

There are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[22] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[41] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[22] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[22] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[24] while the northern hemisphere demonstrated increasing activity,[28] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[20] This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[39]

Greens[edit | edit source]

This is a visual image of Uranus in the green. Credit: Heidi Hammel (Massachusetts Institute of Technology), NASA.

"The "aqua" image (on the right) is taken at 5,470 Angstroms, which is near the human eye's peak response to wavelength. Color has been added to the image to show what a person on a spacecraft near Uranus might see. Little structure is evident at this wavelength, though with image-processing techniques, a small cloud can be seen near the planet's northern limb (rightmost edge)."[44]

Oranges[edit | edit source]

This is a Hubble Space Telescope image at 619.0 nm of Uranus. Credit: Heidi Hammel (Massachusetts Institute of Technology), NASA.
The picture of Uranus is a composite of images taken through blue, green and orange filters. Credit: .

"Spring has finally come to the northern hemisphere of Uranus. The newest images, both the visible-wavelength ones described here and those taken a few days earlier with the Near Infrared and Multi-Object Spectrometer (NICMOS) by Erich Karkoschka (University of Arizona), show a planet with banded structure and detectable clouds."[44]

"The "red" image (on the right) is taken at 6,190 Angstroms, and is sensitive to absorption by methane molecules in the planet's atmosphere. The banded structure of Uranus is evident, and the small cloud near the northern limb is now visible."[44]

The picture of Uranus in true color was "compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometers (5.7 million miles) from the planet, several days from closest approach. The picture [...] has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The picture is a composite of images taken through blue, green and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. Beyond this boundary lies the hidden northern hemisphere of Uranus, which currently remains in total darkness as the planet rotates. The blue-green color results from the absorption of red light by methane gas in Uranus' deep, cold and remarkably clear atmosphere."[45]

Yellows[edit | edit source]

This image depicts changes of the visible magnitude of Uranus in the two spectral bands (upper graph), of the effective microwave temperature (middle graph) and of the stratospheric temperature in the pressure range 20–50 μbar. Credit: Ruslik0.

The image on the right depicts changes of the visible magnitude, or brightness, of Uranus in the yellow and blue bands (upper graph), the effective microwave temperature of the deep troposphere (middle graph) and stratospheric temperature in the pressure range 20–50 μbar (lower graph).

The data for the plots are

  1. yellow intensity and blue intensity, both as visible magnitude (Tables 1-3),[46]
  2. microwave brightness temperature (Tables 1-2),[47] and
  3. stratospheric temperature.[48]

Infrareds[edit | edit source]

This Hubble Space Telescope image shows Uranus in the near-infrared. Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI).
This is an infrared composite of Uranus obtained with Keck Observatory adaptive optics. Credit: Lawrence Sromovsky, University of Wisconsin-Madison/ W. M. Keck Observatory.
A 1998 false-colour near-infrared image of Uranus showing cloud bands, rings, and moons obtained by the Hubble Space Telescope's [Near Infrared Camera and Multi-Object Spectrometer] NICMOS camera. Credit: Hubble Space Telescope - NASA Marshall Space Flight Center.
This is an image taken in near infrared wavelengths by the Gemini North Telescope. Credit: NASA/ESA and H. B. Hammel, Gemini North Telescope.
Using infrared filters, Hubble captured detailed features of three layers of Uranus' atmosphere. Credit: NASA/JPL/STScI.

At right is an infrared image of Uranus "showing Ariel in transit across Uranus, taken on July 26, 2006. Due to the planet's extreme axial tilt--carried through in the tilt of its satellites's orbits--transits are only possible near the equinoxes. Taken in the near-infrared, atmospheric banding and the planet's oblateness are readily apparent."[49]

The second pair of images on the left are an "infrared composite ... of the two hemispheres of Uranus obtained with Keck adaptive optics. The component colors of blue, green, and red were obtained from images made at near infrared wavelengths of 1.26, 1.62, and 2.1 microns respectively. The images were obtained on July 11 and 12, 2004. The representative balance of these infrared images which were selected to display the vertical structure of atmospheric features gives a reddish tint to the rings, an artifact of the process. The North pole is at 4 o'clock."[50]

The lower right image shows an apparent linear arrangement of clouds in the southern hemisphere using the Hubble Space Telescope in 1998.

The second image at the left is in the "near infrared wavelengths by the Gemini North Telescope [revealing] what scientists are calling an “anvil cloud of methane” rising up from the depths into the sunshine. Reflections from methane ice crystals are supposed to be causing the bright patch."[51]

"Over the years, the Hubble Space Telescope has observed many bright spots on Uranus. They appear similar to the bright spots seen in Jupiter’s southern latitudes, as well as in its polar aurorae."[51]

On Saturn, a “great white spot” periodically appears in its southern latitudes."[51]

“Saturn occasionally ‘burps’, creating a great white spot 3 times the size of the Earth. It is inexplicable on standard models. However, it is the kind of thing to be expected following an exceptionally powerful lightning discharge deep into Saturn’s atmosphere. The discharge forms a vertical jet of matter from the depths that spouts into the upper atmosphere.”[52]

"Hubble Space Telescope has peered deep into Uranus' atmosphere to see clear and hazy layers created by a mixture of gases. Using infrared filters, Hubble captured detailed features of three layers of Uranus' atmosphere."[53]

"Hubble's images are different from the ones taken by the Voyager 2 spacecraft, which flew by Uranus 10 years ago. Those images - not taken in infrared light - showed a greenish-blue disk with very little detail."[53]

"The infrared image [third down on the right] allows astronomers to probe the structure of Uranus' atmosphere, which consists of mostly hydrogen with traces of methane. The red around the planet's edge represents a very thin haze at a high altitude. The haze is so thin that it can only be seen by looking at the edges of the disk, and is similar to looking at the edge of a soap bubble. The yellow near the bottom of Uranus is another hazy layer. The deepest layer, the blue near the top of Uranus, shows a clearer atmosphere."[53]

"Image processing has been used to brighten the rings around Uranus so that astronomers can study their structure. In reality, the rings are as dark as black lava or charcoal."[53]

"This false color picture was assembled from several exposures taken July 3, 1995 by the Wide Field Planetary Camera-2."[53]

Submillimeters[edit | edit source]

This is an ALMA submillimeter image of Uranus. Credit: ALMA (ESO/NAOJ/NRAO).

For observations on "UT 1991 November 19 and 20[, the] primary pointing and calibration source was Uranus, for which we used S0.8 = 73.5 Jy (TB = 84 K) and S1.1 = 43.4 Jy (TB = 93 K)."[54]

"As a demonstration of ALMA's new short wavelength capabilities, the commissioning team released a new image of planet Uranus as it appears in submillimetre wavelength light. The image — obtained with ALMA's shortest wavelength, Band 10 receivers — reveals the icy glow from the planet's atmosphere, which can reach temperatures as low as -224 C (giving Uranus the coldest atmosphere in the Solar System). ALMA's now broader range of capabilities will enable astronomers and planetary scientists to study and monitor temperature changes at different altitudes above the clouds of Uranus and other giant planets in our Solar System."[55]

Astrognosy[edit | edit source]

Uranus has an interior. Credit: WolfmanSF.

Uranus has a core and a mantle as shown in the model on the right.

Astrochemistry[edit | edit source]

In January 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km[56] 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.[57] The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. Only a limited number of discrete clouds were observed.[17]

Atmospheres[edit | edit source]

This graph shows temperature profile in the troposphere and in the lower stratosphere of Uranus. Heights are also indicated. Credit: Ruslik0.
Colorized infrared image of Uranus is obtained on August 6, 2014, with adaptive optics on the 10-meter Keck telescope, where white spots are large storms. Credit: Imke de Pater, University of California, Berkeley / Keck Observatory.

Uranus's atmosphere has a primary composition of hydrogen and helium and contains more "ices" such as water, ammonia, and methane, along with traces of hydrocarbons.[15] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[58] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[15][59] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[15] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.[15][60][61] The mixing ratio is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[62] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. Along with methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[63] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).[62][64][65] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.[64][65][66]

“The weather on Uranus is incredibly active.”[67]

“This type of activity would have been expected in 2007, when Uranus’s once-every-42-year equinox occurred and the Sun shined directly on the equator. But we predicted that such activity would have died down by now. Why we see these incredible storms now is beyond anybody’s guess.”[68]

Eight "large storms [were detected] on Uranus when observing the planet with the Keck Observatory on August 5 and 6, 2014."[67]

Atmospheric astrognosy[edit | edit source]

The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[15] The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.[69][61] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.[15][26] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[26][59]

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.[70] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,[71] which form in this part of the atmosphere as a result of methane photolysis.[63] Heat is also conducted from the hot thermosphere.[71] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300 km corresponding to a pressure range of 10 to 0.1 mbar (1000 to 10 kPa) and temperatures of between 75 and 170 K.[62][64] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10−7 relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.[62][64][66] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.[64] The abundance ratio of water is around 7×109.[65] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10 mBar level) forming haze layers,[63] which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.[62][13]

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.[15][13] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too.[70][13] In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass together with the high temperatures explain why the corona extends as far as 50 000 km or two Uranian radii from the planet.[70][13] This extended corona is a unique feature of Uranus.[13] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[70] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[61] Observations show that the ionosphere occupies altitudes from 2 000 to 10 000 km.[61] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.[13][72] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[73] Auroral activity is insignificant as compared to Jupiter and Saturn.[13] [14]

It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.[15][59]

Coronal clouds[edit | edit source]

The upper part of the thermosphere, where the mean free path of the molecules exceeds the scale height, [The scale height sh is defined as sh = RT/(Mgj), where R = 8.31 J/mol/K is the gas constant, M ≈ 0.0023 kg/mol is the average molar mass in the Uranian atmosphere,[15] T is temperature and gj ≈ 8.9 m/s2 is the gravitational acceleration at the surface of Uranus. As the temperature varies from 53 K in the tropopause up to 800 K in the thermosphere, the scale height changes from 20 to 400 km.] is called the exosphere.[74] The lower boundary of the Uranian exosphere, the exobase, is located at a height of about 6,500 km, or 1/4 of the planetary radius, above the surface.[74] The exosphere is unusually extended, reaching as far as several Uranian radii from the planet.[74][13] It is made mainly of hydrogen atoms and is often called the hydrogen corona of Uranus.[70] The high temperature and relatively high pressure at the base of the thermosphere explain in part why Uranus's exosphere is so vast. The corona contains a significant population of supra-thermal (energy of up to 2 eV) hydrogen atoms. Their origin is unclear, but they may be produced by the same mechanism that heats the thermosphere.[74][13] The number density of atomic hydrogen in the corona falls slowly with the distance from the planet, remaining as high a few hundred atoms per cm3 at a few radii from Uranus.[74] The effects of this bloated exosphere include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings. The infalling dust in turn contaminates the upper atmosphere of the planet.[70]

Ancient history[edit | edit source]

This central part of a large floor mosaic depicts Aion-Uranus personifying the sky. Credit: Bibi Saint-Pol.
This is an image of a painting by artist Giorgio Vasari (1511–1574). Credit: Dodo Vasari.

The ancient history period dates from around 8,000 to 3,000 b2k.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star[75][76]

Uranus is named after the ancient Greek deity of the sky Uranus, the father of Cronus (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit.[77]

The second image at right is a painting by artist Giorgio Vasari (1511–1574). The main focus is on Cronus (Saturn) castrating Uranus (the Greek sky god). As both Uranus and Cronus are represented by men, this suggests that they were similar in nature. "[T]he ancients’ religions and mythology speak for their knowledge of Uranus; the dynasty of gods had Uranus followed by Saturn, and the latter by Jupiter. ... It is quite possible that the planet Uranus is the very planet known by this name to the ancients. The age of Uranus preceded the age of Saturn; it came to an end with the “removal” of Uranus by Saturn. Saturn is said to have emasculated his father Uranus."[78]

Ouranos[edit | edit source]

“Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.[79]

Uranus ... , Ouranos meaning "sky" or "heaven") was the primal Greek god personifying the sky. His equivalent in Roman mythology was Caelus. In Ancient Greek literature, Uranus or Father Sky was the son and husband of Gaia, Mother Earth. According to Hesiod's Theogony, Uranus was conceived by Gaia alone, but other sources cite Aether as his father.[80]

According to the poet Alcman, Aether was the father of Ouranos, the god of the sky. While Aether was the personification of the upper air, Ouranos was literally the sky itself, composed of a solid dome of brass.[81]

Caelus[edit | edit source]

Caelus appears at the top of the cuirass of the Augustus of Prima Porta, counterposed to Earth at the bottom. Credit: Sailko.

Caelus or Coelus was a primal god of the sky in Roman myth and theology, iconography, and literature (compare caelum, the Latin word for "sky" or "the heavens", hence English "celestial").

The name of Caelus indicates that he was the Roman counterpart of the Greek god Uranus, who was of major importance in the theogonies of the Greeks. Varro couples him with Terra (Earth) as pater and mater (father and mother), and says that they are "great deities" (dei magni) in the theology of the mysteries at Samothrace.[82]

According to Cicero and Hyginus, Caelus was the son of Aether and Dies ("Day" or "Daylight").[83] Caelus and Dies were in this tradition the parents of Mercury.[84] Caelus was the father with Hecate of the distinctively Roman god Janus, as well as of Saturn and Ops.[85] Caelus was also the father of one of the three forms of Jupiter, the other two fathers being Aether and Saturn.[86]

Aion[edit | edit source]

This image shows a detail from the Parabiago plate depicting Aion. Credit: Giovanni Dall'Orto.

Aion (Greek Αἰών) is a Hellenistic deity associated with time, the orb or circle encompassing the universe, and the zodiac. The "time" represented by Aion is unbounded, in contrast to Chronos as empirical time divided into past, present, and future.[87] He is thus a god of eternity, associated with mystery religions concerned with the afterlife, such as the mysteries of Cybele, Dionysus, Orpheus, and Mithras. In Latin the concept of the deity may appear as Aevum or Saeculum.[88] He is typically in the company of an earth or mother goddess such as Tellus or Cybele, as on the Parabiago plat].[89] The picture at page right top is of Aion-Uranus.

Physics[edit | edit source]

Uranus has an equatorial radius of 25,559 ± 4 km and a polar radius of 24,973 ± 20 km.[90]

Hypotheses[edit | edit source]

  1. Uranus was closer to the Sun about 40,000 b2k.
  2. What was to become Uranus may have been in a binary system with the Sun.

See also[edit | edit source]

References[edit | edit source]

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External links[edit | edit source]