What is the underlying nature of the dark spot found on Uranus?

What is the underlying nature of the dark spot found on Uranus?

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What is the nature of bright spots found on Uranus? actually quotes's Uranus Has a Dark Spot which says:

During the past decade, many bright spots have been seen on Uranus in both red and near-infrared filters. But this is the first dark spot ever seen on the planet.

Question: So I'd like to know if there is any science-based thinking on the underlying nature of the dark spot on Uranus beyond "it looks dark, like the ones on Neptune". Has this all been sorted out or is it still mostly a mystery?

Wikipedia's Climate of Uranus; Uranus Dark Spot gives a lot of descriptive information and relates the spot to similar things seen on Neptune, and describes some speculation, but this article does not seem explain much.

Just as we near the end of the hurricane season in the Atlantic Ocean, winds whirl and clouds churn 2 billion miles away in the atmosphere of Uranus, forming a dark vortex large enough to engulf two-thirds of the United States. (Image credit: NASA, ESA, L. Sromovsky and P. Fry / University of Wisconsin)

There is a science-based approach to explaining the dark spot on Uranus. In 2009 (the image and sighting of the spot in question are from 2006) a paper was published titled: The Dark Spot in the atmosphere of Uranus in 2006: Discovery, description, and dynamical simulations by H.B. Hammel a, L.A. Sromovsky et. al…

In this work they say that the dark spot is a wind vortex. Quoting the paper:

Regardless of how it is computed, the dark features's zonal velocity is higher than all other velocities measured to date near that latitude.

Using computer simulations they are able to predict possible initial velocities, lifespan of the Vortex, center latitude of the initial and center latitude when it reached the end of its lifespan. They go further and state that the mere existence of the vortex shows the dynamic and evolving seasonal nature of Uranus's atmosphere.

So I'd like to know if there is any science-based thinking on the underlying nature of the dark spot on Uranus beyond "it looks dark, like the ones on Neptune".

The abstract explains that the spot has a markedly different spectral character than the one on Neptune:

The dark feature's contrast and extent varied as a function of wavelength, with largest negative contrast occurring at a surprisingly long wavelength when compared with Neptune's dark features: the Uranus feature was detected out to 1.6 μm with a contrast of −0.07, but it was undetectable at 0.467 μm; the Neptune GDS seen by Voyager exhibited its most prominent contrast of −0.12 at 0.48 μm, and was undetectable longward of 0.7 μm. Computational fluid dynamic simulations of the dark feature on Uranus suggest that structure in the zonal wind profile may be a critical factor in the emergence of large sustained

Climate of Uranus

The climate of Uranus is heavily influenced by both its lack of internal heat, which limits atmospheric activity, and by its extreme axial tilt, which induces intense seasonal variation. Uranus' atmosphere is remarkably bland in comparison to the other giant planets which it otherwise closely resembles. [1] [2] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet. [3] [4] Later observations from the ground or by the Hubble Space Telescope made in the 1990s and the 2000s revealed bright clouds in the northern (winter) hemisphere. In 2006 a dark spot similar to the Great Dark Spot on Neptune was detected. [5]


The outer solar system contains the four giant planets: Jupiter, Saturn, Uranus, and Neptune. The gas giants Jupiter and Saturn have overall compositions similar to that of the Sun. These planets have been explored by the Pioneer, Voyager, Galileo, and Cassini spacecraft. Voyager 2, perhaps the most successful of all space-science missions, explored Jupiter (1979), Saturn (1981), Uranus (1986), and Neptune (1989)—a grand tour of the giant planets—and these flybys have been the only explorations to date of the ice giants Uranus and Neptune. The Galileo and Cassini missions were long-lived orbiters, and each also deployed an entry probe, one into Jupiter and one into Saturn’s moon Titan.

11.2 The Giant Planets

Jupiter is 318 times more massive than Earth. Saturn is about 25% as massive as Jupiter, and Uranus and Neptune are only 5% as massive. All four have deep atmospheres and opaque clouds, and all rotate quickly with periods from 10 to 17 hours. Jupiter and Saturn have extensive mantles of liquid hydrogen. Uranus and Neptune are depleted in hydrogen and helium relative to Jupiter and Saturn (and the Sun). Each giant planet has a core of “ice” and “rock” of about 10 Earth masses. Jupiter, Saturn, and Neptune have major internal heat sources, obtaining as much (or more) energy from their interiors as by radiation from the Sun. Uranus has no measurable internal heat. Jupiter has the strongest magnetic field and largest magnetosphere of any planet, first discovered by radio astronomers from observations of synchrotron radiation.

11.3 Atmospheres of the Giant Planets

The four giant planets have generally similar atmospheres, composed mostly of hydrogen and helium. Their atmospheres contain small quantities of methane and ammonia gas, both of which also condense to form clouds. Deeper (invisible) cloud layers consist of water and possibly ammonium hydrosulfide (Jupiter and Saturn) and hydrogen sulfide (Neptune). In the upper atmospheres, hydrocarbons and other trace compounds are produced by photochemistry. We do not know exactly what causes the colors in the clouds of Jupiter. Atmospheric motions on the giant planets are dominated by east-west circulation. Jupiter displays the most active cloud patterns, with Neptune second. Saturn is generally bland, in spite of its extremely high wind speeds, and Uranus is featureless (perhaps due to its lack of an internal heat source). Large storms (oval-shaped high-pressure systems such as the Great Red Spot on Jupiter and the Great Dark Spot on Neptune) can be found in some of the planet atmospheres.

Whoa, Uranus Looks Totally Messed Up Right Now

The appearance of a massive white cap on Uranus may seem alarming, but as planetary scientists are learning, this is what a prolonged summer looks like on the remote ice giant.

Ice giants Uranus and Neptune have water-rich interiors coated with hydrogen, helium, and a pinch of methane, the latter of which gives these outer planets their distinctive cyan complexion. Unlike Earth, where seasons last just a few months, Neptune and Uranus experience seasons that last for decades, resulting in strange and intense atmospheric phenomena.

New images released by the Outer Planet Atmospheres Legacy (OPAL) program highlight a evolving atmospheric events on both ice giants, namely an extended white cap over Uranus’ north pole and a new dark vortex on Neptune. A long-term side project of the Hubble program, OPAL is an annual effort to map these two planets when their orbital paths bring them closest to Earth. The new data, captured during the autumn of 2018, are providing important new insights into the seasonal variations on both Neptune and Uranus.

“The yearly observations are helping us to understand the frequency of storms, as well as their longevity,” Amy Simon, a scientist at NASA’s Goddard Space Flight Center who leads the OPAL mission, told Gizmodo. “That’s important because these planets are quite far from the Sun, so this will help constrain how they are forming and more about the internal heat and structure of these planets. Most of the extrasolar planets that have been found are this size of planet, though at all sorts of distances from their parent stars.”

The large white cap strewn over the north pole of Uranus is particularly dramatic. The likely cause of this feature has to do with the planet’s unique tilt, which causes sunlight to shine directly onto the north polar regions for an extended period of time during the summer. It’s currently mid-summer at Uranus’ north pole, resulting in the protracted white cap.

“The November 2018 image of Uranus occurs at a time 10 years after the equinox, when the northern hemisphere was just emerging into spring sunlight after spending decades in polar winter,” Leigh Fletcher, an astronomer at the University of Leicester, told Gizmodo. “Back in 2007, there didn’t appear to be anything like this polar cap over the springtime pole. But as time progressed, a reflective band—whitish against Uranus’ blue hues—began to appear encircling the north pole. And now, 10 years on, that band has turned into a thick polar cap of aerosols that’s hiding the deeper polar region from view.”

Fletcher said it’s a “spectacular example of seasonal change” on this ice giant, with “the aerosol cap evolving as spring becomes summer.” The exact causes of these aerosol changes, he said, remain a mystery, with possibilities including warming temperatures, unusual chemistry, some large- scale atmospheric circulation pattern, or a combination of all these.

“Thankfully we’re not too far away from having an answer, as the James Webb Space Telescope will be able to diagnose the temperatures and chemistry responsible for these reflectivity changes that Hubble has been monitoring,” added Fletcher.

Patrick Irwin, a planetary scientist at Oxford University, said the phenomenon is not a storm, as NASA described it in its release. Rather, “it’s caused mainly—at least in our models —by a lowering of the methane abundance above the main cloud deck accompanied by a possible slight increase in the haze opacity,” he told Gizmodo.

Simon thought the expanded Uranian polar feature was cool, but “more interesting to me is that bright storm just below it,” she told Gizmodo. “That particular storm had flared up and was visible in small ground-based telescopes just prior to these observations, which shows how quickly they can change.”

Looking at the new Neptune image, it appears that a dark vortex has once again reared its ugly—yet fascinating—head. The new anti-cyclonic storm, seen at the top center of the photo above, is about 11,000 kilometers (6,800 miles) across. This is now the fourth dark vortex observed by Hubble since 1993. Two of these storms were observed by the Voyager 2 probe during its flyby of the system in 1989. Taken together, these observations affirm the transient and recurring nature of these storms. A polar vortex observed in 2016, for example, has largely faded away .

“The Neptune dark spot is much larger than the one we saw a few years ago, and is comparable in size to the Voyager Great Dark Spot seen in 1989,” said Simon. “This is also the first time we could see the region before a storm of that size formed, so that will help us in modeling the formation process.”

The causes of these dark spots is a mystery, but because they’re only seen at the bluest wavelengths, “my money is on some sort of coloration of the clouds,” said Irwin.

Often overshadowed by their larger cousins, Jupiter and Saturn, these distant ice giants are proving to be fascinating in their own right. We now await next year’s OPAL observations with much anticipation.

Astronomers Detect Hydrogen Sulfide in Uranus’ Upper Atmosphere

Astronomers using the Gemini North Telescope on Hawaii’s Maunakea have detected hydrogen sulfide, the gas that gives rotten eggs their distinctive odor, in Uranus’ cloud tops. The discovery is described in a paper published in the journal Nature Astronomy.

Uranus in natural colors. Image credit: NASA / ESA / Hubble Team / Erich Karkoschka, University of Arizona.

Scientists have long debated the composition of Uranus’ clouds and whether hydrogen sulfide or ammonia dominates the cloud deck, but lacked definitive evidence either way.

“Now, thanks to improved hydrogen sulfide absorption-line data and the wonderful Gemini spectra, we have the fingerprint which caught the culprit,” said lead author Dr. Patrick Irwin, from the Department of Physics (Atmospheric, Oceanic and Planetary Physics) at the University of Oxford, UK.

The Gemini data, obtained with the Near-Infrared Integral Field Spectrometer (NIFS), sampled reflected sunlight from a region immediately above the main visible cloud layer in Uranus’ atmosphere.

“While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of NIFS on Gemini, combined with the exquisite conditions on Maunakea,” Dr. Irwin said.

The detection of hydrogen sulfide high in Uranus’ cloud deck — and presumably Neptune’s — contrasts sharply with the inner gas giant planets, Jupiter and Saturn, where no hydrogen sulfide is seen above the clouds, but instead ammonia is observed.

The bulk of Jupiter and Saturn’s upper clouds are comprised of ammonia ice, but it seems this is not the case for Uranus. These differences in atmospheric composition shed light on questions about the planets’ formation and history.

“The differences between the cloud decks of the gas giants (Jupiter and Saturn), and the ice giants (Uranus and Neptune), were likely imprinted way back during the birth of these worlds,” said co-author Dr. Leigh Fletcher, from the University of Leicester, UK.

“During our Solar System’s formation the balance between nitrogen and sulfur (and hence ammonia and Uranus’ newly-detected hydrogen sulfide) was determined by the temperature and location of planet’s formation.”

Another factor in the early formation of Uranus is the strong evidence that our Solar System’s giant planets likely migrated from where they initially formed. Therefore, confirming this composition information is invaluable in understanding Uranus’ birthplace, evolution and refining models of planetary migrations.

“When a cloud deck forms by condensation, it locks away the cloud-forming gas in a deep internal reservoir, hidden away beneath the levels that we can usually see with our telescopes. Only a tiny amount remains above the clouds as a saturated vapor,” Dr. Fletcher said.

“And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulfide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break.”

“We’ve strongly suspected that hydrogen sulfide gas was influencing the millimeter and radio spectrum of Uranus for some time, but we were unable to attribute the absorption needed to identify it positively. Now, that part of the puzzle is falling into place as well,” said co-author Dr. Glenn Orton, from NASA’s Jet Propulsion Laboratory.

“While the results set a lower limit to the amount of hydrogen sulfide around Uranus, it is interesting to speculate what the effects would be on humans even at these concentrations.”

“If an unfortunate human were ever to descend through Uranus’ clouds, they would be met with very unpleasant and odiferous conditions.”

“But the foul stench wouldn’t be the worst of it. Suffocation and exposure in the minus 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell,” Dr. Irwin said.

“The new findings indicate that although the atmosphere might be unpleasant for humans, this far-flung world is fertile ground for probing the early history of our Solar System and perhaps understanding the physical conditions on other large, icy worlds orbiting the stars beyond our Sun.”

Physical characteristics

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/cm 3 makes Uranus the second least dense planet, after Saturn. [10] [11] This value indicates that it is made primarily of various ices, such as water, ammonia, and methane. [15] 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. [15] [73] Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5   Earth masses. [15] The remainder of the non-ice mass (0.5 to 3.7   Earth masses) is accounted for by rocky material. [15]

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. [15] [74] The core is relatively small, with a mass of only 0.55   Earth masses and a radius less than 20% of Uranus' 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. [15] [74] Uranus's core density is around 9   g/cm 3 , with a pressure in the centre of 8   million   bars (800 GPa) and a temperature of about 5000   K. [73] [74] 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. [15] [74] This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. [75]

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. [76] [77] [78] 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'. [79] [80] Scientists also believe that rainfalls of solid diamonds occur on Uranus, as well as on Jupiter, Saturn, and Neptune. [81] [82]

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. [83]

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 of which model is correct. [73] The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers. [15] 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. [10] This surface is used throughout this article as a zero point for altitudes.

Internal heat

Uranus's internal heat appears markedly lower than that of the other giant planets in astronomical terms, it has a low thermal flux. [23] [84] Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun, [23] but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere. [16] [85] Uranus's heat flux is only 0.042 ± 0.047   W/m 2 , which is lower than the internal heat flux of Earth of about 0.075   W/m 2 . [85] The lowest temperature recorded in Uranus's tropopause is 49   K (�.2   °C �.5   °F) , making Uranus the coldest planet in the Solar System. [16] [85]

One of the hypotheses for this discrepancy suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature. [86] This impact hypothesis is also used in some attempts to explain the planet's axial tilt. Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface. [15] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport [16] [85] perhaps double diffusive convection is a limiting factor. [15]

In a recent study, the ice giants' interior conditions were mimicked by compressing water containing minerals like olivine and ferropericlase. It showed that much magnesium could be dissolved in the liquid interiors of Uranus and Neptune. A thermal insulation layer made of dissolved magnesium in Uranus due to a larger quantity in Uranus than Neptune was proposed as a possible explanation of Uranus's low temperature. [87]


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. [16] 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) . [89] The tenuous thermosphere extends over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar. [90] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of � and 50   km (� 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 󔼒   bar (10   kPa to 10   µPa) and the thermosphere extending from 4,000   km to as high as 50,000   km from the surface. [16] There is no mesosphere.


The composition of Uranus's atmosphere is different from its bulk, consisting mainly of molecular hydrogen and helium. [16] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03 [19] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05 . [16] [85] This value is close to the protosolar helium mass fraction of 0.275 ± 0.01 , [91] indicating that helium has not settled in its centre as it has in the gas giants. [16] The third-most-abundant component of Uranus's atmosphere is methane ( CH
4 ). [16] Methane has prominent absorption bands in the visible and near-infrared (IR), making Uranus aquamarine or cyan in colour. [16] 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. [16] [18] [92] The mixing ratio [lower-alpha 9] is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out. [93] The abundances of less volatile compounds such as ammonia, water, and hydrogen sulfide in the deep atmosphere are poorly known. They are probably also higher than solar values. [16] [94] 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. [95] They include ethane ( C
2 H
6 ), acetylene ( C
2 H
2 ), methylacetylene ( CH
3 C
2 H ), and diacetylene ( C
2 HC
2 H ). [93] [96] [97] Spectroscopy has also uncovered traces of water vapour, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets. [96] [97] [98]


The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude. [16] The temperature falls from about 320   K (47   °C 116   °F) at the base of the nominal troposphere at �   km to 53   K (�   °C �   °F) at 50   km. [89] [92] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57   K (� and �   °C � and �   °F) depending on planetary latitude. [16] [84] The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of 59.1   ±   0.3   K (�.1   ±   0.3   °C �.3   ±   0.5   °F) . [84] [85]

The troposphere is thought to have a highly complex cloud structure water clouds are hypothesised to lie in the pressure range of 50 to 100   bar (5 to 10   MPa) , ammonium hydrosulfide clouds in the range of 20 to 40   bar (2 to 4   MPa) , ammonia or hydrogen sulfide clouds at between 3 and 10   bar (0.3 and 1   MPa) and finally directly detected thin methane clouds at 1 to 2   bar (0.1 to 0.2   MPa) . [16] [18] [89] [99] The troposphere is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes. [23]

Upper atmosphere

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53   K (�   °C �   °F) in the tropopause to between 800 and 850   K (527 and 577   °C 980 and 1,070   °F) at the base of the thermosphere. [90] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons, [101] which form in this part of the atmosphere as a result of methane photolysis. [95] Heat is also conducted from the hot thermosphere. [101] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300   km corresponding to a pressure range of 1000 to 10 Pa and temperatures of between 75 and 170   K (� and �   °C � and �   °F) . [93] [96] 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. [93] [96] [98] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower. [96] The abundance ratio of water is around 7 × 10 − 9 . [97] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10   mBar level) forming haze layers, [95] 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. [93] [102]

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850   K. [16] [102] The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor the auroral activity can provide the necessary energy to maintain these temperatures. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1   mBar pressure level may contribute too. [90] [102] In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass and high temperatures explain why the corona extends as far as 50,000   km (31,000   mi) , or two Uranian radii, from its surface. [90] [102] This extended corona is a unique feature of Uranus. [102] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings. [90] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus. [92] Observations show that the ionosphere occupies altitudes from 2,000 to 10,000   km (1,200 to 6,200   mi) . [92] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere. [102] [103] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity. [104] Auroral activity is insignificant as compared to Jupiter and Saturn. [102] [105]


Before the arrival of Voyager   2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, scientists had expected the magnetic field of Uranus to be in line with the solar wind, because it would then align with Uranus's poles that lie in the ecliptic. [106]

Voyager ' s observations revealed that Uranus's magnetic field is peculiar, both because it does not originate from its geometric centre, and because it is tilted at 59° from the axis of rotation. [106] [107] In fact the magnetic dipole is shifted from Uranus's centre towards the south rotational pole by as much as one third of the planetary radius. [106] 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). [106] The average field at the surface is 0.23   gauss (23   µT). [106] Studies of Voyager   2 data in 2017 suggest that this asymmetry causes Uranus's magnetosphere to connect with the solar wind once a Uranian day, opening the planet to the Sun's particles. [108] 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. [107] The dipole moment of Uranus is 50   times that of Earth. [106] [107] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants. [107] 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. [75] [109] Another possible explanation for the magnetosphere's alignment is that there are oceans of liquid diamond in Uranus's interior that would deter the magnetic field. [79]

The magnetic field of Uranus
(animated 25 March 2020)

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail, and radiation belts. [106] [107] [110] Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's. [106] [107] Uranus's magnetotail trails behind it into space for millions of kilometres and is twisted by its sideways rotation into a long corkscrew. [106] [111]

Uranus's magnetosphere contains charged particles: mainly protons and electrons, with a small amount of H2 + ions. [107] [110] Many of these particles probably derive from the thermosphere. [110] The ion and electron energies can be as high as 4 and 1.2   megaelectronvolts, respectively. [110] The density of low-energy (below 1   kiloelectronvolt) ions in the inner magnetosphere is about 2   cm 𕒷 . [112] The particle population is strongly affected by the Uranian moons, which sweep through the magnetosphere, leaving noticeable gaps. [110] The particle flux is high enough to cause darkening or space weathering of their surfaces on an astronomically rapid timescale of 100,000   years. [110] This may be the cause of the uniformly dark colouration of the Uranian satellites and rings. [113] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles. [102] Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere. [105]

In March 2020, NASA astronomers reported the detection of a large atmospheric magnetic bubble, also known as a plasmoid, released into outer space from the planet Uranus, after reevaluating old data recorded by the Voyagerق space probe during a flyby of the planet in 1986. [114] [115]

Jupiter Meets Uranus

Object : Planet Jupiter
Date : September 13, 2010
Time : 12:00-12:45 LST/ 07:00-07:45 UT
Location : Surprise, Arizona USA
Medium : white paper, colored pencils, paint brush # 4 and #10 used as a
Instruments : CPC 1100 SCT/ 25mm Plossl/ No filters/ Binoculars 25X100
Magnitude : -2.9
Weather : calm winds, clear skies, temp- mid to upper 80’s

There is no better time than right now! As the summer parade of planets bid
farewell and disappear into the western horizon,(Venus,Mars & Saturn)
Jupiter steps up to the spotlight on the East side of town. Jupiter has an
ongoing list of activities happening on and off its surface. Physically, the
sheer size of its disk is expected to reach 50″ as it nears opposition on
the 20th of this month. As of the time of this sketch, it had a disk size of
49.7″. Although not as bright as Venus(-4.7), It’s pretty shiny for being
the only contender on the lonely Southeastern front of the night sky.

On its surface or close to its Jovian atmosphere, Jupiter was recently
recorded to have been struck by some sizeable bolides. Meteors that burst
into fireballs while getting pulled by the gravity of the gas giant. While I
did not notice any of those fireballs(would’ve been cool), I did notice
other features.Through the scope the most obvious is that Jupiter is
spinning with only one of the two major belts. Only the North Equatorial
Belt is clearly visible. Last spring, the South Equatorial Belt just
disappeared before our averted eyes. It’s believed to be hiding under a
thick blanket of ammonia clouds. Previous circumstances have shown that the
SEB will resurface sometime soon. For now, a slight grayish hue is all that
remains visible of the SEB ocassionally highlighted by darker shades of eddy
currents. The Great Red Spot is easy to ‘spot’ since the lack of the SEB
doesn’t mask it from view, it seems to ride adjacent to the South Temperate
Belt. The GRS is not alone, it was found to have an oval reddish storm about
half its size keeping company just South of its perimeter. Under steady sky
conditions, the designated ‘Oval BA’ or ‘Red Spot Jr” was barely discernible
to the Southwest of the GRS. A more pronounced white oval storm was embedded
and riding high on the westernmost edge of the NEB. For added effect, the
Galilean satellite Europa was just coming out of occultation on the Eastern
limb next to the North Temperate Belt.

From a different perspective, through the binoculars, Jupiter is not exactly
all alone. In the same field of view Uranus is not far away from its big
brother. During my observation both planets were a separated by less than 1
and 1/2 degrees. Uranus will also reach opposition hours later after
Jupiter. Uranus’ disk is very tiny in comparison with Jupiter but you can
still get a pastel lightgreen color out of it. I tried to locate with the
naked eye and had some slight success but I believe its because I knew where
to look. Other than that I think I would have a hard time picking it out-I
was in Surprise I have to admit, not exactly dark skies.

I hope you enjoyed this little report, wishing you all dark and clear skies!

Moon over Armagh

Moon over Armagh on Christmas Eve
Sketch and Details by Miruna Popescu

This painting depicts how the southern sky looks on 24 December 2009 at 5.30 pm, when the Moon’s phase reaches first quarter. The next brightest celestial object at this time is the planet Jupiter, which this year is the “Christmas Star” for the Royal School, Armagh. Jupiter is seen here just before it disappears behind the school. The painting shows stars in Pisces, Pegasus, Aquarius and other constellations, and the location of the planet Uranus (visible through a telescope) about a third of the way from the Moon to Jupiter. Uranus was found in 1781 (seven years after the old building of the Royal School was completed) by the astronomer and musician William Herschel, the discovery constituting the first identification of a planet since ancient times and earning Herschel the post of King’s astronomer from George III.

In 1609, the year after the founding of the Royal School, Galileo Galilei used an early telescope to map the Moon and discover satellites of Jupiter. To mark the 400th anniversary of Galileo’s first use of the telescope to observe the sky, 2009 is being celebrated worldwide as the International Year of Astronomy.

Dr Miruna Popescu from Armagh Observatory is the coordinator for the International Year of Astronomy 2009 in Ireland.

Autumn Skywatching Treat: See Uranus and Neptune in the Night Sky

Midautumn places the two outermost planets in excellent position for viewing.

We often speak of the five "naked-eye" planets (Mercury, Venus, Mars, Jupiter and Saturn), but in actuality, there is a sixth that can be glimpsed with the unaided eye if you know precisely where to look — and another that can be seen when you use a good pair of binoculars. Uranus can also be seen by a sharp-eyed observer who knows where to look for it Neptune is the only planet that requires optical aid in order to be seen.

Both planets were discovered after the invention of the telescope. Uranus was discovered more or less by accident in 1781. Uranus' failure to follow its predicted orbit seemed to be due to the gravitational pull of a planet farther out in space. Two astronomers independently calculated the position of the undiscovered planet, and when telescopes were turned to this region in 1846, Neptune was found. [Stargazing Maps: Best Night Sky Events of October 2017]

So, while our evening sky will soon be devoid of bright planets (Saturn will depart the scene by early December), Uranus and Neptune will be in excellent positions to be seen.

Of course, the trick is that you'll have to know exactly where to look!

Uranus, the green planet

Barely visible to the unaided eye on very dark, clear nights, the planet Uranus — currently shining at magnitude 5.7 — is now visible during the evening hours among the stars of the constellation Pisces, the fishes. Pisces is shaped like two fishing lines tied together in a knot, with one fish dangling from each line it resembles the shape of the letter V, tilted on its side. The star that marks the knot is known as Al Risha, a fourth-magnitude star. Above Al Risha is a star of similar brightness, known as Omicron Piscium.

The next step is to carefully study a star chart, then scan that region with binoculars. Uranus should be evident, set off by its greenish tint. Uranus just passed its opposition to the sun (on Oct. 19) and is currently visible in the sky all through the night. Right now, it appears at its highest at around midnight local daylight time, when it will stand roughly 60 degrees above the southern horizon roughly two-thirds up from the horizon to the point directly overhead (the zenith).

Using a magnification of 150x with a telescope of at least three-inch aperture, you just might be able to resolve Uranus into a tiny, pale-green, featureless disk. While observing Uranus from the Susan F. Rose Observatory at the Custer Institute in Southold on Oct. 21, New York amateur astronomer Bart Fried wrote to New York's Amateur Observers' Society (NYAOS): "[At] 180-power for Uranus . that's a speck!" Unless the seeing, or blurring and twinkling caused by Earth's atmosphere is "total chaos," Fried suggested trying a 300x telescope, "and next time, it will actually look like a planet. And maybe with [a larger aperture], some mottling or cloud feature will be visible."

Indeed, larger instruments will better resolve this planet's verdant disk.

Uranus currently is 1.85 billion miles (2.97 billion kilometers) from the sun and 1.76 billion miles (2.83 billion km) from Earth. It has a diameter of 31,518 miles (50,712 km), and according to flyby magnetic data from Voyager 2 in January 1986, it has a rotation period of 17.4 hours.

At last count, Uranus has 27 moons. They are all in orbits that lie in the planet's equator, in which there is also a complex of nine narrow, nearly opaque rings, which were discovered in 1978. Uranus likely has a rocky core surrounded by a liquid mantle of water, methane and ammonia, encased in an atmosphere of hydrogen and helium.

A bizarre feature is how far over Uranus is tipped. Its north pole lies 98 degrees from being directly up and down to its orbit plane. Thus, its seasons are extreme: When the sun rises at its north pole, it stays up for 42 Earth years then it sets, and the North Pole is in darkness for 42 Earth years.

Neptune, the blue planet

Neptune, on the other hand, is much too faint to be perceived with the unaided eye. With Pluto's demotion to dwarf planet status in 2006, Neptune is now recognized as the most distant planet in the solar system. Currently, it lies at a distance of 2.78 billion miles (4.48 billion km) from the sun and 2.73 billion miles (4.39 billion km) from Earth.

It is slightly smaller than Uranus, with a diameter of 30,598 miles (49,232 km). Currently at magnitude 7.8, it's more than six times dimmer than Uranus. At this moment in time, Neptune can be found in the constellation of Aquarius, the Water Carrier.

You might try using the 3.7-magnitude star Lambda Aquarii to steer you toward Neptune. Currently, Neptune is only about a half-degree (about a full moon's width) to the south of this star. Neptune should be recognizable, thanks to its bluish color. If you have access to a dark, clear sky and carefully examine a star chart of this region, you should have no trouble in finding it with a good pair of binoculars.

From Long Beach, New York, amateur astronomer Larry Gerstman wrote to NYAOS: "For the last couple of months I have been following the motion of the planet Neptune in several of my binoculars. Its motion is like watching that of an asteroid, and much of the apparent motion is really more about the motion of the Earth. I have been using mostly my 20x60 Bushnell binoculars, which I just took out of my closet after many years of non-use since I have larger pairs, but I'm rediscovering how great they are — especially with their wide apparent field of 70-degrees (which is an actual field of 3.5-degrees at 20x) and crisp sharpness in a compact size."

Neptune is currently at its highest point in the sky at around 9 p.m. local daylight time, climbing about 40 degrees above the southern horizon nearly halfway from the horizon to the zenith. [How to Measure Distances in the Night Sky]

With a telescope, trying to resolve Neptune into a disk will be more difficult for observers to do than it will be with Uranus. You're going to need at least an 8-inch telescope with a magnification of no less than 200x, just to turn Neptune into a tiny blue dot of light.

One of Neptune's 14 moons, Triton, has a tenuous atmosphere of nitrogen, and at 1,680 miles (2,703 km) in diameter, it's larger than Pluto. Because it is moving in a retrograde (backward) orbit, there has been some suggestion that Neptune's strong gravitational pull may actually have captured it in the distant past. Those who have access to a large telescope of 12 inches or more might even be able to get a glimpse of Triton, very close to Neptune itself.

Voyager 2 passed Neptune in August 1989 and relayed its possession of a deep-blue atmosphere, with rapidly moving wisps of white clouds. Also evident was a Great Dark Spot, rather similar in nature to Jupiter's famous Great Red Spot. Observations of Neptune using the Hubble Space Telescope suggest that the dark spot seen by Voyager 2 has dissipated yet it has apparently been replaced by another. The atmosphere of Neptune is apparently chiefly composed of hydrocarbon compounds. Based on the rotation rate of its magnetic field, a rotation rate of 16.1 hours has been assigned to Neptune. Voyager 2 also revealed the existence of at least three rings around Neptune, composed of very fine particles.

What Uranus Cloud Tops Have in Common With Rotten Eggs

Hydrogen sulfide, the gas that gives rotten eggs their distinctive odor, has been verified as one of the key components of clouds at Uranus.

Even after decades of observations and a visit by NASA's Voyager 2 spacecraft, Uranus held on to one critical secret -- the composition of its clouds. Now, one of the key components of the planet's clouds has finally been verified.

A global research team that includes Glenn Orton of NASA's Jet Propulsion Laboratory in Pasadena, California, has spectroscopically dissected the infrared light from Uranus captured by the 26.25-foot (8-meter) Gemini North telescope on Hawaii's Mauna Kea. They found hydrogen sulfide, the odiferous gas that most people avoid, in Uranus' cloud tops. The long-sought evidence was published in the April 23rd issue of the journal Nature Astronomy.

The detection of hydrogen sulfide high in Uranus' cloud deck (and presumably Neptune's) is a striking difference from the gas giant planets located closer to the Sun -- Jupiter and Saturn -- where ammonia is observed above the clouds, but no hydrogen sulfide. These differences in atmospheric composition shed light on questions about the planets' formation and history.

"We've strongly suspected that hydrogen sulfide gas was influencing the millimeter and radio spectrum of Uranus for some time, but we were unable to attribute the absorption needed to identify it positively. Now, that part of the puzzle is falling into place as well," Orton said.

The Gemini data, obtained with the Near-Infrared Integral Field Spectrometer (NIFS), sampled reflected sunlight from a region immediately above the main visible cloud layer in Uranus' atmosphere.

"While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of NIFS on Gemini, combined with the exquisite conditions on Mauna Kea," said lead author Patrick Irwin of the University of Oxford, U.K.

No worries, though, that the odor of hydrogen sulfide would overtake human senses. According to Irwin, "Suffocation and exposure in the negative 200 degrees Celsius [minus 328 degrees Fahrenheit] atmosphere made of mostly hydrogen, helium and methane would take its toll long before the smell."

Read more on the news of Uranus' atmosphere from Gemini Observatory here.

Watch the video: Revealing the Nature of Dark Matter (May 2022).