Atmosphere of Uranus
The atmosphere of Uranus 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. Uranus's atmosphere is the coldest of all the planets, with its temperature reaching as low as 49 K.[1]
The Uranian atmosphere can be divided into five main 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 (and exosphere) extending from an altitude of 4,056 km to several Uranian radii from the nominal surface at 1 bar pressure.[2] Unlike Earth's, Uranus's atmosphere has no mesosphere.
The troposphere hosts four cloud layers: methane clouds at about 1.2 bar, hydrogen sulfide and ammonia clouds 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 planet's interior. Nevertheless, observations of such clouds were used to measure the planet's 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, obtained some valuable compositional data. The Uranus Orbiter and Probe is scheduled to launch in 2031, arriving at Uranus in 2044. Its primary science objectives include a detailed study of Uranus' atmosphere.
Observation and exploration
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope (the region accessible to
The observational history of the Uranian atmosphere is long and full of error and frustration. Uranus is a relatively faint object, and its visible angular diameter is smaller than 5″.[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.[4] 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.[4][5] 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.[6] The nature of the broad dark bands in its visible spectrum remained unknown until the fourth decade of the twentieth century.[4]
Although Uranus is presently largely blank in appearance, it has been historically shown to have occasional features, such as in March and April 1884, when astronomers
The key to deciphering Uranus's spectrum was found in the 1930s by
In January 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km[13] 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.[14] The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. Only a limited number of discrete clouds were observed.[15]
In the 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.[16] Tracking them has allowed astronomers to re-measure wind speeds on Uranus, known before only from the Voyager 2 observations, and to study the dynamics of the Uranian atmosphere.[17]
Composition
The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of
The third most abundant constituent of the Uranian atmosphere is
Knowledge of the isotopic composition of Uranus's atmosphere is very limited.[29] 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.[30] The deuterium is found almost exclusively in hydrogen deuteride molecules which it forms with normal hydrogen atoms.[31]
Infrared spectroscopy, including measurements with
Structure
The Uranian atmosphere can be divided into three main 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 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.[2][39]
Troposphere
The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude.[2] The temperature falls from about 320 K at the base of the troposphere at −300 km to about 53 K at 50 km.[3][19] The temperature at the cold upper boundary of the troposphere (the tropopause) actually varies in the range between 49 and 57 K depending on planetary latitude, with the lowest temperature reached near 25° southern latitude.[40][41] The troposphere holds almost all of the mass of the atmosphere, and the tropopause region is also responsible for the vast majority of the planet's thermal far infrared emissions, thus determining its effective temperature of 59.1±0.3 K.[41][42]
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 300 bar, ammonium hydrosulfide clouds in the range of 20 and 40 bar, ammonia or hydrogen sulfide clouds at between 3 and 10 bar and finally thin methane clouds at 1 to 2 bar.[3][24][27] Although Voyager 2 directly detected methane clouds,[25] all other cloud layers remain speculative. The existence of a hydrogen sulfide cloud layer is only possible if the ratio of sulfur and nitrogen abundances (S/N ratio) is significantly greater than its solar value of 0.16.[24] Otherwise all hydrogen sulfide would react with ammonia, producing ammonium hydrosulfide, and the ammonia clouds would appear instead in the pressure range 3–10 bar.[28] The elevated S/N ratio implies depletion of ammonia in the pressure range 20–40 bar, where the ammonium hydrosulfide clouds form. These can result from the dissolution of ammonia in water droplets within water clouds or in the deep water-ammonia ionic ocean.[27][28]
The exact location of the upper two cloud layers is somewhat controversial. Methane clouds were directly detected by Voyager 2 at 1.2–1.3 bar by radio occultation.[25] This result was later confirmed by an analysis of the Voyager 2 limb images.[24] The top of the deeper ammonia/hydrogen sulfide clouds were determined to be at 3 bar based on the spectroscopic data in the visible and near-infra spectral ranges (0.5–1 μm).[43] However a recent analysis of the spectroscopic data in the wavelength range 1–2.3 μm placed the methane cloudtops at 2 bar, and the top of the lower clouds at 6 bar.[44] This contradiction may be resolved when new data on methane absorption in Uranus's atmosphere are available.[b] The optical depth of the two upper cloud layers varies with latitude: both become thinner at the poles as compared to the equator, though in 2007 the methane cloud layer's optical depth had a local maximum at 45°S, where the southern polar collar is located (see below).[47]
The troposphere is very dynamic, exhibiting strong zonal winds, bright methane clouds,[48] dark spots[49] and noticeable seasonal changes. (see below)[50]
Stratosphere
The
Hydrocarbons heavier than methane are present in a relatively narrow layer between 160 and 320 km in altitude, corresponding to the pressure range from 10 to 0.1 mbar and temperatures from 100 to 130 K.
In addition to hydrocarbons, the stratosphere contains carbon monoxide, as well as traces of water vapor and carbon dioxide. The mixing ratio of carbon monoxide—3 × 10−8—is very similar to that of the hydrocarbons,[38] while the mixing ratios of carbon dioxide and water are about 10−11 and 8×10−9, respectively.[36][58] These three compounds are distributed relatively homogeneously in the stratosphere and are not confined to a narrow layer like hydrocarbons.[36][38]
Ethane, acetylene and diacetylene condense in the colder lower part of stratosphere[34] forming haze layers with an optical depth of about 0.01 in visible light.[59] Condensation occurs at approximately 14, 2.5 and 0.1 mbar for ethane, acetylene and diacetylene, respectively.[60][d] The concentration of hydrocarbons in the Uranian stratosphere is significantly lower than in the stratospheres of the other giant planets—the upper atmosphere of Uranus is very clean and transparent above the haze layers.[55] This depletion is caused by weak vertical mixing, and makes Uranus's stratosphere less opaque and, as a result, colder than those of other giant planets.[55][61] The hazes, like their parent hydrocarbons, are distributed unevenly across Uranus; at the solstice of 1986, when Voyager 2 passed by the planet, they were concentrated near the sunlit pole, making it dark in ultraviolet light.[62]
Thermosphere and ionosphere
The outermost layer of the Uranian atmosphere, extending for thousands of kilometres, is the
The thermosphere and upper part of the stratosphere contain a large concentration of
One of the sources of information about the ionosphere and thermosphere comes from ground-based measurements of the intense
The upper atmosphere of Uranus is the source of the
Hydrogen corona
The upper part of the thermosphere, where the mean free path of the molecules exceeds the scale height,[g] is called the exosphere.[75] 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.[75] The exosphere is unusually extended, reaching as far as several Uranian radii from the planet.[76][77] It is made mainly of hydrogen atoms and is often called the hydrogen corona of Uranus.[78] The high temperature and relatively high pressure at the base of the thermosphere explain in part why Uranus's exosphere is so vast.[h][77] 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.[80] 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.[78]
Dynamics
Uranus has a relatively bland appearance, lacking broad colorful bands and large clouds prevalent on Jupiter and Saturn.[16][62] Discrete features were only once observed in Uranus's atmosphere before 1986.[12][7] The most conspicuous features on Uranus observed by Voyager 2 were the dark low latitude region between −40° and −20° and bright southern polar cap.[62] The northern boundary of the cap was located at about −45° of latitude. The brightest zonal band was located near the edge of the cap at −50° to −45° and was then called a polar collar.[81] The southern polar cap, which existed at the time of the solstice in 1986, faded away in 1990s.[82] After the equinox in 2007, the southern polar collar started to fade away as well, while the northern polar collar located at 45° to 50° latitude (first appeared in 2007) have grown more conspicuous since then.[83]
The atmosphere of Uranus is calm compared to those of other
Uranus exhibits a considerable seasonal variation over its 84-year orbit. It is generally brighter near solstices and dimmer at equinoxes.[50] The variations are largely caused by changes in the viewing geometry: a bright polar region comes into view near solstices, while the dark equator is visible near equinoxes.[86] Still there exist some intrinsic variations of the reflectivity of the atmosphere: periodically fading and brightening polar caps as well as appearing and disappearing polar collars.[86]
See also
- Magnetosphere of Uranus
Notes
- ^ a b Negative altitudes refer to locations below the nominal surface at 1 bar.
- ^ Indeed, a recent analysis based on a new data set of the methane absorption coefficients shifted the clouds to 1.6 and 3 bar, respectively.[45][46]
- ^ In 1986 the stratosphere was poorer in hydrocarbons at the poles than near the equator;[26] at the poles the hydrocarbons were also confined to much lower altitudes.[56] Temperatures in the stratosphere may increase at the solstices and decrease at equinoxes by as much as 50 K.[57]
- ^ At these altitudes the temperature has local maxima, which may be caused by absorption of solar radiation by haze particles.[18]
- ^ The total power input into the aurora is 3–7 × 1010 W—insufficient to heat up the thermosphere.[70]
- near-infrared part of the spectrum (1.8–2.5 μm) with the total emitted power of 1–2 × 1010 W. The power emitted by molecular hydrogen in the far infrared part of the spectrum is about 2 × 1011 W.[72]
- ^ 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,[18] 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.
- eV) hydrogen atoms. Their origin is unclear, but they may be produced by the same mechanism that heats the thermosphere.[79]
Citations
- ^ Williams, Matt (December 16, 2014). "What is the average surface temperature of the planets in our solar system?". phys.org. Retrieved 2022-04-20.
- ^ a b c d Lunine 1993, pp. 219–222.
- ^ a b c de Pater Romani et al. 1991, p. 231, Fig. 13.
- ^ a b c d e f g Fegley Gautier et al. 1991, pp. 151–154.
- ^ Lockyer 1889.
- ^ Huggins 1889.
- ^ a b Perrotin, Henri (1 May 1884). "The Aspect of Uranus". Nature. 30: 21. Retrieved 4 November 2018.
- ^ a b Adel & Slipher 1934.
- ^ Kuiper 1949.
- ^ Herzberg 1952.
- ^ Pearl Conrath et al. 1990, pp. 12–13, Table I.
- ^ a b Smith 1984, pp. 213–214.
- ^ Stone 1987, p. 14,874, Table 3.
- ^ Fegley Gautier et al. 1991, pp. 155–158, 168–169.
- ^ Smith Soderblom et al. 1986, pp. 43–49.
- ^ a b c Sromovsky & Fry 2005, pp. 459–460.
- ^ Sromovsky & Fry 2005, p. 469, Fig.5.
- ^ a b c d e f g Lunine 1993, pp. 222–230.
- ^ a b c Tyler Sweetnam et al. 1986, pp. 80–81.
- ^ Conrath Gautier et al. 1987, p. 15,007, Table 1.
- ^ Lodders 2003, pp. 1, 228–1, 230.
- ^ Conrath Gautier et al. 1987, pp. 15, 008–15, 009.
- ^ NASA NSSDC, Uranus Fact Sheet Archived 2011-08-04 at the Wayback Machine (retrieved 7 Oc 2015)
- ^ a b c d Lunine 1993, pp. 235–240.
- ^ a b c d Lindal Lyons et al. 1987, pp. 14, 987, 14, 994–14, 996.
- ^ a b c d Bishop Atreya et al. 1990, pp. 457–462.
- ^ a b c Atreya & Wong 2005, pp. 130–131.
- ^ a b c de Pater Romani et al. 1989, pp. 310–311.
- ^ Encrenaz 2005, pp. 107–110.
- ^ Encrenaz 2003, pp. 98–100, Table 2 on p. 96.
- ^ Feuchtgruber Lellouch et al. 1999.
- ^ Burgdorf Orton et al. 2006, pp. 634–635.
- ^ a b Bishop Atreya et al. 1990, p. 448.
- ^ a b c Summers & Strobel 1989, pp. 496–497.
- ^ Encrenaz 2003, p. 93.
- ^ a b c d e f Burgdorf Orton et al. 2006, p. 636.
- ^ Encrenaz 2003, p. 92.
- ^ a b c Encrenaz Lellouch et al. 2004, p. L8.
- ^ Herbert Sandel et al. 1987, p. 15,097, Fig. 4.
- ^ Lunine 1993, pp. 240–245.
- ^ a b Hanel Conrath et al. 1986, p. 73.
- ^ Pearl Conrath et al. 1990, p. 26, Table IX.
- ^ Sromovsky Irwin et al. 2006, pp. 591–592.
- ^ Sromovsky Irwin et al. 2006, pp. 592–593.
- ^ Fry & Sromovsky 2009.
- ^ Irwin Teanby et al. 2010, p. 913.
- ^ Irwin Teanby et al. 2007, pp. L72–L73.
- ^ Sromovsky & Fry 2005, p. 483.
- ^ a b Hammel Sromovsky et al. 2009, p. 257.
- ^ a b Hammel & Lockwood 2007, pp. 291–293.
- ^ a b c Herbert Sandel et al. 1987, pp. 15, 101–15, 102.
- ^ a b c d Lunine 1993, pp. 230–234.
- ^ Young 2001, pp. 241–242.
- ^ a b Summers & Strobel 1989, pp. 497, 502, Fig. 5a.
- ^ a b c d e Herbert & Sandel 1999, pp. 1, 123–1, 124.
- ^ Herbert & Sandel 1999, pp. 1, 130–1, 131.
- ^ Young 2001, pp. 239–240, Fig. 5.
- ^ Encrenaz 2005, p. 111, Table IV.
- ^ Pollack Rages et al. 1987, p. 15,037.
- ^ Lunine 1993, p. 229, Fig. 3.
- ^ Bishop Atreya et al. 1990, pp. 462–463.
- ^ a b c Smith Soderblom et al. 1986, pp. 43–46.
- ^ a b Herbert & Sandel 1999, pp. 1, 122–1, 123.
- ^ Miller Aylward et al. 2005, p. 322, Table I.
- ^ Herbert Sandel et al. 1987, pp. 15, 107–15, 108.
- ^ a b Tyler Sweetnam et al. 1986, p. 81.
- ^ a b Lindal Lyons et al. 1987, p. 14,992, Fig. 7.
- ^ a b c Trafton Miller et al. 1999, pp. 1, 076–1, 078.
- ^ Encrenaz Drossart et al. 2003, pp. 1, 015–1, 016.
- ^ a b Herbert & Sandel 1999, pp. 1, 133–1, 135.
- ^ Lam Miller et al. 1997, pp. L75–76.
- ^ a b Trafton Miller et al. 1999, pp. 1, 073–1, 076.
- ^ Miller Achilleos et al. 2000, pp. 2, 496–2, 497.
- ^ Herbert & Sandel 1999, pp. 1, 127–1, 128, 1, 130–1, 131.
- ^ a b Herbert & Hall 1996, p. 10,877.
- ^ Herbert & Hall 1996, p. 10,879, Fig. 2.
- ^ a b Herbert & Sandel 1999, p. 1,124.
- ^ a b Herbert Sandel et al. 1987, pp. 15, 102–15, 104.
- ^ Herbert & Hall 1996, pp. 10, 880–10, 882.
- ^ Herbert & Hall 1996, pp. 10, 879–10, 880.
- ^ Rages Hammel et al. 2004, p. 548.
- ^ a b c Sromovsky & Fry 2005, pp. 470–472, 483, Table 7, Fig. 6.
- ^ Sromovsky Fry et al. 2009, p. 265.
- ^ Sromovsky & Fry 2005, pp. 474–482.
- ^ Smith Soderblom et al. 1986, pp. 47–49.
- ^ a b Hammel & Lockwood 2007, pp. 293–296.
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External links
Media related to Uranus (atmosphere) at Wikimedia Commons