Climate of Uranus

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Uranus' southern hemisphere in approximate natural colour (left) and in higher wavelengths (right), showing its faint cloud bands and atmospheric "hood" as seen by Voyager 2

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's 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]

Banded structure, winds and clouds

Uranus in 2005. Rings, southern collar and a light cloud in the northern hemisphere are visible.
Hubble images showing the seasonal changes in the atmosphere of Uranus. The south of Uranus is at the upper right and north is at the lower left. The south polar cap disappears between 2007 and 2011 and the north polar cap appears between 2010 and 2015.

The first suggestions of bands and weather on Uranus came in the 19th century, such as an observation in March and April 1884 of a white band circling partially around Uranus's equator, only two years after Uranus's "spring" equinox.[6]

In 1986

latitudinal structure of Uranus is different from that of Jupiter and Saturn, which demonstrate multiple narrow and colorful bands.[1]

In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[3] In all other respects Uranus looked like a dynamically dead planet in 1986. However, in the 1990s the number of observed bright cloud features grew considerably.[1] The majority of them were found in the northern hemisphere as they started to become visible.[1] The common though incorrect explanation of this fact was that bright clouds are easier to identify in its dark part, whereas in the southern hemisphere the bright collar masks them.[10] Nevertheless, there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[11] They appear to lie at a higher altitude, which is connected to fact that until 2004 (see below) no southern polar cloud had been observed at the wavelength 2.2 micrometres,[11] which is sensitive to the methane absorption, whereas northern clouds have been regularly observed in this wavelength band. The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, whereas at least one southern cloud has persisted since the Voyager flyby.[1][4] Recent observation also discovered that cloud-features on Uranus have a lot in common with those on Neptune, although the weather on Uranus is much calmer.[1]

Uranus Dark Spot

The first dark spot observed on Uranus. Image was obtained by ACS on HST in 2006.

The dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged.[12] In that year observations from both Hubble Space Telescope and Keck Telescope revealed a small dark spot in the northern (winter) hemisphere of Uranus. It was located at the latitude of about 28 ± 1° and measured approximately 2° (1300 km) in latitude and 5° (2700 km) in longitude.[5] The feature called Uranus Dark Spot (UDS) moved in the prograde direction relative Uranus's rotation with an average speed of 43.1 ± 0.1 m/s, which is almost 20 m/s faster than the speed of clouds at the same latitude.[5] The latitude of UDS was approximately constant. The feature was variable in size and appearance and was often accompanied by a bright white cloud called Bright Companion (BC), which moved with nearly the same speed as UDS itself.[5]

The behavior and appearance of UDS and its bright companion were similar to Neptunian

orographic clouds).[5] UDS is supposed to have a similar nature, although it looked differently from GDS at some wavelengths. Although GDS had the highest contrast at 0.47 μm, UDS was not visible at this wavelength. On the other hand, UDS demonstrated the highest contrast at 1.6 μm, where GDS were not detected.[5] This implies that dark spots on the two ice giants are located at somewhat different pressure levels—the Uranian feature probably lies near 4 bar. The dark color of UDS (as well as GDS) may be caused by thinning of the underlying hydrogen sulfide or ammonium hydrosulfide clouds.[5]

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

The emergence of a dark spot on the hemisphere of Uranus that was in darkness for many years indicates that near equinox Uranus entered a period of elevated weather activity.[5]

Winds

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper

meridional winds.[1]

Seasonal variation

Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere has existed for less than one full Uranian year (84 Earth years).[15] A number of discoveries have however 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.[16] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[17] Stratospheric temperature measurements beginning in the 1970s also showed maximum values near 1986 solstice.[18]

HST images show changes in the atmosphere of Uranus as it approaches its equinox (right image)

The majority of this variability is believed to occur due to changes in the viewing

polar region of Uranus is much brighter than the equatorial bands.[3] In addition, both poles demonstrate elevated brightness in the microwave part of the spectrum,[19] whereas the polar stratosphere is known to be cooler than the equatorial one.[18] So seasonal change seems to happen as follows: poles, which are bright both in visible and microwave spectral bands, come into the view at solstices resulting in brighter planet, whereas the dark equator is visible mainly near equinoxes resulting in darker planet.[10] In addition, occultations at solstices probe hotter equatorial stratosphere.[18]

The visible magnitude of Uranus in two spectral bands (upper graph)[16] adjusted for the distance, effective microwave temperature (middle graph) and the stratospheric temperature (lower graph).[17] Blue band is centered at 470 nm, yellow at 550 nm.

However, there are some reasons to believe that seasonal changes are happening in Uranus. Although Uranus 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.

polar collar present in its southern hemisphere at −45° was expected to appear in its northern part.[20] This indeed happened in 2007 when Uranus passed an equinox: a faint northern polar collar arose, whereas the southern collar became nearly invisible, although the zonal wind profile remained asymmetric, with northern winds being slightly slower than southern.[9]

The mechanism of physical changes is still not clear.

circulation, because thick polar clouds and haze may inhibit convection.[19]

For a short period in the second half of 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a

seasonal variations in its weather.[12][20]

Circulation models

HST image of Uranus taken in 1998 showing clouds in the northern hemisphere
The greenish color of Uranus's atmosphere is due to methane and high-altitude photochemical smog. Voyager 2 acquired this view of the seventh planet while departing the Uranian system in late January 1986. This image looks at Uranus approximately along its rotational pole.

Several solutions have been proposed to explain the calm weather on Uranus. One proposed explanation for this dearth of cloud features is that Uranus's

thermal flux.[1][13] Why Uranus's heat flux 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.[1] Uranus, by contrast, 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.[22][23] In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m2, which is lower than the internal heat flux of Earth of about 0.075 W/m2.[22] The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System, colder than Neptune.[22][23]

Another hypothesis states that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface.[24] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.[22][23]

References

  1. ^ a b c d e f g h i j k l m n o p q Sromovsky & Fry 2005.
  2. ISBN 9781139495066
    . Retrieved 19 November 2014.
  3. ^ a b c d e Smith Soderblom et al. 1986.
  4. ^ a b c d Lakdawalla 2004.
  5. ^ a b c d e f g h Hammel Sromovsky et al. 2009.
  6. ^ Perrotin, Henri (1 May 1884). "The Aspect of Uranus". Nature. 30: 21. Retrieved 4 November 2018.
  7. ^ a b c d e f Hammel de Pater et al. ("Uranus in 2003") 2005.
  8. ^ a b c d e Rages Hammel et al. 2004.
  9. ^ a b Sromovsky Fry et al. 2009.
  10. ^ a b c Karkoschka ("Uranus") 2001.
  11. ^ a b c d Hammel de Pater et al. ("Uranus in 2004") 2005.
  12. ^ a b Sromovsky Fry et al. 2006.
  13. ^ a b Hanel Conrath et al. 1986.
  14. ^ Hammel Rages et al. 2001.
  15. ^ Shepherd, George (1861). The Climate of England. Longman, Green, Longman, and Roberts. p. 28. Retrieved 19 November 2014. The planet Uranus completes her revolution round the sun in 84 years.
  16. ^ a b c d Lockwood & Jerzykiewicz 2006.
  17. ^ a b Klein & Hofstadter 2006.
  18. ^ a b c Young 2001.
  19. ^ a b c Hofstadter & Butler 2003.
  20. ^ a b c d e f Hammel & Lockwood 2007.
  21. ^ Devitt 2004.
  22. ^ a b c d Pearl Conrath et al. 1990.
  23. ^ a b c Lunine 1993.
  24. ^ Podolak Weizman et al. 1995.

Sources

External links
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