Vad har uranus för atmosfär
Trots att Neptunus ligger längre bort ifrån solen än vad Uranus gör har den en mycket livligare atmosfär. The atmosphere of Uranus is composed primarily of hydrogen and helium.
Fakta om Uranus
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. The troposphere hosts four cloud layers: methane clouds at about 1. 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.
Little is known about the Uranian atmosphere. To date, only one spacecraft, Voyager 2 , which passed by the planet in , obtained some valuable compositional data. The Uranus Orbiter and Probe is scheduled to launch in , arriving at Uranus in Its primary science objectives include a detailed study of Uranus' atmosphere. Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope the region accessible to remote sensing is called its atmosphere.
The observational history of the Uranian atmosphere is long and full of error and frustration. Although Uranus is presently largely blank in appearance, it has been historically shown to have occasional features, such as in March and April , when astronomers Henri Joseph Perrotin , Norman Lockyer , and Charles Trépied observed a bright, elongated spot presumably a storm circling the equator of the planet.
The key to deciphering Uranus's spectrum was found in the s by Rupert Wildt and Vesto Slipher , [ 8 ] who found that the dark bands at , , , and nm belonged to gaseous methane. In January , the Voyager 2 spacecraft flew by Uranus at a minimal distance of , km [ 13 ] providing the first close-up images and spectra of its atmosphere. Only a limited number of discrete clouds were observed. In the s and s, 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.
The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of molecular hydrogen and helium. The third most abundant constituent of the Uranian atmosphere is methane CH 4 , [ 23 ] the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Knowledge of the isotopic composition of Uranus's atmosphere is very limited.
It appears to be higher than the protosolar value of 2. Infrared spectroscopy, including measurements with Spitzer Space Telescope SST , [ 32 ] and UV occultation observations, [ 33 ] found trace amounts of complex hydrocarbons in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by solar UV radiation. There is no mesosphere. The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude.
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 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. These can result from the dissolution of ammonia in water droplets within water clouds or in the deep water-ammonia ionic ocean.
The exact location of the upper two cloud layers is somewhat controversial. Methane clouds were directly detected by Voyager 2 at 1. The troposphere is very dynamic, exhibiting strong zonal winds, bright methane clouds, [ 48 ] dark spots [ 49 ] and noticeable seasonal changes. The stratosphere is the middle layer of the Uranian atmosphere, in which temperature generally increases with altitude from 53 K in the tropopause to between and K at the base thermosphere.
Hydrocarbons heavier than methane are present in a relatively narrow layer between and km in altitude, corresponding to the pressure range from 10 to 0. In addition to hydrocarbons, the stratosphere contains carbon monoxide, as well as traces of water vapor and carbon dioxide. Ethane, acetylene and diacetylene condense in the colder lower part of stratosphere [ 34 ] forming haze layers with an optical depth of about 0.
Fakta om Uranus
The thermosphere and upper part of the stratosphere contain a large concentration of ions and electrons , forming the ionosphere of Uranus. This phenomenon, which occurs in the thermospheres of all giant planets and was mysterious for a time after its discovery, is interpreted as a UV fluorescence of atomic and molecular hydrogen excited by solar radiation or by photoelectrons. The upper part of the thermosphere, where the mean free path of the molecules exceeds the scale height , [ g ] is called the exosphere.
The infalling dust in turn contaminates the upper atmosphere of the planet. Uranus has a relatively bland appearance, lacking broad colorful bands and large clouds prevalent on Jupiter and Saturn. The atmosphere of Uranus is calm compared to those of other giant planets. Only a limited number of small bright clouds at middle latitudes in both hemispheres [ 16 ] and one Uranus Dark Spot have been observed since Uranus exhibits a considerable seasonal variation over its year orbit.