Atmosphere of Uranus - Structure - Troposphere

Troposphere

The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude. The temperature falls from about 320 K at the base of the troposphere at −300 km to 53 K at 50 km. 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. 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.

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. Although Voyager 2 directly detected methane clouds at 1.2–1.3 bar via a radio occultation experiment, 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. 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. 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.

The exact location of the upper two cloud layers is somewhat controversial. As was said above, the methane clouds were directly detected by Voyager 2 at 1.2–1.3 bar by radio occultation. This result was later confirmed by an analysis of the Voyager 2 limb images. 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). 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. This contradiction may be resolved when new data on methane absorption in Uranus's atmosphere are available. 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).

The troposphere is very dynamic, exhibiting strong zonal winds, bright methane clouds, dark spots and noticeable seasonal changes. (see below)