(4 ±0.5) x 10"11 at 0.1 mbar

Spitzer 10-20 ^m

Burgdorf et al. (2006)

a Fraction of H2 with eqm ortho/para.

a Fraction of H2 with eqm ortho/para.

Mole fraction

Figure 4.21. Observed and modeled abundance profiles in the atmosphere of Uranus.

Mole fraction

Figure 4.21. Observed and modeled abundance profiles in the atmosphere of Uranus.

ever, the observed spectrum may also be explained by the presence of a deep, very optically thick cloud, such as the expected water cloud. Hence, whether or not the deep atmosphere of Uranus is really isothermal is debatable. In Figure 4.20 we have assumed that the temperature profile follows an SALR at depth.

The low IR flux of Uranus means that spectral features of possible disequilibrium species such as phosphine, germane, and arsine are very difficult to detect. Ground-based observations of phosphine lines in the very far IR between 1 mm and 1.5 mm suggest the P/H to be <6x the solar value, while HST observations at 5 ^m (Encrenaz et al., 2004) revise the v.m.r. (at the 3 bar level) to be less than 8 x 10—7 (1.3 x the solar value). We might expect the deep P/H ratio to be somewhat higher, given the high C/ H ratio, and its low abundance suggests sluggish vertical mixing in Uranus' atmosphere. This sluggish motion is supported by the low detected abundance of CO at the cloud tops from the same 5 ^m HST observations of less than 1.8 x 10—8 increasing to ^2.5 x 10—8 at the tropopause, pointing to an additional external supply of oxygen atoms.

The sluggish motion of Uranus' atmosphere is also indicated by the observation by Voyager IRIS (Conrath et al., 1998) that the ortho:para hydrogen ratio appeared to be close to equilibrium at all altitudes, although the ratio was found to be lowest at southern latitudes and highest at northern latitudes, suggesting upwelling in the south and subsidence in the north during the Voyager 2 flyby. The near-equilibrium of the ortho:para hydrogen ratio in Uranus' stratosphere was also concluded by ISO (Fouchet et al., 2003), who further point out that, in addition to sluggish motion, the ortho-para conversion process is efficiently catalyzed by small aerosols, which are thought to be very abundant in Uranus' stratosphere.

Clouds and hazes

Using the ECCM described earlier a massive water cloud is expected to condense anywhere between 100 bar and 1,000 bar (the exact level depends on the H2O/H2 ratio and the deep temperature profile, both of which are uncertain); an ammonia hydrosulfide cloud somewhere around 40 bar; either an ammonia ice cloud, or a hydrogen sulfide ice cloud somewhere around 8 bar (which depends on whether the deep v.m.r. of ammonia is greater than or less than that of hydrogen sulfide, assuming that the minor species is effectively mopped up in the NH4SH cloud leaving just the more abundant species to condense at lower temperatures); and a methane ice cloud at approximately 1.5 bar (de Pater et al., 1991). As has already been discussed, the apparent depletion of tropospheric ammonia suggests that almost all the ammonia that is not dissolved in an aqueous ammonia cloud or water-ammonia ionic ocean at deeper levels reacts with H2S to form NH4SH leaving an H2S cloud to condense near 2 bar to 8 bar.

Observationally only two convective cloud decks were observed in the Uranian atmosphere during and immediately after the Voyager 2 flyby in 1986 (Figure 4.13): a thin cloud near the 1.5 bar level, and an optically thick cloud beneath with a cloud top at approximately 2.7 bar to 3.1 bar, detected using observations of the hydrogen 4-0 and 3-0 quadrupole lines, and the 681.8 nm methane line (Baines and Bergstralh,

1986; Baines et al., 1995b). The observed abundance of methane rapidly reduces with height at the 1.5 bar level indicating that this is indeed the methane cloud, but the visible optical depth of the methane cloud was found to be very thin (0.4 < t < 0.7) at low latitudes, indicating either very weak vertical mixing at this level or that particles rapidly grow and precipitate in this cloud leading to low visible reflectance. The visible opacity of the methane haze was found to increase to approximately 2.4 at 65°S in Uranus' "bright" South Polar zone and the mean particle size in the methane layer was estimated to be of the order of 1 ^m at all latitudes observed (Rages et al., 1991). The lower cloud is probably the top of the expected hydrogen sulfide cloud, although no positive spectral identification has been made. All we do know is that the aerosols in this cloud appear bright in the blue-green, but darken significantly at wavelengths longer of 0.6 ^m (Baines and Bergstrahl, 1986; Baines and Smith, 1990). This, combined with methane gas becoming increasingly absorbing at longer wavelengths in the visible spectrum leads to Uranus' dominant blue-green color. The identity of this chromophore material is unknown but may arise from "tanning" of aerosols by incident UV sunlight.

Disk-averaged observations of the near-IR spectrum of Uranus from the 1970s (Fink and Larson, 1979) were used by Sromovsky et al. (2006) to show that reflectance from the expected methane cloud at 1.5 bar remains small in the 1 ^m to 2 ^m range, but rather than finding a single cloud at bar, reflectance was needed from two pressure levels: ^2 bar and 6-8 bar. This conclusion was also reached from near-IR observations recorded in 2006 by UKIRT (Irwin et al., 2007), and the Keck Observatory in 2004 (Sromovsky and Fry, 2007) and in 2006 (Sromovsky and Fry, 2008). Reflection from the upper cloud was determined to increase towards the southern hemisphere, reaching a peak at 45°S, where a bright polar collar has been visible in the last decade.

As mentioned earlier it is believed that methane observed in the stratosphere arrives there almost entirely through eddy mixing from the tropopause. Although small convective clouds are seen (Karkoschka, 1998b) these are not as bright as the similar small clouds seen on Neptune, although they have become increasingly common and brighter in the lead-up to, and during, Uranus' Northern Spring Equinox in 2007 (Hammel et al., 2005a,b; Sromovsky et al., 2007). The pressure at the tops of these discrete clouds is estimated to vary from 0.5 bar to as little as 0.2 bar, with particle sizes of the order of 1 ^m, similar to the properties of the methane haze layer. Hence, like Neptune, these clouds are thought to be convective methane cumulus clouds. However, unlike Neptune (as we will see), these clouds are not thought to be vigorous enough to penetrate the tropopause and thus increase the stratospheric methane to levels greater than the "cold trap value'' of the saturated v.m.r. at the tropopause. In fact, the stratospheric abundance of methane in Uranus' stratosphere is the lowest of any of the giant planets indicating very weak vertical mixing. Methane photochemistry, which is important between 10mbar and 0.1 mbar, produces hydrocarbons such as ethane (C2H6), acetylene (C2H2), ethylene (C2H4), and polyacetylenes (C2nH2, n = 2,3,4). These products diffuse through the atmosphere via eddy mixing, but unlike Jupiter and Saturn where the products diffuse through the tropopause without further processing, the temperature in Uranus'

stratosphere is so low that these products condense to form stratospheric hydrocarbon haze layers at lower altitudes. Diacetylene ice (C4H2) is expected to start condensing at p > 0.1mbar, acetylene ice at p > 2.5mbar, and ethane ice haze at p > 15mbar. Once haze particles start to condense, they begin to coalesce to form larger particles, which may then gravitationally sediment out of the atmosphere. Hence the haze layers gradually thin out at pressures greater than roughly 30 mbar. The mean haze particle size is estimated to be of the order of 0.1 ^m (West et al., 1991) and the visible optical depth of the combined haze layers is estimated to be very low at only 0.01. The main component of the haze is modeled to be acetylene ice, although the haze particles are found not to be the pure white hydrocarbon condensates that are expected, but instead are quite dark (imaginary refractive index of 0.01 in the visible). The cause of this may possibly be UV-induced polymerization or "tanning", which appears consistent with the dark particles that are also found in Neptune's stratosphere. The submicron size of the haze particles means that they precipitate out of the stratosphere only very slowly on timescales of 10 to 100 years. Once they reach the troposphere they are evaporated and eventually pyrolyzed back to methane at sufficiently high temperatures. No very great change in the stratospheric haze optical depth with latitude has been found, which is in stark contrast to the stratospheric hazes of Jupiter and Saturn, which are strongly UV-absorbing near the poles. Presumably auroral processes are not so important in Uranus' atmosphere.

The disk-averaged albedo of Uranus has been monitored at visible wavelengths since the 1950s. The light curve is found to be approximately sinusoidal with peaks at the solstices (Lockwood and Jerzykiewicz, 2006) and is well fitted by a simple model where the albedo of Uranus at latitudes equatorwards of 45°S and 45°N is darker than that polewards of 45°S and 45°N by a factor of f (Hammel and Lockwood, 2007). Different parts of the observed light curve at 472 nm and 551 nm require different f factors of between 0.75 and 0.98, pointing to considerable interseasonal variability, probably caused by variations in convective activity at midlatitudes. Variations in stratospheric temperature have also been monitored by a number of methods (Hammel and Lockwood, 2007) and temperatures were seen to increase up to the solstice in 1986, but have since decreased. Variations in microwave brightness have also been monitored since the early 1970s (Klein and Hofstadter, 2006) and the variations do not appear to be due to geometric effects alone, but are also due to temporal variations in temperature or ammonia abundance down to pressure levels of tens of bars. In particular, there is evidence of a rapid planetary-scale change from 1993 to 1994. These changes coincided with significant changes seen in the cloud and haze layers at visible/near-IR wavelengths by HST (Rages et al., 2004), where the South Pole faded during 1994-2002 and a bright zone appeared at 45°S, together with a less bright zone appearing in the intervening years at 70°S.

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