Uranus deep atmosphere revealed

Abstract

We have examined Uranus' radio spectrum and the latitudinal variation in its radio brightness temperature. We conclude that Uranus' spectrum cannot be explained with models based upon thermochemical equilibrium. The spectrum can only be matched if there is a low ammonia volume mixing ratio (a few times 10 -7 between roughly 150 < T < 240 K), distributed uniformly over a large range of altitude. To remove sufficient NH3 at deep tropospheric levels through the formation of an NH4SH-solid cloud, a large concentration of H2S gas is needed, which implies that sulfur must be enhanced relative to its solar value by a factor of a few hundred. We further can constrain the S/N ratio to be at least live times the solar value if the H20 volume mixing ratio is <100x solar and equal to or larger than 3x solar if H20 is >~500x solar. The elemental ratios derived from our work support the theory of planetary accretion by Pollack and Bodenheimer (1989, in Origin and Evolution of Atmospheres (S. Atreya, Ed.)). The constant mixing ratio of NH3 in the 145 < T ~< 240 K range implies either vertical mixing on time scales much shorter than expected from reasonable values of the eddy diffusion coefficient, or supersaturation of NH4SH at levels where T ~ 240 K. The pole-equator gradient in Uranus' radio brightness temperature implies a latitude-dependent variation in the ammonia mixing ratio, ranging from a few times 10 -6 at altitudes where the temperature is 145 < T < 240 K in the equatorial region (latitudes <30-40°), a few times 10 7 at 145 < T < 220 K at midlatitudes, and --10 -7 down to 280 K in the polar region. Whereas the values in the equator and midlatitudes can be explained by condensation theories, the polar value can only be explained by invoking strong downdrafts of dry air (gas from which most of the ammonia vapor has been removed by condensation). A comparison of images taken at 2-and 6-cm wavelength show a difference in the spatial distribution of the brightness temperature. In addition, images at both wavelengths show time variability in the zonal brightness distribution.

Clouds and aerosols on Uranus: Radiative transfer modeling of spatially-resolved near-infrared Keck spectra

“…Sromovsky et al, 2011), suggest that methane is increasingly depleted toward high latitudes. This result agrees with microwave observations of the deep atmosphere, which find the greatest volatile concentration in the equatorial region (de Pater et al, 1989;Hofstadter and Butler, 2003).…”

“…Depletions down to 10 bars or more, as we find here and as have been previously indicated (Sromovsky et al, 2014), must be associated with deep circulation cells, but microwave observations have revealed a symmetric deep atmosphere (Hofstadter and Butler, 2003;Hofstadter et al, 2007). In addition, there is an inconsistency between the deep latitudinal variations revealed by microwave observations (de Pater et al, 1989;Hofstadter and Butler, 2003), and models of deep depletion gradients that predict wind speeds much higher than we observe (Sromovsky et al, 2011). A more complete understanding of Uranus' atmospheric circulation is clearly needed to simultaneously explain the range of observations.…”

“…They argue that depletion must occur only at shallow depths because a latitudinal gradient in mixing ratio at great depths would produce winds inconsistent with what is observed on Uranus. However, this is seemingly at odds with microwave observations that imply depletions and latitudinal differences down to tens of bars (de Pater et al, 1989;Hofstadter and Butler, 2003). Irwin et al (2011Irwin et al ( , 2012a and Tice et al (2013) also found evidence for an enrichment of methane at the equator over mid-latitudes, and note that the apparent variation in cloud top pressure with latitude may disappear when this methane gradient is considered.…”

New Measurements of the Winds of Uranus

“…Current diabatic-circulation models are preliminary and do not yet suggest whether the profiles would change smoothly with time, whether the peak velocities at the solstices are similar, nor whether there is a temporal phase lag with season. Earlier studies of the seasonal variations in energy balance and thermal structure of Uranus also indicated asymmetry and/or temporal variability (Wallace 1983;Friedson and Ingersoll 1987), as have long-term microwave observations of the deeper Uranian atmosphere (de Pater et al 1989;Hofstadter 1992). Conrath et al (1990) suggest any seasonally forced winds would be weak because of long radiative and frictional time constants.…”

Time Variability in the Radio Brightness Distribution of Saturn

“…2-5 are essentially those by de Pater and Mitchell (1993) and are based on a thermochemical equilibrium atmosphere after Romani (1986). For details about the construction of such model atmospheres see de Pater et al (1989). As in Fig.…”