Vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ northern spring equinox from 2006 to 2008
ABSTRACT Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s northern spring equinox in December 2007.The observed reflectance spectra in the Long J (1.17–1.31 μm) and H (1.45–1.65 μm) bands, obtained with the slit aligned along Uranus’ central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6–8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6–8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J.  Icarus, 181, 309–319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E.  Planet. Space Sci., 56, 624–647; Karkoschka, E., Tomasko, M.  Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L.  Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A.  vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488–489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R.  Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M.  Icarus, 192, 527–557;Karkoschka, E., Tomasko, M.  Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus’ atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6–8 bar is unclear, but the absence of the expected methane cloud at 1.2–1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P.  J. Geophys. Res., 92, 14987–15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M.  Icarus, 182, 577–593; Sromovsky, L.A., Fry, P.M.  Icarus, 192, 527–557; Sromovsky, L.A., Fry, P.M.  Icarus, 193, 252-266; Karkoschka, E., Tomasko, M.  Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M.  Icarus).The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M.  Icarus) find may vary from ∼1–2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions.
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ABSTRACT: Prior to Uranus' northern spring equinox in 2007 December, near-infrared spectra and images of Uranus were obtained in 2006 August and September using the UIST instrument on the United Kingdom Infrared Telescope (UKIRT). Latitudinally resolved spectra were recorded between 1 and 2.5 μm at a resolving power varying between 550 and 2000 with the instrument in long slit mode and with the slit aligned with the planet's central meridian to determine the north-south variations of Uranus' cloud structure. Our observations appear to be the first latitudinally resolved complete (1-2.5 μm) near-IR spectra of Uranus ever recorded, and we present initial determinations of the latitudinal variation of Uranus' vertical cloud structure from these data. We find two main cloud decks of similar density, one based near the 2 bar level and one based in the 8-10 bar region. The upper cloud is found to extend from 50° south to 45° north, increasing in thickness and altitude toward the south, especially in the south circumpolar collar at 45° south, but clearing toward the poles. However, we find that the deeper cloud layer is thickest at the equator and thins symmetrically toward both poles. We also report the first-ever observation that the bright south circumpolar collar is, at some wavelengths, actually darker than other latitudes and provide an explanation of this phenomenon in terms of the latitudinal variations in cloud structure.The Astrophysical Journal 12/2008; 665(1):L71. · 6.73 Impact Factor
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ABSTRACT: The Uranian atmosphere is investigated on the basis of S-band and X-band occultation observations (including measurements of Doppler frequency perturbations) obtained during the Voyager 2 encounter with Uranus in January 1986. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is assumed to have an H2/He abundance ratio of about 85/15, but also to contain small amounts of CH4 at above-cloud relative humidity 30 percent, cloud-base relative humidity 78 percent, and below-cloud mixing ratio 2.3 percent by number density. Other parameters estimated include magnetic-field rotation period 17.24 h, 1-bar equatorial radius 25,559 + or - 4 km, polar radius 24,973 + or - 20 km, equatorial acceleration of gravity 8.69 + or - 0.01 m/sec sq, and atmospheric temperature 76 + or - 2 K (assuming 85 + or - 3 percent H2).Journal of Geophysical Research 01/1988; · 3.17 Impact Factor