AAS 201st Meeting, January, 2003
Session 45. The Solar System
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[45.14] Seasonal Change in the Deep Atmosphere of Uranus

M.D. Hofstadter (JPL/Caltech), B.J. Butler (NRAO)

We have analyzed microwave maps of Uranus made with the VLA in 1981, 1985, 1989, 1994, and 2002. The observations, at wavelengths of 2 and 6 cm, are sensitive to atmospheric composition and temperature between 5 and 50 bars. Over this time Uranus moved from early southern summer to early fall. All maps show the planet to be strongly bi-modal: a region around the South Pole is consistently much brighter than all other latitudes, with the transition always centered near -45 degrees. The contrast between bright and dark regions, however, increased significantly between 1989 and 1994. The planet appeared stable both before and after this time. Disk-averaged measurements at a wavelength of 3.5 cm (M. Klein et al. 2002 Bull. Am. Astron. Soc. 34, 874, and this conference), also show a rapid change occuring in the atmosphere around 1992. It is surprising that planetary-scale changes occur this deep and this rapidly in a giant planet.

The most likely explanation for brightness features on the planet is spatial variations in the abundance of absorbers such as NH3 and H2O. Bright regions are depleted in absorbers by more than an order of magnitude. Since these species are condensable, atmospheric circulation and cloud formation can create the observed spatial variations. The changes we see over time might therefore be related to seasonal variations in the deep circulation. The dynamical model of Friedson and Ingersoll (1987, Icarus 69,143-156) does predict a bi-modal atmosphere, with some latitudes being convective and others stabily stratified, but it does not predict meridional variability as deep as we have seen.

We will continue to observe Uranus throughout its equinox passage in 2007 in anticipation of further changes. We are coordinating our work with observers at visible and infrared wavelengths in hopes of developing a comprehensive picture of the troposphere, and are working with dynamicists to better constrain atmospheric models. We are also working with the Goldstone-Apple Valley Radio Telescope science education partnership, which has a single-dish radio telescope operated by students in support of professional astronomers.

The work of M. Hofstadter was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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