Seasonal Changes on Uranus: Analysis of Imaging Data 1986-2006
Uranus is in the midst of a unique era from an observational perspective: the planet's northern hemisphere has come into view for the first time in decades and is now revealing a surprising array of tantalizing detail, while regions of the southern hemisphere also display levels of activity never previously observed with modern detectors. Uranus as a whole has changed noticeably since Voyager was there in 1986, darkening since 1988 after brightening steadily from 1972 to 1985. Hubble Space Telescope (HST) observations since 1994 show changes in the banded structure of Uranus' bright south polar cap. Keck observations using adaptive optics (AO) have recorded unprecedented levels of activity near the southern polar collar. A tremendous amount of data has been obtained over the past few years, and preliminary analyses of the individual data sets has started. The recent Uranus data include: images from the HST Wide-Field Planetary Camera 2 (WFPC2), Advanced Camera for Surveys (ACS), and Near Infrared Camera/Multi-Object Spectrometer (NICMOS); near-infrared images from Keck Observatory using AO to obtain spatial resolution comparable to or better than HST; and SpeX infrared spectra obtained at the Infra-Red Telescope Facility (IRTF) on Mauna Kea.
This study constitutes a coordinated analysis of these diverse sets of Uranus observations and the original Voyager data set to investigate the nature of long-term (seasonal) changes on Uranus over the past 20 years. We model the full time span of Uranus observations since the Voyager era to seek out significant temporal changes in such parameters as (1) the methane cloud's altitude, optical thickness, and single scattering albedo; (2) aerosol absorption below the base of the methane cloud; and (3) the height of the putative H2S cloud at >-3 bars. This study addresses a number of questions regarding the Uranian atmosphere. How do the northern and southern hemispheres differ in vertical aerosol structure? Is Uranus' southern hemispheric haze abundance varying? Is the asymmetry of the planet's atmosphere time-variable? (Models suggest that significant changes in the radiation balance of the atmosphere occur on seasonal cycles; the north-south dichotomy could be a detectable manifestation of such changes.)