Monitoring Active Atmospheres on Uranus and Neptune
Snapshot observations of Uranus and Neptune to monitor changes in their atmospheres on time scales of weeks and months, as we have been doing for the past seven years. Previous Hubble Space Telescope observations (including previous Snapshot programs 8634, 10170, 10534, and 11156), together with near-IR images obtained using adaptive optics on the Keck Telescope, reveal both planets to be dynamic worlds which change on time scales ranging from hours to (terrestrial) years. Uranus equinox occurred in December 2007, and the northern hemisphere is becoming fully visible for the first time since the early 1960s. Cycle l6 Snapshot observations in September 2008 (GOlll56) show that the very long-lived bright feature seen near 35'S for the past decade or so has now moved north to -20oS, and may be rapidly heading toward extinction.
HST observations during the past several years (Hammel et al. 2005, Icarus 175,284 and references therein) have also revealed strongly wavelength-dependent latitudinal structure, numerous visible wavelength cloud features in the northern hemisphere, and in 2006 the first clearly defined dark spot seen on Uranus. Long-term ground-based observations (Lockwood and Jerzekiewicz 2006, Icarus 180,442; Hammel and Lockwood 2007, Icarus I 86, 291) reveal seasonal brightness changes that seem to demand the appearance of a bright northern polar cap within the next few years. Recent HST and Keck observations of Neptune (Sromovskye t al. 2003, Icarus 163, 256 and references therein) show a general increase in activity at south temperate latitudes until 2004, when Neptune returned to a rather Voyager-like appearance with discrete bright spots rather than active latitude bands. This study's additional Snapshot observations of these two dynamic planets elucidate the nature of longterm changes in their zonal atmospheric bands and clarify the processes of formation, evolution, and dissipation of discrete albedo features.