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From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity., Comment: 20 pages, 19 figures, 4 tables, on-line supplemental material available

Hubble Space Telescope imaging of Uranus in 1994, 1997, 1998, and 2000 revealed 13 cloud features, allowing the first measurements of wind velocities at northern latitudes not accessible to the Voyager cameras and new measurements of southern-latitude wind velocities determined during the 1986 Voyager encounter. Images acquired with the Keck 10-meter telescope adaptive optics system in June 2000 also showed some of the same features. Wind speeds inferred from feature motions—along with additional measurements by Karkoschka (1998, Science 280 ) and Voyager measurements (Smith et al . 1986, Science 246 )—indicate a zonal wind profile that is asymmetric with respect to the equator. Small but consistent differences are seen between the newer data and a profile determined from Voyager data: nearly all the newer measurements have slightly slower velocities than expected. We cannot yet determine whether the source of these differences is a slowly changing Uranian wind profile or subtle latitudinal structure in a temporally constant profile. If Uranus' winds vary with time, this may indicate unusual atmospheric dynamics created by the 98° tilt of the planet's rotation axis.

We present phase curves of Jupiter from 0-140 degrees as measured in multiple optical bandpasses by Cassini/ISS during the Millennium flyby of Jupiter in late 2000 to early 2001. Phase curves are of interest for studying the energy balance of Jupiter and understanding the scattering behavior of Jupiter as an exoplanet analog. We find that Jupiter is significantly darker at partial phases than an idealized Lambertian planet by roughly 25% and is not well fit by Jupiter-like exoplanet atmospheric models across all wavelengths. We provide analytic fits to Jupiter's phase function in several Cassini/ISS imaging filter bandpasses. In addition, these observations show that Jupiter's color is more variable with phase angle than predicted by models. Therefore, the color of even a near Jupiter-twin planet observed at a partial phase cannot be assumed to be comparable to that of Jupiter at full phase. We discuss how WFIRST and other future direct-imaging missions can enhance the study of cool giants., Comment: 29 pages, 9 figures, accepted to AJ

Limits on the physical properties of the scattering haze near the top of Titan's atmosphere are derived from data obtained from seven high-phase-angle images from Voyager 1 and 2. From the ratio of the intensities observed at two different high phase angles, an estimate can be made of the forward-scattering lobe of the single-scattering phase function. Comparing the forward-scattering estimate with diffraction lobes from particles of different radii, it is concluded that the average radius of the particles found in the upper few tenths of an optical depth exceeds 0.19 micron. Judging from data observed at four different phase angles, the haze particles probably have a refractive index near 1.6 and a mean size of about 0.5 micron, if the widths of their diffraction peaks are close to those for equal-volume spheres. However, the highly polarizing nature of the particles over a broad wavelength-bandpass (Tomasko and Smith, 1982; West et al, 1983) combined with their forward-scattering behavior makes it very unlikely that the particles are spherical. The nonsphericity contributes to the uncertainty about the radii of the particles, but it is thought that the average radius is several tenths of a micron.