Post-equinox dynamics and polar cloud structure on Uranus

Abstract

Post equinox imaging of Uranus by HST, Keck, and Gemini telescopes has enabled new measurements of winds over previously sampled latitudes as well as measurements at high northern latitudes that have recently come into better view. These new observations also used techniques to greatly improve signal to noise ratios, making possible the detection and tracking of more subtle cloud features. The 250 m/s prograde jet peaking near 60• N was confirmed and more accurately characterized. Several long-lived cloud features have also been tracked. The winds pole-ward of 60• N are consistent with solid body rotation at a westward (prograde) rate of 4.3• /h with respect to Uranus' interior. When combined with 2007 and other recent measurements, it is clear that a small but well-resolved asymmetry exists in the zonal profile at middle latitudes, peaking at 35• , where southern winds are 20 m/s more westward than corresponding northern winds. High S/N Keck II imaging of the north polar region of Uranus reveals a transition from streaky bands below 60• N to a region from 60• to nearly the north pole, where widely distributed small bright spots, resembling cumulus cloud fields, with several isolated dark spots, are the dominant style of cloud features. This presents a stark contrast to 2003 detailed views of the south polar region of Uranus when no discrete cloud features could be detected in comparable Keck II near-IR images. The pressure levels of discrete clouds estimated from spatial modulations in H and Hcont images indicate that the polar cloud features are generally in the 1.3 to 2-3 bar range, as are equatorial and several mid-latitude features. Several of the brighter mid latitude features are found above the 1.2-bar level of methane condensation.

Thermal imaging of Uranus: Upper-tropospheric temperatures one season after Voyager

“…We also note that neither the increase of dynamical activity noted recently in Uranus (e.g. Sromovsky et al 2012;de Pater et al 2015) nor longer--term changes noted by Irwin et al (2012) can be attributed to large--scale changes of temperature field, at least at the resolution of our measurements. The ultimate test of the meridional variability of TE could be addressed by a spacecraft that could determine not only the full bolometric output of the atmosphere as a function of latitude -including the long wavelengths at the peak of the Planck function that the Voyager--2 IRIS experiment did not sample -but also the meridional variability of the Bond albedo -including an estimate of differences in the phase functions characteristic of each latitude.…”

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

“…The Voyager Ultraviolet Spectrometer (UVS) observations (e.g., Broadfoot et al, 1986;Herbert et al, 1987;Yelle et al, 1987Yelle et al, , 1989Bishop et al, 1990) first made it clear that K zz was comparatively small on Uranus, at least at summer southern solstice at the time of the Voyager encounters. Our Spitzer observations confirm that stratospheric K zz values are still low on Uranus in 2007, despite several other observations that suggest that stronger tropospheric convective activity or other dynamical changes on Uranus may have occurred from the 1986 Voyager encounter to at least the 2007 equinox over a wide range of latitudes (e.g., Karkoschka, 2001;Rages et al, 2004;Klein and Hofstadter 2006;Hammel and Lockwood 2007;Norwood and Chanover 2009;Irwin et al, 2008Irwin et al, , 2011Irwin et al, , 2012Sromovsky et al, 2009;Sromovsky et al, 2012a;Sromovsky et al, 2012b).…”

OBSERVED VARIABILITY AT 1 and 4 μm IN THE Y0 BROWN DWARF WISEP J173835.52+273258.9

“…The variability in this case is predominantly due to Jupiter's Great Red Spot. Sromovsky et al (2012) present observations of episodic bright and dark spots on Uranus. One and then two bright spots were seen on the planet's surface in 2011, drifting at very different rates, and …”