High S/N Keck and Gemini AO imaging of Uranus during 2012-2014: New cloud patterns, increasing activity, and improved wind measurements

摘要

We imaged Uranus in the near infrared from 2012 into 2014, using the Keck/NIRC2 camera and Gemini/NIRI camera, both with adaptive optics. We obtained exceptional signal to noise ratios by averaging 8-16 individual exposures in a planet-fixed coordinate system. These noise-reduced images revealed many low-contrast discrete features and large scale cloud patterns not seen before, including scalloped waveforms just south of the equator, and an associated transverse ribbon wave near 6 degrees S. In all three years numerous small (600-700 km wide) and mainly bright discrete features were seen within the north polar region (north of about 55 degrees N). Two small dark spots with bright companions were seen at middle latitudes. Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. Winds at high latitudes were measured with great precision, revealing an extended region of solid body rotation between 62 degrees N and at least 83 degrees N, at a rate of 4.08 +/- 0.015 degrees/h westward relative to the planet's interior (radio) rotation of 20.88 degrees/h westward. Near-equatorial speeds measured with high accuracy give different results for waves and small discrete features, with eastward drift rates of 0.4 degrees/h and 0.1 degrees/h respectively. The region of polar solid body rotation is a close match to the region of small-scale polar cloud features, suggesting a dynamical relationship. The winds from prior years and those from 2012-2014 are consistent with a mainly symmetric wind profile up to middle latitudes, with a small asymmetric component of similar to 0.09 degrees/h peaking near +/- 30 degrees, and about 60% greater amplitude if only prior years are included, suggesting a declining mid-latitude asymmetry. While winds at high southern latitudes (50-90 degrees S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by Karkoschka (Karkoschka [2015]. Icarus 250, 294-307) has revealed an extremely large north-south asymmetry in this region, which might be seasonal. Greatly increased activity was seen in 2014, including the brightest ever feature seen in K' images (de Pater et al. [2015]. Icarus 252, 121-128), as well as other significant features, some of which had long lives. Over the 2012-2014 period we identified six persistent discrete features. Three were tracked for more than 2 years, two more for more than I year, and one for at least 5 months and continuing. Several drifted in latitude towards the equator, and others appeared to exhibit latitudinal oscillations with long periods. We found two pairs of long-lived features that survived multiple passages within their own diameters of each other. Zonally averaged cloud patterns were found to persist over 2012-2014. When averaged over longitude, there is a brightness variation with latitude from 55 degrees N to the pole that is similar to effective methane mixing ratio variations with latitude derived from 2012 STIS observations (Sromovsky et al. [2014]. Icarus 238, 137-155). (C) 2015 Elsevier Inc. All rights reserved.