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

TL;DR

This study used high-quality adaptive optics imaging from Keck and Gemini telescopes between 2012-2014 to uncover new cloud patterns, measure wind speeds, and analyze seasonal activity variations on Uranus.

Contribution

It provides the first detailed wind measurements and cloud pattern observations of Uranus during this period, revealing a solid body rotation region and seasonal asymmetries.

Findings

Discovered new low-contrast cloud features and large-scale patterns.

Measured wind speeds indicating a solid body rotation at high northern latitudes.

Observed increased atmospheric activity and persistent cloud features over two years.

Abstract

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. 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. 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\deg N). Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. These revealed an extended region of solid body rotation between 62\deg N and at least 83\deg N, at a rate of 4.08$\pm0.015$\deg/h westward relative to the planet's interior (radio) rotation of 20.88\deg/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\deg/h and 0.1\deg/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. While winds at high southern latitudes (50\deg S - 90\deg S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by 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). Over the 2012-2014 period we identified six persistent discrete features. Three were tracked for more than two years, two more for more than one year, and one for at least 5 months and continuing.