Evidence of a Polar Cyclone on Uranus From VLA Observations

Abstract

We present observations of Uranus in northern spring with the Very Large Array from 0.7 to 5 cm. These observations reveal details in thermal emission from Uranus' north pole at 10s of bars, including a dark collar near 80°N and a bright spot at the polar center. The bright central spot resembles observations of polar emission on Saturn and Neptune at shallower pressures. We constrain the variations in temperature and NH3/H2S abundances which could explain these features. We find that the brightness temperature of the polar spot can be recreated through 5 K temperature gradients and/or 10× depletion of NH3 or H2S vapor between 10 and 20 bars, both consistent with the presence of a cyclonic polar vortex. The contrast of the polar spot may have increased since 2015, which would suggest seasonal evolution of Uranus' polar circulation at depth.