Evidence of a Polar Cyclone on Uranus From VLA Observations.

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. Plain Language Summary: New radio telescope observations of Uranus several interesting features, including a compact feature at the center of the North Pole which appears warmer than its surroundings. This feature likely indicates the presence of a polar cyclone and shows similarities to polar features observed on other giant planets in the solar system. Key Points: Very Large Array observations in 2021 and 2022 reveal a bright, compact spot centered at Uranus' pole at several wavelengthsConstraints on horizontal temperature and composition gradients necessary to explain the polar emission structure are derivedInferred patterns in temperature, zonal wind speed and trace gas variations are consistent with the presence of a compact cyclonic vortex [ABSTRACT FROM AUTHOR]