Latitudinal Variations in Uranus' Vertical Cloud Structure from UKIRT UIST Observations

Abstract

Prior to Uranus' northern spring equinox in 2007 December, near-infrared spectra and images of Uranus were obtained in 2006 August and September using the UIST instrument on the United Kingdom Infrared Telescope (UKIRT). Latitudinally resolved spectra were recorded between 1 and 2.5 μm at a resolving power varying between 550 and 2000 with the instrument in long slit mode and with the slit aligned with the planet's central meridian to determine the north-south variations of Uranus' cloud structure. Our observations appear to be the first latitudinally resolved complete (1-2.5 μm) near-IR spectra of Uranus ever recorded, and we present initial determinations of the latitudinal variation of Uranus' vertical cloud structure from these data. We find two main cloud decks of similar density, one based near the 2 bar level and one based in the 8-10 bar region. The upper cloud is found to extend from 50° south to 45° north, increasing in thickness and altitude toward the south, especially in the south circumpolar collar at 45° south, but clearing toward the poles. However, we find that the deeper cloud layer is thickest at the equator and thins symmetrically toward both poles. We also report the first-ever observation that the bright south circumpolar collar is, at some wavelengths, actually darker than other latitudes and provide an explanation of this phenomenon in terms of the latitudinal variations in cloud structure.