Stellar Occultation Observations of Saturn's North-Polar Temperature Structure

Abstract

We have observed a stellar occultation of GSC5249-01240 by Saturn's north polar region on November 20, 1995 from NASA's Infrared Telescope Facility (IRTF). This is the first recorded occultation by the polar region of a giant planet. The occulted region extends 88 km in vertical height and 660 km in horizontal length, over a region from 82.5° to 85° in planetocentric latitude and from 20° to 30° in planetocentric longitude. Based on isothermal model fits to the light curve, we find an equivalent isothermal temperature of 130 ± 10 K at a pressure level of 1.6 ± 0.1 μbar, which corresponds to a half-light latitude of 83.2 ± 0.2° and longitude of 24.1 ± 0.5°. Using numerical inversion procedures, we have retrieved the temperature profile of the occulted region, which suggests an increase in temperature (with radius) of 14.5 K between 6 and 10 μbar. We also find temperature fluctuations of 1 to 5 K along the path probed by the occultation; if the observed temperature gradients of these fluctuations apply to the vertical direction only, then this region is super-adiabatic. More likely, these thermal gradients are due to a combination of diffractive scintillations and horizontal temperature variations. Given that isothermal model fits and numerical inversions cannot separate individual contributions to observed temperature gradients, such as from vertical variations, horizontal variations, and scintillations, this occultation requires further study.