Haze and cloud structure of Saturn's North Pole and Hexagon Wave from Cassini/ISS imaging

Abstract

In this paper we present a study of the vertical haze and cloud structure in the upper two bars of Saturn's Northern Polar atmosphere using the Imaging Science Subsystem (ISS) instrument onboard the Cassini spacecraft. We focus on the characterization of latitudes from 53º to 90º N. The observations were taken during June 2013 with five different filters (VIO, BL1, MT2, CB2 and MT3) covering spectral range from the 420 nm to 890 nm (in a deep methane absorption band). Absolute reflectivity measurements of seven selected regions at all wavelengths and several illumination and observation geometries are compared with the values produced by a radiative transfer model. The changes in reflectivity at these latitudes are mostly attributed to changes in the tropospheric haze. This includes the haze base height (from 600±200 mbar at the lowest latitudes to 1000±300 mbar in the pole), its particle number density (from 20±2 particles/cm 3 to 2±0.5 particles/cm 3 at the haze base) and its scale height (from 18±0.1 km to 50±0.1 km). We also report variability in the retrieved particle size distribution and refractive indices. We find that the Hexagonal Wave dichotomizes the studied stratospheric and tropospheric hazes between the outer, equatorward regions and the inner, Polar Regions.

Saturn's Atmosphere at 1–10 Kilometer Resolution

“…Sanz‐Requena et al () used VIO, BL1, MT2, CB2, and MT3 images from 26 June 2013 to study haze and cloud heights around Saturn's north pole. They use a full radiative transfer model, which is beyond the scope of the present paper largely because we have only broad‐band CB2 and CLR filters and little or no angular coverage.…”

“…The greatest sensitivity to cloud properties comes where the methane filters have intermediate optical depths ‐ where the transmission to space is around 0.5, for example. Sanz‐Requena et al () estimate that the MT2 and MT3 filters have greatest sensitivity to cloud properties at altitudes between 60 mbar and 250 mbar, and they cite West et al () and Garcia‐Melendo et al (, ) for earlier estimates. They also state that the CB2 filter is sensitive to an intermediate region of tropospheric haze at 350–700 mbar as well as the region down to the tropospheric cloud deck at 1.4 bars.…”

Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

“…Under hydrostatic and geostrophic balance, the vertical structure of the horizontal wind field and the horizontal temperature gradient are related by the thermal wind equation: where y is the meridional coordinate. In this study, we have assumed that the cloud level at which winds have been measured corresponds to a pressure level of 500 mbar (Sanz‐Requena et al, ), which corresponds to a zonally averaged isentropic surface of θ ~137 K. In order to evaluate the zonal wind profile with the same latitudinal resolution as the thermal structure and obtain smooth distributions, we have binned the zonal wind profile in 0.5° latitude, and then, we have used data separated by 2° latitude. We have then used the wind values at this level as a boundary condition for integration of the thermal wind equation.…”

“…In this study, we compute zonally averaged quasi‐geostrophic potential vorticity maps of both Saturn's polar regions at latitudes higher than 68° planetographic and heights between the ammonia cloud tops (~500 mbar; Sanz‐Requena et al, ), where winds have been measured, and 1‐mbar pressure levels. In our maps, we combine wind profiles measured from Cassini ISS by cloud tracking (Antuñano et al, ; García‐Melendo et al, ; Sánchez‐Lavega et al, ), with high‐resolution zonally averaged temperature profiles retrieved from Cassini CIRS data (Fletcher et al, ).…”

Thermal Emission From Saturn's Polar Cyclones

“…The south polar vortex has two distinct eyewalls, at 89.1° S and 88.3° S, with cloud altitudes changing abruptly by 40 and 70 km, respectively (Dyudina et al, ). The north polar vortex has a more complicated, and temporally variable, cloud structure, with a relatively bright region of spiraling clouds between 89.3°N and 89.7°N surrounded by a dark ring extending to 89.1°N (Antuñano et al, ; Sanz‐Requena et al, ; Sayanagi et al, ). Images in methane band filters, which are sensitive to the amount of upper tropospheric haze, show a dark spot at both poles extending to 88.7°, indicating a hole in the tropospheric haze (Sayanagi et al, ).…”