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Multilayer hazes over Saturn’s hexagon

Multilayer hazes over Saturn’s hexagon

Saturn’s hexagon is a distinct and long-lasting wave. It was discovered in the upper cloud level by the Voyager 1 and 2 spacecraft during their 1981 flybys of the planet. It has been photographed since 1991 with the Hubble Space Telescope (HST) and ground-based observatories, as well as between 2006 and 2017, with the Cassini Imaging Science Subsystem (ISS) and Visual and Infrared Mapping Spectrometer (VIMS).

The Cassini images have enabled us to discover that, just like a sandwich, the hexagon has a multi-layered system of at least seven mists that extend from the summit of its clouds to an altitude of more than 300 km above them

The Cassini spacecraft took pictures of the planet’s limb in June 2015 with a high spatial resolution of 1–2 km/pixel, encompassing a broad wavelength range from 225 to 950 nm and including methane absorption bands. These pictures indicated the presence of at least six stacked haze layers above the top cloud deck, south of the hexagon. The presence of such hazes, as well as knowledge of their physical features, can give crucial new insights to our understanding of Saturn’s atmosphere, particularly the hexagonal wave and its interaction with upper layers of the atmosphere.

Fig. 1
This graph shows the absolute reflectivity (I/F times 1000) as a function of the vertical distance across the limb in the 890-nm methane absorption band filter. The inset shows an image of the haze layers in this filter and their identification (L0–L7) in the reflectivity scan across Saturn’s limb. The haze layers extend meridionally at least from latitude +75.8°N to +77.1°N.

Their nature is consistent with their production at higher altitudes by condensation of hydrocarbon ices like as acetylene and benzene. Individual layers ranged in vertical thickness from 7 to 18 km, and they extended in height beyond the pressure level of 0.5 bar up to 0.01 bar (130 km in vertical extent), with a thin but prolonged aerosol layer above them up to an altitude of 340 km (pressure 0.4 mbar).

The researchers also looked at the regularity of the hazes’ vertical distribution. The proposed idea is that the hazes are organized by the vertical propagation of gravity waves, which cause oscillations in the density and temperature of the atmosphere, a well-known occurrence on Earth and other planets. The researchers speculate that the creation of these gravity waves might be caused by the dynamics of the hexagon itself and its strong jet stream. On Earth, waves of this sort have been observed created by the undulating jet stream traveling at velocities of 100 km/h from West to East in the mid-latitudes.

Although Saturn’s characteristics suggest that it is the sole example in the solar system, the phenomena might be comparable on both planets. This is an area that will require further investigation in the future.

Multilayer hazes over Saturn’s hexagon from Cassini ISS limb images, A. Sánchez-Lavega, A. García-Muñoz, T. del Río-Gaztelurrutia, S. Pérez-Hoyos, J. F. Sanz-Requena, R. Hueso, S. Guerlet & J. Peralta

Published: May, 2020

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