Hexagon found around Saturn's north pole.

Hexagon found around Saturn's north pole 

The bands, whorls and wavy lines decorating the atmosphere of Saturn have fascinated planetary scientists since the Voyager 1 and 2 spacecraft flew by in 1980 and 1981. Even now, study of the photos they took continues to reveal surprises. One striking example is a gigantic hexagon, about a fourth the diameter of Saturn itself, on the clouds around the planet's north pole.

Neither Voyager craft flew over the pole, so their photos were not taken from a down-looking viewpoint that would reveal the hexagon at a glance. Instead, the cameras faced Saturn's lower latitudes, where the hexagon was not immediately apparent. "A full view of the complete polar region was obtained by making projections of several images taken as the planet rotated and mosaicking them together," reports David Alan Godfrey of the National Optical Astronomy Observations in Tucson, Ariz.

Strictly speaking, Godfrey notes in the November ICARUS, it is not so much a hexagon as an atmospheric feature consisting of a wave pattern that is repeated six times around the planet, ranging between a few degrees above and below 76[deg.]N latitude.

A number of researchers have shown interest in the feature. "It's got to be high in the atmosphere, a pattern that forms from a coupling between circulation and rotation," says Reta Beebe of New Mexico State University in Las Cruces. "It appears to be in a region of stratus cloud -- above the region where there is upward mixing."

"Earth has a similar wave pattern with three cycles in it instead of six," notes Michael Allison of the Goddard Institute for Space Studies in New York City. "But because the waves diffuse so rapidly to the north and south, there is not the appearance of a pole-centered triangle. Those disturbances develop into our major weather patterns."

The meteorology is complex, Allison says, but its implications are significant. "The hexagonal cloud feature may exist as a 'stationery wave,' driven by the perturbation caused by the anticyclonic oval which resides at the same latitude [see left photo]." The hexagon, he suggests, may be "a planet-encircling wave, maintained by the sharp latitudinal variation in the 100-meter-per-second eastward jet in which it is embedded. With a longitudinal scale corresponding to a pattern of six repetitions -- which is what we see -- we calculate that the wave has a westward drift relative to the eastward flow of the background wind, thereby holding the hexagon in a fixed longitudinal position on the planet."

That the hexagon remains at a fixed longitude also means it rotates at about the same rate as Saturn itself, as defined by the rotation of the planet's radio emissions, although the connection between the two phenomena is unclear. "Although several uncertainties remain," Allison says, "the clearly defined horizontal scale and the implication of the hexagon's longitudinally fixed position make it a wonderful template for understanding the vertical structure of the atmosphere beneath it."