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Hexagon on Saturn mystifies astronomers

March 27, 2007
Courtesy NASA
and World Science staff

An odd, six-sided, honeycomb-shaped fea­ture en­cir­cling Sat­urn’s north pole has cap­tured the in­ter­est of sci­en­tists with NASA’s Cas­si­ni space­craft mis­sion. 

This night­time view of Sat­urn's north pole shows a bi­zarre six-sided hex­a­gon fea­ture en­cir­cling the north pole. (Cour­te­sy: NASA/J­PL/U­ni­ver of Ar­i­zo­na)

The agency’s Voy­ag­er 1 and 2 space­craft im­aged the hex­a­gon over two dec­ades ago, but its ap­pear­ance in new Cas­si­ni im­ages shows it’s long lived, height­en­ing the rid­dle, sci­en­tists said.

“We’ve nev­er seen an­y­thing like this on any oth­er plan­et,” said Kev­in Baines of the Cas­si­ni team at NASA’s Je­t Pro­pul­sion Lab­o­ra­to­ry in Pas­a­de­na, Ca­lif. In­deed, Sat­urn’s thick at­mos­phere “is per­haps the last place you’d ex­pect to see such a six-sided ge­o­met­ric fig­ure, yet there it is.”

Nor­mal­ly, in the ringed plan­et’s at­mos­phere, cir­cle-shaped waves and for­ma­tions called con­vec­tive cells pre­dom­i­nate, he said. A con­vec­tive cell is a pat­tern cre­at­ed by an up­draft of warmed flu­id and sink­ing of cooled flu­id.

A sec­ond hex­a­gon, sig­nif­i­cantly darker than the brighter his­tor­i­cal fea­ture, is al­so seen in the Cas­si­ni pic­tures. 

An in­stru­ment on the craft, called the vis­u­al and in­fra­red map­ping spec­trom­e­ter, is the first to cap­ture the whole hex­a­gon in one im­age, in­ves­ti­ga­tors said. “This is a very strange fea­ture, ly­ing in a pre­cise ge­o­met­ric fash­ion with six near­ly equal­ly straight sides,” said Baines, an at­mos­pher­ic ex­pert who is part of the team man­ning the in­stru­ment.

The hex­a­gon is si­m­i­lar to Earth’s po­lar vor­tex, which has winds blow­ing in a cir­cu­lar pat­tern around the po­lar re­gion, re­search­ers said. But on Sat­urn, the vor­tex is hex­a­gonal rath­er than cir­cu­lar. The hex­a­gon is near­ly 25,000 kilo­me­ters (15,000 miles) across. Near­ly four Earths could fit in­side it. 

The new im­ages are tak­en in in­fra­red light, a lower-energy form of light than the vis­i­ble type. These im­ages show the hex­a­gon ex­tends much deeper down in­to the at­mos­phere than pre­vi­ously ex­pected, some 100 kilo­me­ters (60 miles) be­low the cloud tops, ac­cord­ing to sci­en­tists. The hex­a­gon, they added, con­tains clouds that seem to be whip­ping around the hex­a­gon like cars on a race­track.

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An odd, six-sided, honeycomb-shaped feature encircling Saturn’s north pole has captured the interest of scientists with NASA’s Cassini spacecraft mission. NASA’s Voyager 1 and 2 spacecraft imaged the hexagon over two decades ago, but its appearance in new Cassini images shows it’s long lived, heightening the riddle, scientists said. “We’ve never seen anything like this on any other planet,” said Kevin Baines of the Cassini team at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Indeed, Saturn’s thick atmosphere “is perhaps the last place you’d expect to see such a six-sided geometric figure, yet there it is.” Normally, on the ringed planet’s atmosphere, circularly-shaped waves and formations called convective cells predominate, he said. A convective cell is a pattern created by an updraft of warmed fluid and sinking of cooled fluid. A second hexagon, significantly darker than the brighter historical feature, is also seen in the Cassini pictures. An instrument on the craft, called the visual and infrared mapping spectrometer, is the first to capture the whole hexagon in one image, invest igators said. “This is a very strange feature, lying in a precise geometric fashion with six nearly equally straight sides,” said Baines an atmospheric expert who is part of the team manning the instrument. The hexagon is similar to Earth’s polar vortex, which has winds blowing in a circular pattern around the polar region, researchers said. But on Saturn, the vortex is hexagonal rather than circular. The hexagon is nearly 25,000 kilometers (15,000 miles) across. Nearly four Earths could fit inside it. The new images are taken in infrared light, a lower-energy form of light than the visible type. These images show the hexagon extends much deeper down into the atmosphere than previously expected, some 100 kilometers (60 miles) below the cloud tops, according to scientists. The hexagon, they added, contains clouds that seem to be whipping around the hexagon like cars on a racetrack.