Monday, December 3, 2007

Saturn si al sau hexagon


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