Saturn’s ‘ab­so­lutely gar­gan­tuan’ ring

Los Angeles Times - - THE NATION - By Amina Khan amina.khan@la­

Saturn’s big­gest ring just broke its own record.

Sci­en­tists have found that the planet’s Phoebe ring is more than twice the thick­ness once thought — even though it can’t be seen with the hu­man eye.

The study, pub­lished Thurs­day in the jour­nal Na­ture, also found that the over­sized ring is made mostly of tiny par­ti­cles.

“I was giv­ing talks say­ing Saturn has a gi­ant in­vis­i­ble ring, which makes you sound just per­fectly crazy,” said lead au­thor Dou­glas Hamil­ton, a plan­e­tary sci­en­tist at the Uni­ver­sity of Mary­land, Col­lege Park. “It is 10 to 20 times larger than the sec­ond-big­gest ring, so this thing is ab­so­lutely gar­gan­tuan.

Many of Saturn’s rings seem to hug the planet, form­ing the beau­ti­ful striped disc for which the gas gi­ant is known.

But in 2009, NASA’s in­fra- red-sens­ing Spitzer Space Tele­scope picked up an­other ring so large that the en­tire planet and its main rings ap­pear as a small dot in the mid­dle.

The ring is dif­fi­cult to de­tect in vis­i­ble light (NASA’s Cassini space­craft did so a few years later), but it can be clearly seen as a ghostly halo at infrared wave­lengths.

Hamil­ton and col­leagues first went look­ing for this “gi­ant in­vis­i­ble ring” be­cause of the strange two-tone col­or­ing of Iape­tus, one of Sat- urn’s moons.

One side is pale, but the other side of the moon is dark, as if cov­ered with soot. The sci­en­tists the­o­rized that un­seen par­ti­cles from far­ther out were hit­ting Iape­tus and black­en­ing one side. (Iape­tus is tidally locked with Saturn and can’t ro­tate fast enough to get an all-around coat­ing.)

“If that ma­te­rial was com­ing in­ward and coat­ing the face of Iape­tus, then maybe we could see the ring,” Hamil­ton said, and the team started search­ing.

At the time, they es­ti­mated that the ring stretched from 4.8 mil­lion to 7.76 mil­lion miles, though with Spitzer’s rel­a­tively nar­row field of view, they couldn’t say how thick it was

Now, with NASA’s Wide­field Infrared Sur­vey Ex­plorer tele­scope, they have found that it stretches from about 3.75 mil­lion to 10.1 mil­lion miles — a lit­tle be­yond Phoebe, a moon that is the same color as the stuff in the ring, and is prob­a­bly the source for the dark ma­te­rial.

While the Phoebe ring is enor­mous, the par­ti­cles within are sur­pris­ingly tiny — the sci­en­tists es­ti­mate that the vast ma­jor­ity of them are smaller than the size of a soc­cer ball.

Un­der­stand­ing why the par­ti­cle size dis­tri­bu­tion is so skewed could of­fer re­searchers “a win­dow into the his­tory of the Saturn sys­tem,” Hamil­ton said.

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