Saturn’s rings

Sat­urn sits high in the south right now. The best time to look is around 11 p.m. when the planet is high­est in the sky and less affected by the tur­bu­lent motion of Earth’s atmos­phere. Look for a pale yel­low point of light much brighter than any of the fainter stars that sur­round it.

If humans ever travel reg­u­larly among the plan­ets, the top tourist attrac­tion in the solar sys­tem will surely be Saturn’s rings. Even a small department-store tele­scope reveals them, even though the planet is cur­rently 800 mil­lion miles away from Earth.

We can see them from Earth because of their size and reflec­tiv­ity. The rings are huge — 200,000 miles from side to side. Sat­urn would just fit in the space between Earth and its moon. The rings are also exceed­ingly shiny because they are com­posed of a thin plane of a bil­lion tril­lion hunks of ice and ice and rock mixed together. Some of the hunks are the size of grains of sand and oth­ers as big as icebergs.

The rings are thus not really rings at all but a vast col­lec­tion of moons orbit­ing Sat­urn and arranged in over a thou­sand con­cen­tric bands.

All the gigan­tic gas plan­ets — Jupiter, Uranus, Nep­tune, and Sat­urn — have some sort of ring sys­tem, but the oth­ers are pale and puny shad­ows of Saturn’s dis­play. Jupiter’s ring of dusty mate­r­ial is barely there. Uranus has a dozen or so dark and nar­row bands. Neptune’s few rings are dense over parts of their arcs and nearly invis­i­ble elsewhere.

Astronomers don’t really under­stand the dynam­ics of plan­e­tary rings very well. Why, for exam­ple, do they stay in dis­creet bands instead of spread­ing out?

A pos­si­ble answer is the exis­tence of moons near some of the rings. These so-called “shep­herd moons” may pro­vide enough grav­ity to keep the rings cor­ralled. The trou­ble is that some of the rings don’t seem to have shep­herds — at least we can’t see any. How­ever, our lack of vision is not sur­pris­ing. The shep­herds are so tiny that they are invis­i­ble to Earth-based tele­scopes. They had to be dis­cov­ered by pass­ing space­craft, and per­haps even those space­craft don’t have the resolv­ing power to see the smaller ones.

The ques­tion that has plagued astronomers since the dis­cov­ery of Saturn’s rings still puz­zles them today. Why do the gas plan­ets have rings at all? Some of the rings, the ones you can¹t se in a tele­scope, were formed by ice gey­sers on some of the moons of Sat­urn. Oth­ers were formed by the con­stant bom­bard­ment of microm­e­te­orites on those moons. These meth­ods form nearly invis­i­ble rings around the other gas giant plan­ets. But Sat­urn is clearly spe­cial. Its rings are huge and eas­ily vis­i­ble. What gives?

One the­ory sug­gests that the larger bod­ies, like plan­ets and moons, formed from smaller par­ti­cles that clumped together. The rings are com­posed of those small par­ti­cles. They could not form into moons because their prox­im­ity to the enor­mous grav­ity of the nearby gas planet pre­vented them from coalescing.

Another pos­si­bil­ity is that the rings are frag­ments of icy moons that formed far­ther away from the gas plan­ets. If these moons strayed too close to their plan­e­tary brethren, they would have been torn to pieces, and the debris would have been stretched into ring-like cir­cles around the plan­ets. A third alter­na­tive is that the rings were cre­ated by col­li­sions of two moons that had already formed. Or per­haps, in an even more likely sce­nario, a pass­ing aster­oid slammed into one of Saturn’s moon. The icy and rocky frag­ments were then drawn into rings by the grav­ity of the planet. If such cos­mic train wrecks occurred in the dis­tant past, a planet’s grav­i­ta­tional pull would even­tu­ally drag most of the frag­ments into a fiery col­li­sion with the planet’s atmos­phere. Slowly, the rings would dis­ap­pear. Plan­ets like Jupiter might have had exten­sive ring sys­tems bil­lions of years ago, but time has reduced them to the rem­nants we see today.

Saturn’s impres­sive rings would be the result of more recent col­li­sions, per­haps as late as 100 mil­lion years ago. Given enough time, they will slowly dis­ap­pear. In a bil­lion years or so, Saturn’s rings may look more like Jupiter’s, and our solar system’s most impres­sive tourist attrac­tion will fade from view.

Tom Burns is the direc­tor of Ohio Wes­leyan University’s Perkins Obser­va­tory, and he would be very happy to answer your ques­tions or sell you a ticket to one of its upcom­ing Friday-night pro­grams. He can be reached at tlburns@owu.edu or 740–363-1257.

Posted by Tom Burns on Apr 15 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.