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Study tracks “rain” from Saturn’s rings

April 11, 2013
Courtesy ofthe Jet Propulsion Laboratory
and World Science staff

Sat­urn’s rings give off a form of rain that falls on to the plan­et be­low, cov­er­ing larg­er ar­eas than pre­vi­ously thought, new re­search finds.

Based on NASA-funded ob­serva­t­ions and anal­y­sis led by the Uni­vers­ity of Leices­ter, U.K., the study found that the rain af­fects the make­up and tem­pe­r­a­ture struc­ture of parts of Sat­urn’s up­pe­r at­mos­phere.

This di­a­gram il­lus­trates how charged wa­ter par­t­i­cles are thought to flow in­to the Sa­tur­ni­an at­mos­phere from the rings, re­duc­ing at­mos­pher­ic bright­ness. (Cred­it: NA­SA/JPL-Caltech/Space Sci­ence In­st./U. of Leices­ter)


“Sat­urn is the first plan­et to show sig­nif­i­cant in­ter­ac­tion be­tween its at­mos­phere and ring sys­tem,” said James O’Dono­ghue, a post­grad­u­ate re­searcher at Leices­ter and lead au­thor of a pa­pe­r on the find­ings in this week’s is­sue of the jour­nal Na­ture.

He added that the rain’s main ef­fect is to re­duce the num­ber of elec­trons, or charged sub­a­tom­ic par­t­i­cles, per a giv­en amount of space in Sat­urn’s ion­o­sphere, or up­pe­r at­mos­phere. That ex­plains why those elec­tron lev­els are meas­ured to be un­usu­ally low at some Sa­tur­ni­an lat­i­tudes, he added; the work al­so helps clar­i­fy the de­vel­op­ment of the ring sys­tem and changes in the plan­et’s at­mos­phere.

“It turns out that a ma­jor driv­er of Sat­urn’s ion­o­spheric en­vi­ron­ment and cli­mate across vast reaches of the plan­et are ring par­t­i­cles lo­cat­ed some 36,000 miles [60,000 kilo­me­ters] over­head,” ex­plained Kev­in Baines, a co-au­thor on the pa­pe­r, based at NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry, in Pas­a­de­na, Ca­lif. “The ring par­t­i­cles af­fect both what spe­cies of par­t­i­cles are in this part of the at­mos­phere and where it is warm or cool.”

In the early 1980s, im­ages from NASA’s Voy­ag­er space­craft showed two to three dark bands on Sat­urn. Sci­en­tists the­o­rized that wa­ter could have been show­er­ing down in­to those bands from the rings. The hard-to-discern bands weren’t seen again un­til the team con­duct­ing the new work looked at Sat­urn in near-in­fra­red light with the Keck Observato­ry on Mauna Kea, in Ha­waii, in April 2011.

In Sat­urn’s ion­o­sphere, charged par­t­i­cles form when the oth­er­wise neu­tral at­mos­phere is ex­posed to a flow of en­er­get­ic par­t­i­cles from the Sun. When the sci­en­tists tracked the pat­tern of emis­sions from a charged mol­e­cule called tri­a­tom­ic hy­dro­gen, they ex­pected to see a un­iform plan­et-wide glow of in­fra­red light. In­stead they saw a se­ries of “light” and “dark” bands in this form of light, which is too low-energy to be seen by the un­aided eye. Dark ar­eas cor­re­sponded to wa­ter-rich parts of Sat­urn’s rings and light ones to gaps in the rings.

The sci­en­tists sur­mised that charged wa­ter par­t­i­cles from the plan­et’s rings were be­ing drawn to­wards the plan­et along Sat­urn’s mag­net­ic field lines and were neu­tralizing the glow­ing tri­a­tom­ic hy­dro­gen. This leaves “shad­ows” in what would oth­er­wise be a plan­et-wide glow. These shad­ows cov­er an es­ti­mat­ed 30 to 43 pe­rcent of the plan­et’s up­pe­r at­mos­phere sur­face from around 25 to 55 de­grees lat­i­tude—a good deal more ar­ea than that sug­gested by Voy­ag­er im­ages.

Both Earth and Ju­pi­ter have un­iformly glow­ing equa­to­ri­al re­gions. Sci­en­tists ex­pected the same at Sat­urn, but they in­stead saw dra­mat­ic dif­fer­ences at dif­fer­ent lat­i­tudes.

“Sat­urn has dark bands where the wa­ter is fall­ing in, dark­en­ing the ion­o­sphere,” said Tom Stal­lard, a pa­pe­r co-au­thor at Leices­ter. “We’re now al­so try­ing to in­ves­t­i­gate these fea­tures with an in­stru­ment on NASA’s Cas­si­ni space­craft. If we’re suc­cess­ful, Cas­si­ni may al­low us to view in more de­tail the way that wa­ter is re­mov­ing ion­ized [charged] par­t­i­cles, such as any changes in the al­ti­tude or ef­fects that come with the time of day.”


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Saturn’s rings give off a form of rain that falls on to the planet below, covering larger areas than previously thought, new research finds. Based on NASA-funded observations and analysis led by the University of Leicester, U.K., the study found that the rain affects the makeup and temperature structure of parts of Saturn’s upper atmosphere. “Saturn is the first planet to show significant interaction between its atmosphere and ring system,” said James O’Donoghue, a postgraduate researcher at Leicester and lead author of a paper on the findings in this week’s issue of the journal Nature. He added that the rain’s main effect is to reduce the number of electrons, or charged subatomic particles, per a given amount of space in Saturn’s ionosphere, or upper atmosphere. That explains why those electron levels are measured to be unusually low at some Saturnian latitudes, he added; the work also helps clarify the development of the ring system and changes in the planet’s atmosphere. “It turns out that a major driver of Saturn’s ionospheric environment and climate across vast reaches of the planet are ring particles located some 36,000 miles [60,000 kilometers] overhead,” explained Kevin Baines, a co-author on the paper, based at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif. “The ring particles affect both what species of particles are in this part of the atmosphere and where it is warm or cool.” In the early 1980s, images from NASA’s Voyager spacecraft showed two to three dark bands on Saturn. Scientists theorized that water could have been showering down into those bands from the rings. The hard-to-discern bands weren’t seen again until the team conducting the new work looked at Saturn in near-infrared light with the Keck Observatory on Mauna Kea, in Hawaii, in April 2011. In Saturn’s ionosphere, charged particles form when the otherwise neutral atmosphere is exposed to a flow of energetic particles from the Sun. When the scientists tracked the pattern of emissions from an unusual, charged molecule called triatomic hydrogen, they expected to see a uniform planet-wide glow of infrared light. Instead they saw a series of “light” and “dark” bands in this form of light, which is too low-energy to be seen by the unaided eye. Dark areas corresponded to water-rich parts of Saturn’s rings and light ones to gaps in the rings. The scientists surmised that charged water particles from the planet’s rings were being drawn towards the planet along Saturn’s magnetic field lines and were neutralizing the glowing triatomic hydrogen. This leaves “shadows” in what would otherwise be a planet-wide glow. These shadows cover an estimated 30 to 43 percent of the planet’s upper atmosphere surface from around 25 to 55 degrees latitude—a good deal more area than that suggested by Voyager images. Both Earth and Jupiter have uniformly glowing equatorial regions. Scientists expected the same at Saturn, but they instead saw dramatic differences at different latitudes. “Saturn has dark bands where the water is falling in, darkening the ionosphere,” said Tom Stallard, a paper co-author at Leicester. “We’re now also trying to investigate these features with an instrument on NASA’s Cassini spacecraft. If we’re successful, Cassini may allow us to view in more detail the way that water is removing ionized [charged] particles, such as any changes in the altitude or effects that come with the time of day.”