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The Weird World of PHOEBE Scientists Are Finding That Saturn’S Strange Moon Isn’T Like the Other Satellites in the Solar System

The Weird World of PHOEBE Scientists Are Finding That Saturn’S Strange Moon Isn’T Like the Other Satellites in the Solar System

Planetary science The weird world of Scientists are finding that ’s strange isn’t like the other satellites in the . by Michael Carroll

The interiors of the and the Planetary evolution solar system’s largest seemed seemed so straightforward. The pro- predictable, too: Due to a phenom- cess began, so the story went, with a enon called differentiation, heavy ele- disk of dust and gas called the solar ments within such a world sank to nebula. At the center of this rotating become a core while lighter ones rose disk, a dense clump condensed and to form a less dense crust. The smaller became a star — the . It pulled in bodies, the ones with less and more material while its gravity grew. thus lower gravities like nuclei And just as an ice skater pulls her and smaller moons, carried out no arms in to spin faster, the disk’s rota- such settling and retained their well- tion speed began to increase. mixed interiors. Irregular clumps of gas and dust Scientists have long considered this that moved throughout the disk grew scenario their standard model. Then into , comet nuclei, and, ulti- came Saturn’s moon Phoebe. mately, planets. The rocky terrestrial planets — , , , and Surprisingly spherical — established themselves near As natural satellites go, Phoebe, with a the Sun; the infant star’s heat burned diameter of 132 miles (213 kilometers), away many of their (elements falls below the midrange size of more and compounds with low boiling familiar moons like or . points). Out where it was colder and This number is significant: Other sat- calmer, the bodies that would become ellites of similar sizes are not large Phoebe is an oddball moon of Saturn. and Saturn pulled enough for their weak gravities to pull Even though it’s small, it has a spherical and helium toward themselves from them into spherical shapes. They are shape and other -like characteristics. Scientists think volatiles breaking through the surrounding solar nebula. Farther probably cold clumps of rock and ice. the surface in Phoebe’s early history (as out still, and were Not so with Phoebe. The strange illustrated here) might be responsible for its rounded shape and craters. MICHAEL CARROLL able to hold onto water and eventually moon is round, leading scientists to FOR became the ice we see today. hypothesize that internal heat may

© 2014 Kalmbach Publishing Co. This materialWWW.ASTRONOMY.COM may not be reproduced in any29 form without permission from the publisher. www.Astronomy.com The Nice model Onset of Uranus Saturn

0° west longitude 90° west longitude

Jupiter Neptune Disk of

Just before scattering starts GEORGE GRANTHAM BAIN COLLECTION (LIBRARY OF CONGRESS) OF (LIBRARY COLLECTION BAIN GRANTHAM GEORGE William Henry Pickering discovered Phoebe — Saturn’s ninth satellite — in 1899 after studying photographs he took in August 1898.

180° west longitude 270° west longitude most likely idea is that it got warmed up so Although Phoebe has bumpy, irregular topography, it has a fairly spherical shape. Scientists created this the [surface] ice could melt and flow. The digitally rendered model of Phoebe’s shape using imaging data taken when NASA’s Cassini spacecraft shape is globally relaxed. That’s the most flew by the saturnian satellite June 11, 2004. NASA/JPL/SPACE SCIENCE INSTITUTE important observation, and it comes from analyzing photos from many angles.” Saturn’s moon has a low , giving it weak and high porosity. Scientists But smaller bodies hold heat for less would expect Phoebe to have a similar appearance due to its comparable size; Hyperion is about 220 miles have softened it early in its history. Phoebe’s to equilibrium; it had to be partially molten time. Where could Phoebe’s heat have come (360 kilometers) across its longest axis, while Phoebe is 132 miles (213km) wide. NASA/JPL/SPACE SCIENCE INSTITUTE Just after scattering starts density is also higher than typical icy satur- for this to happen.” from? Bonnie Buratti of JPL suggests one nian satellites. Both of these pieces of infor- When a planet settles, or relaxes, into a scenario: “Phoebe may have formed very mation suggest that the tiny satellite may be spheroid, it reaches gravitational equilib- early in the solar system, when there were through the surface. According to Buratti, If Phoebe came from somewhere other differentiated with a dense rocky core, put- rium — its shape is stable under the influ- isotopes with a short life.” As radioactive “Their shape could also be due to mass wast- than the saturnian system, its early wan- ting it in the small club of planet-like bod- ence of its internal gravity. Phoebe seems isotopes decay, they lose radiation and/or ing where the sides have caved in. These derings reinforce the chinks in the standard ies. “It’s very different from the rest of the to have reached this point. If it has, says particles and release heat. For any given craters are cone-shaped — very strange.” model’s armor. Many researchers believed saturnian system,” says planetary scientist Castillo-Rogez, its core likely heated the amount of such radioactive material, the Castillo-Rogez agrees, adding, “The largest something might be awry with their theory Julie Castillo-Rogez of the Jet Propulsion outer layers to give the moon its shape. time it takes for half of the mass to decay is crater has practically lost her head!” long before they got a close look at Phoebe, Laboratory (JPL) in Pasadena, California. “Phoebe should be like [saturnian satellite] called its half-life. Phoebe is not the only oddball in the says Hal Levison of the Southwest Research Researchers can learn about a solar sys- Hyperion, with lots of porosity throughout,” One isotope that may be responsible for saturnian system, and that’s what makes Institute in Boulder, Colorado. In its forma- tem object’s interior and thus if it is differen- she adds. “The fact that it has a high density Phoebe’s heat specifically, Buratti says, is its study so fascinating to researchers like tive years, he explains, “the Sun shed angu- tiated using spacecraft flybys. Such a world — three times greater than Hyperion — aluminum-26, which has a half-life of about Castillo-Rogez. “You have such a variety of lar momentum by creating a disk of is much more massive than any probe, so tells us that something happened to cause 720,000 years. “It produces a lot of heat in a bodies, and they probably don’t come from material. Grains of ice and rock were float- when a craft passes by a planetary body, the the collapse of the porous [outer layers]. The few thousand years,” she says. “That heat the same place,” she says. “We think the ing around in the disk.” Those particles 200 million years later massive object’s gravity bends its course. may have been responsible for hydrother- inner satellites formed from the rings of slowly settled, stuck together, and thus Flight engineers can chart this path. If the mal activity and other processes. Small Saturn. Hyperion may be a fragment of grew into larger objects: the solar system’s pass is close enough to the planet or moon bodies with very rich chemistry were heated another [inner] body or a captured object. four terrestrial planets, Uranus, Neptune, or if there are enough passes, scientists can by these isotopes with short half-lives. [The Iapetus may be original to the system, but and what scientists think are the cores of determine the larger object’s internal struc- objects] need to form very early for that its density is very weird. Then you have Jupiter and Saturn. But, in the standard ture. While researchers have used data from heat to be available. Formation within the Phoebe and other outer satellites that are model, those bodies formed where they NASA’s Cassini spacecraft to employ this first 3 million years [of the solar system] probably captured.” currently . The problem, says Levison, method to learn about and , gives us a way to relax Phoebe’s shape.” Phoebe’s orbit provides a hint that it is that this depiction simply cannot be true.

the probe did not fly close enough to Phoebe winged its way in from afar: The moon cir- ET AL. GOMES, AFTER KELLY, ROEN : to permit such estimates, Castillo-Rogez From outside or within? cles Saturn in a retrograde course, opposite Was upheaval needed? That relaxed surface also affects the shape of The standard model of planet formation

says. The calculation has “a huge error bar,” the direction that the saturnian system natu- ASTRONOMY she continues. “But the one thing that is craters. Phoebe’s crater rims seem to be com- rally rotates. Its orbit is also eccentric — not faces two major problems. The first is what NASA/JPL/SPACE SCIENCE INSTITUTE SCIENCE NASA/JPL/SPACE The Nice model is scientists’ leading theory robust is that the shape of Phoebe is close The crater at top center displays alternating layers posed of only ice in certain places, and the circular — and inclined at an extreme angle. Levison calls the meter barrier. “Imagine of the early solar system’s evolution. It main- of bright and dark material. Researchers think its craters themselves are oddly shaped. “If you Phoebe travels far from its parent world, these floating dust grains,” he says. “These tains that around 4 billion years ago, Jupiter’s Michael Carroll, a science writer and astronom- appearance might be the result of subsurface light- look at the craters on Phoebe, they don’t look circling the planet in a great loop some 16 stick together and grow like dust bunnies. and Saturn’s passed through a reso- ical artist, won the Jonathan Eberhart Planetary colored icy material thrown out (perhaps from in- nance, flinging Uranus and Neptune farther ternal venting) that buried the darker surface layer. like impact craters,” says Buratti. Instead, to million miles (26 million km) across. These In objects 1 to 10 kilometer-size [0.6 to 6 out and into a disk of protoplanetary bodies. Sciences Journalism Award for his August 2011 Based on this and similar evidence, scientists think scientists they appear to result from inter- clues point to a capture of the moon early in miles], if they hit gently enough, they’ll The upheaval might have thrown Phoebe Astronomy article, “Storm warning.” Phoebe has a dusty surface covering an icy interior. nal forces, such as gas or volatiles escaping the evolution of our solar system. stick because of gravity.” But scientists inward, where Saturn’s gravity held onto it.

30 ASTRONOMY • MARCH 2014 WWW.ASTRONOMY.COM 31 PHOEBE FEEDS SATURN’S MOST DISTANT RING other. It looks like somebody took the disk of material in the outer solar system and shook it real hard. The planets in their cur- rent configuration just can’t do that.” Some- Saturn Titan thing had to change in the structure of our planetary system. Dynamicists now suspect that the giant planets began in a compact configuration and then migrated out. This scenario is the heart of the Nice model, named after the town in France where Levi- son and three colleagues first combined Iapetus forces to craft it (see illustration on p. 31). The Nice model proposes two extreme alternatives to the standard model. One of these scenarios describes a smooth migra- Phoebe tion where Uranus and Neptune slowly spi- NASA/JPL/SPACE SCIENCE INSTITUTE SCIENCE NASA/JPL/SPACE ral out through the solar nebula. The other The Cassini spacecraft captured high-resolution photographs of Phoebe during a 2004 flyby, and scien- g idea maintains that the four giant planets tists combined six to create this mosaic. This image shows detail as small as 243 feet (74 meters) wide. rin (and possibly a fifth, which was later ejected ust from our solar system) huddled much D nearer to the Sun until a global instability A captured interloper probably lost a lot of volatiles. When it caused Uranus and Neptune to take off in The violent past that Levison and others formed, it probably had methane and nitro- crazy trajectories. Their orbits crossed each point to may have left fingerprints on Phoe- gen ice.” Theory says that the moon lost other’s and even intersected those of Jupiter be’s surface. Thanks to Cassini’s spectrom- those volatiles as it was flung from the and Saturn. Gravity from the two gas giants eter, an instrument that breaks a target’s outer depths of the solar system. sent Uranus and Neptune packing, where radiation into its constituent wavelengths, For Phoebe to have ended up where it is, In 2009, scientists discovered a new ring around Saturn that they think is fed by dust from Phoebe. This ring is the largest yet known in the solar system. they settled into a disk of planetesimals that Buratti and colleagues see -dioxide its travels from the may have ASTRONOMY: ROEN KELLY; NASA/JPL/SPACE SCIENCE INSTITUTE (IAPETUS; PHOEBE; SATURN; TITAN) no longer exists. This population of outer ice. “If you look at the visible spectrum, taken it closer to the Sun, perhaps as close asteroids and comet nuclei, which would there’s a kind of upturn toward the ultra- as Jupiter, before capture. Once in Saturn’s eventually give rise to the Kuiper Belt, was violet [that may be] due to scattering from vicinity, there may have been even more While astronomers have known about Saturn’s characteristic bright ring million km). It’s some 40 times as thick as the planet’s radius, which “a place where thousands of Earth-like iron particles,” she says. “There’s also a con- fireworks: Phoebe may have had a sister. system for centuries, they identified an enormous dust ring around the matches Phoebe’s vertical motion along its orbit. The ring seems to be allowed things like and tending theory that the surfaces are covered Levison puts it this way: “There is no planet in 2009. Phoebe orbits within the ring, so astronomers think the inclined 27° to Saturn’s equatorial plane; the moon’s orbit follows a 30° Phoebe to grow,” says Levison. with organic molecules — polycyclic aro- way that you could just have Saturn sitting moon contributes material to it. This is the largest known planetary ring angle to the planet’s plane. in the solar system. The newly discovered ring is about 100 times larger than the planet’s The giant planets later destroyed the matic hydrocarbons.” These hydrocarbons there and have Phoebe come in by itself and Using NASA’s to detect radiation, main . That system is also aligned with Saturn, which means a disk, but its mass and interaction with the contain hydrogen, oxygen, nitrogen, and become captured. Gravity is time-reversible. Anne Verbiscer of the University of Virginia in Charlottesville and col- different mechanism must have helped form this newly found ring. Scien- worlds closer to the Sun essentially saved the carbon — the building blocks of life. If something can get in, something can get leagues found the thick ring. Its inner edge starts around 4.8 million miles tists think particle impacts with Phoebe produced debris that remains in a giant planets, preventing them from being Asteroids and known as car- out.” In other words, for Saturn to capture (7.7 million kilometers) from Saturn and extends to 7.8 million miles (12.5 thick disk around the satellite’s orbit, creating this huge ring. — Liz Kruesi ejected from the solar system. Not every- bonaceous chondrites form in the outer Phoebe, a third object was necessary. In the body accepts the Nice model, but, Levison portion of the solar system’s main encounter, Phoebe’s companion would have asserts, “No one has developed a model that belt or beyond, where the environment is been ejected and the lost energy would have have a problem with the bodies between Some researchers propose that turbu- throws them out of the solar system. You is even close to being an alternative.” cool enough for complex hydrocarbons to slowed Phoebe enough for capture. dust-grain-sized and kilometer-sized. lence in the solar nebula may have con- simply cannot get these guys to accrete [in form. “There are these wet, hydrocarbon- Another intriguing possibility is that Levison points to two coffee mugs on his centrated meter-sized objects in “eddies” the current Uranus-Neptune region].” filled objects that appear in the main belt, Phoebe was a moon of either Uranus or desk. “If I bring these two cups together, — regions where fluid or gas flows differ- The Kuiper Belt of thousands of objects and some get transported into the inner Neptune. In those wild days of planetary no matter how gently, they aren’t going to ently from other nearby environments. beyond Neptune poses another mystery. system,” says Buratti. While Phoebe’s sur- migration, one of the ice giants may have stick together,” he says. “There is no good The Sun’s disk may have hosted swarms Beyond the eighth planet, bodies move with face shows a history of impactors — that come close enough to Saturn to shed some force to hold together things that are a of boulder-sized rocks, herded into these high inclinations and high eccentricities in likely originated from the main of its own natural satellites, perhaps actu- meter- to a decimeter-size. Another prob- vortices, which then gradually interacted to resonances with Uranus and Neptune. — Buratti believes that observations of spe- ally trading a few with the larger planet. lem is that there is aerodynamic drag on form larger objects. (One example of such an object is Pluto, cific materials on the moon’s surface might Wherever Phoebe came from, its charac- the particles. They are feeling the force of A second substantial change in scien- which completes two orbits for every three provide some evidence that it migrated in teristics continue to intrigue. With its cone- the gas they are moving through — feeling tists’ view of planetary evolution — and one of Neptune — a 2:3 resonance.) But because from an outer region, like the Kuiper Belt. shaped craters, rich mineralogy, spherical the headwind.” that is better established — is that the giant of the way planets develop, these objects Cassini’s instruments have detected vol- shape, high density and perhaps great age, Small particles (micron-sized) can be planets did not form in their current orbits. had to have formed in circular, low- atiles that could indicate just that. Phoebe’s Phoebe seems more planet than moon. And suspended in the gas of the primordial solar “You cannot make Uranus and Neptune inclination, low-velocity orbits. That’s the organic surface composition is unlike any yet this bizarre little world ranks small in nebula without settling, while drag doesn’t where they are,” Levison declares — they only way to get planets to accrete, yet surface yet observed within Neptune’s the solar system’s retinue of satellites. Says affect large bodies because of their mass. must have formed nearer to the Sun. “The observers find quite a different picture in orbit. Its complex makeup causes Castillo- Castillo-Rogez, “I like the smaller bodies of NASA/JPL/SPACE SCIENCE INSTITUTE SCIENCE NASA/JPL/SPACE But those in-between objects, in the meter problem we find is that if you put a bunch the Kuiper Belt. As loose fragments of ejecta run down the side of Rogez to wonder, “What happens when the solar system. They don’t get the public range, should migrate into the Sun before of Earth-sized objects out in that region, “We see objects on inclined, high- this crater on Phoebe’s surface, they create streaks. you take a Kuiper Belt object and bring it excitement that Titan or do, but they grow, effectively preventing any planets they don’t hit each other,” he explains. eccentricity orbits,” says Levison. “We see The crater is 8 miles (13 kilometers) wide. closer to the Sun?” She adds: “Phoebe has they have their own story to tell.” from forming. “This has been a problem the “They gravitationally scatter one another multiple populations that clearly came from field has faced since the ’70s,” Levison says. into Jupiter-crossing orbits, and Jupiter different locations superimposed on each LEARN ABOUT SATURN’S BIGGEST MOON, TITAN, AT www.Astronomy.com/toc.

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