Planetary science The weird world of PHOEBE Scientists are finding that Saturn’s strange moon isn’t like the other satellites in the solar system. by Michael Carroll The interiors of the planets and the Planetary evolution solar system’s largest moons 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 Sun. It pulled in bodies, the ones with less mass and more material while its gravity grew. thus lower gravities like comet 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 asteroids, comet nuclei, and, ulti- came Saturn’s moon Phoebe. mately, planets. The rocky terrestrial planets — Mercury, Venus, Earth, and Surprisingly spherical Mars — 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 volatiles (elements falls below the midrange size of more and compounds with low boiling familiar moons like Iapetus or Mimas. 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. Jupiter and Saturn pulled hydrogen 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 planet-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, Uranus and Neptune 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 ASTRONOMY became the ice giants 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 planetary migration Uranus Saturn 0° west longitude 90° west longitude Jupiter Neptune Disk of planetesimals 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 Hyperion has a low density, giving it weak surface gravity 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 orbit. The problem, says Levison, method to learn about Enceladus and Titan, 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.