Cassini Finds Saturn's Moon Phoebe has Planet-Like Qualities

Posted on April 27, 2012

Data from NASA's Cassini mission reveals that Phoebe, one of Saturn's many moons, has more planet-like qualities than previously thought. Phoebe has been called a planetesimal, or remnant planetary building block. Cassini images suggest Phoebe originated in the Kuiper Belt, a region of ancient, icy, rocky bodies beyond Neptune's orbit. Data shows Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. NASA researchers say its average density is about the same as Pluto, another object in the Kuiper Belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif, says, "Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out. Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of planets and their moons."

The researchers say the data suggests Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40% higher than the average inner Saturnian moon. Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time. This could explain the signature of water-rich material on Phoebe's surface previously detected by Cassini.

Jonathan Lunine, a co-author on the study and a Cassini team member at Cornell University, Ithaca, N.Y., says, "By combining Cassini data with modeling techniques previously applied to other solar system bodies, we've been able to go back in time and clarify why it is so different from the rest of the Saturn system."

This panel of images shows the nearly spherical shape of Saturn's moon Phoebe, as derived from imaging obtained from NASA's Cassini spacecraft. Each image represents a 90-degree turn. You can see a larger version of this image here.

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