Saturn Moon Has Planet-Like Properties
New analysis of images of Phoebe lead scientists to theorize how the 140-mile-in-diameter rock started life. They suggest that it began its life as a planetoid in the Kuiper belt, a much larger version of the asteroid belt that sits on the outer edge of our planetary system.
The scientists actually call Phoebe a planetesimal — a small celestial body that existed during the evolution or formation of the planets, or a remnant building block of planets.
“Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out,” said Julie Castillo-Rogez, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL). “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 giant planets and their moons.”
Describing the data in the April issue of Icarus, NASA scientists said Saturn’s moon is more planet-like than moon-like, and has more planet-like qualities than previously thought.
One tell-tale sign is the moon’s shape. “From the shape seen in Cassini images and modeling the likely cratering history, we were able to see that Phoebe started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts,” said Peter Thomas, a Cassini member from Cornell University, and co-author of the paper.
Data from Cassini also show that Phoebe was hot early in its life. That heat, scientists speculate, may have come from radioactive elements which generated internal heat, helping the body to form. Internal warming may account for water signatures found by Cassini, suggesting that the planetoid may have been warm enough to host liquid water at one time in its history.
Phoebe also has much denser rock-rich material concentrated near its core. It’s average density is close to that of Pluto, which lies within the Kuiper belt, and may have had similar origins. And because Phoebe orbits backwards, unlike the other moon’s in Saturn‘s grasp, scientists believe it may have been captured by Saturn’s gravity as it flew through space. Furthermore, most of the other moon‘s in Saturn‘s orbit appear to be formed from gas and dust, unlike Phoebe.
“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,” said Jonathan Lunine, a co-author on the study and a Cassini team member from Cornell.
The scientists say that Phoebe was born within the first 3 million years of the birth of our solar system, which occurred 4.5 billion years ago. The team believe that Phoebe may have been porous at one time, and as it warmed up it collapsed in on itself giving it its odd shape and density seen today.
The scientists conjecture that several hundred years after Phoebe cooled down, it drifted toward the inner solar system in a solar-system-wide rearrangement. Phoebe was large enough to survive this turbulent rearrangement.
The Cassini-Huygens mission is joint partnership between NASA, the European Space Agency, and the Italian Space Agency. The mission is managed by JPL for the Science Mission Directorate in Washington. The California Institute of Technology in Pasadena manages JPL for NASA.
Image 1: Phoebe’s true nature is revealed in startling clarity in this mosaic of two images taken during Cassini’s flyby on June 11, 2004. Image Credit: NASA/JPL/Space Science Institute
Image 2: 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. Image credit: NASA/JPL-Caltech/SSI/Cornell