Cassini Orbiter Reveals Just What Saturn’s Moon Is Made Of

The debate about the origin of Saturn’s moon Phoebe is over. Although it may look like an asteroid, the tiny moon actually is an icy interloper from the fringes of the solar system, according to new results from NASA’s Cassini orbiter.

Phoebe is “a frozen time capsule” from 4 billion years ago, “waiting for Cassini to come along and open it up,” imaging scientist Torrence Johnson of Pasadena’s Jet Propulsion Laboratory said Wednesday.

Scientists made their determination based on spectroscopic measurements of Phoebe taken when Cassini swooped to within 1,280 miles of the battered and beaten moon June 11.

“In two short weeks, we have added more to what we know about Phoebe than we had learned about it since it was discovered 100 years ago,” project scientist Dennis Matson said.

The images and measurements taken during the fly-by indicate that Phoebe is lighter than rock but heavier than ice -- similar in density to Neptune and its moon, Triton. That suggests Phoebe has the same origin as those bodies, in the Kuiper Belt on the edge of the solar system.

“We believe the [early] solar system was full of Phoebes,” Johnson said. “As the big planets formed, that material was either swept into those planets or swept out of the solar system into the Kuiper Belt. Phoebe apparently stayed behind, trapped in orbit around the young Saturn.”

Spectroscopic data indicate that Phoebe’s surface is made of water ice, water-bearing minerals, carbon dioxide, possible clays and “primitive organic chemicals,” said Roger N. Clark of the U.S. Geological Survey in Denver.

“We don’t see carbon dioxide in the asteroid belt,” said Bonnie Buratti, an instrument specialist at JPL. “That means [Phoebe] is definitely not an asteroid. Phoebe was formed beyond the orbit of Jupiter ... in the Kuiper Belt.”

Infrared measurements indicate Phoebe’s surface is very cold, about minus 261 degrees Fahrenheit in sunlight, according to John Pearl of NASA’s Goddard Space Flight Center in Greenbelt, Md. On the night side of the moon, the temperature drops to minus 325 degrees.

The relatively small change in temperature between night and day indicates that the surface is covered with a thin layer of ice or dust, he added.

The $3.3-billion Cassini spacecraft, launched in 1997, will fire its rocket next week to enter orbit around Saturn, where it will spend four years exploring the planet, its moons and rings. The craft also will launch the Huygens probe to the surface of Saturn’s moon Titan in December.