Images snapped by NASA’s Cassini spacecraft show that Saturn’s moon, Phoebe, is not an asteroid but a 4.5-billion-year-old primordial body from the solar system’s outer reaches, scientists said on Wednesday.
The images of Phoebe’s pitted surface gave scientists their first close look at a planetesimal, small bodies from an area at the fringe of the solar system known as the Kuiper Belt that may have provided the building blocks of the Milky Way.
High-definition photographs as well as spectrographic and thermal images taken during Cassini’s June 11 fly-by revealed that Phoebe likely is made up of ice, rock and carbon compounds similar to those seen in Pluto and Neptune’s moon, Triton.
“We believe the solar system was full of Phoebes,” scientist Torrence Johnson said at a news briefing at NASA’s Jet Propulsion Laboratory in Pasadena, California.
“As the big planets formed, that material was either swept into those planets or swept out of solar system into the Kuiper Belt,” Johnson said. “Phoebe apparently stayed behind, trapped in orbit around the young Saturn ... a frozen time capsule waiting for Cassini to open it up.”
Phoebe’s core -- too dense for pure ice and too light for pure rock -- is a combination of the two whose ratio resembles that of comets and other Kuiper belt bodies, scientists said.
Cassini’s spectrometer picked up signs of water-bearing minerals, carbon dioxide and organic material on Phoebe’s heavily cratered surface, scientist Bonnie Buratti said.
The small moon also yielded spectral signatures of minerals that scientists so far have been unable to identify, she said.
The pass by Phoebe was part of Cassini’s mission to take a four-year look at Saturn and the objects around it. The robotic spacecraft began its journey seven years ago and is scheduled to begin orbiting the ringed planet on June 30.
Cassini, a cooperative project of NASA, the European Space Agency and the Italian Space Agency, is expected to conduct 76 orbits around the Saturn system and undergo 52 close encounters with seven of Saturn’s 31 known moons.
More information and images from the Phoebe fly-by are available online at http://saturn.jpl.nasa.gov.