Image: Phoebe's surface
NASA/JPL/Space Science Institute
Small bright craters on Phoebe's surface may have been created by collisions that exposed material from below, possibly water ice.
By Senior science writer
updated 6/15/2004 7:08:00 PM ET 2004-06-15T23:08:00

You have to appreciate Phoebe's rugged mug, a face weathered by time, with deep and shadowed sockets complemented by sharp ridges, a brooding darkness accented by highlights visible upon close inspection. It is a look of survival etched over 4.5 billion years.

And if preliminary analyses of new images are correct, this tiny moon of Saturn is quite the wanderer, a vagabond of the frigid fringes of the solar system that's been lured inward, grabbed by Saturn's mighty gravity, and turned into an awkward satellite.

Phoebe was visited Friday by the Cassini spacecraft. The successful flyby exposed the small world in unprecedented detail and portends a rich survey of the Saturnian system, a mission that will include 52 flybys of seven moons.

Way back
Phoebe looks a lot like an asteroid. It's just 137 miles (220 kilometers) wide and riddled with craters. It is roughly spherical but by no means perfect.

Scientists have long suspected Phoebe is instead an object captured from the Kuiper Belt, a more distant region of icy bodies beyond Neptune. There were already clues: Phoebe is oddly dark for a moon and it orbits backward compared to the rotation of Saturn and the paths of the planet's larger moons.

If Phoebe is from the Kuiper Belt, it could be a very useful, unsullied specimen of the early solar system, an object that gathered itself together before the planets had finished forming and which has changed little since.

While most asteroids are chips of larger objects, Kupier Belt Objects probably collided less often -- Phoebe may never have been any bigger than it is now. That's important, because it would mean Phoebe is probably too small to have ever had a melted core, so its contents could be pristine, a window to the chemicals and minerals of the solar system at birth.

The new pictures hint at Phoebe's primordial nature.

Mostly ice
Surface features and the way in which light reflects are "subtly different" from asteroids, said Peter Thomas, a senior research associate at Cornell University and a specialist in the study of small satellites. "I can't quantify it yet," Thomas told yesterday. "It's just sort of an impression."

Some of the bright areas on the small moon may be material from below -- possibly water ice -- that's been exposed by impacts. Thomas said it's too early to know for sure. But if a full analysis shows Phoebe to be composed largely of ice, that would almost guarantee it came from beyond. Asteroids contain little ice and are made mostly of rock and metals.

"If it turns out there's a lot of ice in this thing, then it's safe to say its not asteroidal," Thomas said. He points out, though, that nobody really knows what a Kuiper Belt Object should look like.

Carolyn Porco, Cassini Imaging Team leader at the Space Science Institute in Boulder, Colorado, also sees hints of a lot of ice beneath the surface of Phoebe in layers evident along the crater walls.

"The dark stuff we're seeing is just a coating on the surface," Porco said in a telephone interview. "We think the body is probably ice-rich."

Firm answers await a full analysis of the many tests run on Phoebe during the flyby.

Working 'like a charm'
Meanwhile, as one of the most complex, multifaceted space probes ever built, the $3.3-billion Cassini mission has shown it can deliver on one of its main tasks. The size of a small school bus, it flew within 1,285 miles (2,068 kilometers) of Phoebe at a relative speed of 13,000 mph (20,900 kilometers per hour) while 11 separate instruments gathered data.

An onboard autonomous target tracker "seemed to work like a charm," Porco said.

Radar probed the surface down to nearly 8 inches (20 centimeters). Cassini also recorded ultraviolet and infrared light. Phoebe's gravitational effect on the spacecraft was noted. From all this and more, astronomers expect to determine the moon's exact mass and density, create a detailed global surface map, and figure out a lot about what it's made of.

"The best thing is the spacecraft actually did this complicated encounter," said Thomas, the Cornell researcher. "We're hoping to do this off-and-on for four years. It's very reassuring to know we can."

Cassini's next major milestone will come at the end of this month, when the plutonium-powered probe settles into its first of 76 planned orbits around Saturn. Early next year it will release the Huygens probe, which will land on the surface of Titan, Saturn's largest Moon.

The Cassini-Huygens mission is a project of NASA, the European Space Agency and the Italian Space Agency.

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