Buzzing the Dark Moon


Cassini will release its Huygens probe in December towards the largest Saturnian moon, Titan Image Credit: NASA/JPL

The most complex interplanetary mission ever launched is about to meet one of the solar system’s enigmatic moons. Cassini will fly by Phoebe, Saturn’s largest outer moon, on Friday, June 11th. The closest approach is at approximately 4:56 p.m. EDT.

A final trajectory correction maneuver is scheduled for June 16. On arrival date, June 30, Cassini will become the first spacecraft to orbit Saturn. Once in orbit it will conduct an extensive, four-year tour of the Saturn system, including its majestic rings and many known moons.

"The arrival date and trajectory to Saturn were specifically selected to accommodate this flyby, which will be the only opportunity during the mission to study Phoebe at close range," said Dave Seal, mission planner for the Cassini- Huygens mission at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. "Phoebe’s orbit is simply too far from Saturn, at almost 13 million kilometers (about 8 million miles), nearly four times as far as the next closest major satellite, Iapetus. A later encounter is not feasible."

"The last time we had observations of Phoebe was by Voyager in 1981," said Dr. Torrence Johnson, former Voyager imaging team member, Galileo project scientist and Cassini imaging team member. "This time around, the pictures of the mysterious moon will 1,000 times better, as Cassini will be closer," he said.

Voyager 2 captured images of Phoebe from about 2.2 million kilometers (about 1.4 million miles) away. Cassini will obtain images from a mere 2,000 kilometers (about 1,240 miles) above the moon’s surface. Cassini will also collect spectroscopic and radar data that could decipher the composition and origin of this distant moon. Cassini’s Phoebe images, already twice as good as returned by Voyager 2, show large craters and variations in surface brightness.

On June 11, the Cassini orbiter will fly by Phoebe, and data and images will be returned on June 12.

"We anticipate that Phoebe will be heavily cratered in the higher resolution images we expect to see in the next few days," said Dr. Peter Thomas, a member of the imaging team and a senior research associate at Cornell University, Ithaca, N.Y., who specializes in studies of small satellites. "The hints of different brightness also suggest the highest resolution images, several hundred times better, will show a variety of materials," he said.


Close approach image of Phoebe taken June 10 Image Credit: NASA/JPL

Discovered in 1898 by American astronomer William Henry Pickering, Phoebe is of great interest to scientists. "With the instruments Cassini carries, we might learn more about Phoebe’s internal structure and composition. What we have are many unanswered questions: Did it ever melt? Does it have evidence of past interior melting? Was it ever an icy body? Why is Phoebe in such an odd orbit?" said Dr. Dennis Matson, project scientist for the Cassini-Huygens mission at JPL.

Phoebe has a diameter of 220 kilometers (about 136.7 miles), which is equal to about one-fifteenth of the diameter of Earth’s moon. Phoebe rotates on its axis every nine hours and 16 minutes, and it completes a full orbit around Saturn in about 18 months. Its elliptical orbit is inclined approximately 30 degrees to Saturn’s equator. Phoebe’s retrograde orbit means it goes around Saturn in the opposite direction of the larger interior Saturnian moons. Previous ground-based observations have shown water ice present on its surface.

Phoebe is also unusual as it is very dark. It reflects only six percent of the sunlight it receives. Phoebe’s darkness and retrograde orbit suggest it is most likely a captured object. A captured object is a celestial body caught by the gravitational pull of a much bigger body, generally a planet. Some scientists believe Phoebe might even be an object from the outer solar system, similar to the objects found in the Kuiper Belt. The Belt is a collection of small icy bodies beyond Pluto that were never drawn together by gravity to form a planet.

"The dark and odd-shaped Phoebe may be a piece of the building blocks from which some of the planets formed," said Dr. Bonnie Buratti, scientist on the Cassini-Huygens mission at JPL. "It might hold clues about the early formation of our solar system."

The spectral data and images obtained from the June 11 flyby will help scientists determine the icy moon’s surface composition and properties.

"This is a unique opportunity," said Dr. Dale Cruikshank, co-investigator for the Cassini Visual and Infrared Mapping Spectrometer (VIMS), an instrument that will measure the chemical signatures of Phoebe’s surface. "We’ve never had a close-up look at an irregular, low-reflective moon of any planet before, so we are prepared to be surprised," he said.

Cruikshank will study the VIMS high-resolution spectral data to determine the distribution of recently observed water ice on Phoebe’s surface. He also will use the data to determine the ability of Phoebe’s surface to reflect light (known as its ‘albedo’) and the source of Phoebe’s mysterious dark color. "This odd moon of Saturn has a little ice and a lot of black material on its surface, but beyond that, we know very little," Cruikshank noted.

Phoebe’s surface color appears almost black when observed by powerful telescopes, scientists say. The moon, which is about 130 miles in diameter, reflects only 6 percent of the sunlight it receives.

Among the 30-plus moons of Saturn are a menagerie of surfaces. Scientists consider Saturn, its rings and moons, to be akin to a miniature solar system unto itself. Image Credit: NASA/JPL

Because of its dark color, and because Phoebe’s orbit is irregular (elliptical, outside the plane of Saturn’s equator and retrograde), scientists think the moon is probably a captured object, possibly a comet, asteroid or Kuiper Belt Object (KBO).

KBOs are lumps of ice, rock and black material in the outer solar system that were never drawn together by gravity to form a planet. They are of great interest to scientists because they are believed to be primordial, which means they probably date back to the formation of the solar system some 4.6 billion years ago. About half of the comets that occasionally come near the Earth and sun are KBOs.

One theory of Phoebe’s mysterious dark color, which also is shared by the forward face of Iapatus, another nearby Saturn moon, is that it is due to the abundance of an organic material called tholin. Tholin is a sticky, waxy, dark red residue whose tiny particles cause the brownish haze of Saturn’s largest moon, Titan.

The tholin that may cover Phoebe is thought by Cruikshank and others to be abiotic, which means it is not made from living organisms. Scientists hypothesize the tholin is a natural by-product of the organic chemistry of the carbonaceous materials that make up Phoebe. Comet dust is an example of abiotic organic material. Cruikshank specializes in icy bodies in the outer solar system and the composition of small satellites, including all the satellites of Saturn.

Since its discovery in 1898, Phoebe has been of interest to astronomers because it is so different from Saturn’s other large moons. If Cassini finds that its surface is really made of carbonaceous organic material, scientists can use that information to learn about our solar system’s formation and early history. Phoebe’s surface material may even include amino acids, the building blocks of life.

Related Web Pages

Chronology of a Scientific Safari
Long, Strange Trips
Titan’s Icy Bedrock
Saturn– JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame