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Dark side of a Saturnian moon: Iapetus is coated with foreign dust

Date:
December 11, 2009
Source:
Cornell University
Summary:
Iapetus is often called Saturn's most bizarre moon, due to its starkly contrasting hemispheres -- one black as coal, the other white as snow. Images taken by the Cassini-Huygens spacecraft, orbiting Saturn since 2004, offer the most compelling evidence to date of why and how the moon got its yin-yang appearance, as well as clues to how other such satellites might have formed in the early universe.
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Cassini-Huygens spacecraft images of Iapetus' dark, leading side and its bright, trailing side. The high-resolution images shed new light on the long-standing puzzle of how Iapetus got its unusual coloration.
Credit: Cassini Imaging Team

Iapetus is often called Saturn's most bizarre moon, due to its starkly contrasting hemispheres -- one black as coal, the other white as snow.

Images taken by the Cassini-Huygens spacecraft, orbiting Saturn since 2004, offer the most compelling evidence to date of why and how the moon got its yin-yang appearance, as well as clues to how other such satellites might have formed in the early universe. Analyzed by a research team that includes Cornell scientists, the images are detailed in the Dec. 10 online edition of the journal Science.

"This is not the most fundamental problem in the world," said research team member Joseph A. Burns, Cornell's Irving Porter Church Professor of Engineering and professor of astronomy. "But it's an enigma that's been puzzling astronomers for centuries."

Since pictures of Iapetus from the Voyager mission 30 years ago confirmed its intriguing color scheme, scientists have puzzled over whether Iapetus' dark-light contrast was the result of external debris hitting some of the moon, or whether the dark dust was the result of internal activity. Now they know the dust came from elsewhere.

Using pictures taken by Cassini, particularly during a September 2007 close fly-by, the scientists assert that Iapetus' darker half, called Cassini Regio, is the result of the planet's leading side getting bombarded by dusty debris from another Saturnian moon, Phoebe, which orbits in the opposite direction beyond Iapetus.

It is a longstanding theory, but in a paper published in the journal Nature in October, three Cornell-trained astronomers announced the discovery of an enormous ring of debris ¬- 10,000 times the area of Saturn's famous main ring system -- around Saturn, near Phoebe's location, pointing to it as the ring's source. Burns calls this ring the "smoking gun" supporting dust hitting Iapetus and other moons around Saturn.

"The ring of collisional debris that has come off Phoebe and its companion moons is out there, and now we understand the process whereby the stuff is coming in," Burns said. "When you see the coating pattern on Iapetus, you know you've got the right mechanism for producing it."

Small, white craters that dot Iapetus' darker half indicate a veneer of dark dust, only meters deep, covering a white, icy surface that matches the rest of the satellite. The imaging data also revealed that all the materials on the leading side are much redder than the shielded and brighter trailing side -- another indication that the leading side's dust came from elsewhere.

Other pictures revealed that the transition from the dark to light hemispheres is not a solid line, but rather a mottled, patchy array of bright and dark spots.

The pattern, the scientists say, supports a theory described in a companion paper in Science that the darker parts of the moon tend to heat up when struck by sunlight, encouraging the ice to evaporate underneath.

This causes any dark spots to get even darker, creating the mottled look.

The research team includes Paul Helfenstein and Peter C. Thomas, both senior research associates at Cornell. The paper's first author is Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. The Cassini program is an international cooperative effort involving NASA, the European Space Agency, and the Italian space agency.


Story Source:

The above post is reprinted from materials provided by Cornell University. The original item was written by Anne Ju, Cornell Chronicle. Note: Materials may be edited for content and length.


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Cornell University. "Dark side of a Saturnian moon: Iapetus is coated with foreign dust." ScienceDaily. ScienceDaily, 11 December 2009. <www.sciencedaily.com/releases/2009/12/091210173611.htm>.
Cornell University. (2009, December 11). Dark side of a Saturnian moon: Iapetus is coated with foreign dust. ScienceDaily. Retrieved July 30, 2015 from www.sciencedaily.com/releases/2009/12/091210173611.htm
Cornell University. "Dark side of a Saturnian moon: Iapetus is coated with foreign dust." ScienceDaily. www.sciencedaily.com/releases/2009/12/091210173611.htm (accessed July 30, 2015).

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