Credit: NASA/JPL/Space Science Institute

NASA's Cassini spacecraft recently relayed hundreds of images of Iapetus, Saturn's third largest moon, as the spacecraft flew by the mysterious icy moon on 10 September 2007.

Iapetus (1,436 km across) is a two-faced cosmic wonder. One hemisphere of the moon is as bright as snow, while the other is as dark as tar. The images show both hemispheres. Interestingly, Iapetus is likened to the famous yin-yang symbol of the Far Eastern thought, due to the sharp contrast in surface brightness between its two hemispheres.

Images show numerous craters, along with the strange mountain ridge that runs along the moon's equator. Many of the close-ups targeted the rugged 20-km high mountain ridge that renders the walnut-shaped appearance of Iapetus.

Iapetus, Walnut-shaped Moon
Credit: NASA/JPL/Space Science Institute/Gordan Ugarkovic

"The images are really stunning," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany, who led the imaging observation planning. "Every new picture contained its own charm. I was most pleased about the images showing huge mountains rising over the horizon. I knew about this scenic viewing opportunity for more than seven years, and now the real images suddenly materialized."

The spacecraft flew approximately 1,640 km from the surface of Iapetus. Therefore, this flyby was nearly 100 times closer to Iapetus than the 2004 flyby.

Aided by Cassini's data, scientists are now tackling fascinating topics, such as the moon's irregular shape, the mountain ridge that lies almost directly on the equator and Iapetus' drastic brightness contrast.

"There's never a dull moment on this mission," said Bob Mitchell, Cassini Program Manager. "We are very excited about the stunning images being returned. There is plenty here to keep many scientists busy for many years."

"Our flight over the surface of Iapetus was like a non-stop free fall, down the rabbit hole, directly into Wonderland! Very few places in our Solar System are more bizarre than the patchwork of pitch dark and snowy bright we have seen on this moon," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute.

"Iapetus provides us a window back in time, to the formation of the planets over four billion years ago. Since then its icy crust has been cold and stiff, preserving this ancient surface for our study," said Torrence Johnson, of the Cassini imaging team.

Cassini's observations of Iapetus will help to determine the chemical composition of the surface; look for evidence of a tenuous atmosphere or erupting gases and map the surface temperature of the nightside. The data obtained will be analyzed in the coming weeks.

Further Reading

The Cassini-Huygens Mission to Saturn
http://saturn.jpl.nasa.gov
http://www.nasa.gov/cassini

Aymen Mohamed Ibrahem
Senior Astronomy Specialist