The New Year's resolution for Cassini scientists is to figure out why Iapetus has a bulging waistline and if this anatomical feature has anything to do with its strange dark/light appearance.
Images returned by the Cassini spacecraft cameras during a New Year's Eve flyby have unveiled several unique surface features on Iapetus that have modified discussions about the origin of the extremely dark region on Iapetus, known as Cassini Regio.
One of these features is a startling, long narrow ridge that lies almost exactly on the equator of Iapetus, bisects its entire dark hemisphere and reaches 20 kilometers high (12 miles). It extends over 1,300 kilometers (808 miles) from side to side, along its midsection. No other moon in the solar system has such a striking geological feature. In places the ridge is comprised of mountains. Some are bright white, while others have exactly the same brownish color as the dark material. In height, they rival Olympus Mons on Mars, which is surprising for a small body like Iapetus. Mars is nearly five times the size of Iapetus.
Dr. Tilmann Denk, imaging team associate at Freie Universitaet in Berlin, Germany, and the planetary satellite expert responsible for planning the Iapetus imaging observations, was delighted with the results of the flyby and amazed at the new findings. "When we first saw hints of an equatorial feature within the dark terrain in early images, we did not realize that this would turn into one of the major discoveries on Iapetus. This is the prize after more than two years of detailed planning. The Iapetus images alone will keep us busy for years."
The flyby images, which revealed a region of Iapetus never before seen, show feathery-looking black streaks at the boundary between dark and bright hemispheres, that indicate dark material has fallen onto Iapetus. They also show craters near this boundary with bright walls facing towards the pole and dark walls facing towards the equator.
"We can now say with reasonable certainty that the leading hemisphere of Iapetus was coated by a dark material falling onto it from a particular direction," said Dr. Carolyn Porco, imaging team leader and director of the Cassini Imaging Central Laboratory for Operations in Boulder, Colo. "What isn't clear yet is where the material came from. However, that equatorial ridge is looking awfully suspicious."
The leading theories are material originating from either within or outside Iapetus. Opinions differ on which is the correct explanation.
Dr. Gerhard Neukum, a professor at Freie Universitaet and an imaging team member said, "I think there is evidence now that a genetic relationship exists between the dark material in Cassini Regio and the newly detected equatorial bulge. The process responsible for placement of the dark coating could be plume-style eruptions at the equatorial bulge where dark material accumulated at the surface as fallout."
Dr. Alfred McEwen, a professor in the Department of Planetary Sciences at the University of Arizona, Tucson, takes the opposing view. "There are no clear volcanic landforms on Iapetus," he said. "The surface is entirely heavily cratered, so any volcanism must have been minor, there is no known source of energy for volcanism, and the body has an irregular shape, so it probably isn't differentiated from internal heat. This is about the least likely place in the solar system to find volcanism, especially post-dating all the cratering."
Other scientists who thought the material originated from the outside of Iapetus are now waffling as a result of the new images. "A week or so ago, I would have said "outside" without batting an eyelid," said Dr. Paul Helfenstein, an imaging team associate and an expert in icy satellites from Cornell University, Ithaca, New York. "I'm still inclined to lean that way, but now with the discovery of the bizarre equatorial ridge I'm non-plussed! I kind of like it that way... makes the investigation all the more fun. I suspect that our data will soon reveal some more secrets."
Cassini's next close encounter with Iapetus will occur in September 2007. The resolution of that flyby could be 100 times better than the ones currently being analyzed. The hope is that the increased detail may shed light on the whether or not the equatorial bulge on Iapetus has been volcanically active in the past.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The European Space Agency built and manages the development of the Huygens probe and is in charge of the probe operations. The Italian Space Agency provided the high-gain antenna, much of the radio system and elements of several of Cassini's science instruments. The imaging team is based at the Space Science Institute, Boulder, Colo.