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Scientists at Freie Universität Solve Centuries-old Mystery of Saturn’s Moon Iapetus

Published in the Prestigious Journal "Science"

№ 361/2009 from Dec 11, 2009

Scientists at Freie Universität Berlin published on December 10 in the prestigious journal Science, together with colleagues from the United States and the German Aerospace Center (DLR) in Berlin-Adlershof, two papers containing the most plausible explanation given thus far for the mystery, unsolved for more than three centuries, of the extreme brightness dichotomy (two completely different hemispheres) of Saturn’s moon Iapetus. To this end, the researchers used image and temperature data recorded and measured by instruments of the international Cassini-Huygens Saturn mission. Lead author on the German side is Cassini imaging team associate and researcher at the Planetary Sciences and Remote Sensing division at Freie Universität Berlin, Tilmann Denk. The co-authors are Cassini imaging team member, Prof. Dr. Gerhard Neukum, head of Planetary Sciences and Remote Sensing at Freie Universität, and two other researchers in the Cassini group.

The research on Saturn's moon Iapetus is documented in a number of publications in the journals Science and Nature, where for several years, the latest findings from the cameras on the Cassini-Huygens spacecraft of Saturn and the Mars Express of Mars, have been made accessible to scientists worldwide. In the exploration of Saturn's moons, the scientists in Berlin are not only involved in the data analysis, but also crucially involved in the observation planning. For example, practically all the camera shots of Saturn's moon Iapetus were planned and prepared in Berlin.

Images from the Voyager 1 and Voyager 2 spacecrafts in 1980 and 1981 and the Cassini spacecraft since 2004 show the exact course of the brightness dichotomy on the surface. According to these images, the dark region near the equator, called the Cassini Regio, extends far into the trailing side (antapex) of Iapetus, while bright material near the pole is also found on the leading side (apex). Tilmann Denk from Freie Universität Berlin explains: "Slight temperature differences promote the sublimation of water ice, especially on the leading side. In the process dark, low-volatile material is left behind, which is further heated by solar radiation. The process reinforces itself, and after about 1 to 2 billion years the uppermost decimeters are virtually ice-free and very dark."

Critical to the emergence of the brightness dichotomy in the observed shape is the interaction with a second effect that was discovered in the image data. Due to a minimal but persistent incidence of infalling dust on the leading side of Iapetus, resulting in a light asymmetry in color and brightness, the thermally induced redistribution of water ice is not only a function of the local angle of incidence of solar radiation (thus of the latitudes of the moon), but also dependent on the longitudes and, for that reason, primarily on Iapetus's leading side. These effects are described in the two publications.

The Cassini-Huygens mission to Saturn is a joint endeavor of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Italian Space Agency (ASI).

The Cassini research of the Remote Sensing and Planetary Sciences division of the Institute of Geological Sciences at Freie Universität Berlin is funded by the German Federal Ministry of Economics and Technology / German Aerospace Center (DLR Space Agency).

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Website of Planetary Sciences and Remote Sensing at Freie Universität Berlin:

For more information about the Cassini-Huygens Mission, see: