Cassini scientists wring out the details on spongy Hyperion
Thu Jul 5, 2007 at 07:50 UTC
Scientists on NASA's Cassini mission to Saturn now have a better understanding of why the odd moon Hyperion has such an unusual appearance.
The crucial factor in creating the strange, sponge-like appearance of Hyperion appears to be its extremely low density, say Cassini scientists in a research paper being published in the July 5 issue of the journal Nature. The researchers examined Cassini spacecraft images and other data on the moon's mass acquired during encounters with Hyperion over the past three years.
Hyperion is covered by a large number of medium-sized, well-preserved craters. "Careful mapping of features showed that its bizarre appearance probably results from a convergence of rather routine effects," said Dr. Peter Thomas, a Cassini Imaging Team member working at Cornell University in Ithaca, New York.
Cassini's closest encounter with Hyperion, in September 2005, allowed accurate measurement of the moon's mass and size, which showed it is only slightly more than half as dense as water.
"The close flyby produced a tiny but measurable deflection of Cassini's orbit. Therefore, the orbit determination, carried out by our Italian colleagues, allowed us to estimate the mass with fairly good accuracy," said Cassini radio science deputy team leader Nicole Rappaport of the Jet Propulsion Laboratory in Pasadena, Calif. "Combined with the determination of Hyperion's volume from imaging data, this provided an accurate computation of its density."
Initial conjectures about the origin of Hyperion's strange surface invoked dark materials accumulating on crater floors that subsequently got warmed by sunlight and melted deeper into the surface. However, the new analyses have shown that the moon's low density, and therefore low surface gravity, may cause craters to form differently from those on other, denser bodies that have been explored in the Solar System.
Image Credit: NASA/JPL/Space Science Institute
High resolution image
This false-color view of Saturn's moon Hyperion reveals crisp details across the strange, tumbling moon's surface. Differences in color could represent differences in the composition of surface materials. The view was obtained during Cassini's close flyby on Sept. 26, 2005.
Hyperion has a notably reddish tint when viewed in natural color. The red color was toned down in this false-color view, and the other hues were enhanced, in order to make more subtle color variations across Hyperion's surface more apparent.
According to the researchers, impactors smacking into Hyperion's porous outer layers form craters more by compressing the surface than by blasting out material, as they do on denser bodies. Additionally, the moon's low gravity means that any material ejected from craters on Hyperion has a good chance of escaping completely and not re-impacting the surface, making Hyperion's craters look sharper and less blanketed by debris than on other bodies.
These new results about Hyperion's craters accompany another paper in the same journal issue which details the moon's composition. Both are important to understanding this unique moon's history and evolution. (Cassini spacecraft finds hydrocarbons on Saturn's moon Hyperion)
"With time, we are coming to understand the various planetary processes that shape the bodies in our solar system," said imaging team leader Carolyn Porco. "And this latest work on Hyperion is a beautiful demonstration of that."
