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Spongy-Looking Hyperion Tumbles Into View

July 12, 2005

JPL — Two new Cassini views of Saturn’s tumbling moon Hyperion offer the best looks yet at one of the icy, irregularly-shaped moons that orbit the giant, ringed planet.

The views were acquired between June 9 and June 11, 2005, during Cassini’s first brush with Hyperion.

Hyperion is decidedly non-spherical and its unusual shape is easy to see in the movie, which was acquired over the course of two and a half days. Jagged outlines visible on the moon’s surface are indicators of large impacts that have chipped away at its shape like a sculptor.

Preliminary estimates of its density show that Hyperion is only about 60 percent as dense as solid water ice, indicating that much of its interior (40 percent or more) must be empty space. This makes the moon more like an icy rubble pile than a solid body.

In both the movie and the 3D image, craters are visible on the moon’s surface down to the limit of resolution, about 1 kilometer (0.6 mile) per pixel. The fresh appearance of most of these craters, combined with their high spatial density, makes Hyperion look something like a sponge.

The moon’s spongy-looking exterior is an interesting coincidence, as much of Hyperion’s interior appears to consist of voids. Hyperion is close to the size limit where, like a child compacting a snowball, internal pressure due to the moon’s own gravity will begin to crush weak materials like ice, closing pore spaces and eventually creating a more nearly spherical shape.

The images used to create these views were obtained with Cassini’s narrow-angle camera at distances ranging from approximately 815,000 to 168,000 kilometers (506,000 to 104,000 miles) from Hyperion. Cassini will fly within 510 kilometers (317 miles) of Hyperion on Sept. 26, 2005.

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 imaging team is based at the Space Science Institute, Boulder, Colo.

Hyperion Movie

Play Quicktime movie showing Saturn’s moon Hyperion

This movie sequence provides the record of Cassini’s first close brush with Hyperion, Saturn’s chaotically tumbling moon. As the spacecraft whizzes past, Hyperion’s unusual shape is most apparent. The jagged outlines are indicators of large impacts chipping away at Hyperion’s shape as a sculptor does to marble.

Hyperion’s irregular dimensions are 164 by 130 by 107 kilometers (102 by 81 by 66 miles).

These Cassini images are the best views yet of one of the large, low-density objects that orbit Saturn. Hyperion is close to the size limit where, like a child compacting a snowball, internal pressure due to the moon’s gravity will begin to crush weak materials like ice, closing pore spaces and eventually creating a more spherical shape.

However, this moon has a very irregular shape and preliminary estimates of its density show that it is only about 60 percent as dense as solid water ice. This suggests that much of its interior (40 percent or more) must be empty space.

The low density further suggests that Hyperion is mostly made of water ice, with a low rock and metal content. If the moon had significant higher density components, its implied porosity would be significantly higher than 50 percent. The dark material on the surface is therefore likely a minor component, possibly originating from impacts of dark material, as seen on Iapetus.

Hyperion’s elliptical orbit and irregular shape influence its chaotic tumbling. Further, because it is in a resonance orbit with the giant moon Titan, impact debris ejected with sufficient energy does not come to rest again on Hyperion. Instead, debris is tugged gravitationally into Titan’s orbit, where it impacts the large smoggy moon.

This series of 25 images was taken over a period of nearly two and a half days, between June 9 and June 11, 2005, as Cassini’s orbit took it close to Hyperion.

Cassini will have one close, targeted flyby of Hyperion on September 26, 2005.

At the beginning of the movie Cassini was approximately 815,000 kilometers (506,000 miles) from Hyperion; at the end, the spacecraft was 327,000 kilometers (203,000 miles) distant. The closest image was acquired from a distance of 168,000 kilometers (104,000 miles).

The images were taken using the narrow-angle camera and a spectral filter sensitive to ultraviolet wavelengths centered at 338 nanometers. Image scale ranges from 5 kilometers (3 miles) per pixel at most distant to 1 kilometer (0.6 mile) at best. The images have been enhanced to improve the visibility of surface features.

A stereo anaglyph (3D) image from this encounter is also available (see PIA06244).

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Spongy-Looking Hyperion Tumbles Into View


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