Saturn's Sponge: The mysteries of Hyperion revealed
(Courtesy of: NASA/JPL/ University of Arizona/Ames/Space Science Institute)
Saturn's small moon, Hyperion, doesn't quite live up to the ringed majesty of its planetary parent. Nor is it at all like the luminous orb that gazes down from Earth's night sky. With its deeply perforated surface and ovoid body, Hyperion resembles nothing so much as a giant "cosmic sponge," says planetary researcher Carolyn Porco of the Space Science Institute in Boulder, Colorado. Porco coauthored one of two new papers published July 5, 2007 in the journal Nature that examine the moon's bizarre appearance in detail and reveal its blotchy surface chemistry.
Saturn watchers have known Hyperion was a bit of an odd duck since NASA's
Cassini orbiter snapped the first close-range portraits of the moon in
September 2005. It is just one of 60 intriguing known satellites that swarm
the ringed planet, but Hyperion still manages to distinguish itself. Although
a fair number of this crowd forgo the spherical shape of our moon, Hyperion
is the largest of its misshapen siblings. Hyperion also reflects little
light, despite its icy exterior—especially from the centers of the dark
craters that pock every inch of its surface. Add to this the fact that Hyperion
is a "tumbling moon"—rotating chaotically through space rather
than spinning sedately—and the strange body is hardly recognizable as a
moon at all.
These latest analyses bring the details of this Bizarro world into even
more sharp relief, quite literally. Unlike the soft-looking rings and mounds
decorating the surface of our own moon, Hyperion's craters appear sharp-edged,
deep, and freshly made. Missing is the splash of ejected material that
blankets craters on other planets and moons, as if objects hitting Hyperion
tunneled right into the surface.
Porco and her colleagues suggest that this is exactly what happened. Instead of blasting material outward, asteroids and other impactors hitting Hyperion simply compress its porous outer layers. Analysis of Cassini's data indicate that Hyperion is so porous, in fact, that it has half the mass expected for an icy body of it's size. Not only do collisions compress the surface like so much meringue, the lightweight moon doesn't have enough gravity to hold onto much of the ejecta produced by impact. Instead of falling back onto the craters, it just floats off into space.
The resulting depth of the craters casts their centers into shadow. Reddish-black deposits inside enhance the effect, giving rise to initial speculations that dark material had somehow pooled in the basins. "The surface of Hyperion is made of dirty-water ice," says Dale Cruikshank of the NASA-Ames Research Center and coauthor of the other Nature paper.
Various hydrocarbon combinations dot the icy surface of Hyperion. (
Courtesy of : NASA/JPL/ University of Arizona/Ames/Space Science Institute)
He and his colleagues determined the chemical composition of the surface,
the entirety of which is coated with a fine organic dust. Using Cassini's
Visual and Infrared Mapping Spectrometer they mapped the unevenly distributed
materials and color-coded the results: blue = exposed frozen water; red
= carbon dioxide ice ("dry ice"); magenta = a mix of water ice
and carbon dioxide ice; and yellow = a mix of carbon dioxide ice and an
The material of least reflectivity, or albedo, on Hyperion closely resembles
the surface composition of two other Saturnian moons, Pheobe and one hemisphere
of Iapetus. This complex of organics forms a common thread between these
moons and the interstellar dust and comets, says Cruikshank. Hyperion may
be unique, he notes, but it is made of "the same organic stuff, not
only in our solar system, but throughout our galaxy and other galaxies."
It seems oddball Hyperion isn't so odd afterall.
—Jessa Forte Netting
Thomas,P. C.,…Porco,C., et al., Hyperion's sponge-like appearance. , Nature, 448: 50-56, (5 July 2007) [DOI:10.1038/nature05779]
Cruikshank, D. P., et al.Surface composition of Hyperion. , Nature, 448: 54-56, (5 July 2007) [DOI:10.1038/nature05948]
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Related Web Sites:
Cassini-Huygens Mission to Saturn & Titan,
NASA Solar System Exploration - Hyperion,
Cassini Visual and Infrared Mapping Spectrometer,
CICLOPS - Cassini Imaging Central Laboratory for Operations
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