Hyperion
Published on Jan 28, 2007 at 2:04 pm.
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Filed under extraterrestrial moons.
I saw that today’s Astronomy Picture of the Day was Hyperion, so I thought that I’d say a bit about this moon. In September, 2005, the Cassini spacecraft flew by Saturn’s moon Hyperion, providing us our best view of this enigmatic body. And, enigmatic it certainly is. Discovered in 1848, little was known of Hyperion until the Voyager missions. Voyager 2 actually photographed Hyperion in 1981, but from such a great distance that little detail was possible. However, we did find that Hyperion is non-spherical, somewhat unusual for a body this size. Hyperion is shaped sort of like a slightly flattened potato, 360 kilometers long and 280 or 225 kilometers wide (depending on which way you measure it).
The Voyager studies did show that Hyperion rotates chaotically. Chaotic rotation means that not only does the rate of rotation change, but so does the axis of rotation. Hyperion tumbles around Saturn rather than spinning on an axis. If you were on Hyperion, you’d never know when or where the Sun would rise. Furthermore, the rotation is, in fact, chaotic, meaning not just complicated, but unpredictable. You can not calculate its rotation more than a number of cycles into the future. The chaotic rotation is likely due to gravitational torques that Hyperion experiences from Saturn’s largest moon, Titan. Hyperion has a 3:4 orbital resonance with Titan (three orbits of one equals 4 orbits of the other), and that means that it gets similar tugs at the same time every few orbits. Its elongated shape means that there will be greater torque on it than if it were spherical.
But, Cassini’s 2005 pass revealed even more startling features. For one thing, the surface of Hyperion looks like no other body that we have yet seen in the Solar System. It is covered in deep, but narrow, craters, and looks remarkably like a giant sponge. Hyperion has a low mass, and it is believed to be mostly made of water ice, like many of Saturn’s moons. It has a darker albedo (reflectivity) than similar objects, so it is believed to be covered in a thin layer of something dark. It isn’t clear what that something is, but frequently in the outer Solar System dark means organic (carbon bearing) compounds, but normally you get those farther from the Sun than Saturn. There seems to be more dark material at the base of the distinctive narrow craters than elsewhere. Those narrow craters are as of yet unexplained, though there have been some pretty speculative suggestions. Hyperion has a very low density, too, coming in at only 0.6 g/cm3. That is far lower than other moons, and it suggests that the entire moon migh be riddled with caverns, so its spongelike features may continue throughout. That is even harder to explain.
Some speculations that I have heard for the craters is that perhaps the dark material absorbs sunlight and warms up. So, whenever the Sun shines into the craters, the material at the bottom warms that warms the ice beneath it. There is no atmosphere, so the ice won’t melt, but it can sublimate. But, it seems unlikely that enough heating could happen in the brief time that the Sun shines into the craters. And besides, at that distance from the Sun, the amount of solar heating would be prettly low, anyway. Other suggestions are that perhaps Hyperion was originally composed of a mixture of water ice and frozen carbon dioxide (dry ice). But, that would suggest that Hyperion is a captured body, such a giant comet or a Kuiper Belt object. There is little indication of that. Besides, its orbit is very close to that of the other Moons, and it is not terribly eccentric (elliptical) as would normally be expected of a captured body. Other suggestions are that it is a piece of a larger body broken up by an impact. That still doesn’t explain everything though.
So, Hyperion remains a mystery. But, Cassini is still in orbit around Saturn, and there will be more studies of Hyperion, so perhaps in the future we’ll have a better idea how this strange body came to be this way.
-Astroprof
(Images courtesy of NASA, JPL)
