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Mike Malaska • Mar 23, 2011

LPSC 2011: Sponge-moon Hyperion

Almost two weeks since the Lunar and Planetary Science Conference, and I'm still getting notes on talks! This writeup came from Mike Malaska. By day, Mike is a Ph.D. organic chemist leading drug discovery projects for a pharmaceutical company. By night, he is a space enthusiast fascinated by the surface geology of Titan and other planetary bodies. I thank him for these notes on Hyperion! --ESL

Saturn's moon Hyperion has a bizarre sponge-like appearance that is in dramatic contrast to other heavily cratered bodies in the solar system. In the view below, a relatively massive 130-kilometer diameter crater can be seen to have been blown out of the 270-kilometer moon, followed by further impacts that peppered the surface.

Color mosaic of Hyperion
Color mosaic of Hyperion Color mosaic of Hyperion from Cassini's flyby on Sept. 26, 2005, at about 00:40 UTC.Image: NASA / JPL-Caltech / SSI

A presentation by Alan Howard and coauthors attempted to explain the appearance of Hyperion through a surface evolution model. Their model assumes that Hyperion is not composed of solid ice, but is instead composed of a matrix of water ice interspersed with a frozen volatile component. Carbon dioxide (CO2) was mentioned as a possibility for the volatile component, but other molecules are possible.

Evolution of an impact crater on Hyperion
Evolution of an impact crater on Hyperion An explanation for how Hyperion got its "spongy" appearance, based on a model proposed at the 2011 Lunar and Planetary Science Conference by Alan Howard and coauthors. Due to Hyperion's low gravity, most of the debris tossed outward from an impact crater (A & B) would be lost from Hyperion, going instead into orbit around Saturn. Whereas craters on other bodies, like the Moon, Mars or Mercury, initially have a rim and ejecta deposits outside the crater, Hyperion would lack these. Also, since not much ejecta falls back to the surface following an impact, there would not be an intracrater plain created on the surface. So crater saturation will occur relatively quickly on Hyperion compared to larger bodies.The release (uncapping) of volatile matrix materials along the crater walls weakens the exposed "bedrock" and causes mass-wasting (landslides) into the bottom of the crater, creating crenulated rim shapes and a debris pile at the bottom of the crater (C). Throw in a later addition of dark-colored material that collects in the unlit depths of the crater, and you get the surface we see today (D).Image: Mike Malaska (based on a model proposed by Howard et al. 2011)

The release (uncapping) of volatile matrix materials along the crater walls weakens the exposed "bedrock" and causes mass-wasting (landslides) into the bottom of the crater, creating crenulated rim shapes and a debris pile at the bottom of the crater (C). Throw in a later addition of dark-colored material that collects in the unlit depths of the crater, and you get the surface we see today (D).

The actual view of the surface of Hyperion (Figure 1) compares well to the projected appearance based on their evolution model (Figure 2).

Actual and simulated surface of Hyperion
Actual and simulated surface of Hyperion A closeup view of Saturn's moon Hyperion (fig. 1) is compared with a simulated Hyperion surface, based upon a model proposed by Alan Howard and coworkers at the 2011 Lunar and Planetary Science Conference. In this model, Hyperion's low gravity is the key to its strange appearance. The width of the figure is about 50 kilometers; vertical relief is about 2.6 kilometers.Image: NASA / JPL-Caltech / SSI / Howard et al. (2011)

This scenario portrays Hyperion as a comet-like object that was never warmed by passage in the inner solar system, but was instead subjected over the eons to bombardment followed by localized volatilization of the matrix near the impact. One question asked during the Q and A session: Why don't some of the other small ice moons of Saturn look like Hyperion? One answer given by a member of the audience was that some of the inner moons may have "something else going on," such as overall deposition from ring debris building a layered mantle atop the surface. (This is an explanation proposed for the weird saucer shapes of Pan and Atlas). Hyperion can now be placed at a midpoint in a spectrum that has icy moons at one end, and comets at the other.

Atlas in Color, 8 June 2005
Atlas in Color, 8 June 2005 Cassini captured this view of Atlas on 8 June 2005, from a distance of 162,000 kilometers. Three images through red, green, and blue filters have been combined to create this color view, which is also enlarged by a factor of two. The view of Atlas shows a saucer shape, 46 by 20 kilometers in size. Atlas' equator appears to have swept up fine ring dust, obscuring any impact craters and bulging its equator. Image data is from the official Cassini data release on the Planetary Data System.Image: NASA / JPL-Caltech / SSI / Emily Lakdawalla

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