On 1 July 2004, seven years after its launch from Earth, NASA's Cassini orbiter fired its main engine, throttled back its speed and allowed itself to be pulled in by Saturn's gravity. In the three years since then, the spacecraft has provided snapshots of the strange and fascinating worlds that inhabit the Saturn system: the eerily Earth-like Titan, for instance, the rain on whose plains is mainly methane; or Enceladus, from near whose 'tiger-striped' southern pole an icy plume spouts into space.
No moon, however, seems quite as odd as the body that hove into Cassini's close view on 25 September 2005. Irregularly shaped and chaotically rotating, Hyperion, the eighth-largest saturnian moon, looks for all the world like a bathroom sponge.
Two papers in this issue take a detailed look at this odd character. In the first of these, Thomas et al. (P. C. Thomas et al. Nature 448, 50–53; 2007) conclude that Hyperion's sponge-like appearance (Fig. 1 on page 51) comes from an unusually high density of well-preserved craters between 2 and 10 kilometres across. According to the authors' theory, these discrete craters survived, rather than having been eroded away or filled in by material ejected in the impacts that formed them, because Hyperion's interior is indeed porous. A meteorite hitting a significantly porous body will compress its surface rather than excavate it, and produce much less ejected material.
The authors arrive at this explanation by first calculating Hyperion's mean radius — no mean feat, as its irregular shape and random spinning make imaging it in one shot impossible. They were also able to compute the moon's mass through a dynamical model requiring that Hyperion maintain a stable orbit in the complex gravitational environment of the Saturn system. This mass was only just over half that expected for a body of Hyperion's 135-km calculated mean radius, if it were made of water ice — and considerably less if higher-density materials are present. In other words, there is far less within Hyperion than had been expected.
In the second of the papers, Cruikshank et al. (D. P. Cruikshank et al. Nature 448, 54–56; 2007) delve more deeply into Hyperion's make-up. Using data from ultraviolet and infrared spectrometers onboard Cassini, they divide the moon's surface into distinct areas of high reflectivity, seemingly dominated by water ice, and low reflectivity, concentrated at the craters' bases. These latter areas have a considerably diminished water-ice signature, but a prominent absorption band best explained by the presence of solid carbon dioxide in complex with a further, unknown material.
Hyperion's spectra, like those of other saturnian satellites such as Phoebe and Iapetus, also provide evidence for irregularly scattered, nitrogen-rich, organic molecules. The image shown here of three surveys overlaid on the moon's surface topography (blue, H2O; red, CO2; green, carbon nitrides, CN) gives an idea of the complexity of the picture. Cyan indicates areas with strong H2O and CN signals, but little CO2; yellow areas are dominated by CO2 and CN, with little H2O; and magenta equates to CO2 and H2O, but little organic material.
What these discoveries tell us of Hyperion's origin and the wider history of the Saturn system is as yet unclear. But as the pictures from Cassini continue to roll in, the varied worlds they depict do not cease to surprise.