Close up view of Prometheus

On 27 January 2010, the Cassini spacecraft acquired this close-up view of the small 60km Saturnian satellite Prometheus. Shadows cast by its irregular surface suggests that this satellite might have the `flying saucer' shape that is also exhibited by Saturn's other small satellites Pan and Atlas (as detailed in Porco et al 2007). Note that this peculiar shape can result when a satellite forms while orbiting inside a dense planetary ring (such as perhaps Pan, which does orbit in Saturn's main A ring) while also under the influence of Saturn's strong gravitational tide. This in turn suggests that Prometheus (as well as its cohorts Atlas and Pandora) might actually have formed at the outer edge of the A ring and then migrated to its current position just beyond the A ring's outer edge. This kind of radial migration is in fact expected to be driven by the strong gravitational torques that the A ring exerts on the nearby satellites. Check the CICLOPS website for more details.

Iapetus' dichotomy explained

Iapetus is Saturn's icy yin-yang satellite. This Cassini image shows that the side of this satellite that leads in its orbit about Saturn is 10 times darker than its polar regions. This darkening was long thought to be due to contamination from Saturn's even more distant satellite Phoebe. Meteorites crashing into Phoebe will launch small debris that goes into orbit about Saturn, resulting in a vast dust ring that also contaminates Iapetus. This dust ring was only recently discovered by the Spitzer Space Telescope. Some of that dark dust gets deposited at Iapetus' leading face, which is then warmed by sunlight. Recent work by John Spencer and Tilmann Denk show that this can cause surface ice there to evaporate at the equator. That water vapor can migrate towards and then freeze out at the poles, brightening the satellite there, and giving it its yin-yang appearance. Additional images can be found at Cassini's CICLOPS website.