Iapetus – The strange moon of Saturn
Iapetus was discovered by Giovanni Domenico Cassini, an Italian astronomer, in October 1671.
Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System, and the largest body in the Solar System known not to be in hydrostatic equilibrium. Iapetus is best known for its dramatic “two-tone” coloration.
Scientists have long wondered why one hemisphere of Iapetus is so dark in comparison to its other hemisphere, and in comparison to other surfaces in the Saturn system. Iapetus may be sweeping up particles from the more-distant dark moon, Phoebe. If that is the darkening mechanism, it should be steadily renewing the dark surface because very few fresh bright craters are detected within the dark terrain. An alternate theory is that there might be ice volcanism distributing darker material to the surface. Volcano-like eruptions of hydrocarbons might form the dark surfaces, particularly after chemical reactions caused by solar radiation.
Discoveries by the Cassini mission in 2007 revealed several other unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon.
While Cassini keep docked Iapetus, the starry sky runs on the bottom. Among all the stars, the brightest, eclipsed for 7 frames, is Bellatrix, in the Orion constellation.
Image credit: NASA/ JPL/ CICLOPS / Space Science Institute/ Kevin Gill