The Cassini Spacecraft of the National Aeronautics and Space Administration (NASA) has revealed surface characteristics of one of Saturn's moon, the Iapetus.

Dubbed as "Yin-Yang" moon, Iapetus is quite unique because its leading region exudes a reflectivity that is so dark, it resembles a coal. The hemisphere that follows, on the other hand, is significantly brighter. 

The difference between the hemispheres was observed by Giovanni Cassini in 1671 when he discovered the said moon. He said that he could observe Iapetus only on the west part of Saturn. Such observation was an accurate conclusion that one side of Iapetus had a darker side than the other and that it was tightly cinched with Saturn.

The reason why the hemispheres of Iapetus are different has long baffled scientists. Aside from that, the dark part of the moon is said to be darker than any other surfaces in the entire Saturn system.

One theory is that Iapetus may be brushing off particles from the distant dark moon, Phoebe. Should this be accurate, the dark surface of Iapetus should be constantly renewed due to the low amounts of fresh bright craters identified within the dark terrain.

Another angle being looked at in explaining the surface of Iapetus is the presence of a possible ice volcanism that disburse darker particles to the surface. The dark surfaces may be created by hydrocarbons that resemble volcanic eruptions. "Volcano-like eruptions of hydrocarbons might form the dark surfaces, particularly after chemical reactions caused by solar radiation," NASA explained.

In the Cassini flyby of Iapetus, which flew in September 2007, a process called thermal segregation was found to be the most probable explanation for the moon's dark hemisphere.

Lapetus rotates very slowly such that it can take up to about 79 days for one rotation to complete. The slow rotation could signify that the cycle of temperature everyday is notably long so that the dark object, which absorbs more heat than bright icy objects, can take in heat from the Sun. Such warming mechanism will trigger any transient or icy materials situated in the dark to step up and move out to the colder parts of the moon. The dark materials on the moon would turn darker, causing the adjacent bright regions to turn brighter.

Dark materials from external sources may have potentially flocked to the Iapetus in small amounts, which could have caused the process of thermal segregation.

Lapetus measures 914 miles in diameter and has a density of about 1.2 times more than that of water. In the past, experts have suggested that the moon is comprised of three quarters of ice and one quarter of rock.