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The mystery of Iapetus
SINCE the late 17th century, astronomers have been puzzled by Saturn’s moon Iapetus (eye-AP-I-tus). The mystery is that half of Iapetus is bright whereas the other half is absolutely dark. It was named after a Greek Titan, who, as legend has it, was imprisoned by Zeus after the war against the gods. According to the Voyager 2 image taken on August 22, 1981, Iapetus has a bright icy terrain with a heavily cratered surface and a dark terrain, which is probably covered by some dark material.
Its dark side is the one that faces Saturn. Except at the poles, where the material is bright, the entire side that faces Saturn is dark. The orbit and rotation of this large outer moon is the same as Earth’s Moon which is in “lockstep” with Earth’s orbit. Thus we always see the same side of our Moon.
Iapetus is 907 miles wide and orbits Saturn at 2.2 million miles or 3.6 million kilometres. Both Saturn and its moon take 79 Earth days to complete their orbits. Though the Voyager 2 spacecraft took the interesting image, it does not explain the dark side.
A split personality
Astronomers feel that the reason for the split personality of Iapetus could be that the dark side picks up debris. A good example would be the front of a race-car, which is continuously hit with the dirt, and tire rubber that other cars leave off. The debris as far as Iapetus is concerned, could be tiny pieces of Phoebe, another moon, which could be impacted by meteors. But the puzzle is not solved as the dark Phoebe, which has a backward orbit, does not have the same dark colour as Iapetus. And interestingly, the dark material on Iapetus is more concentrated in the bottom of the craters. Thus the theory evolved that whatever the strange dark material is, it oozes from under the surface.
Recently, Iapetus was scanned by radar by scientists who used the Arecibo Observatory situated in Puerto Rico. Though the picture is not very much clearer, some new facts have come to light.
According to Gregory Black of the University of Virginia, “it is known that the bright side is mostly water-ice, but we find it does not reflect the radar like other icy satellites that we’ve studied with the radar before. The ice on Iapetus appears much less reflective.”
The reason for the assumption is that clean water ice is radar-bright so the ice must contain some material that is radar darkening. Black thinks, “most likely this is a bit of ammonia mixed with the water ice.”
These results were reported last month in the journal Science. Though astronomers have theorized that ammonia might be present on Saturn’s moons, there is no direct evidence. Black also states, “another surprise is that the radar system sees Iapetus as a uniform object, meaning no difference between the light and dark sides.”
This could mean that there might be a thin coating of some dark material on the ammonia-laden water ice, like dirt on clean ice. Black further explains, “a thin coating would not have much effect on the radar reflection, which sees the underlying ice, and therefore both sides would look the same in radar but different optically. This interpretation depends somewhat on what the dark material is made of, but we are not able to answer that question.”
This summer, the Cassini spacecraft will be conducting a thorough examination of the Saturnain system and we might then get a better picture of what’s going on. It is scheduled to make one or more close flybys of Iapetus and make fresh observations.
“That will be better able to determine compositions, in particular that of the dark material, or directly detect ammonia,” believes Black. In addition, the space probe will also closely observe the boundary between the dark and the bright side which will help solve the mystery of whether the dark material oozes from underneath or comes from above.
“Cassini will certainly help enormously. Although I’m not sure I can say if Cassini will solve these issues completely,” says Black. In case Cassini does solve this intriguing phenomenon, it will help solve a centuries-old investigation of the mystery. Cassini is named after Giovanni Domenico Cassini, the man who actually discovered Iapetus in 1671. He also saw that the though the moon was visible when it was on one side of Saturn, it disappeared when it went around the other side. It was then that he discovered that Iapetus had a split personality.
What’s a moon?
Even after years of discovery and theories, the definition of a moon is still hazy. More so now as so many kinds of celestial rocks around more rocks have complicated the issue. And no one seems to be in a hurry to specify what exactly we can call a moon. Small moons, larger moons and even minuscule moons are all being discovered by the dozens in our own solar system. The tally is way beyond a hundred and is expected to double if not triple in a few years.
From the smallest (1-km) satellite of Jupiter to the large moons that orbit the near and far regions of space, they are sources of constant mystery and wonder. Some orbit backwards, some have strange far-out elliptical orbits while some follow more conventional behaviour. And some hold shaky positions and are suspected to be captured asteroids, such as Phobos, Mars’ largest moon.
“There’s no accepted definition,” says Alan Stern of the Southwest Research Institute, (SwRI).
So how do we differentiate the many tiny and larger strange orbiting rocks from moons? Inside Saturn’s rings are numerous chunks of rocks. Embedded in those rings are also moons. Then there are also invisible moons. So small that they cannot be detected easily, like the Hydrogen atoms that circle Earth’s outer fringes. Their behaviour is moonlike but they can’t be moons. So size does play a role. In other words, what would we call a 6-inch rock that orbits a planet?
Stern calls it a 6-inch rock but “it’s technically a satellite,” which even he has acknowledged. He also suggests a category called “minor moons” but that is not a very serious suggestion.
Sooner or later the definition will have to be more specific, agrees Joe Burns, a mathematical theorist and astronomer from Cornell University, and who is on the Cassini imaging team.
There is also another problem. While countries around the world keep sending huge chunks of metal objects into space, the term “satellite” confuses the public like never before. Now mostly, the term is thought to be a man made object that orbits. Thus leading to a loose classification of the term. “The one property that I think a satellite must posses to be a satellite is an orbit that encircles its planet,” says Burns.
At present, the types of moons that orbit a planet are classified as: irregular moons, regular moons and inner moons, which are also called “ring moons.” The irregular ones have a shady origin. They could either be asteroids, comets or pieces of debris left over from planet formation.
In addition they also have strange trajectories. They sometimes zoom close to the planet and speed off into far out regions. In a research that Burns carried out, these unstable orbits are due to the Sun’s gravity. In which case, the so-called moon would either crash into the planet or fleet past it. He also states that it is very difficult for a planet to lure a moon into its orbit.
“If it comes from infinity, its going back to infinity,” he explains. It could be that these irregulars ran into a stretched area of gas around a giant planet 4.5 billion years ago. As yet, scientists are not even sure how planets were formed so having an idea of how moons were formed would even help them in learning about planet formation.
When Nasa’s New Horizons mission launches in 2006, to study the Pluto-Charon system, experts might have a better idea. But whatever definition the future holds of these “rocks around rocks’ will certainly change man’s perspective of what we can call a moon. One thing, though remains the same from times immemorial. What we can look up and see in the sparkling sky, the alluringly familiar sight of our beautiful “Moon.”
The writer
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