On October 25, 1671, Giovanni Domenico Cassini, an Italian astronomer, mathematician, astrologer and engineer, discovered Iapetus (pronounced eye-app-eh-tuss), one of Saturn’s many moons. He also discovered Tethys, Dione and Rhea (moons of Saturn), Cassini Division in the rings of Saturn and numerous other things, but we will deal only with Iapetus this week.
The third-largest and third-most massive of Saturn’s moons, Iapetus is also the most distant of Saturn’s major moons. It has an equatorial ridge - a chain of 10-kilometre high mountains - running across the moon’s equator and an orbit that is inclined at a little over 15 degrees to Saturn’s equator. We will, however, be focussing on its two faces, a feature that has been posing a question ever since Iapetus was discovered.
Hide and seekAfter discovering Iapetus, Cassini decided to continue observing it. To his astonishment, he noticed that the moon was completely invisible when it had to be on Saturn’s eastern side. Not one to give up easily, Cassini followed up the next year by searching for the moon again. Again, while the moon was visible on Saturn’s western side, it went on to disappear in the eastern side. It was as if Iapetus had decided to play with Cassini... play a game of hide and seek.
Decades passed and so did the telescopes with which he worked with, as they improved constantly. Finally, early in the 18th century, Cassini was able to observe Iapetus on both sides of Saturn, but found out that it was much fainter on the eastern side.
Cassini made two conclusions based on his observations, both of which turned out to be correct. He suggested that Iapetus was two-toned — one side much darker than the other — and that it was tidally locked with Saturn. But the question remained as to how Iapetus became two-toned. In fact, it remained a question and puzzled scientists for over 300 years.
Mystery solvedIt was only early in this century that NASA’s Cassini space probe was able to observe Iapetus up close and confirm that the trailing hemisphere is in fact 10 to 20 times more reflective than the leading hemisphere.
The reason for this has been found to be a combination of the moon Phoebe’s feature and thermal segregation. A moon of Saturn that is a captured object from the Kuiper belt, Phoebe is not only the only moon of Saturn that orbits in the opposite direction, but is also far more distant than that of Iapetus and is also very, very dark.
Over time, dark particles emitted by Phoebe as a steady stream has accumulated on one side of Iapetus and not the other. Thermal segregation then explains why the stuff from the bright side, which is ice, doesn’t simply cover up the dark material.
With a rotational period of over 79 days, the slow rotation implies daily temperature cycles are long, allowing accumulated dark material to absorb more heat - the same way a black surface left out in the Sun becomes hotter to touch than a white surface. This heating causes the icy, volatile content within the dark material to sublime out and move to colder regions on Iapetus.
The dark regions of Iapetus thus become even darker and the brighter, colder regions end up becoming even more reflective. The result, in essence, is a yin and yang style moon for Saturn that is unique to the entire solar system.
Write to A.S.Ganesh at ganesh.a.s@thehindu.co.in
