Under the rings of Saturn

Kamal Lodaya, The Institute of Mathematical Sciences, Chennai

Two years ago in 2011 the planet Saturn was at opposition. At that time James O'Donoghue and his colleagues at the University of Leicester made observations using the large telescopes at the Keck Observatory in Hawaii, USA. The Keck observatory has two telescopes with mirrors of diameter 10 meters (each mirror is the size of a classroom).
"Opposition" means that Saturn is opposite to the Sun as seen by us. In other words, the Earth is between the Sun and Saturn. When the Earth is between the Sun and the Moon, the Moon is at opposition. We usually call it "full moon". Saturn was at opposition again on April 15, 2012. Then again this year on April 28, 2013.
A question to think about: why does Saturn come back to opposition every 378 days?

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Saturn: A gas giant with rock-and-ice rings
Saturn is the sixth planet in the solar system, after Mercury, Venus, Earth, Mars and Jupiter. It orbits the Sun at around ten times the distance of the Earth. It takes about 30 years for Saturn to complete one revolution around the Sun. Saturn is huge: its diameter is almost ten times that of the Earth. Its mass is nearly a hundred times that of the Earth. If you think about this, you will see something interesting. The volume of a sphere is proportional to the cube of its diameter, and in fact Saturn's volume is almost 760 times the Earth's. But the mass is not bigger by the same factor, so the density of Saturn must be less than that of the Earth. This is not surprising, because unlike the Earth, Saturn is made up almost entirely of hydrogen gas (94 per cent).
The rings are made of crores of particles of water ice and some rocky material, all orbiting the planet. The innermost particles revolve around Saturn the fastest; the outermost ones the slowest. Some of these particles could be the size of dust; the largest ones are as big as houses. If we imagine putting all the material in the rings together (at least all the material we see now), it would form a "moon" about 400 kilometres in diameter. That is one-tenth the diameter of our own Moon.
Over the last three years the spacecraft Cassini orbiting Saturn photographed evidence of "meteors" hitting the rings. It is thought that a meteor (which is itself made of rock and ice, coming perhaps from outside the Saturnian system) hits the ring material, making smaller pieces which then orbit Saturn. But these smaller bits hit the rings again. The whole sequence should form little clouds of dust. When sunlight strikes the rings at a shallow angle, they can hardly be seen from the Earth. But at this angle the cloud of dust thrown up can be caught by Cassini's cameras and this is what is seen (not the meteor, which is too small).
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Full moon means that the Moon rises in the evening, is overhead at midnight, and sets in the morning. We see it all night. When Saturn is at opposition, it also rises in the evening, is overhead at midnight and sets in the morning, and can be seen all night.
When we are seeing the full moon from the Earth at night, the Sun is behind us, so that what we see is the entire sunlit half of the Moon. Running from its north pole to its south pole is the "noon line", those parts of the Moon where, if you were standing there, the Sun would be overhead. In the same way, at opposition we can see from Earth the entire day side of Saturn and the full "noon line" from its north pole to its south pole.
O'Donoghue and his colleagues made observations of the "noon line" of Saturn in 2011, using a spectrograph. They made measurements of the spectrum in the infrared region of the electromagnetic spectrum. (See the article on Planets for more explanations of these terms.) They noticed small variations in the spectrum of Saturn over all its latitudes.
In particular, they made detailed observation of one of the lines in the spectrum and compared it with that from different parts of Saturn (at different latitudes) taken by space missions. They found that the spectra from the different regions are different. These differences had nothing to do with the angle at which the Sun is above Saturn. Instead, the way the spectrum varied with the latitudes on Saturn matched the angle at which the rings are above Saturn.
We all know that Saturn is the planet with rings around it. O'Donoghue and his team suggested that there may be a "rain" of ice from the rings onto the planet's outer atmosphere. This is an astonishing connection, and it was made because the way the spectrum varies with the latitudes on Saturn.
Could there be a different explanation? Perhaps the differences could be because of different weather in different parts of Saturn? Weather changes can be assumed to depend on the rotation of the planet and so will be symmetric in both the hemispheres. But the observed latitude variation is different between the northern and southern hemispheres, so weather patterns can be ruled out.
Way back in the 1980s when the Voyager spacecraft flew past Saturn, their photographs showed there were dark "spokes" at some places in Saturn's rings. Some scientists had suggested that the rings might be "eroding". The theory of these scientists now explains how the rings may be eroding, simply by falling as "rain" on the planet!
If this theory is true it suggests that the rings of Saturn change with time, and may have been formed long after the planet itself formed. For example, they could be about 10 crore years old when one of Saturn's moons was broken apart by Saturn's tidal forces. 
It is predicted that Mars's nearest moon, Phobos, will get broken up in another 4 or 5 crore years, since it is slowly coming closer and closer to the planet. Then Mars may grow rings too!
The opposition of Saturn on April 28 this year provided Earth-based astronomers an opportunity to confirm the earlier observations by the Leicester team and look for evidence of other kinds of "raining" phenomena.
NASA's Cassini spaceship was launched in 1997, reached Saturn in 2004, and is now in orbit around the planet. Observations from Cassini can be combined with Earth-based observations to suggest other interesting theories.
NASA has planned that Cassini will be allowed to fall into Saturn's atmosphere in 2017. We do not know how long the spaceship will survive the fall, but as it keeps falling, it will send back precious data about the composition of the planet's atmosphere.