Saturn, the sixth planet from the Sun in our solar system and the second largest planet after Jupiter, is beautiful as well as mysterious. It is also a source of intense radio emissions that spacecrafts and satellites have been capturing for a very long time. Many such Saturn’s radio emissions are being recorded by the satellite Cassini-Huygens, and the scientists have compressed them to audio frequencies for normal ears to hear.
The Cassini-Huygens spacecraft, which arrived at Saturn in 2004 and the first to enter orbit, has been receiving radio signals since 2002 when it was 234 million miles from the planet.
Other spacecrafts had studied the magnetosphere of Saturn before Cassini, including the Voyager spacecraft in November 1980 and August 1981. The first to study the magnetosphere was Pioneer 11, in September 1979, which made some measurements of the plasma parameters. They were also able to measure the planetary magnetic field, plasma composition and density, high energy particles and spatial distribution, plasma waves and radio emissions. The Cassini spacecraft itself was launched in 1997, and arrived in orbit in 2004, making the first measurements after more than two decades and continues to provide information about the magnetic field and plasma parameters of the planet’s magnetosphere.
The radio emissions from Saturn are very powerful and low frequency emissions called Saturn kilometric radiation (SKR).
The frequencies of SKR lie in the range of 10 to 1300 kHz, that is the wavelength is a few kilometers, and with the maximum around 400 kHz. The power of these emissions is strongly modulated by the rotation of Saturn and is also dependent on the solar wind pressure. The total power of the SKR is around 1 GW, though it was known to drop completely when Saturn was immersed into the giant magnetotail of Jupiter in 1981. The radiation is thought to be generated by an instability of electrons moving along magnetic field lines related to the auroral field lines of the planet.
The time on the recording of the radio waves was compressed so that 13 seconds correspond to 27 seconds, about 2 times the real time and the frequencies were shifted down by a factor of 260 since they are well above the audio frequency range.
Cassini spacecraft has been detecting these radio emissions from the planet using the Cassini Radio and Plasma Wave Science (RPWS) instrument since 2002. In has provided the first high resolution observations of these emissions showing an array of variations in frequency and time. The complex radio spectrum with rising and falling tones, as shown in the picture, indicate a very complicated interaction between the waves in the planet’s radio source region, which is actually very similar to Earth’s auroral radio emissions. It also shows that there are numerous small radio sources moving along the magnetic field lines of Saturn’s auroral region.
The plasma of Saturn’s magnetosphere consists of a large number of water group ions that come from Enceladus, one of its moons, which ejects as much as 1,000 kg/s of water vapor from its geysers.
The emissions can only be observed from the outer space because Earth’s ionosphere absorbs them before they reach the Earth. Since Saturn’s magnetosphere was first discovered in 1979 by Pioneer 11, a few other spacecrafts were made to detect the radio emissions from the planet in the outer space, and the planet now has its own dedicated satellite, Cassini, that does the same. The planet’s magnetosphere is filled with plasmas originating from both the planet and its numerous moons, which is composed of various water group ions, other hydrogen and oxygen based ions, nitrogen ions and protons. A portion of the water vapor ejected by Enceladeu’s geysers is ionized and forced to rotate with the planet’s magnetic field.
Here is the audio of the radio emissions received from Saturn.[sources:cassini.physics.uiowa.edu, wikipedia.org]