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Saturn's rings rain on its atmosphere


Saturn by night

Charged ice particles in Saturn's rings have been found to flow along the planet's magnetic field lines onto the ionosphere (Source: NASA/JPL-Caltech/Cassini)

Saturn's ring rain Saturn's magnetic field is causing particles from its icy rings to rain on to the planet, according to scientists.

The discovery of this so-called 'ring rain' reported in the journal Nature, may help researchers understand the processes which formed the majestic ring system seen around Saturn today.

Astronomers, including the study's lead author James O'Donoghue of the University of Leicester, used the 10-metre Keck telescope's infrared spectrometer to study spectrum emissions from Saturn's upper atmosphere.

The emissions are caused by the excitation of atoms and molecules at specific wavelengths.

O'Donoghue and colleagues detected emissions of tritium, an isotope of hydrogen that can be used as a proxy for water, in a wide band across Saturn's upper atmosphere (called the ionosphere) extending from about 25° to 60° latitude.

The band is caused by charged water particles raining out of the rings along Saturn's magnetic field lines, leaving a ring imprint on the planet's ionosphere.

"This pattern implies the transfer of charged particles derived from water from the ring-plane to the ionosphere ... on a global scale, flooding between 30 to 43 per cent of the surface of Saturn's upper atmosphere," the authors write.

Saturn's rings are made of particles of nearly pure water-ice, ranging from tiny sub micrometre-sized grains to small moons several kilometres wide.

Ionisation

These particles can become ionised when hit by electromagnetic radiation, usually from the Sun or by exposure to dense plasma from micrometeorite impacts.

O'Donoghue and colleagues say Saturn's magnetic field acts as a conduit, channelling ionized particles from the ring, along the magnetic field lines and on to the ionosphere.

They found peaks in emission readings could be traced back along the planetary magnetic field lines to gaps in Saturn's rings.

Although the patterns appear symmetrical on both sides of the equator, they actually occur at higher latitudes in the north than in the south.

O'Donoghue and colleagues say the lack of latitudinal symmetry indicates the phenomenon is not caused by weather patterns or other atmospheric processes.

The findings may explain why the properties seen in Saturn's ionosphere at lower latitudes don't match predicted models.

The magnetic connection between Saturn and its rings could explain these hemispheric discrepancies.

Mystery of the rings

"The study helps explain what's eroding the rings and what we see in the ionosphere says Macquarie University astronomer Dr Craig O'Neill.

"We know surprisingly little about Saturn's rings, we're not even sure how old they are."

O'Neill says Saturn's rings should only be about 100 million years old, "because they would have been disrupted and dispersed by now if they were older".

However he says recent studies suggest they could be as old as the solar system, 4.5 billion years.

"Understanding a mechanism like this, and how effective it is and how it works helps constrain the dynamics and evolution of the rings, says O'Neill.

Tags: planets-and-asteroids