Astronomers Observe “Ring Rain” Falling on Saturn

Ionised water from Saturn’s rings is flooding the planet’s upper atmosphere, say astrophysicists

It’s easy to imagine that rain is a uniquely terrestrial phenomenon. But astronomers have evidence of rain in a number of other exotic places. On Saturn’s moon Titan, for example, they can see a steady drizzle of methane and on Venus, clouds of sulphuric acid create acidic downpours, although this rain never reaches the ground.

Now astronomers can add another extraterrestrial wet spot to this list. James O’Donoghue at the University of Leicester in the UK and a few pals say that it rains on Saturn too. These guys have used the 10-metre Keck Telescope in Hawaii to take infra-red images showing water streaming down from the and flooding Saturn’s upper atmosphere.

And they say this could explain a long-standing puzzle about the ringed-planet—why the temperature of the upper atmosphere is much higher than astronomers expect.

Astronomers have long noticed that Saturn’s ionosphere is unusual. This is the upper part of the atmosphere that becomes charged when bombarded with solar radiation.

On Jupiter, the ionosphere produces a weak infrared glow that is more or less uniform across the planet. That’s the result of the Sun’s even illumination, which ionises the atmosphere producing a plasma of ions and electrons.

But Saturn’s ionosphere is more interesting. It too produces an infrared glow but not an even one. In fact, Saturn glows more brightly at certain latitudes and these peaks and troughs are asymmetric above and below the equator.

How this happens is something of a mystery. One idea is that Saturn’s rings shade the upper atmosphere from the Sun’s radiation and so prevent it from being ionised.

But astronomers say the peaks and troughs do not coincide with the rings’ shadow. “The shadow cast by the rings is known to create variations in the ion density with latitude, but this is unable to explain the features seen,” say O’Donoghue and co.

Now they think they have the answer—ring rain. The Keck observations show ionised water streaming into the upper atmosphere where it quenches the ionosphere by mopping up any free electrons.

The flow of water follows Saturn’s magnetic field lines which are influenced by the complex series of gaps in Saturn’s rings. It is this ring rain that causes the asymmetric infrared emissions above and below the equator. “This ring ‘rain’ plays a fundamental role in modulating ionospheric emissions,” say O’Donoghue and pals.

Ring rain could also explain another puzzle: why Saturn’s upper atmosphere is much hotter than expected. Ring rain could be heating the upper atmosphere.

However, O’Donoghue and co say more observations and better atmospheric modelling will be needed to settle the questions once and for all.

The discovery of ring rain will certainly whet astronomers’ appetites for more data, either from ground-based telescopes or from NASA’s Cassini spacecraft now in orbit around the Saturn.

And if you ever get a chance to visit, don’t forget an umbrella.

Ref: arxiv.org/abs/1310.7293 or Nature 496, 193–195 (11 April 2013) : The Domination Of Saturn’s Low Latitude Ionosphere By Ring ‘Rain’