Rings revealed

LAST week I wrote about some of the results from Cassini’s final few months in orbit around Saturn.

Cassini was a spacecraft in orbit around the ringed planet between 2004 and 2017. It was deliberately crashed into Saturn on September 15 to avoid the possibility that it would eventually collide with one of Saturn’s moons.

Enough information has come back from Cassini to keep scientists busy for years, and I have already mentioned some results in earlier articles.

However, there are many more findings, and one of them is related to Saturn’s magnetic field. These results actually pose quite a problem.

In the case of Jupiter, the largest of the sun’s planets, its magnetic field is inclined about 10 degrees to the plane of its axis and equator. In other words, if you imagine a giant bar magnet inside Jupiter, its north and south poles are tipped over by about 10 degrees to the north and south “geographic” poles of the planet. This is, however, a very general result: the Juno spacecraft, currently in orbit around Jupiter, has shown us that the field is complex, with components of the field also generated relatively close to the surface.

There is no significant tilt in the case of Saturn. We have known for some time that its magnetic field is tilted by less than half a degree, but we now know, from Cassini, that it is quite insignificant: less than 0.016 degrees.

A tilted field would normally be expected, based on scientists’ understanding of the way in which a planetary magnetic field is produced.

Although more work will be done on Cassini’s results, it is possible that we shall have to wait for future spacecraft visits to Saturn before we understand its magnetic field, and interior rotation, properly.

Cassini’s proximity to the rings of Saturn in its final months gave us a huge advantage in our study of this fascinating and best-known feature of the planet.

As I have mentioned in earlier articles, the rings are made up of countless pieces of material, mostly water ice. As such, they are a major target for collisions with interplanetary objects. Cassini has proved this, having detected the results of several collisions, which have produced “clouds” of material.

Of special interest, however, are the several quite odd patterns in the rings that have been called “propellers”. This year, before it took its death plunge into Saturn, Cassini took the best-ever image of one of these features, called Bleriot.

The features are called propellers simply because they do seem to resemble an aeroplane propeller. They are thought to exist because of the effects of tiny moonlets embedded within the ring system, which affect the distribution of ring material in their vicinity.

The feature called Bleriot is named after the aviator who, in 1909, was the first to fly across the English Channel. He took off from Calais on July 25, for a flight lasting just over half an hour.

Other “propeller” features in Saturn’s ring system have also been named after famous aviators, including Amelia Earhart, who in 1932 was the first woman to fly solo across the Atlantic Ocean.

Currently, we can see Saturn clearly in our Tasmanian evening sky. If you go out tonight as the twilight fades, about 9pm, you will see the planet towards the west. It is a whitish point of light to the right of the constellation of Scorpius, The Scorpion, and it is almost as bright as the brightest stars.

There is another planet to watch for in the early evenings: tiny Mercury, which can be seen much closer to the horizon, below and to the left of Saturn. Tonight, you will see Mercury just to the right of the bright reddish-orange star Antares, which forms the Scorpion’s “heart”.

Enjoy your planet gazing!