Oh My!

The solar system is full of amazing sights; from aurora on Earth, to giant solar flares, to the great red spot there are countless objects to fascinate us. every now and then comes along something you just don’t expect. Often these are from objects you just wouldn’t imagine to be interesting (which just proves the old adage; ‘never judge a book by its cover.’)

The Cassini mission to Saturn (launched on October 15th. 1997 and arrived seven years later) is starting to wind down (sadly) and will soon be sent on a course that will see it fly through Saturn’s rings and eventually into the planet’s atmosphere to be destroyed. Despite that it is still providing us with surprises; the most recent being its images of the little shepherd moon Pan.

759Raw image of Pan. NASA. The grey lines either side of Pan are rings of Saturn!!

Looking through a telescope at Saturn from Earth the rings look quite simple; there


appear to be two sets separated by  black gaps or ‘divisions’. the Encke division is very difficult to see and cuts around the A ring. The second gap is the Cassini division and is much easier to see, lying between the A and B ring. The image below also shows the C ring or crepe ring which is very difficult to see in amateur telescopes and is only hinted at in the above Nordic telescope image.

post-6974-0-24780700-1405571735Stargazer’s lounge image. (Just for clarity and because you can never have too many pictures of Saturn!)

This begs the question: how are the rings able to stay so neat? could it be gravity and that he material that makes up the rings be in just the right place that they don’t change?

Pioneer 11  became the first spacecraft to visit Saturn. It was launched from Cape Canaveral  on the 5th. April 1973. Pioneer 11’s path through Saturn’s outer rings took it within 21,000 km of the planet, where it discovered two new moons (almost colliding with one of them in September 1979) and a new “F” ring. Saturn from Pioneer looked very similar to Saturn through a telescope.

739508main_739460main_AC79-9107_3-full_full Pioneer 11 image of Saturn.

It wasn’t until the Voyager spacecraft flew past Saturn ( Voyager 1 in November 1980 and Voyager 2 in August 1981) that we began to realise what a complicated and far more extensive system the rings formed.

8bg                       Saturn’s rings with “spoke” features in B-ring. Aug. 22, 1981. Distance 2.5 million miles.

The spokes created much excitement and discussion and were endlessly replayed in a video sequence on TV…and to be honest I did and still could watch them happily for hours.

10bg The F-ring (discovered by Pioneer 11.) Two braided but separate orbit rings. Nov. 12, 1980. Range 750,000 km.

A clue to the structure of the rings was found by Voyager 1 when  it discovered three new moons, Prometheus, Pandora, and Atlas. Prometheus and Pandora are shepherding moons of the F-rings, and Atlas is a shepherd of the A-rings. After a lot of analysis and some careful thinking it was realised that these moons are able to control the movement (or shepherd) of material making up the rings by either pushing the material into place in the rings or ejecting stray material from the system all together. All this is done by the gravitational force of the much larger satellites on the smaller material that makes the rings. Pan creates stripes called “wakes” ( which are places where ring material has collected in an orderly manner) in the ring material on either side of it. Since ring particles closer to Saturn than Pan move faster in their orbits, these particles pass the moon and receive a gravitational “kick”. This kick causes waves to develop in the gap and also throughout the ring, extending hundreds of miles into the rings. These waves intersect downstream to create the wakes.

Pan has a similar shape to Atlas and that unusual shape may be the result of fine material from the rings aggregating on the surface of the moon.

A little more about Pan before we go: it was discovered on the 16th. July 1990 by Mark Showalter who was working on Voyager data. It is approximately 35 kilometres across and 23 km wide. It lies within the Encke Gap in Saturn’s A Ring. It orbits Saturn every 13.8 hours, at a distance of 134,000 kilometres and is responsible for keeping the 325 kilometre wide Encke Gap open.

There are many fascinating worlds around Saturn – have a look for Daphnis another shepherd moon as well as the ones mentioned above.

To give you an idea of the complexity of the ring system here is another image from Cassini: