Archive | August 2012

Why doesn’t Saturn’s ring aggregate to form moons?

This is a question that I was asked to answer for the Naked Astronomy podcast. I thought that, like last time, it would be nice to give a short article on this topic with a few more visual parts. If you want to listen to my answer either listen to the full podcast or the snippet of me giving the answer.

Saturn’s rings from the HST

So, lets start of by looking a bit at the rings of Saturn. The rings extend from about 7,000 to 80,000 km above Saturn’s equator. The thickness of the rings changes quite a bit over them from about 10m up to 1km. The rings are mostly made out of pure water ice. The total mass of all of the material in the rings is about that of Saturn’s moon Mimas, that’s the one that looks a bit like the death star. That is 0.000001% of the mass of Saturn, so quite a tiny fraction.

The largest gaps in the rings, such as the Cassini division, can be seen in your back garden with a small telescope – really worth a look. The smaller and more intricate structures to the rings wasn’t found until the Voyager spacecraft orbited around Saturn. The gaps are thought to be due to the gravitational pull of Saturn’s tiny moon which shepherd the material around. A good example of this is Prometheus and Pandora which orbit inside/outside of a ring of material. Though some of the rings are also due to resonances from the motion of the more massive moons.

Pandora (left) and Prometheus (right) orbit on either side of the F ring;

Before we look into the creation of icy moons it is worth thinking about how the rings were created. It is thought they were either created by:

– a small moon that was shattered by meteor impacts, though the rock content doesn’t really match
– tidal destruction of a comet that came too close to Saturn though this would lead to a mix distribution of ice and rock that is not seen and its more likely that this sort of action would occur around Jupiter
– or a Titan-sized moon with a rocky core and an ice mantle. As the moon spirals into Saturn early in the solar system’s history tidal forces rip off the icy mantle, distributing it into the rings. The rocky core holds together until it hits Saturn’s surface.

Going back to the main point in this article and the question at hand. The large moons of the Solar System are thought to have finished forming at about the same time as their host planets, some 4.5 billion years ago. The smaller moons are different. The smaller moons of Saturn have bright nearly pure ice surfaces that looks undamaged from meteoroid impacts. Their ages are estimated to be only 10 million years old. So new objects are still forming in the Solar System. It is thought hat once the rings are formed the particles of ice start to clump together in the outskirts of the ring. There is a distance from the planet, the Roche limit, at which a moon could form. Inside of this limit the tidal force from the planet is too strong and causes the material to break up and hence no moons can form. Once these moons form they migrate inwards, closer to the planet, and eventually end up into the stable orbits we see them in now.

More info: “The recent formation of Saturn’s moonlets from viscous spreading of the main rings”, Nature 465, 752-754 (10 June 2010), doi:10.1038/nature09096

Summers day walk around the MRAO

We took a stroll (and drive as the site is large and I was feeling lazy) around the Mullard Radio Astronomy Observatory (MRAO) this afternoon. It was the first time I’d taken Elizabeth out to the site so it was nice to show her the things I constantly go on about when I’m doing tours. It was a lovely day and I managed to get a couple of nice shots of the arrays out there…

AMI on a sunny summers day

MERLIN dish at Cambridge

Anglesey Abbey

On Saturday we took a nice slow walk around Anglesey Abbey. Its the second time we have been to this lovely National Trust place – we are slowly getting our money’s worth out of our annual membership. The grounds of Anglesey Abbey are lovAnglesey Abbeyely. Our last visit was great too but the house itself was a bit disappointing. The Lode mill however was a treat. This time we avoided the house and the mill was looking a bit full when we walked past. Instead we took a nice walk around the “nature trail”. We didn’t see much wildlife until we got to the viewing cabin they had. The view was full of lovely butterflies (such as red admirals, peacocks and white), dragonflies and some great tits. Definitely worth a 20 minute sit and watch. Using the binoculars provided (once again I left my nice ones at home) I managed to get a shot of one of the great tits.

Red Dragonfly at Anglesey Abbey

I was also pleased to get a shot, with me moving quite close and slowly, of this red dragonfly.

The moral of the story is I need to remember my big camera… though it was nice to be an opportunist with my mobile phone whilst enjoying nature. Overall a lovely wonder around the gardens.


A week in the life of AMI

One of my jobs as the outreach officer at the Cavendish is to take people out on tours at of the Mullard Radio Astronomy Observatory (MRAO). This is something i very much enjoy but alas quite often none of the telescopes appear to move. This is due to them normally tracking a source for quite some time. They will slowly move with the sky but unless they move to another source, which they do for calibration purposes, you don’t see this happen. Quite often the time spent at one of our telescopes is too short to see this movement. So to illustrate this I’ve made a couple of time-lapse movies to show the Arcminute Microkelvin Imager (AMI) large and small arrays actually moving over the course of a week.

The Large Array consists of the eight 13-m dishes:

The Small Array consists of ten 3.7-m dishes:

I hope this nicely shows that they do indeed move about! If you can’t see the videos above please try: large array, small array.

Oh and for those of you who are interested, these videos were made with a cron job automatically grabbing the webcam feed with wget every hour and then using ffmpeg to do the conversion into a movie:

x=1; for i in *jpg; do counter=$(printf %04d $x); mv "$i" img"$counter".jpg; x=$(($x+1)); done
ffmpeg -r 6 -b 3200 -i img%04d.jpg output.mp4

Magnetic field of Ganymede

As part of this months Naked Astronomy podcast I answered a question on “How does Ganymede’s magnetic field arise?”. This is a rather interesting question and I thought this would be an interesting blog post too. If you want to listen to my answer on the podcast go here.

Lets start of with a bit of background; Ganymede is the 7th moon of Jupiter. It is also the largest and most massive moon in the Solar System. It has a mass that is about 2 times that of the Earth’s Moon. Ganymede is mostly made out of silicate rock and ice water, though it has an Iron-rich liquid core.


This natural color view of Ganymede was taken from the Galileo spacecraft during its first encounter with the Jovian moon. North is to the top of the picture and the sun illuminates the surface from the right. The dark areas are the older, more heavily cratered regions and the light areas are younger, tectonically deformed regions.

Ganymede orbits around Jupiter, which  has a strong magnetic filed and it was only due to observations by the Galileo spacecraft (between 1995-2000) that we were able to determine that Ganymede has it own magnetic field. This magnetic filed is 3 times stronger than Mercury. This creates a tiny magnetosphere inside Jupiters.

There is even some evidence for aurora being observed around Ganymede’s poles.

The magnetic field is likely generated in a similar way to the Earth’s. To generate this magnetic field there must be: a conducting fluid, enough energy to cause the fluid to move, and a “seed” magnetic field. In the case of the Earth there is, a molten iron core which is a great conductor, convective flow from heat with the Earth’s rotation moving it in the right manner, and an existing magnetic field from the Sun.

As molten Iron steams through the magnetic field an electric current is formed (this is called magnetic induction). Due to the presence of an electric field a magnetic field is created and continues assuming there is a flow of fluid.

So Ganymede needs to have a conducting material moving in the interior!


HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede (Feldman et al. 2000)


How this magnetic field is generated is still a bit of a mystery. There is a thought that the Iron core will have cooled and fluid motions and hence the magnetic field will have not been sustained. There are other suggestions that the tidal forces from Jupiter disrupted the cooling. The tidal energy being dissipated into the ice shell / silicate mantle, thus insulating the core.

Much more will be learnt about the composition and magnetic filed of Ganymede with JUICE (Jupiter Icy Moons Explorer) which has a planned launch data of 2022 and arrival at Ganymede for 2033.