The Cassini Mission: Part II – The Moons

July 19th was ‘Wave at Saturn’ day, the day Cassini took our picture from in orbit around Saturn (so I do hope you waved). I feel, therefore, it’s time to post the promised and hotly anticipated Part II of my favourite pictures from the mission so far. Part I can be found here.



Geysers making rings. Credit: NASA/JPL-Caltech/Michael Benson, Kinetikon Pictures

Before the Cassini mission, the Saturnian moon Enceladus wasn’t particularly noted for anything much. But things have changed, and it must now be regarded as one of the most interesting bodies in the Solar System. An icy world, it has the brightest surface of any object in the Solar System, scarred by great fissures all across it. At the south pole, however, is the killer feature — a series of fissures warmer than the surrounding ice. These have become known as the Tiger Stripes and in this picture we see the heat map highlighting them, as well as yellow stars indicating the locations of some features. But what are they?

The beautiful image on this page was compiled by Michael Benson, who has a book called Planetfall: New Solar System Visions with other such gorgeous gems. Here, the limb of Enceladus is lit by the Sun while the rest is bathed in the yellow glow of ‘saturnshine’. The south pole lies at the top, and from this region of the surface we can see plumes of ice streaming out into space. It turns out those yellow stars mark the sites of ice geysers!

On Earth, we tend to see geysers in areas of volcanic activity. Water trickles its way through the rock, collecting in underground chambers where it is superheated before escaping explosively as steam and hot water through gaps at the surface. The presence of these geysers on Enceladus hints that there may be warm, slushy ice and liquid water deep beneath the surface. The expelled material is now known to be responsible for Saturn’s tenuous outer ring, the E-ring, which Enceladus orbits within. This might also explain why the surface is so bright and relatively crater-free; it’s constantly snowing tiny ice crystals onto the surface.

On top of all this, Cassini also flew through the plumes, measuring their chemical composition. While the majority is water ice, there are also traces of nitrogen, methane, carbon dioxide, and hydrocarbons like propane, ethane and acetylene. This mixture, coupled with the warm internal conditions, promotes Enceladus right to the top of possible hosts of life within the Solar System, and is definitely somewhere to return to in the future.


Iapetus is a world of contrasts. When first observed through a telescope by Cassini (that’s the 17th century Italian-French astronomer Giovanni Cassini, not the spacecraft), the astronomer was perplexed that he was able to see the moon when it was west of Saturn, but not when it was to the east. It took him many years before finally managing to see its other face, and he noted that it was indeed much fainter. In the pictures here we can see why. It’s not because of a star gate complex (sorry Arthur C. Clarke). It’s actually due to one segment being icy and white and the other dirty and brown. These different regions fit together much like the segments of a tennis ball.

The moon spins once on its axis in the same time it takes to orbit Saturn (tidal locking), so one face is always pointing in the direction of the orbit while the other is facing where the moon has come from. For Giovanni Cassini, this meant that when Iapetus was west of Saturn and moving away from the Earth, he saw the bright white surface shining back; but when it was east and moving towards him, the darker surface thwarted him.

The two faces of Iapetus. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

The reason for the contrast is fascinating. One clue is that the leading face (left) is generally redder than the trailing face (right), as seen in these enhanced colour snaps. Why might this be? Well, Saturn has some moons that orbit retrograde, meaning they do so in the opposite direction to the way the planet spins, which is also the direction most moons orbit their planets. These could be captured asteroids or comets. Impacts on these moons and moonlets could create very diffuse rings of retrograde debris that could eventually land on the surface of other moons. Perhaps some reddish dust is being created in such a way and swept up by Iapetus as it orbits, accumulating on the leading side. Indeed, other moons show signs of this too and we have a rather guilty looking culprit in the retrograde outer moon Phoebe, which is just one moon out from Iapetus and has an observed ring associated with it (as with Enceladus and the E-ring).

Iapetus’ long day (79 Earth days) means the day and night temperatures vary wildly. Once a build up of darker, reddish material was present on the leading face, it will have absorbed more heat than the lighter trailing face during the day. On average, the equator at the leading face would stay hotter than the equator at the trailing face. More ice would sublime off the darker surfaces and voilà; we have a positive feedback loop. Over a long period of time we’re left with the contrast we see today.

As if it wasn’t weird enough, the moon also has a very distinctive ridge around its equator (fly-by video here), giving it a walnut-like shape. The ridge is around 13km high, making the peaks on it some of the highest mountains in the Solar System (about 1.5 times as high as Everest/Qomolangma on Earth). How it formed is still a mystery. Perhaps it was formed via centrifugal forces when Iapetus spun far faster in the past, or maybe through geological convection of the warmer interior pushing the crust upwards, but no hypothesis is widely accepted yet.


The spongy face of Hyperion. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Hyperion is one of the oddest looking moons in the Solar System, resembling a tumbling sponge. It’s shape is irregular, meaning it hasn’t settled into a spheroid or ellipsoid shape through its self-gravity, which is unique for such a large moon. It appears to be very porous indeed (around 40% of its volume is empty space) and might be simply a clump of dirty chunks of ice held together relatively loosely.

The surface appears to be coated in the same reddish dust that we see on Iapetus, which lends support to the idea that this material comes from Phoebe.

Uniquely in the Solar System, Hyperion rotates chaotically. This means it spins in an unpredictable manner as it orbits in its rather elliptical orbit. It’s possible that Titan could be partly responsible for this, since the two moons orbit in a resonance of 3:4 (Titan orbits 4 times for every 3 of Hyperion). But much is still unknown about this moon. One thing we can say is that it looks cool.



The first image from the surface of Titan. Credit: ESA/NASA/JPL/University of Arizona

Saturn’s largest moon, Titan, is a fascinating place. It has a thick, hazy blanket of an atmosphere — coloured orange from organonitrogen compounds — which makes it unique among the natural satellites in the Solar System. In fact, it has a more substantial atmosphere than the Earth! Coupled with its low surface gravity, this means humans would be able to fly by flapping big wings strapped to their arms… seriously.

On Christmas Day 2004, the Huygens probe said farewell to Cassini and separated for its ultimate descent to the surface of Titan. A few weeks later it entered the atmosphere and slowly parachuted through the hundreds of kilometres of haze until it glimpsed the surface for the first time before coming to a gentle landing. It’s amazing that this little lander took a voyage of more than 7 years and a billion miles and made it safely to the surface without a hitch.

This is the first image taken from the surface, and it shows pebble-like rocks, only they’re not made of rock. The surface of Titan is composed of water ice mixed with some hydrocarbons, but it’s so cold — about -180°C — that this dirty ice is as hard as rock. Those in the foreground of the image are about 5 – 15cm across, and they sit on a darker surface containing more hydrocarbons with signs of being smoothed and undercut by erosion.

On Titan, methane and other hydrocarbons fall as rain, erode gullies and valleys, and fill lakes and seas. There’s a full hydrological cycle at play, except the chemicals involved are not what we’re used to on Earth. What we’re looking at here is probably an old, dried up riverbed similar to those on Earth.

The methane in the atmosphere should break down on timescales of millions of years, but it seems to have been present for a long period of the moon’s history. There is speculation that cryovolcanoes spew methane out from the moon’s interior, replenishing the atmospheric content and allowing methane clouds to form and rain to fall now as it has for eons. Mountain ranges featuring volcano-like craters are known from radar, with peaks over a kilometre tall, so this could well be true. Additionally, the mountain ranges on Titan are all named after those in Tolkien’s Middle Earth, which is one of my favourite facts ever!

If you have a few minutes spare, I’ve embedded a narrated video of the probe’s descent and landing below, which contains actual footage! It’s well worth watching to experience the first and as yet only landing performed in the outer Solar System.

Hello, World

Finally, here’s the photo that motivated these two posts; the Earth from Saturnian orbit at a distance of 1.45 billion km. I don’t think there’s much more to be said about it, apart from that there’s also a narrow-angle shot showing the Earth and Moon here. Pretty awesome.

You are here. Credit: NASA/JPL-Caltech/Space Science Institute



  1. Writewireless

    That last shot (Earth seen from Saturn’s orbit) kind of puts you in your place, doesn’t it? Thanks for translating scientist-ese into English and taking us where no man has gone before. Love your blog.

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