A few days ago I read an article about plans for NASA’s & ASI’s Cassini probe to take a picture of the Earth from its orbit around Saturn, in the same vein as Carl Sagan’s Pale Blue Dot taken by the Voyager 1 probe. This type of picture really drives home how insignificantly small and delicate the Earth is as it floats through the Universe, and I’m sure the new picture will instil people with the same sense of wonder and/or horror that its predecessor did. The Cassini-Huygens mission has expanded our knowledge of the Solar System so much during its time that I’ve wanted to write about it for a while. And by “write about it” I obviously mean “post nice pictures”.
So here is Part I of my favourite images taken by Cassini, and its companion on the long journey to Saturn, ESA’s Huygens probe. All are click-able if you want to open them up for full-screen viewing.
The Road Goes Ever On
Launched in 1997, Cassini-Huygens took 7 years to reach the Saturnian system. On the way the spacecraft carried out fly-bys of Venus (twice), the Earth and Jupiter in order to gravitationally slingshot them out to Saturn. As they passed Jupiter, Cassini took many pictures and this is probably my favourite.
The icy moon Europa is completely dwarfed here by the gas giant planet; in fact, you could fit about 93,000 Europas inside Jupiter! You also get a sense of how close in Europa orbits, going around its parent planet in only 3.5 days. Just imagine standing on the surface and looking up to see the sky filled by those orange and cream clouds! As a result of this close orbit, the tidal forces exerted on little Europa are believed to stretch and squeeze it so much that, beneath the granite-hard surface of ice, there could lie an ocean of water of perhaps twice the volume of the oceans here on Earth. This makes Europa one of the most interesting places in the Solar System in the search for extra-terrestrial life.
If you click the image to enlarge it, you may even be able to see that the surface is scarred by cracks called ‘lineae’ (here’s a clearer picture of them). These are thought to be caused by warmer ice emerging when slabs of the outer crust move apart, similar to magma seeping out at the continental ridges on Earth, but caused by the tidal forces not tectonic activity.
Behind it we can see the cloud tops of Jupiter. The Great Red Spot dominates, an anticyclone big enough to swallow the Earth twice over, which has lasted for more than 180 years — perhaps far longer. The brown colour is thought to be caused by chemicals in the atmosphere changing colour when exposed to ultraviolet light from the Sun at the cloud tops, although it’s not known exactly what all these chemicals are. The majority of the clouds we see are made of ammonia crystals, and the white clouds are generally higher than the darker ones.
I think this brilliantly shows how dynamic and physically imposing Jupiter really is. Please check out the Flickr page of Gordan Ugarkovic, who did the image processing on this and many other beautiful images — you won’t regret it.
After swinging on by and picking up the last boost of momentum, the spacecraft floated onward to their final destinations.
There But Not Back Again
Saturn is the jewel of the Solar System and this portrait is simply stunning. This was taken on October 6 2004, only 3 months after Cassini-Huygens entered orbit, and captures mostly the southern face of Saturn. This was during southern summer, almost 5 years before the equinox when the Sun illuminates both north and south equally, so the southern side of the rings are well-lit and they cast shadows onto the northern hemisphere. The scattering of the Sun’s light also results in the northern hemisphere looking blue, compared to the usual yellowish colour elsewhere. There are spot-like storms dotted across the band near the mid latitudes. Also note how transparent the rings appear, especially the inner edge. They can be only metres thick in places, yet are more than 120,000 kilometres wide.
Saturn’s atmosphere is mostly hydrogen with a little helium, much like Jupiter’s, and the upper clouds are mostly ammonia ice just like Jupiter’s too. Beneath this layer are clouds made of water ice, and beneath this is a layer where the pressure and temperature allows liquid water drops to form. There’s actually a lot of water in the Saturn system; the rings are almost entirely water ice — great, glistening space icebergs — and many of the moons are also mostly water ice. However, deep inside the planet things are a lot less chemically familiar. Helium and hydrogen can take liquid forms, and deeper still may lie metallic hydrogen, which can only form at extreme pressures impossible to replicate in a lab. Interestingly, Saturn radiates approximately 2.5 times as much energy into space as it receives from the Sun, and the heat responsible for this is thought to be generated by the planet contracting slowly and, inside the warm interior, helium rain falling towards the core causing frictional heating of the surrounding material. Then, at the very centre, there is thought to be a dense rocky core, although you could never hope to land on it.
Although the Voyager probes flew past Saturn on their… voyages, Cassini was the first, and still the only, probe to orbit the planet. In the 9 years since it got there we’ve learned an awful lot about it and its system of moons and rings, as well as gotten jaw-dropping shots like this.
One Moon to Pull Them All
The August 2009 equinox on Saturn gave the orbiting Cassini a unique opportunity to view the rings lit edge-on, something that happens only every 15 Earth years. One of the really interesting things that it saw were vertical structures in the rings themselves, created by the gravitational tugging of little moons and moonlets on the smaller icy ring particles.
In this photo, the 8 kilometre-wide Daphnis is seen doing just this. Cassini discovered Daphnis in 2005, although its existence was predicted due to the previously discovered Keeler Gap. This is a thin gap in Saturn’s A ring, only 42 km (26 mile) wide. It was hypothesised that a shepherd moon was responsible for keeping this band clear of debris and, sure enough, Daphnis is that very shepherd.
The moon orbits within the gap, but on a slightly different plane to most of the particles in the rings. This means the gravitational force it exerts on the ring material lifts it up and out of the ring-plane in great ripples, which can be over a kilometre tall. The rings themselves are only about 10m thick at this point, so these waves are very considerable structures. Due to the edge-on sunlight, we can see the shadows cast by these waves onto the surrounding ring, and the shadow of the shepherd Daphnis as he tends his flock.
Much of the structure we see in the rings comes about as a result of gravitational interaction between the many millions of ring particles and the swarm of Saturn’s moons, and this picture captures the dynamic relationship perfectly.
The Eye of Saturn
As the equinox passed, Cassini got its first glimpse of the north pole in direct sunlight. It found that, right at the very pole itself, there lies raging an immense hurricane-like storm. In this amazing picture we see the eye of the storm. Each pixel in this image covers 2 km of clouds, yet it looks to be very high-definition. This is because the eye is 2,000 km across! Winds can be in excess of 500 km/hr (330 mph), and the eye is hundreds of kilometres deep. Here it is in context, showing the hexagonal jet stream feature that encircles the polar region and the far-off rings in vivid blue hues.
The image is in false colour of course. This brings out the depth by highlighting clouds at different altitudes in different colours. Three filters were used to image the storm in near-infrared wavelengths — light with slightly longer wavelengths than red visible light — before they were added together. The image filtered at a wavelength of 890 nanometers (billionths of a metre) is coloured blue, that filtered at 728 nanometers is coloured green, and that filtered at 752 nanometers is coloured red. This means that red indicates low clouds and green indicates high ones.
In the Land of Shadow
This is one of the most surreal images Cassini has snapped over its 15+ years. It’s actually very much like the picture the mission controllers plan to take, as reported in the news story at the beginning of this post.
With the planet eclipsing the Sun, the rings and outer atmosphere are backlit eerily to stunning effect. The diffuse, outermost band of the rings is the E ring, which has an interesting relationship with the moon Enceladus (I’ll say more about this in Part II). Inside this we can make out the thin and relatively dim G ring and, just inside this at about 10 o’clock, a pale blue dot. Yes, that’s us — good old Earth, only 1.5 billion km away!
This image isn’t just beautiful and humbling, but also shows us how complex and varying the rings are. Different bands exhibit different scattering properties and are different thicknesses. Questions surrounding the mystery of how they formed, how they change over time, and how long they might last into the future are still unanswered. In 2017, in its final act, Cassini will be fired through the rings and down into Saturn, collecting vital information even as it carries out a kamikaze dive toward its demise. But before that, there’s much more science to be done and, I’m sure, plenty more to be learned.