Two moons: shrouded Titan and snowball Rhea

This image taken by NASA's Cassini spacecraft shows Titan and Rhea - two of the largest of the moons known to be orbiting the ringed world of Saturn.

In the foreground is the smaller moon Rhea - an ancient and heavily cratered body, bearing numerous pocks and scars from past meteor impacts. It has a diameter of 1528 kilometres.

Rhea's pitted surface contrasts sharply with the smooth glow of Titan behind it. Despite the difference in appearance the two moons are quite similar in composition, with both containing a mixture of rock and water ice.

Rhea is thought to be made of three quarters ice and one quarter rock. However, Cassini observations have determined that it doesn't contain a distinct rocky core - instead, it is made up of rock and ice mixed together, giving it the consistency of a giant dirty snowball.

In the background is the shrouded world of Titan which has a diameter of 5150 kilometres, making it 50 per cent larger than the Earth's own moon.

But, unlike Earth's grey desolate-looking companion, Titan is shrouded in a thick atmosphere giving it a creamy orange hue.

Titan is the only body in the Solar System other than Earth, to have a thick, nitrogen-rich atmosphere.

However, Titan's atmosphere contains hydrogen, methane, ethane, and other hydrocarbons - but no oxygen.

These molecules react with sunlight to form thick layered smog.

One of these smog layers can be seen in the image as a hazy band of blue encircling Titan and brightening into two crescent-shaped hoods over the moon's polar regions.

These polar hoods contain furiously swirling high-altitude areas of dense gas that grow and dissipate with the changing seasons, which on Saturn and its moons last for around seven Earth years.

When Cassini arrived in the Saturnian system back in 2004, Titan already had a thick hood above its north pole, which was experiencing winter.

After the Saturnian equinox in August 2009, Titan's northern hemisphere began moving into spring, while its southern latitudes headed into autumn, producing a polar vortex above Titan's south pole which takes just nine hours to complete a full rotation.