Cassini At Saturn: Five Extraordinary Moments

The Cassini spacecraft has been locked in an intricate dance around Saturn for the past thirteen years and now nears its fateful descent. On 15 September 2017 at 11:45 BST, Cassini will make a final dive into the atmosphere of Saturn and the mission will end.

The gas giant Saturn sits nine times further from the Sun than Earth and is the second largest planet in the Solar System. Its iconic rings have captured our imagination ever since Galileo Galilei dubbed them ‘ears’ in 1613, but until Cassini, no spacecraft had entered into orbit around Saturn.

Cassini is a joint NASA and European Space Agency (ESA) mission that launched on 15 October 1997. It was originally designed to study the Saturnian system for just four years, but after two extensions and over 300,000 raw images, Cassini has made some truly fascinating discoveries.

1. First liquid found on a body other than Earth

Titan is Saturn’s largest moon and the second largest in the Solar System, but up until 2005, it remained shrouded in mystery.

Then Cassini flew by Titan from 2005-2006 and carried out RADAR scans by bouncing radio waves off its surface. When the scans came back, there appeared to be darker regions and lighter ones.

The darker regions fascinated the Cassini team as these were regions where the radio waves didn’t return. This meant that the regions were flat compared to the rougher terrain of the lighter regions. In 2006, Cassini imaged the north pole of Titan, and again, similar dark regions emerged.

This led the Cassini team to conclude that the dark regions were liquid-filled lakes. However, the surface temperature on Titan is about -180°C – way too cold for liquid water to exist.

After further analysis, the lakes were found to contain liquid methane along with other hydrocarbons.

This was the first time that a liquid had ever been discovered on a world other than the Earth.

2. Saturn’s dynamic rings

Saturn’s rings are one of the most iconic features in our Solar System and Cassini has had a fantastic opportunity to study them throughout its mission.

In 1859 Scottish physicist James Clerk Maxwell correctly predicted that the rings of Saturn could not be solid, but up until Cassini, nobody knew what the rings were made of and how dynamic the rings were. Cassini captured extraordinary views of Saturn’s rings and discovered that the sizes of individual ring particles range from a single grain of sand to several kilometres wide. Saturn’s rings are made from pieces of water ice and they constantly collide with each other to reveal clean, reflective surfaces. This reflection of sunlight is what allows us to see Saturn’s rings so vividly from Earth.

Cassini also found that some of Saturn’s moons contribute to the ring system, whilst some moons draw material away from the rings through gravity. In fact, the icy plumes from the moon Enceladus are the source of Saturn’s entire E ring.

3. Vertical structures on Saturn’s rings

Another goal in Cassini’s mission was its two-year ‘Equinox Mission’.

Equinox means equal night in Latin and it occurs when a planet’s axis isn’t tilted away or towards the Sun, leading to equal day and night times.  Cassini waited until it was ‘noon’ on Saturn during the equinox to image the planet. Cassini found previously unknown vertical structures on the rings – some of which tower up to one kilometre high!

This was the first time that vertical structures had ever been seen on the rings of Saturn. It’s thought that the structures are caused by shepherd moons that orbit between the rings – their gravitational pull distorts the smooth ring shape.

4. Huygens landing and discoveries

In 2005 ESA’s Huygens probe made history as it became the first spacecraft to land on a world in the outer Solar System. This world was Titan, Saturn’s largest moon. Prior to Cassini it was believed that Titan was a barren and crater-filled world. However, as Huygens flew through the atmosphere, a different story started to emerge.

Watch this NASA video for a recreation of Huygens’ descent.

When the Huygens probe landed, the pictures revealed no craters at all. Instead there were smooth, round rocks as if worn by flowing liquid. The entire sky had a yellow hue due to methane gas and carbon particles in the atmosphere. However, methane should just evaporate away due to the Sun’s harsh radiation, so even at 1.5 million kilometres away from the Sun, Titan shouldn’t have been able to hold on to its methane clouds. So where was this methane coming from? The Cassini orbiter then changed its flight path to scan for sources of methane and found methane lakes and rivers. It’s now known that Titan has a methane cycle similar to the water cycle on Earth.