On 13th April 2017, NASA announced that Enceladus, a small moon orbiting Saturn has almost all of the right conditions for life to evolve - Royal Obeservatory astronomer Affelia Wibisono explains what they've found.

 

A question that I get asked a lot at the Royal Observatory Greenwich is “Do aliens exist?”. Scientists haven’t found any definite evidence for their existence yet, but most astronomers believe that there probably is some kind of extra-terrestrial life somewhere in the Universe, even if they are microscopic germ-like beings. What about the rest of the Solar System? Could there be life on any of the other seven planets or their natural satellites? On 13th April 2017, NASA announced that Enceladus, a small moon orbiting Saturn has almost all of the right conditions for life to evolve, making it the most habitable world for these otherworldly lifeforms elsewhere in our cosmic neighbourhood.

NASA's press release

An oceanic moon

Liquid water is essential to life. Luckily for Enceladus, it has plenty of it. In fact, underneath its thick icy shell is an underground sea of salty water. Scientists first suspected that Enceladus might be hiding this ocean back in 2005. The Cassini spacecraft reached the Saturnian system in 2004 after travelling through space for seven years.

Latest news on the Cassini mission

It saw jets coming out of Enceladus and by 2007, researchers realised that almost all of those jets came out of geysers located on four huge V-shaped cracks, called tiger stripes, at the South Pole of the moon. These cracks, or fissures, are stretched open and shut closed as Saturn squeezes and tugs on Enceladus heating up its insides as the moon makes its elliptical orbit around the ringed planet. The pressure under the surface increases and forces warm salty water through the fractures at a speed of 800 miles per hour (1300 km per hour)! Cassini even managed to fly through these plumes several times! The heavier material falls back down, heating up a small patch of the ground. Lighter material gets shot up above the moon and the lightest materials escape Enceladus’ gravitational pull to form Saturn's E ring.  

Learn about Saturn's E ring

The only way that this could happen is if there was a subsurface reservoir of liquid water. More on these plumes later.

 

Enceladus
The Sun is behind Enceladus and backlights the plumes coming out of Enceladus’ South Pole. The moon itself is being lit up by sunlight that was reflected by Saturn.

Tiger stripes
The tiger stripes on Enceladus have the highest surface temperatures on the moon even though they are at the South Pole!

Definitive proof for a global underground ocean came in 2015 when astronomers noticed that Enceladus wobbles (also known as libration) as it orbits Saturn. A very small wobble means that the moon is completely solid all the way through; a big wobble means that the crust is separated from the rocky core by a layer of liquid. You can see this for yourself – spin a hard-boiled egg and a raw egg and you’ll find that the raw egg with its runny insides wobbles a lot more as it twirls. Enceladus librates by a whopping tenth of a degree. This was enough to prove that there is indeed a global sea under the icy crust of Enceladus!

Enceladus’ interior structure
Artist’s illustration of Enceladus’ interior structure

Chemistry for life

Further studies on the composition of the plumes confirmed that they consist of water vapour, carbon dioxide, methane and ammonia. This means that at the moment, phosphorus and sulphur are the only elements missing in Enceladus’ sea for it to have the complete set of ingredients needed for the kind of life that we are familiar with. However, scientists are confident that they might be present in Enceladus’ rocky core.

Hydrothermal activity and a buffet for organisms?

The amount of methane released by Enceladus’ geysers was higher than what anyone was expecting. This led to the idea that Enceladus might also have hydrothermal vents on its seafloor as similar vents on the Earth’s seabed are known to emit lots of methane. Water from the moon’s sea can seep into the seafloor where it gets heated by the core and reacts with minerals in the rocks. In the case of Enceladus, the minerals are silica. You can find silica in quartz sand. The mixture of hot water and dissolved material can gush back out into the ocean via these vents. As the water carrying the minerals cools, the dissolved silica precipitates to make small grains, which then get ejected out of the surface by Enceladus’ geysers. Cassini discovered these miniscule silica grains in Saturn’s E ring in 2009.

Enceladus
Enceladus is located inside Saturn’s E ring and geysers on the moon release frozen water vapour, carbon dioxide and silica amongst other things and provide material for this ring.

Hydrothermal vents
How the hydrothermal vents (also known as white smokers) form on Enceladus.

During Cassini’s daring final flyby of Enceladus in October 2015, molecular hydrogen (two hydrogen atoms bonded together) was discovered in the plumes. The best explanation to where this molecular hydrogen comes from - those hydrothermal vents. On Earth, microbes that live around hydrothermal vents use hydrogen as a source of energy – they use a chemical reaction to combine hydrogen with carbon dioxide to produce methane. 

Find out more about this chemical reaction

Scientists believe that life on Earth could have started under the sea around a hydrothermal vent. Which leads us to question, is there life right now in Enceladus’ sea? Or if not right now, could life evolve there in the future? There seems to be enough hydrogen and carbon dioxide down there for it. However, the Enceladian ocean might prove to be too acidic for life.

Hydrothermal vents
A white smoker in the Pacific Ocean. Scientists believe Enceladus might have similar hydrothermal vents on its seafloor.

Giant tubeworms
Giant tubeworms are one of the organisms that live around terrestrial hydrothermal vents. Could Enceladus have something similar?

Anyone that has seen my shows at the Peter Harrison Planetarium might be aware that Saturn’s satellites are my favourite bunch in the Solar System and that I have a soft spot for the Cassini mission. After all, I did spend a year trying to understand the connection between the atmosphere and lakes on Titan (another of Saturn’s moons) by using Cassini data. Sadly, its fuel is almost gone and its time is almost up. In September 2017 it will crash into Saturn after 20 years of service, just as we’re getting so tantalisingly close to finding out just how habitable Enceladus is. On the other hand, when Cassini was launched in 1997 we had no idea about the plumes so the instruments on board the spacecraft weren’t designed to see if this incredibly small and icy moon could harbour life. Hopefully, we’ll visit Enceladus again soon to unravel more of its mysteries and build on Cassini’s legacy.