NASA Scientists' Astonishment at Enceladus

Science & Technology

Enceladus is perhaps one of the most intriguing objects in the entire Solar System. And yet, it is only the sixth-largest moon of Saturn, and in natural light, it looks very unassuming. However, there’s a lot more to Enceladus than meets the eye. It’s an active, icy world, with jets of water vapor pouring out from its southern hemisphere.

Thanks to the remarkable Cassini mission, we have studied and observed Enceladus in exquisite detail, and perhaps know more about it than some of the closer and bigger Jovian moons. However, although we’ve seen a lot from the outside, it’s the inside of the moon that still holds so many mysteries.

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On Oct. 9, 2008, just after coming within 25 kilometers (15.6 miles) of the surface of Enceladus.Credit to NASA/JPL/Space Science Institute

we will explore some of the most fascinating details of Enceladus, piecing together photos and data from a variety of different missions, and find out almost everything you could want to know about this special moon. Enceladus is currently Saturn’s 14th closest moon. I say currently, as Saturn has some tiny moonlets hidden in its rings that may or may not be classified as moons in the future.

It is the second closest major moon though, second only to Mimas. That means that its orbit takes it just outside of Saturn’s major rings. Its orbit follows the plane of the rings very precisely, and it only takes 33 hours to orbit Saturn once.

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This is an artist's concept of Saturn's rings and major icy moons.Credit to NASA / JPL-Caltech.

Interestingly, it is in a 2:1 orbital resonance with Dione, Saturn’s 4th closest major moon. In other words, it orbits twice around Saturn in the time Dione orbits once. This orbital resonance is believed to prevent Enceladus’ orbit from ever becoming perfectly circular, which causes Enceladus to undergo tidal deformation. This is significant, as these tidal forces heat Enceladus’ core.

You see, as far as we can tell, Enceladus’ surface is predominately made of clean water, certainly with little to no rocks or much else there. Because Saturn is situated so far away from the Sun, it means the outer layer of water on Enceladus has frozen over. Enceladus is essentially a frozen ocean world, a giant ball of water ice.

Because it is free from other materials on the surface, this moon is one of the whitest objects in the solar system, with a Bond Albedo:0.81, which is pretty much as high as snow. As such, it is one of the coldest satellites of Saturn, with a noon temperature of -200°C, as the white colour of its surface reflects a large percentage of the sunlight reaching it back into space.

However, about 30-40km down under the surface of Enceladus, pressures start building and heat energy generated from the tidal deformation of its orbit has increased the temperature of the water ice to the point where water at this depth can exist in a liquid form.

It could well be that there is an entire mantle or global ocean of water that the ice crust is resting upon, very much like the magma mantle that our rocky crust on Earth rests upon. At the very least, scientists expect there to be a huge pocket of water under the moon’s South Pole.

How do we know this?

Well, the most obvious indicator is the huge plumes of water being ejected from cracks in the crust, something referred to as water or cryovolcanism. These jets are active, consistently blasting around 250kg of water into space every second, at speeds exceeding 2000km/h.

This is powerful enough that most of the water vapour particles escape Enceladus’ weak gravity, and they end up in orbit around Saturn, forming Saturn’s E Ring. This ring around Saturn is very diffuse, and so isn’t visible unless it is backlit by the Sun.

From this angle, the light shining through the water particles make the ring appear exceptionally blue this ring is considered the bluest object in the Solar System, even more so than Neptune, due to the ring’s uniformity. The E ring is Saturn’s second outermost ring, and it is 2000km wide.

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Cassini images of long, sinuous, tendril-like features from Saturn's moon Enceladus.Credit to NASA / JPL-Caltech.

Its shape is also heavily influenced by the orbit of Enceladus. Enceladus’ plumes create Tendril Shapes in the ring as more material erupts out of it, however, these sections of the ring tend to smooth off as Enceladus moves further away along its orbit. During Cassini’s mission, Cassini was able to pass through these Plumes to detect the substances being ejected from them.

Cassini wasn’t designed with this in mind; scientists didn’t even know about the plumes until Cassini got there, however, Cassini was equipped with an instrument called the Cosmic Dust Analyser, designed to detect what the tiny dust grains in orbit around Saturn are made of, and it was able to use it for Enceladus’ plumes too.

As it wasn’t specifically designed with this in mind, it might not have given us the full picture of what’s in these particles, but while water was the predominant substance detected, amino acids, carbon dioxide, nitrogen and methane were also found.

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The Cosmic Dust Analyzer, or CDA, detected dust particles one-thousandth of a millimeter wide.Credit to NASA / JPL-Caltech.

Amino acids are significant as they are the building blocks of life and can be found around thermal vents at the bottom of Earth’s oceans.

Does this mean Enceladus has thermal vents of its own? And if so, do they have ecosystems of life around them?

While evidence for an underground ocean is abundant, scientists still aren’t completely sure about Enceladus’ internal structure. At some points in the past, scientists believed that Enceladus was water all the way through, however, data from Cassini suggests that Enceladus’ mass is greater than previously thought, meaning it must have some amounts of iron or silicate material in its core.

Scientists are starting to lean towards the theory that its internal structure is differentiated, meaning it’s a celestial body with defined layers within it. An object of this size doesn’t have to be differentiated it’s so small at only about 500km across, that it is right on the borderline of being in hydrostatic equilibrium, or in other words, being rounded by its gravity.

There are other objects out there of similar or slightly smaller sizes that are not in hydrostatic equilibrium, like Neptune’s Proteus. In any case, assuming it does have a differentiated interior, this core is likely to be predominately rocky. This is important, as thermal vents in Enceladus’ water ocean would have to come from a rocky core.

A rocky ocean floor would also provide nutrients and minerals essential for what we believe life would need to form and evolve. Thermal activity does exist, due to the way Enceladus has plumes in the first place, and the amino acids detected in the plumes suggest a rocky core.

As Cassini passed over Enceladus, it also mapped out the thermal emissions from the moon. It turned out that the jets line up with what has come to be known as Enceladus’ Tiger Stripes. These are large depressions, roughly 130 km long, 2 km wide and 500m deep.

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South pole of Enceladus spew icy material from the subsurface ocean. Cassini, the tiger stripes are visible in false-blue color.Credit to NASA, ESA, JPL, SSI, Cassini Imaging Team

It is believed that these are tectonic fractures in the moon’s icy crust. What is interesting about the surface features of Enceladus, is that there are virtually no impact craters at all over the southern hemisphere, and not many anywhere else.

This implies that Enceladus’ surface is very young, as while it does have a thin atmosphere made up of the ejected water from the plumes, this isn’t nearly enough to burn up asteroids before they hit the surface. Some water from the plumes settles again on the surface, which smooths it off over time. This is another reason why Enceladus is so round for such a small celestial body.

Apart from the tectonic fractures and a few craters, Enceladus’ topographical variation is quite minimal. There are no mountain ranges to speak of, although there is what you might call a rough terrain around the south pole if you zoom in far enough.

This is perhaps the highest resolution image we have of its surface, and as you can see, it does appear like a giant glacier. It’s interesting to note that even in this small view, there are some smooth sections of ice but also jagged regions. Over the North Pole, the clear difference is the number of craters present there.

While there are tectonic fractures here too, there are no plumes on this side of the moon, so the surface here is a lot older than around the South Pole, which is why it has more of a cratered surface. As Cassini flew through the plumes around the South Pole, they also took the opportunity to image the surface closely around the tiger stripes.

The surface here is pretty incredible, unlike anything you would have seen on Earth. It’s like the surface has been folded, squashed, and shifted around, leaving these remarkable fracture lines and formations in the surface ice.

The fact that Cassini was able to get so close to Enceladus is a feat in and of itself. It’s fantastic that we can have such a close-up view of something so far away. Unfortunately though, since the Cassini mission has ended, we no longer have anything in orbit around Saturn that can study Enceladus further.

There have been plenty of mission proposals in the past, but all of them were cancelled before they came to fruition. What would be incredible is a probe that could either make its way into Enceladus’ ocean somehow or at the very least search the plumes for signs of life. There is already a mission, called Dragonfly, going to the nearby moon of Titan, however, this won’t have anything to do with Enceladus.

So, although a couple of mission proposals are currently under review to go there, that means we are unfortunately still a couple of decades away at least. Which is a shame! Because who knows what secrets still lie in wait under that crust. So, there we have it, a look at the intriguing little moon of Enceladus.

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