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SYFY WIRE Bad Astronomy

A Snail’s Pace Change in Cassini Reveals an Ocean Under Enceladus

By Phil Plait
I’d like to be
Under the sea,
In Enceladus’ ocean
In the shade.
            — with apologies to the Beatles

New results from the Cassini Saturn spacecraft offer further evidence that the tiny moon Enceladus has an undersurface ocean of liquid water. This is pretty exciting news, since liquid water was once thought to be a commodity unique to Earth in the solar system.

We’ve known for years there must be some repository of water under the surface. In 2005, Cassini discovered huge plumes of liquid water erupting from the south pole region of Enceladus, and later pegged them to long cracks in the surface nicknamed “tiger stripes” (technically called “sulci,” they look more like tiger claw marks to me, which I think is a more appropriate metaphor as well).

The new data strongly imply that the water is coming from an ocean under the tiger stripes, some 30-40 kilometers (20-25 miles) under the surface, and reaching up to latitudes of 50° or so (Enceladus is about 500 kilometers (300 miles) across. What’s really cool is that it was found using the cosmic equivalent of a cop’s radar gun.

Cassini is in contact with Earth via radio. The frequency of those radio waves emitted is essentially constant, but as the spacecraft moves toward or away from Earth we see a tiny Doppler shift in the frequency. Think of it as ever so slightly moving a radio dial to a different channel, if you like. These tiny changes due to Cassini’s motion can be measured to incredible accuracy.

Cassini has encountered Enceladus many times over the years. From 2010 to 2012 it flew past at very low altitude over the surface of the moon three times (at 48, 50, and 99 kilometers, close shaves indeed). If Enceladus were a perfectly homogeneous sphere, then we would have seen Cassini’s speed smoothly increasing until it reached closest approach, then slowing smoothly as it drew farther away. That would be the simple effect of the moon’s gravity pulling harder on the spacecraft as it got nearer, then slowing it as it receded.

But that’s not exactly what was seen. Overall, yes, but on top of that were tiny variation’s in the spacecraft’s speed, amounting to a mere 0.2–0.3 millimeters per second—that’s about half an inch every minute, literally slower than a snail’s pace. What caused those?

The smooth acceleration only happens if the moon is a smooth sphere. But it’s not. For example, a mountain on the surface would pull on Cassini a bit harder, changing its velocity a wee bit. A large depression would also affect the velocity, though in the opposite way, because the lack of material pulls on Cassini less.

We know Enceladus is not a smooth sphere; the south polar region is actually a basin, with slightly lower elevation overall. That means there’s less mass there, which in turn means that its gravity is somewhat less. We’d expect it to have less force on the spacecraft, so it doesn’t accelerate on approach and decelerate as it recedes quite so much.

In fact that’s what was seen, but there’s more: given the size of the depression, Cassini wasn’t affected as much as expected. It actually accelerated and decelerated a bit more than what the math showed it should. The most likely cause: something under the surface denser than the ice above it, something with more mass that compensated for the basin’s lack of mass.

Given the fact that the moon is mostly covered in ice, and the plumes are made of liquid water, this dense region deep under the south pole is very likely to be the reported ocean of liquid water (which is denser than ice). 

That’s very cool. It’s important to understand this was expected; if it hadn’t found that anomaly then that would’ve been pretty strange, since we know the plumes must come from somewhere. Given this, the next questions become 1) just how big and deep is this ocean, and b) how does the water get from that ocean up to the surface? We know the plume eruptions are linked to the shape of the orbit of Enceladus around Saturn, which means they erupt due to the influence of Saturn’s gravity on the moon. When Enceladus is farther from Saturn the stress from the planet’s gravity is relieved a bit, and the tiger strip cracks open wider, letting the water through. But how the water is transported the dozens of kilometers up to the surface is still somewhat uncertain.

I’ll note that the existence of this ocean was not really “discovered” in the traditional sense. The plumes from the surface already made it clear liquid water existed under the surface. What makes this great is that it lends a lot of much-needed support to what we already thought was going on. I also suspect that the new data can’t entirely rule out some other reason for the gravitational anomaly, like a large region of rock under the surface. The scientists involved do note that is highly unlikely, and that’s fine by me. I do think the existence of the ocean is the most likely and best explanation of the velocity seen.

And the implications of this are very exciting indeed. Organic (that is, carbon-based) materials have been detected in the plume water, meaning the rock and water under the surface of Enceladus have mixed. And even though the Sun never reaches these icy depths, there’s also clearly a source of energy melting the water (in the form of Saturn’s gravity squeezing the moon). That doesn’t mean there’s life there, of course, but it is pretty provocative. And now we think similar plumes from Jupiter’s moon Europa exist as well.

I’d say it’s about time we got serious about sending a lander to one or both of these small worlds. This is an entirely new window on our solar system that has been thrown open here. At the worst, we’d learn a vast amount about our neighborhood and the solar system in which we live.

And at best, maybe, just maybe, we’d learn about our neighbors, too.

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