Water in the Solar System

Enceladus Has a Watery Ocean

Back when we knew much less about the solar system than we do now (and I’m talking maybe the middle of the last century), people who studied the solar system thought that Earth was about the largest source of water in the solar system. Oh, they knew about comets and their watery makeup, but for the most part, it seemed like Earth had the corner on the water market.

It turns out, Earth is not quite the oasis in a water-poor solar system as we once thought. In fact, water is everywhere, we just have to use the right tools to find it. Today, astronomers on the Cassini Solstice Mission, which is operating out at Saturn, have gathered enough evidence to suggest that the icy moon Enceladus (which is small — about 500 kilometers across) has a subsurface ocean of liquid water. Enceladus is one of Saturn’s moons and is best known for its geysers of particles streaming out from cracks in its surface.

How did they figure out that Enceladus has an ocean? Sami Asmar of NASA’s Jet Propulsion Laboratory explained that it was a combination of Cassini observations and measurements of signals received from the spacecraft by the Deep Space Network. That’s right, it was deduced by using radio signals and how they’re affected  by gravity.

“The way we deduce gravity variations is a concept in physics called the Doppler Effect”, he said. “It’s the same principle used with a speed-measuring radar gun. As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we’re trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system.”

In layman-speak, the Cassini spacecraft “feels” the gravitational tug of Enceladus, and that tug changes depending on what part of Enceladus the spacecraft is closest to. The changing tugs slow down or speed up its orbital velocity just a tiny amount. That change in speed also changes the frequency of the radio transmissions from Cassini. When the ground stations receive the signal, scientists analyze the signals and can tease out the Doppler effect. That gives them a clue as to the gravitational tug the spacecraft feels as it goes past Enceladus. The gravitational tug of a region full of water is different than the tug of a region that is mostly ice.

The gravity measurements suggest a large, possibly regional, ocean about 10 kilometers) deep, beneath the thin icy shell that makes up Enceladus’s surface. That shell is probably around 30-40 kilometers thick in places.

So, Is there Life on Enceladus?

What does this discovery of water on Enceladus mean for the search for life on other worlds? For one thing, it adds to the list of places where water exists in the solar system. We know there was lots of water on Mars in the past and there is still some there today. How much, we don’t know yet. We also know there’s water at the Moon, but the amount of it (particularly water “encased” in rock) is still under debate. Jupiter’s moon Europa has long been suggested as another place for water, and its frigid surface is made largely of water ice. It’s likely there’s a water ocean under that surface, as well. Of course, the comet reservoir out in the Oort Cloud is known to be a “sink” for water ice, given that so many comets show water in their chemical signatures. So, water isn’t an unknown thing beyond Earth anymore.

The first thing that comes to mind when you think about water somewhere on a planet or moon (or even a comet) is life. If there’s water there, could there be life? That’s a reasonable question to ask, actually. Water is always touted as one of the Big Three Ingredients for Life, along with food and heat (energy). We know life happened here on Earth, where there’s lots of water, so it’s logical to assume that it could happen in other wet places. You just need to look for warmth and something for the life to munch on.

At Enceladus, the subsurface ocean evidence puts it on a short list of places in our solar system that could host microbial life. Couple that evidence with the very obvious gushers of water coming from fractures that planetary scientists have been seeing for some years now, and you have a place that could be hospitable to life. That’s IF it has a source of warmth for microbial life. The food comes along in the form of organic materials that have been detected in the salty spray erupting from beneath the Enceladus surface.

There’s no evidence (yet) that life exists in the Enceladan ocean. That’s a story that requires more data, more study, and more analysis. The big news here is that we’re adding yet another watery place to our list of solar system places where life could exist, given the right conditions.