This artist's concept shows a possible scenario for the internal structure of Titan, as suggested by data from NASA's Cassini spacecraft. Scientists have been trying to determine what is under Titan's organic-rich atmosphere and icy crust. Image credit: A. Tavani
The more we explore the outer solar system with probes like the Cassini spacecraft, the cooler things we discover. This week planetary scientists working with data from that spacecraft announced that there’s a good chance Saturn’s moon Titan has a layer of liquid water hidden beneath that desolate icy surface.
The discovery came from the study of tides on Titan. This moon is squeezed and stretched as it orbits Saturn, and that is bound to cause some heating in the core. It’s also a shape-changing process.
The scientists figured out that Titan is not a big rocky ball that would show a slight bulge on its surface as Saturn’s strong gravitational pull tugged on it. The way they did this is quite ingenious. They looked at Titan during its 16-day orbit of Saturn. As it whirls around the huge planet, Titan’s shape changes and scientists could chart those changes. Titan is not a perfectly round sphere. Instead, it’s slightly elongated like a football. As it orbits Saturn, its long axis grows when it’s closer to Saturn. Eight days later, when Titan is farther from Saturn, it’s much less elongated and more nearly round. Cassini measured the gravitational effect of that squeeze and pull. These measurements and the assessment of Saturn’s gravitational pull on Titan provide the best data yet of Titan’s internal structure and what they show is that for the shape to change as much as it does, Titan likely has a an ocean layer. It’s not necessarily a huge or deep one, but the fact that it’s there at all is one more step in learning more about Titan’s structure.
Now, I read a few stories here and there about how this supposed ocean is darn near proof that life could exist on Titan.
Not so fast. The presence of a subsurface layer of liquid water at Titan is not necessarily an indicator for life. There are still a lot of studies to be done before scientists understand what Titan looks like in its interior, and whether or not the conditions are right for life to exist in that ocean, or perhaps at a rock-water interface deep inside.
The implications of an ocean in Titan is an exciting finding, no matter what else is discovered there. This mysterious, cloudy world is slowly yielding up its secrets, and in the process, is opening our minds about what other surprises we’re going to find in, on, or near the worlds of the outer solar system.
As it swooped past the south pole of Saturn's moon Enceladus on 14 July 2005, Cassini acquired high resolution views of this puzzling ice world. Courtesy NASA/ESA/Space Science Institute
Take a look at that moon. It’s called Enceladus and it orbits the planet Saturn. It looks all serene and quiet, in this view. And, probably looks cold and lifeless.
But, in reality, this little world has a hidden ocean beneath that icy surface. And, as on Earth, when we we think “ocean”, we often think “life”.
How can such a cold-looking place support an ocean? Doesn’t it have to be… um… warm? Well, as it turns out, a confluence of gravitational forces (between Saturn, Enceladus and neighboring moons) keep the subsurface water just warm enough to exist as in a liquid-slushy state. Couple that with tectonic actions that crack the surface and allow water under pressure to escape out to space, and suddenly Enceladus is way more than a quiet, icy world. It’s a place that spouts plumes of ice crystals out to space.
And, since the Cassini Solstice mission spacecraft is there studying those worlds, and occasionally sails through these plumes, we get an instant sample of the interior saltwater reservoirs of Enceladus after it has sprayed out to space. The plumes originate from the ‘tiger stripe’ surface fractures at the moon’s south pole, and create the faint E-ring, which traces the orbit of Enceladus around Saturn.
Jets of icy particles bursting from Saturn's moon Enceladus are shown in this NASA/ESA/ASI Cassini image taken on 27 November 2005. This and other recent images of Enceladus backlit by the Sun show the fountain-like sprays of the fine material that tower over the south polar region. This image was taken looking more or less broadside at the 'tiger stripe' fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. Credits: NASA/JPL/Space Science Institute
Today, NASA and ESA released information about the sampling runs that the Cassini spacecraft has been making through the icy plumes that the spacecraft first discovered in 2005. During three passes, done in 2008 and 2009, the mission’s Cosmic Dust Analyzer measured the composition of icy grains that had just been ejected from Enceladus. Close to the moon, the plume particles are large and rich in salt. In fact, about 99 percent of the total mass of solid material ejected through these plumes is very salty — but most of it falls back to the surface of this icy world.
So, how salty are we talking about? It appears that the particles that are the saltiest have what scientists call an “ocean-like” composition. That tells us most of the ice that gets shot out in the plumes comes from liquid salt water inside Enceladus.
Scientists studying these plumes have come up with this scenario to explain the plumes: deep underneath Enceladus’ surface, perhaps 80 km down, there is a layer of water between the rocky core and the icy mantle (structural layers). It is kept liquid by tidal forces generated by Saturn and some neighboring moons, as well as by the heat generated by radioactive decay (from elements that exist inside the moon and have been there since it formed).
Salt in the rock dissolves into the water, which accumulates in liquid reservoirs beneath the icy crust. When the outermost layer cracks open, the reservoir is exposed to space. The drop in pressure causes the liquid to evaporate, with some of it flash-freezing into salty ice grains: together these create the plumes.
Roughly 200 kg of water vapor gets lost every second as these plumes stream out to space. According to the team’s calculations, the water reservoirs must have large evaporating surfaces, otherwise they would easily freeze over, and that would stop the plume action dead.
This is one of those findings that simply amazes me about what we are able to learn from our science missions in space. If I walked up to you on the street and asked you, “How can we figure out whether or not a frozen world out near Saturn has subsurface oceans, and how much salt would they have, if they exist?” you would look at me blankly. It’s not on your daily radar, but that doesn’t make such a discovery unimportant. If you stop to think about why we’d even want to know such a thing, it wouldn’t take long to make a connection between water, salt water, warm salt water and… life. And, finding out places in our solar system where life could survive (and this is NOT a story about whether or not life exists there — that’s a whole other issue), tells us a lot about places where it DOES survive and exist. Mainly, here on Earth. We know of extreme places on Earth that have similar environments to Enceladus — and life exists in those Earthly habitats. So, now we know of at least one other place in the outer solar system where the conditions to harbor life mimic those on Earth. Kinda makes Enceladus seem all warm, fuzzy, and neighborly, all of a sudden. Not that life is there… but, the cradle of life is possible there.
