Cassini Probes Subsurface Oceans of Enceladus

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.

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.