from Gizmodo Website
(Image: NASA/JPL/Space Science Institute)
With surprising frequency, this ice-covered moon spurts a plume of water into space - a telltale sign that a global ocean lies underneath.
Scientists have struggled
to explain how such a tiny moon could sustain enough energy to
maintain a liquid ocean, but new research shows that a porous core
could do the trick, and that Enceladus has been wet for billions of
years - a potential sign of habitability.
Remarkably, the 3D models
used for the study indicate that this process, which requires a wet,
porous core, has been ongoing for potentially billions of years, an
observation that bodes well for astrobiologists in search of
microbial alien life.
At its thickest, this ice runs about 12 to 15 miles (20-25 km) deep, but it thins to just a few miles over the southern polar region.
It's in these southern areas where Enceladus' geysers can be found, spewing jets of water vapor and icy grains (some containing simple organics) through cracks in the ice.
(NASA/JPL/Space Science Institute)
But for these ingredients to exist, the temperature at the bottom of the ocean must be exceptionally hot.
Because of what Cassini uncovered, we know there are some serious chemical reactions happening along the boundary that separates the moon's liquid ocean from its warm, rocky core.
Exactly where Enceladus gets all this crazy amount of energy isn't immediately obvious.
The heat required is about 100 times more than what would be expected through the natural decay of radioactive elements within the core's rocks.
More plausibly, a major part of the process has to do with the moon's host: Saturn.
Enceladus spins around its gas giant along an elliptical orbit, where the constant gravitational pushing and pulling creates a tidal effect. At the core, this tidal effect produces friction, and by consequence warmth.
Yet this is still not enough energy to counterbalance the heat bleeding off the ocean.
By all accounts, this moon should've frozen over after about 30 million years, according to scientists. But it hasn't, and because Enceladus is still extremely wet and active, something else must be going on.
To find out, a team from the US and Europe, led by Gaël Choblet from the University of Nantes in France, ran a series of 3D simulations to see what's going on inside this moon.
(ASA/JPL-Caltech/Space Science Institute;
interior: LPG-CNRS/U. Nantes/U. Angers)
With a highly permeable rocky core featuring upwards of 20 to 30 percent empty space, cool liquid water can rush in and get warmed by the tidal friction (temperatures at the core can reach as much as 363 Kelvin or 90 degrees Celsius).
When the water gets hotter than its surroundings, it rises and gets flushed out of the core via narrow cracks, similar to hydrothermal vents at the bottom of Earth's oceans.
This process repeats itself creating a hydraulic cycle of sorts; every 25 to 250 million years or so, the entire volume of Enceladus' ocean goes through the moon's core.
Incredibly, this activity
can be maintained for billions of years, according to the models.
Indeed, in addition to having warm water, organic molecules, and other "building blocks" of life, it's had an ocean for potentially billions of years - enough time (at least in theory) for simple microbial life to emerge.
But we'll only know by exploring this moon even further.
Likewise, Hunter Waite, the program director for NASA's Space Science and Engineering Division, says the research makes sense, pointing to a study (Cassini finds molecular hydrogen in the Enceladus plume - Evidence for hydrothermal processes) he co-authored earlier this year.
The new study, while it explains Enceladus' liquid global ocean, internal heating, thinner ice at the south pole, and hydrothermal activity, doesn't explain why the northern polar region features ancient ice covered in craters.
The models predict
thinning at both poles, so something else is going on that still
needs to be studied.