I received an email from Tom B. this week asking about this news story claiming that water geysers observed on Saturn's moon Enceladus may be due to friction of ice plates rubbing together:
You may have seen the new suggestion regarding how gas plumes form on Enceladus. It suggests that thick ice plates are rubbing together and creating a vapor that escapes as plumes of water vapor. Would you please provide some explanation of how a place as cold as Enceladus could produce anything so hot as water vapor simply by friction?
I can certainly try to explain it! In the outer reaches of our Solar System, water (usually in the form of ice) is very common. Most of the moons, comets, and even larger objects (like Pluto) are made mostly of water ice. Partly this is because the outer solar system is so cold -- temperatures out there are a chilly -325 degrees Fahrenheit! So if you want to make geysers out of this water, you would need to warm the water up by 350 degrees, right?
Well, not necessarily. In my last post, I was talking about the strange forms of "hot ice" that could exist on a Neptune-mass planet around another star. As part of that post, I included this graph showing the phases of water as a function of temperature and atmospheric pressure.
The moons in the outer solar system (except for Titan) have extremely tenuous atmospheres; essentially they have no air pressure. So, looking at the diagram, you notice that the freezing point of water in very low atmospheric pressures goes way down -- only 200 Kelvin (-100 Fahrenheit). So, on these moons, you don't need to warm up the ice quite as much before it changes into a gas, at which point it will find its way through any cracks of the moon's frozen surface and vent into space.
But we also need to consider these big plates of ice. Enceladus orbits Saturn in a near-perfect circle. But it is not quite perfect -- some times it is slightly closer to Saturn than other times. And when it gets closer to Saturn, it gets squeezed a little more. This energy from squeezing (or gravitational tides) seems to cause plate tectonics on Enceladus, just like on Earth, except with water ice instead of rock making up the plates.
On Earth, plate tectonics is responsible for many volcanoes and for earthquakes. Until this press release, scientists thought that the geysers of Enceladus were like Earth's volcanoes -- ice is melted deep in the moon, and near the edges of these plates, the magma (in this case, water), finds it easier to escape to the surface and be erupted out.
So, finally, to the new idea about the geysers. This new concept blames the friction of the ice plates grinding together for the heating of the ice. We on Earth know of frictional heating -- it is why your hands get warm when you rub them together, and why your car's brakes overheat if you use them too much. Friction happens during earthquakes, too, and during moonquakes on Enceladus. In both cases, tremendous amounts of heat are generated. In an earthquake with a magnitude of 4.0 (enough to cause noticeable shaking, but in most of the U.S., there would be little or no damage -- Californians experience 4.0 earthquakes all the time), there is as much energy released as a ton of TNT. A major earthquake, say magnitude 7.0, releases 32 megatons of energy -- dozens of the largest nuclear weapons worth!
This energy has to go somewhere, and most of it goes into friction and heat. As you might imagine, this heat can be tremendous. But on the Earth, it isn't enough to melt rock, which has to get up to thousands of degrees to melt. So an extra couple of hundred degrees deep in the Earth makes no noticeable change. But on Enceladus, this energy would be enough to change the ice into gas, which would then vent out through the fault line that had the moonquake. Remember, these ice sheets are miles thick and acting much more like rocks on Earth than like the ice floes in our arctic oceans (which do gently rub together, and don't create geysers).
The calculations reported in the news story on Enceladus show that friction alone makes enough energy -- remember we are talking about thousands of tons of ice rubbing against each other. But this isn't proof that the geysers on Enceladus are powered by friction; it just shows that it is possible. Yet one thing we know from decades of modern physics is that when strange things are possible (think of black holes and neutron stars), these things tend to happen.
So, in short, the reason the frigid ice on Enceladus could melt due to friction is (a) you don't need to warm it up too much, and (b) remember that we are talking about tremendous amounts of ice, sheets of ice a dozen miles thick, rubbing against each other in sudden releases of energy. And more tests are needed to show if this idea is correct or not.