Monday, August 24, 2009

The Future for Some Habitable Moons

Exchanging Rings?

I was watching BBC's 1999 miniseries The Planets (shown on the Science channel) recently; they were showing several episodes in a row.

In one episode they discussed how Saturn's rings will, eventually, vanish. But that the moon of another outer planet, Neptune, would break up from tidal forces as it slowly spirals in toward the planet, creating another planet with rings rivaling that of Saturn's lost rings. We would swap one ringed planet for another.

Yes, Neptune already has a ring system, but it is very dark - being comprised mostly of rocky debris while Saturn's bright rings are comprised mostly of ice. Neptune's moons are composed of rock and ice - a break up of, say Triton (one of three moons in the solar system with an atmosphere), would create a ring of ice around the planet, giving it a shimmering, shining ring system.

Spiraling Moons

This got me to thinking about the habitable moon discussions. Tidal forces are what help heat the moons (if Europa has liquid water under its icy surface, for instance, it will be the result of tidal forces). The very tidal forces that would allow for the possibility of life, which would allow for a moon to be habitable, may also signal its early demise - if the moon is slowly spiraling in.

So, for some habitable moons, their lifespans may be shorter than those of habitable planets. Which has a bearing on the rise of high order sentient life. High order sentient life probably needs a very long time to arise. Imagine being an alien race finally creating technology and exploring their solar system, only to find that their world will die long before their sun will.

Of course the flip side of the coin is shown by our moon. The Moon is slowly spiraling away from the Earth (at 3.8 centimeters, or about 2.5 inches, per year). However, while in 500 million years or so there won't be any full eclipses anymore, we won't lose the moon. Eventually the Moon and the Earth will become gravitationally locked together. At some point the Earth and Moon will become locked - the same side of the Earth will always face the same side of the moon. When that happens the Moon will quit spiraling away from us.

Why? It has to do with the fact that the Earth has large oceans. Right now the Earth rotates faster than the Moon revolves around it. The tidal bulge created by the Moon's gravitational pull moves ahead and actually forces the Moon to speed up a tiny bit. This causes its orbit to get larger. When the Earth and Moon are locked, the tidal bulge won't move faster than the Moon. High tide will be permanent in some areas, and low tide permanent in others.

image credit:
"Thetis Moon" ©
I still believe that we may find more habitable moons than habitable planets. But that some moons may spiral into toward their planet billions of years before their system's sun(s) die, will be one reason why it may be harder for high order sentient life to arise on some moons.


Cain, Fraser. "Ep. 17: Where Does the Moon Come From?" Astronomy Cast. 1 Jan. 2007. Web. 24 Aug. 2009. <>

Mihos, Chris. "Neptune's Moons." Journey Through the Galaxy. Astronomy Dept. Case Western Reserve University. 13 Sept. 2006. Web. 24 Aug. 2009. <>

Scharringhausen, Britt. "Is the Moon moving away from the Earth? When was this discovered?" Curious About Astronomy: Ask an Astronomer. Cornell University. 21 nov. 2002. Web. 24 Aug. 2009. <>

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