How Do Moons Have Oceans?

A correspondent emailed to tell me that THIS ARTICLE was interesting and deserved to be in the blog. I concur!

The moons in question are some of those of Jupiter and Saturn. We don't know about those around Uranus and Neptune but stay tuned! 

The obvious one is Enceladus, the ice covered moon of Saturn. It has icy plumes erupting near its south pole. A fly through by the Cassini spacecraft found it to be water containing sodium chloride, quartz and hydrogen.

Read the article to find the moons with oceans, but what interests me and the author is how does liquid water exist in what must be a very cold place. The answer seems to be tidal friction. 

In their orbits the moons are subject to gravitational forces from other moon and their parent planet, and these can vary enormously. Bits of the moons are moving against each other producing enough heat to keep their oceans liquid. The outermost parts are frozen, the interior is liquid.

The presence of liquid water leads to speculation about the presence of life - read about it in the article.

Five images of Saturn’s moon Enceladus, seen in infrared light. Nestled beneath its icy shell is a global ocean — a sea that is erupting into space through fractures in the moon’s south pole, coloured red at bottom right.