Earth's Moon Reveals An Ancient Secret. Many astronomers think that during an ancient era, termed the Late Heavy Bombardment, our young Moon was violently battered by a marauding multitude of invading asteroids that crashed onto its newly formed surface. This attack of pelting objects from space occurred about 4 billion years ago, and the shower of crashing asteroids excavated impact craters, and also slashed open deep fissures, in the lunar crust. This sustained shower of merciless impacts increased lunar porosity, and opened up an intertwining network of large seams under the Moon's surface.
Dr. Porco further believes that Enceladus's orbit could have been much more eccentric in the past. The greater the eccentricity, the greater the tidal squeezing, and the resulting structural variations produce heat. In this case, the heat would have been saved inside the icy moon, melting some of the ice to replenish the liquid water sea. Dr. Porco continued to explain that "(T)he tidal flexing occurring now is not enough to account for all the heat presently coming out of Enceladus. One way out of this dilemma is to assume that some of the heat observed today was generated and stored internally in the past... (N)ow that the orbit's eccentricity has lessened, the heat emanating from the interior is a combination of heat produced today and in the past."
Titan's atmosphere is approximately 95% nitrogen. However, in a way that dramatically differs from Earth's own mostly-nitrogen atmosphere, Titan's atmosphere has very little oxygen. Indeed, the remainder of Titan's atmosphere is almost entirely composed of methane--along with small qunatities of other gases, such as ethane. At the extremely cold temperatures that are found at Saturn's great distance from the heat of our Star, Titan's methane and ethane can accumulate on its icy surface to form pools of liquid.