The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known spectral lines at that time could explain it. There was some speculation that the lines were caused by a new element, and the name nebulium was coined for this mysterious material. With better understanding of atomic physics, however, it was later determined that the green spectrum was caused by a low-probability electron transition in doubly ionized oxygen, a so-called “forbidden transition”. This radiation was all but impossible to reproduce in the laboratory at the time, because it depended on the quiescent and nearly collision-free environment found in the high vacuum of deep space.
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." Despite this oddball moon's many exotic attributes, it actually sports one of the most Earth-like surfaces in our Solar System. Titan may also experience volcanic activity, but its volcanoes would erupt with different ingredients than the molten-rock lava that shoots out from the volcanoes of Earth. In dramatic contrast to what occurs on our own planet, Titan's volcanoes erupt icy water "lava" (cryovolcanism). Titan's entire alien surface has been sculpted by gushing methane and ethane, which carves river channels, and fills its enormous great lakes with liquid natural gas. Moons, Myths, Etc. Our bewitching, and sometimes bewildering Moon, has long been the inspiration for magical myths, weird legends, bedtime stories, and beautiful poetry. Earth's Moon is a very ancient symbol of femininity, as well as for wild bouts of strange madness and romantic love. Some ancient, traditional legends and childhood stories tell of a man's face etched out on its shining surface, while others tell strange tales of a "Moon Rabbit." Lovely, ancient myths, tales, and bedtime stories aside, Earth's Moon is a very real object. It has been a companion-world to our Earth almost from that very ancient era, when our Solar System was first forming, about 4.5 billion years ago.