The potential habitable zone on Saturn's moon Enceladus: recent discoveries from NASA's Cassini spacecraft have led many scientists to believe that numerous microorganisms found on Earth could potentially survive on Saturn's moon Enceladus.

NASA's Cassini spacecraft continues its close flybys of Saturn's moon Enceladus, sampling the enormous plumes of water vapor and icy particles being ejected hundreds of miles into space. The most recent flyby was in October 2008; the next is planned for November 2009. The prospect of near-surface liquid water on another planetary body has never been so promising. Of course water in and of itself is not what makes this discovery so exciting: it's the underlying potential of a habitat conducive to supporting life. Not since the Mars Viking missions of the 1970s has the talk of possible life on another planet buzzed so prolifically. But is this buzz justified? Healthy skepticism abounds as the search goes forward.

"Enceladus represents one of the likely, if there is such a thing, places in our solar system other than the Earth where we might find biological activity," said Andrew J. Dombard, visiting associate research professor, department of Earth and Environmental Sciences, University of Illinois at Chicago.

"Personally, I don't think there is life anywhere in this solar system apart from the Earth, but I do think it's very likely there is life in other solar systems," said Francis Nimmo, associate professor, department of Earth and Planetary Sciences, University of California, Santa Cruz.

Dombard and Nimmo both served on the Science Definition Team for NASA'S Enceladus Flagship Mission Concept Study (NASA Goddardd Space Flight Center 2007).

Many scientists now believe an "Earth-like" planet is not necessary for potential life elsewhere. The so-called "habitable-zone," the distance a planet would have to be from the Sun to ensure life wouldn't get too hot or too cold and the Sun's energy would provide the energy for life through photosynthesis, is being extended. Once thought to only lie somewhere between Earth's closest neighboring planets Venus and Mars, several potential micro-habitable zones have now been identified in our solar system. Many scientists now believe that numerous microorganisms found on Earth would do just fine on several of the moons orbiting Jupiter and Saturn. It is not only discoveries in space that have fueled this paradigm shift-in the 1970s scientists discovered life deep in our oceans, where sunlight is unable to reach, that is not dependent on energy derived from our sun. Instead, it depends on hot, nutrient-rich water emanating from hydrothermal vents. Rather than utilizing photosynthesis, these organisms rely on chemosynthesis to survive.

How life on Earth originated is not known; however, the general theory is that billions of years ago the Earth had the ingredients, the environment, and most important, the time for life to form. The time--the primary limiting factor in laboratory experiments--spanned hundreds of millions of years, during which complex chemistry occurred until finally, in just the right way and under the right environmental conditions, the chemistry came together and voila: life. A very basic system of organic molecules was created within a rudimentary membrane, capable of following a template to replicate and catalyzing other reactions necessary for maintenance (Lunine 2005). Early life would most probably have obtained energy through chemosynthesis, with the more complex photosynthesis evolving later.

A vivid imagination is not necessary when contemplating what sort of life is possible on Enceladus--one need only look at the earthly examples known as extremophiles. These are bacteria capable of living in extreme environmental conditions of temperature, pH, radiation, pressure, salinity, dryness, absence of oxygen, absence of sunlight, and chemicals. Earth is teeming with life, from deep within its crust to high in the atmosphere, and the variety of that life is greater than we could ever imagine. Indeed, to many extremophile microorganisms on Earth Enceladus might be like a balmy vacation resort where life is easy and a party is never far away.