Consider a distant planet somewhere in our galaxy, where a lake is being filled by rains that bring down atmospheric elements that make the lake water rather different from the water on our own planet.

The solar ultraviolet rays beating down upon it form life-supporting compounds. Minerals carried from nearby rocks add themselves to the mixture producing further, more complex reactions. Season upon season, eon upon eon, in lake after lake, the mixture fails to reproduce or even conserve itself before it gets washed away. But with the passage of time, the mixture becomes stable and mutates, and one day something reproduces itself. A new life form has been born, a creature unknown to man but perhaps soon to be discovered.

About 4.5 billion years ago our planet was created. Earth consisted of land and water, nothing else. Astronomer and physicist Roland N. Bracewell believes that the creation of life first took place in shallow seas or in lakes, where molecules formed into compounds. These compounds became an essential ingredient in the next step or chain reaction that would transform the contents of the lake into living organisms.

"The existence of water is undoubtedly the most important solvent for life to evolve, since it permits rapid chemical reactions to take place," Bracewell said.

The availability of carbon, permitting the rich chemistry of molecules containing carbon, is another definite plus. Bracewell explains that when liquid water reacts with carbon, complex compounds are created. Hydrogen, nitrogen, and other atoms stick to carbon, carbon atoms stick to each other, and chains of enormous complexity form. Therefore carbon is considered the basis of all compounds of life.

"Carbon is the fourth most found element in the universe," Bracewell said. "Carbon compounds literally litter the cosmos, drifting through interstellar space in giant molecular clouds, making up a significant percentage, by mass, of comets and asteroids. The basic building blocks of life fell to Earth from space and that could easily happen anywhere in space."

Speculations about new life forms have generally assumed that their basic physical needs will be similar to the needs of life on Earth, composed of carbon compounds and using water as their fluid medium. However, one must be prepared for the possibility that somewhere totally different organisms might exist, organisms not based on organic chemistry.

Physicists and astronomers Michael H. Hart and Ben Zuckerman have considered the requirements for life from a broader point of view. What is required, they say, is a flow of free energy, "a system of matter capable of interacting with the energy and using it to become ordered, and enough time to build up the complexity of order that is connected with life."

These conditions may occur in a variety of environments, some very different than Earth. Hart and Zuckerman continue to speculate that these environments may give rise to life forms using solvents other than water, chemical systems making little use of carbon, and even forms based on "physical interactions," rather than molecular transformations. The three possible situations in which life is most likely to evolve, other than the ones we know about, could be:

1. Ammonia: Liquid ammonia, perhaps with some water, would serve as the solvent. This could occur on a planet with temperatures near minus 50 degrees celsius, like Jupiter’s moon, Europe.

2. Hydrocarbons: A mixture of hydrocarbons would function as the solvent. Life could evolve in a wide range of temperatures, depending on the mixture. Repeated reactions, such as hydrogenation, could be used as an energy source.

3. Silicate life: The element silicon is a possible basis for forms of life that would be different from ours, because it combines with other chemical elements in the same way that carbon does.

Dr. David S. McKay said Mars is the most likely place where life may have once existed in the solar system besides on our planet. While it hasn’t been proven that Mars has ever harbored life, studies of the Red Planet clearly shows that there were warmer and wetter periods in its past in which life may have arisen.

McKay thinks that we should not reject the idea that life might exist in sheltered places on Mars. "We have found evidence on our own planet where life has developed and flourished in very unlikely places where we didn’t suspect it was possible, such as several miles below the surface of the Earth or inside rocks that happened to have liquid water and a source of energy available," McKay said.

In 1997 it was announced that a "complete genetic identification" of an organism was found living at the bottom of the sea in the extreme heat and pressure of a volcanic vent. The headline was that this class of microbe, called Archea, represents a completely new branch of life.

McKay suspects there are sheltered places on Mars – on the polar caps, in the polar caps, under the surface, or in permafrost – that contain sources of liquid water we have not yet discovered, and where life might exist.

Judith Braffman-Miller, reporter and researcher on space for USA Today, argues that although people have long suspected there might be life in places as close as Mars, or on other planets in our solar system, planetary probes and other observations have found no evidence of any alien life forms in our solar system or any other place in the Galaxy.

McKay said this is not surprising, considering that life existed on Earth only as single-celled, microscopic organisms for most of its 3.5 billion-year history, 2.5 Billion years, to be exact. If an alien life form is not more developed than a tiny organism, we will have a hard time detecting it.

Braffman-Miller also argues that if alien life forms exist, and if they’re intelligent, why haven’t they made contact with us?

McKay said the reason is because they probably can’t see us, since our sun is much brighter than our planet, making Earth invisible. In the same way we may have difficulties seeing their planet.

So the question remains, is there the possibility of life, past or present, elsewhere in the universe?

NASA scientist Dr. Sten Odenwald said that in the near future NASA has plans to build telescopes and launch probes that will detect Earth-like planets, but in the meantime, all we can do is wait for what might be found.

The truth is out there, we just need to find it.