EARTH'S MOON is a luminous silver-white charm--a gleaming, beckoning object, as well as an ancient symbol for that which is feminine. As the largest and brightest object in the star-splattered night sky, it long has inspired wonder and curiosity. It also is associated with love and frequently serves as a sign of elusive beauty. Yet, it is the only natural body beyond our Earth that we have set foot upon.
A moon can be defined as a natural body that has attained orbit around a planet It is kept in its orbit by the force of the host planet's gravity and the gravity of the moon itself. Some planets host a moon and some do not. There are a few theories about where Earth's moon came from and how it managed to form. The most credible is termed the giant impact theory, which sometimes is termed the Big Whack or Big Splash theory by astronomers when they are in a playful frame of mind. These impish nicknames arise from the fundamental basis of the theory: a Mars-sized protoplanet, named Theia by astronomers, crashed into the primordial Earth billions of years ago. The collision caused part of the Earth's crust to be blasted off into space. Some of this debris was captured into Earth-or-bit, where it eventually was pulled together by the force of gravity to become our moon.
Most of the Big Whack theory first was suggested back in 1975 by William K. Hartmann and Donald R. Davis of the Planetary Science Institute in Tucson, Ariz. Their theory is based on geological evidence gathered by Apollo astronauts when they made their historic journey to the moon in July 1969. Oxygen isotopes embedded in moon rocks proved to be almost identical to those on Earth. Also, other pieces of evidence indicate that the moon is made up of, at least in part, the same material as Earth's mantle.
Our closest--and very bewitching--companion, however, hardly is unique in the cosmic scheme of things. More than 100 moons circle planets in our solar system. Most of them are frozen bodies, composed of ices and rocky material. Yet, a few may not be lifeless after all. In particular, Europa of Jupiter may harbor a subsurface ocean beneath its cracked icy crust, warmed by tidal flexing into a life-loving, liquid water state. Primitive forms of aquatic life may be swimming around in Europa's still-hypothetical global, subsurface ocean. In addition, the second largest moon in our solar system, Titan of Saturn, possesses an environment hauntingly akin to that of our Earth before life developed here (prebiotic). Large raindrops of liquid hydrocarbons fall to the surface of this cold, tortured moon, creating seas and lakes composed of liquid methane and ethane that play the same role as water on our Earth. It is possible that life, as we do not know it, can develop using liquids other than water.
Meanwhile, the largest moon in our solar system, Ganymede of Jupiter, is bigger than the planet Mercury. Like its sister moon Europa, Ganymede may harbor a global ocean of liquid water beneath its crust of icy rock. Likewise, a tiny frozen moon, Enceladus of Saturn, sprays out geysers of ammonia-laced water from its so-called "tiger stripes." Hence, Enceladus may bear water beneath its devastatingly frigid crust of ice.
We have known since 1995 that our solar system is not the only game in town. There are hundreds of extrasolar planets circling stars other than our own sun. Astronomers believe that our Milky Way could be bursting with billions of planets--and an even greater number of moons. Some of these moons could possess the precious, mysterious recipe that allows them to become bubbling cauldrons of life. "Moons form so commonly in our solar system that it would be ludicrous to think that Ibis is unique," indicates Peter Ward, coauthor of Rare Earth.
As far back as ancient Greece, humanity has suspected that there are other solar systems in the universe in addition to our own. This speculation has not always been met with open arms by the powers that be. …