The Bricks of Life: Exploring the Idea of Alien Chemistry

----snip

Hype aside, as most astrobiologists or any one of a thousand books will tell you, carbon-based life is not simply a provincial conceit. There’s good reason why this element is the basis for life on Earth, and probably on most other worlds that shelter biology.... Carbon, in other words, is adept at making complex structures. And complex structures are the bricks of life.

----snip

Are there other contenders? Is carbon really so special, or did it just get lucky here on Earth? If you have a periodic table handy, you’ll note that the element situated under carbon is silicon, which also has four electrons in its outer shell. Ergo, silicon might also seem to be an obvious basis for life, a point that was first made at the end of the nineteenth century by the German astrophysicist, Julius Scheiner. The optimistic Scheiner was certain that other planets in our solar system (including roasty toasty Mercury) sported life.

But his sunny attitude was misplaced when it comes to silicon-based beings. Silicon may be carbon’s chemical cousin, but it’s a poor relation. Because the silicon atom is larger, its bonds with other elements are weaker. While carbon hooks up with two oxygen atoms to make carbon dioxide, a nice waste product for both humans and SUV’s, the silicon equivalent, silicon dioxide, quickly assembles itself into a crystalline lattice. It’s better known as sand, and would make exhaling a gritty experience. The weaker bonds of silicon also preclude the easy formation of those long, same-atom molecular chains that underlie many biological compounds. A slew of complex carbon-based molecules are easily produced in comets, interstellar dust, and university glassware. But if you check out nature’s chemistry lab for silicon (consider volcanic lava), the products are far less interesting.



http://www.space.com/searchforlife/seti_carbon_040415.html