...the Viking expansion is a good example of what is termed an auto-catalytic process. In chemistry the term catalysis means the speeding-up of a chemical reaction by an added ingredient, such as an enzyme. Some chemical reactions produce a product that also acts as a catalyst, so that the speed of the reaction starts from nothing an then runs away as some product is formed, catalyzing and driving the reaction faster and producing more product which drives the reaction still faster. Such a chain reaction is termed auto-catalytic, the prime example being the explosion of an atomic bomb when neutrons in a critical mass of uranium split uranium nuclei to release energy plus more neutrons, which split still more nuclei.

Jared Diamond