Frontiers | Pages 164-165 | See Linked Version | ||||||||||||
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As you undoubtedly recall, one bowl of porridge was too hot, another was too cold, but the third was just right. Temperatures vary on the surfaces of planets as well. Thermometer readings on other worlds depend mostly on their distances from their parent stars and the compositions of their atmospheres. Planets very close to their stars fail the Goldilocks test, as do those orbiting in the deep freeze far away from their suns. For life as we know it, the "just right" range encompasses the temperatures at which liquid water can exist. At ordinary atmospheric pressures, that range is 32 to 212 degrees Fahrenheit. This emphasis on liquid water is not arbitrary. Hydrogen and oxygen--the ingredients of the water molecule--are the first and third most common elements in the universe. Liquid water provides a medium within which chemical reactions can occur stably and quickly. That's not the case for the other forms of matter that water usually assumes, ice and vapor (or steam). The crucial chemical reactions involve substances that contain carbon, the fourth most common element. (The second most common, helium, is inert and plays no role in living things.) Carbon-based chemistry is the most versatile chemistry in the universe. The element's chemical structure allows it to bond readily and strongly with itself and with many other elements. As a result, "organic" carbon-bearing compounds form the basis of all life with which we are familiar. It's encouraging that we see organic compounds everywhere we look. They exist in interstellar gas, dust grains, comets, meteorites, and on some moons in our solar system. Space is chock-full of the raw materials of life. We must be careful not to rule out other possible building blocks of life. Perhaps silicon-based life covers a distant planet, powered by electricity rather than blood or sap. Indeed, our biggest challenge in the search for extraterrestrial life may be to shed our preconceived notions of what forms life must assume. But for now we know of no basic recipe for living things other than liquid water and carbon-based molecules. With that in mind, one look around Earth is enough to see that we are safely within the Sun's "just right" range for life--a region that we more properly call the habitable zone. Rain falls, the oceans ebb and flow, and life teems within any (continued) |