Page Content (OCR)
27 survive in a subsurface “hydrosphere.” Others too believe that organisms may still cling to life, hibernating during the cold spells and thawing out when climate conditions improve.’ Mind-Stretcher. Life may be a temporary phenomenon on many worlds, limited to a particular eon of a planet’s history. Living things may arise from inanimate matter, survive for millions of years, and then die out.® We may find only their traces, frozen, scorched, or crushed by climatic and geological change. The question of life on Mars—past or present—remains open. Three- quarters of the planetary scientists informally polled in 2005 believed that Mars once had conditions hospitable to life. One-quarter thought that it still does. Others argue that life might exist in a dark ocean under the icy surface of Jupiter’s moon Europa.’ Finding an independent origin of biology on another world in our solar system would have momentous implications. As planetary scientist Chris- topher McKay put it, the search for life on Mars is a search for a second Genesis. Discovering a second origin in our own small solar system would imply a fertile universe, one that might produce other minds. Detecting Remote Life How can we detect alien forms of life when they are very far away? By the effects they have on planetary atmospheres. “Except by a purpose- ful act of camouflage,” wrote Gaia theorist James Lovelock, “any life system will reveal its presence through the chemical disequilibria caused by its contrivances.” In the case of Earth, the simultaneous presence of molecular hydrogen, methane, and ammonia in significant quantities would be strong evidence for a dynamic system—life. Would we recognize other signs? If we encounter alien forms of life, Simpson warned, we might not perceive them as living—or we might have to revise our conception of life. On Earth we make a distinction between living matter and nonliving matter. What of life as we do not know it? If we recognized it at all, we might have to place it in a third category." The boundary between life and nonlife has become blurred, even on Earth. In the case of terrestrial biology, viruses often are thought of as being in a gray area between living and nonliving. Scientists and medical experts have debated for years whether viruses can be called life, as they apparently can exist only as parasites on living cells.'? The border may be even less clear when we encounter truly alien biology. Disappointment and Revival