The search for life on other planets is a quest of vast intrinsic interest, but of extreme difficulty in practice. Its root logic, though, is simple enough: If life was brewed on the planet Earth in a primordial soup of chemicals 4 billion to 4.5 billion years ago, why shouldn't it have happened on at least one other planet in the universe, too? Astronomers calculate that hundreds of millions of Earth-like planets must exist throughout the universe. Space exploration has not yet turned up life elsewhere: Mars is dry and sterile, Venus is a raging inferno, and Jupiter is a ball of gas. Scientists who are skeptical about life's ever having arisen elsewhere have had a strong case, given the precise conditions that used to seem essential for life to develop and survive. Planets that are too small cannot retain an atmosphere at all. Those that are too hot or cold have no liquid water. And without water, the chemical process of photosynthesis, which converts sunlight into chemical energy, is impossible. Without an atmosphere of oxygen, the chemical reactions that allow a cell to extract energy from other chemicals cannot normally run, either. But recent findings from our own planet have led scientists to wonder if they have taken too parochial view of life. Microbes have been discovered thriving under circumstances once thought impossible: in volcanic vents, in hot springs, in geysers. The rocks and the cold waters deep beneath the surface of Antarctica and the subterranean ground water in the Columbia River Basin may harbor terrestrial models for the rise of life on Mars and other planets. And some tantalizing, if still highly controversial, evidence from Mars at least hints that some similar forms of life may indeed have arisen there, even if they subsequently died out. Evidence of life in inhospitable environments began to pile up in recent years as highly sensitive gene-sequencing techniques made it possible to identify microorganisms that could not grow in laboratory conditions. Deep-diving submarines have found organisms whose metabolism is powered by heat from geothermal sources. Geologists boring two miles below the Columbia River Basin have found organisms that propagate using hydrogen―created from a chemical reaction between basalt and ground water as their energy source. Some researchers argue that many of these microbes belong to a distinct and previously unrecognized branch of life called “archaea,” perhaps the oldest life forms on Earth. If they are correct, then life did not absolutely need the warm, hospitable primordial soup to form, but could have formed in extreme, hostile environments, too. This would make it possible for archaea-like microbes to exist in extreme circumstances below the surface where water would remain in a liquid state―and that would mean that there could be life on Mars.
If they are correct, then life did not absolutely need the warm, hospitable primordial soup to form, but could have formed in extreme, hostile environments, too.
This would make it possible for archaea-like microbes to exist in extreme circumstances below the surface ↑ (where water would remain in a liquid state)