Passage IV

NATURAL SCIENCE: This passage is adapted from the article “ls There Life on Other Planets?” by Victoria Pope (©2002 by U.S.News and World Report, Inc.).

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.

Early space exploration didn't turn up life. When NASA's Viking landers examined Mars two decades ago, they found a dry, sterile environment. Venus was a blazing inferno. It was only the images from the Galileo spacecraft that raised a slight hope of finding suitable conditions for life elsewhere in our solar system: Jupiter's moon Europa appeared to have an ocean of liquid water covered with pack ice.

Scientists who are skeptical about life's having arisen elsewhere have had a strong case, given the precise conditions that seemed essential for life to develop and survive. Planets that are too small cannot retain an atmosphere at all. Those too large turn into gas balls, like Jupiter. Those that are too hot or too 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've taken too parochial a view of life. Microbes have been discovered thriving under circumstances once thought impossible: in volcanic vents, in hot springs, in geysers. Antarctica's rocks, the cold waters deep beneath the surface of its frozen lakes, and the subterranean ground water below the basalt flows of Washington State's 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. A Martian meteorite found in Antarctica contains microscopic wormlike structures, dating from at least 3.6 billion 45 years ago, that resemble fossilized bacterial forms found on Earth.

The earthly microbes that seem to defy the odds are members of a category loosely called "extremophiles." Among their hardy ranks are the thermophiles, which thrive in searing heat; the halophiles, which love salt; and the barophiles, bacteria often found in the deep sea that live under high pressure.

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 wouldn't grow in petri dishes in the lab. Before these techniques were available, scientists were able to examine only about 1 percent of the real microbial population. New exploration technologies have helped, too. Deep-diving submarines found "cryptoendolithic" ("hiding within rock") organisms whose metabolism is driven by heat from a geothermal source.

Although the issue is highly contentious, some researchers argue that many of these microbes belong to a distinct and previously unrecognized branch of life. Supporters of this idea say that these one-cell throwbacks, now dubbed "archaea" are genetically different from bacteria and appear to be the oldest life forms on Earth. If that's so, it means that life did not necessarily need some warm primordial soup to form hospitable but could have originated in a far greater range of environments—including some downright hostile ones.

One of the first Martian rocks to be analyzed by the rover from NASA's Pathfinder spacecraft appears to be andesite, a type of lava found in the Columbia Basin, an area some view as an apt geological analog to Mars. Scientists are especially on the lookout for any rocks that show signs of water deposits that could have supported life. And microbiologists are hoping to find direct traces of archaealike microbes under the surface of Mars to support the idea that Martian life could have existed on the surface and then retreated underground—just like earthly microorganisms found in the volcanic vents, salt mines, and other hard-to-take environments. Guided by what they know from our planet, researchers now have their eyes set on a costly and complex mission: drilling into the Martian permafrost, which could yield the firmest answers yet about the origins of life.