Passage 2Life Beyond Earth　　A  We all have our suppositions, our scenarios. The late astronomer Carl Sagan estimated that there are a million technological civilizations in our galaxy alone. His more conservative colleague Frank Drake offers the number 10,000. John Oro, a pioneering comet researcher, calculates that the Milky Way is sprinkled with a hundred civilizations. And finally there are skeptics like Ben Zuckerman, an astronomer at UCLA, who thinks we may as well be alone in this galaxy if not in the universe.　　B  All the estimates are highly speculative. The fact is that there is no conclusive evidence of any life beyond Earth. Absence of evidence is not evidence of absence, as various pundits have wisely noted. But still we don’t have any solid knowledge about a single alien microbe, a solitary spore, much less the hubcap from a passing alien starship.　　C  Our ideas about extraterrestrial life are what Sagan called “plausibility arguments,” usually shot through with unknowns, hunches, ideologies, and random ought-to-bes. Even if we convince ourselves that there must be life out there, we confront a second problem, which is that we don’t know anything about that life. We don’t know how truly alien it is. We don’t know if it’s built on a foundation of carbon atoms. We don’t know if it requires a liquid-water medium, if it swims or flies or burrows.　　D  Despite the enveloping nebula of uncertainties, extraterrestrial life has become an increasingly exciting area of scientific inquiry. The field is called exobiology or astrobiology or bioastronomy—every few years it seems as though the name has been changed to protect the ignorant.　　E  Whatever it’s called, this is a science infused with optimism. We now know that the universe may be aswarm with planets. Since 1995 astronomers have detected at least 22 planets orbiting other stars. NASA hopes to build a telescope called the Terrestrial Planet Finder to search for Earth-like planets, examining them for the atmospheric signatures of a living world. In the past decade organisms have been found thriving on our own planet in bizarre, hostile environments. If microbes can live in the pores of rock deep beneath the earth or at the rim of a scalding Yellowstone spring, then they might find a place like Mars not so shabby.　　F  Mars is in the midst of a full-scale invasion from Earth, from polar landers to global surveyors to rovers looking for fossils. A canister of Mars rocks will be rocketed back to Earth in the year 2008, parachuting into the Utah desert for scrutiny by scientists in a carefully sealed lab. In the coming years probes will also go around and, at some point, into Jupiter’s moon Europa. That icy world shows numerous signs of having a subsurface ocean—and could conceivably harbor a dark, cold biosphere.　　G  The quest for an alien microbe is supplemented by a continuing effort to find something large, intelligent, and communicative. SETI—the Search for Extraterrestrial Intelligence—has not yielded a confirmed signal from an alien civilization in 40 years of experiments, but the signal-processing technology grows more sophisticated each year. The optimists figure it’s only a matter of time before we tune in the right channel.　　H  No one knows when—or if—one of these investigations might make a breakthrough. There’s a fair bit of boosterism surrounding the entire field, but I’d bet the breakthrough is many years, if not decades, away. The simple truth: Extraterrestrial life, by definition, is not conveniently located.　　I  But there are other truths that sustain the search for alien organisms. One is that, roughly speaking, the universe looks habitable. Another is that life radiates information about itself—that, if nothing else, it usually leaves a residue or an imprint. If the universe contains an abundance of life, that life is not likely to remain forever in the realm of the unknown.　　J  Contact with an alien civilization would be an epochal and culturally challenging event, but exobiologists would settle gladly for the discovery of a tiny fossil, a mere remnant of extraterrestrial biochemistry. One example. One data point to add to the one we have—Earth life. That’s what we need to begin the long process of putting human existence in its true cosmic context.　　Complete the sentences.　　Choose NO MORE THAN THREE WORDS from the passage for each answer. 　　Write your answers in boxes 1-5 on your answer sheet.　　1. Experts are approaching Mars extensively in the search of ______.　　2 Jupiter’s moon Europa will be in the analysed for it is conceived to embrace a ______ probably hostile to life.　　3. Although no clear signals have been received by human being, advances made in ______ has provided some optimism after 40 years’ experiments.　　4. The reason why extraterrestrial life cannot be immediately found is that it is not readily ______.　　5. The look for alien organism can still be sustained because it would still give off traces such as a ______.