托福阅读材料推荐:距离土星第七远的土星卫星

　　SEEKING SIGNS OF LIFE

　　Once a spacecraft collects a sample from Enceladus, how will scientists test it for life? The process is more complex than simply searching for something that is alive—after all, researchers have argued over the definition of life for years. In the case of hunting for extraterrestrials scientists must get creative. “If you went to Mars and found a dead rabbit on the ground, it’s not alive, but it is compelling evidence of life,” McKay says. “So we’re not searching for something that’s alive but searching for the molecules that life uses. In other words, we’re looking for the body of the dead rabbit.”

　　The molecules that McKay and other scientists consider most important are amino acids—the building blocks for proteins. “They occur on comets and meteorites, so if there’s a primordial soup on Enceladus, it should have amino acids,” McKay says. “They’re so incredibly useful and so good in water that life would be pretty dumb not to use them.” At one point in the Berkeley meeting, however, Porco brought up a critical point: What if Enceladus organisms aren’t made of amino acids? McKay replied, jokingly, “Then we’re sunk and nature is perverse. We should all just give up and become poets.” What he meant is there’s a large consensus in the scientific community that amino acids will be useful in the hunt for life—and if such thinking turns out to be wrong, well “then we’re even dumber than we thought we were,” McKay says.

　　Another important signature scientists want to detect is lipids, which cells use to build their outside walls. “It’s a similar story to amino acids,” explains Alfonso Davila, a research scientist at the SETI Institute and Ames. “They’re something you’d expect to be present at the origin of life and you’d expect cells to use them.” Scientists will need to do more than simply detect amino acids and proteins on Enceladus—both of those molecules exist on their own in many environments, with or without life. But astrobiologists can target distinct structures and distributions of amino acids and lipids they think are unique to life. “We’re looking for molecules and structures that life makes, that are distinctly different from the random mess that chemistry makes,” McKay notes.

　　Some other possibilities on the search list are large organic compounds as well as photos of actual organisms taken by the spacecraft of samples from the plume. That might be, for example, images of an organism swimming or eating. Such a find might be the most direct evidence of something “alive” but many researchers have doubts about the plausibility of imaging organisms, Davila says. “It’s one of those high-risk, high-reward experiments. The likelihood of a negative result is very high,” he adds. “It’s very hard to tell the difference between a cell and a dot that is just a particle.” This issue was hotly debated at the meeting but some scientists sounded more hopeful—they discussed new techniques they are exploring to reliably image Enceladus’s microbes (if they exist).

　　Ultimately, scientists think it will probably take a combination of evidence to show that they have actually found life. And, of course, cost and technology will constrain the experiments they are able to perform. The hunt will be incredibly complicated, especially considering that organisms on Enceladus may not look or operate anything like they do on Earth. “We’re walking a thin line between what we know based on Earth life and what we expect life would be like otherwise,” Davila says. “It’s one of the things that prevents us from coming up with a good strategy.”

　　Or, to reiterate Porco’s observation—“It’s a total bitch of a problem to solve.”

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