As NASA prepares to set twin robots loose

问题：

As NASA prepares to set twin robots loose on the Martian surface and makes plans to send another in 2007, the agency’s long term goal is clear: determine whether the red planet does or ever did harbor life.

But the current search for life is necessarily limited to life as we know it, organisms dependent on liquid water. A SPACE. corn reader recently suggested that "We as humans are arrogant, simply believing that any other form of life will be just like us. "

Researchers devoted to the search for extraterrestrial (ET) have a similar view. "Scientists’ approach to finding life is very Earth-centric," says Kenneth Nealson, a geobiologist at the University of Southern California. "Based on what we know about life on Earth, we set the limits for where we might look on other planets," Nealson said. Within that framework, however, there are extreme cases of life on Earth that suggest the range of places to look on frigid Mars.

Nealson and his colleagues recently found the most extreme sort of organism in a salty liquid lake under the permafrost of Siberia. The organism, named cryopegella, can exist at colder temperatures than any previously discovered. Nealson’s team figures that if the ice at the polar caps of Mars warmed to liquid water, organisms like cryopegella could have awakened and repaired any damage that might have occurred to their various cellular components. That does not mean there are necessarily dormant microbes within the ice caps of Mars. But it does suggest a broader range of potential cradles for life.

Other researchers agree, and a host of so-called "extremophile" discoveries on Earth in recent years indicate the polar regions of Mars might be prime hunting grounds. As on Earth, organisms there might be slathered in natural antifreeze or be able to go dormant for tens of thousands of years, waiting for a brief thaw, their moment in the Sun.

Meanwhile, scientists recognize that there could indeed be life elsewhere in the universe that does not require water. And some astrobiologists are trying to explore the possibilities. But it is a tough problem to approach. In looking for "life as we don’t know it," it’s hard to even imagine what to expect.

Life might or might not exist on Mars. If there are critters there, they might or might not be like bacteria on Earth. In laboratory conditions, scientists in 2001 were able to get one-celled organisms to incorporate an amino acid—a fundamental building block of life—that no other known life uses. The discovery borders on the creation of artificial life, experts said. It also suggests that ET might operate by entirely different rules than those we’re used to.

If life on Mars is fundamentally different from what scientists understand life to be, then current spacecraft and others in the works may well not recognize what’s right under their mechanical noses.

According to Kenneth Nealson, scientists’ current approach to finding life is().

A. limited

B. costly

C. centralized

D. earthly