In 1994, a team of scientists led by David Mckay began studying the meteorite ALH84001, which had beendiscovered in Antarctica in 1984. Two years later, the McKay team announced that ALH84001, which scientists generally agree originated on Mars, contained compelling evidence that life once existed on Mars. This evidence includes the discovery of organic molecules in ALH84001, the first ever found in Martian rock. Organic molecules—complex, carbon-based compounds—form the basis for terrestrial life. The organic molecules found in ALH84001 are polycyclic aromatic hydrocarbons, or PAH's. When microbes die, their organic material often decays into PAH's.

Skepticism about the McKay team's claim remains, however. For example, ALH84001 has been on earth for 13,000 years, suggesting to some scientists that its PAH's might have resulted from terrestrial contamination. However, McKay's team has demonstrated that the concentration of PAH's increases as one looks deeper into ALH84001, contrary to what one would expect from terrestrial contamination. The skeptic's strongest argument, however, is that processes unrelated to organic life can easily produce all the evidence found by McKay' steam, including PAH's. For example, star formation produces PAH's. Moreover, PAH's frequently appear in other meteorites, and no one attributes their presence to life processes. Yet McKay's team notes that the particular combination of PAH's in ALH84001 is more similar to the combinations produced by decaying organisms than to those originating form nonbiological processes.

The passage asserts which of the following about the claim that ALH84001 originated on Mars?

It was initially proposed by the McKay team of scientists.

It is not a matter of widespread scientific dispute.

It has been questioned by some skeptics of the McKay team's work.

It has been undermined by recent work on PAH's.

It is incompatible with the face that ALH84001 has been on Earth for 13,000 years.


登录注册 后可以参加讨论