Line | In 1994, a team of scientists led by David McKay |
began studying the meteorite ALH84001, which had | |
been discovered in Antarctica in 1984. Two years | |
later, the McKay team announced that ALH84001, | |
(5) | 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 | |
(10) | molecules—complex, carbon-based compounds— |
form the basis for terrestrial life. The organic | |
molecules found in ALH84001 are polycyclic | |
aromatic hydrocarbons, or PAHs. When microbes | |
die, their organic material often decays into PAHs. | |
(15) | 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 PAHs might have resulted | |
from terrestrial contamination. However, McKay’s | |
(20) | team has demonstrated that the concentration of |
PAHs increases as one looks deeper into | |
ALH84001, contrary to what one would expect from | |
terrestrial contamination. The skeptics’ strongest | |
argument, however, is that processes unrelated to | |
(25) | organic life can easily produce all the evidence |
found by McKay’s team, including PAHs. For | |
example, star formation produces PAHs. Moreover, | |
PAHs frequently appear in other meteorites, and no | |
one attributes their presence to life processes. Yet | |
(30) | McKay’s team notes that the particular combination |
of PAHs in ALH84001 is more similar to the | |
combinations produced by decaying organisms than | |
to those originating from nonbiological processes. |