QUESTION: 
How does NASA know that the evidence of life in the Mars meteorite was
from Mars and not from organisms here on Earth?

ANSWER from Jack Farmer on November 20, 1996:
This is a good question. David McKay and his colleagues from
Johnson Space Center in Houston, first presented their results for the
meteorite two years ago at a conference and they were strongly challenged
by other scientists to prove that the organic molecules they were seeing
were actually primary to the rock and not Earth-based contamination. This
was a challenge that they met very well using very high spatial-resolution
methods that could localize the organics within certain minerals in the
meteorite that we believe formed on Mars. So, by showing the organics were
inside the carbonate minerals they were able to convince everyone that the
organics were not due to contamination. 

In addition, they showed that the organics actually increased in abundance 
as they moved in toward the interior of the rock, just the opposite of 
what you would expect if they were contaminants!