QUESTION:
How do most scientists explain the success of the three experiments (that
had to do with finding microbial life) conducted by Viking 1?

ANSWER from Bruce Jakosky on December 11, 1997:
The three Viking biology experiments were successful, in that they provided
data that allowed the possible presence of life to be investigated.  They
did not provide a "positive" indication for life.  Two of the experiments
did provide a positive indication initially.  However, when "control"
experiments were performed, one of the positive indications disappearred.
The other, in the so-called "labelled release" experiment, could as easily
be explained by chemical reactions involving surface oxidants that might
form naturally in the atmosphere.

So how do you tell the difference?  In order to reach a conclusion regarding
possible life, one must combine all of the available data from all of the
experiments in order to reach the best answer.  One cannot look at a single
experiment in isolation from the others.  Also, this involves using data
from the GC-MS, which was designed to detect organic molecules in the soil.
The combination of all three biology experiments with the complete absence
of organic molecules at the parts-per-billion level is best explained by the
absence of living organisms at those sites.

This does not rule out martian life, however.  Organisms could exist at
other locations, or they could use different "chemistries" than were
examined by the Viking experiments.  The complete absence of organic
molecules, however, is a difficult constraint to get around.

All in all, the best explanation is that there was no evidence for
biological activity at the Viking landing sites.