Statistics Provide Guidance for Indigenous Organic Carbon Detection on Mars Missions

Data from the Viking and Mars Science Laboratory missions indicate the presence of organic compounds that
are not definitively martian in origin. Both contamination and confounding mineralogies have been suggested as
alternatives to indigenous organic carbon. Intuitive thought suggests that we are repeatedly obtaining data that
confirms the same level of uncertainty. Bayesian statistics may suggest otherwise. If an organic detection method
has a true positive to false positive ratio greater than one, then repeated organic matter detection progressively
increases the probability of indigeneity. Bayesian statistics also reveal that methods with higher ratios of true
positives to false positives give higher overall probabilities and that detection of organic matter in a sample with a
higher prior probability of indigenous organic carbon produces greater confidence. Bayesian statistics, therefore,
provide guidance for the planning and operation of organic carbon detection activities on Mars. Suggestions for
future organic carbon detection missions and instruments are as follows: (i) On Earth, instruments should be tested
with analog samples of known organic content to determine their true positive to false positive ratios. (ii) On the
mission, for an instrument with a true positive to false positive ratio above one, it should be recognized that each
positive detection of organic carbon will result in a progressive increase in the probability of indigenous organic
carbon being present; repeated measurements, therefore, can overcome some of the deficiencies of a less-thandefinitive
test. (iii) For a fixed number of analyses, the highest true positive to false positive ratio method or
instrument will provide the greatest probability that indigenous organic carbon is present. (iv) On Mars, analyses
should concentrate on samples with highest prior probability of indigenous organic carbon; intuitive desires
to contrast samples of high prior probability and low prior probability of indigenous organic carbon should
be resisted.