### Abstract

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.

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.

Original language | English |
---|---|

Pages (from-to) | 706-713 |

Number of pages | 7 |

Journal | Astrobiology |

Volume | 14 |

Issue number | 8 |

Early online date | 25 Jul 2014 |

DOIs | |

Publication status | Published - 7 Aug 2014 |

### Keywords

- Mars—Life-detection instruments—Search for Mars’ organics—Contamination— Biosignatures.

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## Cite this

Sephton, M. A., & Carter, J. (2014). Statistics Provide Guidance for Indigenous Organic Carbon Detection on Mars Missions.

*Astrobiology*,*14*(8), 706-713. https://doi.org/10.1089/ast.2014.1161