The Ladder of Life Detection

Abstract

We describe the history and features of the Ladder of Life Detection, a tool intended to guide the design of investigations to detect microbial life within the practical constraints of robotic space missions. To build the Ladder, we have drawn from lessons learned from previous attempts at detecting life and derived criteria for a measurement (or suite of measurements) to constitute convincing evidence for indigenous life. We summarize features of life as we know it, how specific they are to life, and how they can be measured, and sort these features in a general sense based on their likelihood of indicating life. Because indigenous life is the hypothesis of last resort in interpreting life-detection measurements, we propose a small but expandable set of decision rules determining whether the abiotic hypothesis is disproved. In light of these rules, we evaluate past and upcoming attempts at life detection. The Ladder of Life Detection is not intended to endorse specific biosignatures or instruments for life-detection measurements, and is by no means a definitive, final product. It is intended as a starting point to stimulate discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them.

Keywords: Biomarkers; Biosignatures; Life detection; Life-detection instruments.

FIG. 1.

(a) Any compound for which biology is a source (dark green) or sink (light green) may also have abiotic sources (dark blue) and sinks (light blue). This complicates life detection. (b) This complication is overcome by meeting the criteria of survivability (biological sources > abiotic sinks or biological sinks > abiotic sources) and reliability (biological sources > abiotic sources or biological sinks > abiotic sinks). Examples are provided for each case.

FIG. 2.

In a binary framework, if we attribute a life-detection result of 1 to case A (“the presence of complex organics and large enantiomeric excesses together in a sample cannot be achieved abiotically”), then cases B and C, with additional evidence for life, would also have a result of 1. Therefore, there would be no need to consider each case individually: case A would be a “decision rule” determining the outcome of cases B, C, and all others with complex organics and a large enantiomeric excess. In a probabilistic framework, the result of each measurement matters.