The Ladder of Life Detection
Abstract
Life detection includes measurements seeking to find life: searching for biosignatures (features that evidence ongoing or past biological processes) and establishing context (properties and processes inherent to the sample's current and past settings). To guide the development of missions seeking to find life on Mars, ocean worlds, and exoplanets, we have built a Ladder of Life Detection (https://astrobiology.nasa.gov/research/life-detection/ladder) [1]. This tool, meant to be challenged and upgraded, lists features of life and assesses their potential for providing conclusive evidence for life through the following criteria:* Sensitivity includes all aspects of instrumental performance (e.g. selectivity, dynamic range). The instrument must detect the feature of life within set performance thresholds, to avoid instrumental false negatives.* Discrimination from contamination interfering with the measurement, to avoid instrumental false positives and contamination by hardware or other samples.* Replicability: As many measurements and samples as needed to meet predefined goals and capture the heterogeneity of the setting.* Detectability: Physical, chemical, or geological conditions in the sample's current environment must not prevent the measurement to be made.* Survivability: The feature of life must not have been destroyed in the environments encountered by the sample between its synthesis and its measurement.* Reliability: is the propensity for the feature to be produced by life and distinguished from abiotic backgrounds from any of the environments encountered between feature synthesis and measurement.* Compatibility: The feature must be consistent with what is known of life. This criterion assesses the feature's genericity vs. specificity to life on Earth [2].* Last-resort hypothesis: The measurement (alone or part of a set) must preclude an abiotic origin to a given statistical significance.Features of life: The Ladder details the propensity for each feature to meet the above criteria. We combine this into a subjective "likelihood" that the feature is diagnostic of life. Features below are loosely ranked from highest to lowest likelihood. We encourage the community to refine this ordering.* Darwinian evolution takes place over many generations of organisms as conditions change. There is no widely accepted means to detect or measure it. It is not practical to detect in unexplored natural environments in the time frame of missions.* Growth and reproduction are evidenced by morphologies of concurrent life stages or a reproductive form, such as cell-like structures in multiple stages, including motility.* Metabolism: Organisms derive energy from their environment and convert it to forms used for growth, reproduction, or repair. Conversion is not fully efficient, so metabolism results in waste. Conversion and waste intermediates or products deviate from abiotic thermodynamic equilibrium or kinetically limited abiotic steady states. Such deviations include elemental or isotopic fractionations, co-location of oxidants and reductants, or a response to substrate addition.* Molecules and structures conferring function include polymers that support information storage or transfer (e.g. DNA, RNA) and functions such as catalysis (proteins), specific structural preferences in organic molecules (e.g. homochirality, repeating charge), and pigments (detectable by remote sensing).* Potential biomolecule components comprise organic compounds not found abiotically (e.g. hopanes, ATP, histidine), complex organics (nucleic acid oligomers, peptides, PAHs), and monomeric units of biopolymers.* Potential metabolic byproducts are distributions of species that differ from those resulting solely from abiotic thermodynamic equilibrium or kinetic steady state. They include distributions of elements present at trace levels in biological matter and patterns of complexity in mixtures of organics.* Biofabrics: Microbial communities can affect the morphology of their environment, creating laminations, mounds, or microbially induced sedimentary structures.Beyond measurement interpretations and payload design, we anticipate that the Ladder will spur discussion and progress in life detection, leading to improvements to the Ladder itself.References: [1] Neveu M. et al. Astrobiology , in revision. [2] Baross J. et al. (2007) NRC Report .
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2443N