Life Detection Ladder

The direct detection of extant life has not been attempted by NASA since the Viking Missions in the late 1970s. The Ladder of Life Detection was generated to stimulate and support discussions among scientists and engineers about how one would detect extant life beyond Earth within the practical constraints of robotic space missions. The Ladder draws from lessons learned from previous attempts at detecting life. These past attempts have provided criteria for measurements to constitute convincing evidence for life (columns with blue and orange headers). The Ladder summarizes features of life as we know it, how specific they are to life, and how they can be measured. These features (rows) are sorted in a general sense based on their likelihood of indicating life. More information about the rationale behind the Ladder is provided in a peer-reviewed publication freely available at https://www.liebertpub.com/doi/full/10.1089/ast.2017.1773.

The Ladder of Life Detection











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RUNGFEATUREMEASUREMENTTARGETLIKELIHOODINSTRUMENTAL CRITERIACONTEXTUAL CRITERIA
Roughly, subjectively ordered by (top to bottom):
1. decreasing strength of evidence for life
2. increasing ease of measurement
Listed in no specific order within a given rung…that the feature would be a bio signature, given the criteria to the rightQuantifiable
Detectability
Contamination-free
Likelihood of false positive
RepeatableDetectable
Detectability
Survivable
Likelihood of false negative
Reliable
Ambiguity of feature
Compatible
Specificity to Earth life
Last-resort
Ambiguity of interpretation
Darwinian Evolutionchanges in inheritable traits in response to selective pressuresNot practical under mission contraints
  • In situ
  • Sample return
    No--










    Number of replicates depends on:
  • Characteristics of the instrument.
  • Heterogeneity of the sample.
  • Likelihood of systematic errors
  • Required values of the relevant statistical parameters.
  • Value and cost of information




  • -N/A (extant)---
    Growth and Reproductionconcurrent life stages or identifiable reproductive form, motilitycell(like?) structures in multiple stages
  • In situ
  • Sample return
    LowHardLowMed (don't identify stages, timing off, sample size low)High?Ambiguous. What is a cell? What morphological differences exist?EarthMed/High
    MetabolismMajor element or isotope fractionations indicative of metabolismDeviation from abiotic fractionation controlled by thermodynamic equilibrium and/or kinetics
  • Remote sensing
  • In situ
  • Sample return
    Low/MedEasyHighMediumHighHinges on understanding or contextEarth?Low
    Response to substrate additionWaste output (compound, heat)
  • In situ
  • Sample return
    Low/MedEasyLowHighN/A (extant)Hinges on understanding contextEarth?Medium
    Co-located reductant and oxidantDeviation from abiotic distribution controlled by thermodynamic equilibrium and/or kinetics
  • Remote sensing
  • In situ
  • Sample return
    Med/HighMed (linked to specificity of instrument)Low/MedMed/HighHighMixed reactions, large inventory of chemistriesGenericLow/Med
    Molecules & Structures Conferring FunctionPolymers that support information storage and transfer for terran life (DNA, RNA)Abundance
  • In situ
  • Sample return
  • LowHard (instrument specificity must be high); RNA hard to measure on EarthDNA: high; RNA: low (reactive)Low (technology limited, only terran); RNA highly reactiveLow (hydrolysis in water)ReliableEarthNegligble
    Structural preferences in organic molecules (non-random and enhancing function)Polymer with repeating charge
  • In situ
  • Sample return
  • Low/MedNeed a lot of material and overprinting must be discernableLowMed/HighLow (hydrolysis in water, digenesis)How much preference needed to detect?GenericLow
    Entantiomeric excess >20% in multiple amino acid types
  • In situ
  • Sample return
  • HighHow much excess necessary?LowLowMediumMixed sample both processes presentGenericLow
    Pigments as evidence of non-random chemistries (e.g. specific pathways)Spectral feature and/or color, otherwise see "structural preferences"
  • Remote sensing
  • In situ
  • Sample return
  • Low/MedEasy (fluorescence)LowLow (Limitation of what we are looking for)Low (diagenesis)How to define pigment as we don't know it?Earth (can one abstract?)Very low
    Potential biomolecule componentsOrganics not found abiotically (e.g. hopanes, ATP, histidine)Presence
  • In situ
  • Sample return
  • MediumEasy if enough materialLowHighHighLowEarth?High
    Complex organics (e.g. nucleic acid oligomers, peptides, PAH)Presence
  • Remote sensing (PAH)
  • In situ
  • Sample return
  • HighEasy if enough materialLowHighMediumAbiotic production knownGenericMed/High
    Monomeric units of biopolymers (nucleobases, amino acids, lipids for compartmentalization)Presence
  • Remote sensing
  • In situ
  • Sample return
  • Med/HighLimit of detection, need a lot of materialHighHighMed (diagenesis)Abiotic pathways knownGenericMedium
    Potential metabolic byproductsDistribution of metals e.g. V in oil of Fe, Ni, Mo/W, Co S, Se, PPresence
  • In situ
  • Sample return
  • MediumEasy except background issueLowHighHighBackground knownGenericMedium
    Patterns of complexity (organics)Deviation from equilibrium (P(Poisson distribution of pathway complexity) ,0.01?) or abiotic kinetic distribution
  • In situ
  • Sample return
  • HighBackground issue, material limitedLowHighMedium?Limited documentation of abiotic vs. biotic differencesGenericMedium
    BiofabricsTexturesBiologically mediated morphologies, preferably with co-located composition
  • In situ
  • Sample return
  • MediumMediumLowMediumHigh?Highly ambiguousEarthHigh
    HabitabilityLiquid water, building blocks, energy source, gradients. Redox, temperature, pH, energy, disequilibria
    Rows indicate features of life; columns indicate criteria of Table 1. Features highlighted in green indicate evidence for extant life. Edits to the Ladder are encouraged; see Neveu et al. (2018), doi: 10.1089/ast.2017.1773.

    Column Defintion
    Likelihood - A summary of the factors to the right including specificty, abiguity, false postive, false negative and detectability.
    False Positive - A positive determination based on detection due to contamination or a postive detection caused by an unintended interference with the method of detection.
    False Negative - Lack of detection when the factor is actually present either in the sampe or the system at large. For example, low signal to background or signal below the limit of 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 community discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them. For example, there is room for debate regarding the specific order of each feature in the Ladder, which is highly dependent on the environment in which the measurements would be made. If you would like to suggest any improvements, feel free to contact the Astrobiology Program.