Life Detection Ladder

The direct detection of extant life has not been attempted by NASA since the Viking Missions in the late 1970s. NASA’s Ladder of Life Detection was generated to stimulate and support discussions among scientists and engineers about how one would detect extant life beyond Earth but within our Solar System (particularly on Europa and the other “Ocean Worlds”). In creating the Ladder, we started with the NASA definition of life, “Life is a self sustaining chemical system capable of Darwinian evolution” and considered the specific features of the one life we know —Terran life.

Table: Life Detection Ladder
Ladder RungFeatureMeasurementInstrumentTargetLikelihoodSpecific to Earth Life vs. Potential for Generic LifeAmbiguity of FeatureAmbiguity of InterpretationFalse PositiveFalse NegativeDetectability
Life (metabolism, growth, reproduction)
Darwinian Evolutionchanges in heritable traits in response to selective pressuresnot possibleno~~~~
Growth and Reproductionconcurrent life stages or identifiable reproductive form [growth and reproduction]cell(like?) structures in multiple stagesmicroscopeplume samplelowEarthWhat is a cell? What morphological differences exist?lowhigh (not really a cell)high (don't recognize stages, timing off, sample size low)hard
Metabolismisotopesisotopes indicative of active metabolismirMSplume samplelow/medEarth (can you abstract?)source, sink, contextlowhighloweasy
co-located reductant and oxidant (e.g. persistant H2 +/- CH4 v. O2, nitrate, Fe3+, CO2) [Inferred Persistence]chemical concentrations of substrates and products involved in redox reactionsspectroscopyremote detectionmed/highGenericmixed reactions, large inventory of chemistrieslow-medlow-medmed-highhard (linked to specificity of instrument)
Suspicious biomaterials [not necessarily biogenic]
Functional MoleculesDNAmaterial produced by extraterrestrial lifespectrographic, immunoassay, PCR, hi-prec MSplume samplelowEarthNoneNegligiblehigh (contamination)high (technology limited, only terran)hard (linked to specificity of instrument)
RNAmaterial produced by extraterrestrial lifespectrographic, immunoassay, PCR, hi-prec MSplume samplelowEarthNoneNegligiblelow (RNA reactive, contamination possibility low)high (technology limited, only terran) highly reactivehard to measure on earth
pigmentsmaterial produced by extraterrestrial lifeSpectrometerplume samplelow/medEarth (can you abstract?)How to define if it is not the ones we know?very lowlowhigh (limitation of what we are looking for)easy (fluorescence)
structural preferences in organic molecules [non random and enhancing function)evidence of non random chemistries (such as specific biochemical pathways)LCMSplume samplelow/medEarthHow much of preference is needed to detect?mediumlowhighhard, need a lot of material and overprinting must be discernable]
Potential Biomolecule Componentscomplex organics (peptides, PAH, nucleic acids, hopanes)increasing complexity of potential biomoleculesLCMSplume samplemedGenericabiotic production knownmedium lowloweasy if enough material
amino acids (e.g. glycine, alanine) material produced by extraterrestrial lifeGCMSplume samplehighGenericabiotic production known [glycine not required]mediumlowlow (if only looking for glycine false negative high)easy, if enough material
lipids (fatty acids, esters, carboxylic acids)material produced by extraterrestrial lifeGCMSplume samplemed/highGenericknown abiotic pathways to some productsmed-highhigh (contamination)lowlimit of detection, need a lot of material
General indicatorsdistribution of metals [e.g. vanadium in oil reserves or others like Fe, Ni, Mo/W, Co, S, Se, P]deviation from background bulk concentrations (Preferences)XRFplume samplemedGenericknowledge of backgroundmediumlowhigheasy except background issue
patterns of complexity (organics)deviation from random organic complexity distributionLCMSplume samplehighGenericdocumentation of differences between abiotic and biotic limitedmediumlowhighbackground issue, material limited
chiralitymaterial produced by extraterrestrial lifeLC-MS/MSplume samplehighGenericHow much of an excess is necessary?highlowlowmixed sample both processes present
Habitability
water, presence of building blocks for use, energy source, gradientsenvironments conducive to habitabilityRedox/T/pH/energy/disequilibriahighGenericNonehighlowmedeasy for some measurements, hard for others
Glossary of terms
irMS - Isotope-ratio mass spectrometry
GCMS - Gas chromatography–mass spectrometry
LCMS - Liquid chromatography–mass spectrometry
XRF - X Ray flourescence
MS/MS - tandem mass spectrometry
PCR - polymerase chain reaction

Column Defintion
Likelihood - A summary of the factors to the right including specificty, abiguity, false postive, false negative and detectability.
Functional Molecules - Molecules that are almost certainly produced biologically and are produced because they serve catalytic, metabolic, structural, protective or energy harvesting functions.
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 rungs of the Ladder were assembled from features that can be used to access (1) potential habitability, (2) suspicious biomaterials that could be biogenic or abiogenic, and (3) active processes of life. The lowest rungs are the least directly related to extant life and in some cases are the easiest to measure. For each rung (feature), the target and potential flight instruments for measurement were identified. Our ability to detect and properly interpret a measurement was evaluated in terms of how specific the feature was for Terran-type life, how likely the feature could be produced abiotically (called ambiguity), how likely the measurement would be a false positive due to contamination or measurement interference, how likely the measurement would be a false negative (missing life when it is present), and easy the measurement is to make (detectability). Download this excel spreadsheet here.

What features are you working with? Want to modify or add a rung?Download a blank spreadsheet and fill it out with your data. Then email it to: arc-nai@mail.nasa.gov

Background from The Limits of Organic Life in Planetary Systems.

For generations the definition of life has eluded scientists and philosophers. (Many have come to recognize that the concept of “definition” itself is difficult to define) We can, however, list characteristics of the one example of life that we know—life on Earth:

• It is chemical in essence; terran living systems contain molecular species that undergo chemical transformations (metabolism) under the direction of molecules (enzyme catalysts) whose structures are inherited, and heritable information is itself carried by molecules.
• To have directed chemical transformations, terran living systems exploit a thermodynamic disequilibrium.
• The biomolecules that terran life uses to support metabolism, build structures, manage energy, and transfer information take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and the ability of heteroatoms, primarily oxygen and nitrogen, to modulate the reactivity of hydrocarbons.
• Terran biomolecules interact with water to be soluble (or not) or to react (or not) in a way that confers fitness on a host organism. The biomolecules found in terran life appear to have molecular structures that create properties specifically suited to the demands imposed by water.
• Living systems that have emerged on Earth have done so by a process of random variation in the structure of inherited biomolecules, on which was superimposed natural selection to achieve fitness. These are the central elements of the Darwinian paradigm.