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.
Life Detection Ladder
|Ladder Rung||Feature||Measurement||Instrument||Target||Likelihood||Specific to Earth Life vs. Potential for Generic Life||Ambiguity of Feature||Ambiguity of Interpretation||False Positive||False Negative||Detectability|
|Life (metabolism, growth, reproduction)|
|Darwinian Evolution||changes in heritable traits in response to selective pressures||not possible||no||~||~||~||~|
|Growth and Reproduction||concurrent life stages or identifiable reproductive form [growth and reproduction]||cell(like?) structures in multiple stages||microscope||plume sample||low||Earth||What is a cell? What morphological differences exist?||low||high (not really a cell)||high (don't recognize stages, timing off, sample size low)||hard|
|Metabolism||isotopes||isotopes indicative of active metabolism||irMS||plume sample||low/med||Earth (can you abstract?)||source, sink, context||low||high||low||easy|
|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 reactions||spectroscopy||remote detection||med/high||Generic||mixed reactions, large inventory of chemistries||low-med||low-med||med-high||hard (linked to specificity of instrument)|
|Suspicious biomaterials [not necessarily biogenic]|
|Functional Molecules||DNA||material produced by extraterrestrial life||spectrographic, immunoassay, PCR, hi-prec MS||plume sample||low||Earth||None||Negligible||high (contamination)||high (technology limited, only terran)||hard (linked to specificity of instrument)|
|RNA||material produced by extraterrestrial life||spectrographic, immunoassay, PCR, hi-prec MS||plume sample||low||Earth||None||Negligible||low (RNA reactive, contamination possibility low)||high (technology limited, only terran) highly reactive||hard to measure on earth|
|pigments||material produced by extraterrestrial life||Spectrometer||plume sample||low/med||Earth (can you abstract?)||How to define if it is not the ones we know?||very low||low||high (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)||LCMS||plume sample||low/med||Earth||How much of preference is needed to detect?||medium||low||high||hard, need a lot of material and overprinting must be discernable]|
|Potential Biomolecule Components||complex organics (peptides, PAH, nucleic acids, hopanes)||increasing complexity of potential biomolecules||LCMS||plume sample||med||Generic||abiotic production known||medium||low||low||easy if enough material|
|amino acids (e.g. glycine, alanine)||material produced by extraterrestrial life||GCMS||plume sample||high||Generic||abiotic production known [glycine not required]||medium||low||low (if only looking for glycine false negative high)||easy, if enough material|
|lipids (fatty acids, esters, carboxylic acids)||material produced by extraterrestrial life||GCMS||plume sample||med/high||Generic||known abiotic pathways to some products||med-high||high (contamination)||low||limit of detection, need a lot of material|
|General indicators||distribution 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)||XRF||plume sample||med||Generic||knowledge of background||medium||low||high||easy except background issue|
|patterns of complexity (organics)||deviation from random organic complexity distribution||LCMS||plume sample||high||Generic||documentation of differences between abiotic and biotic limited||medium||low||high||background issue, material limited|
|chirality||material produced by extraterrestrial life||LC-MS/MS||plume sample||high||Generic||How much of an excess is necessary?||high||low||low||mixed sample both processes present|
|water, presence of building blocks for use, energy source, gradients||environments conducive to habitability||Redox/T/pH/energy/disequilibria||high||Generic||None||high||low||med||easy 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
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.
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.