Confidence of Life Detection Scale (CoLD) or the Ladder of Life Detection[1] is a numerical scale developed by NASA astrobiologists to assess possible biosignatures of extraterrestrial life. It was suggested in 2018.[1][2][3][4] The scale is designed similar to NASA’s technological readiness scale.[5]
It is a seven-step scale:
Detect possible signal
Rule out contamination
Make sure biology is possible
Rule out non-biology
Find additional independent signal
Rule out other hypothesis
Independent confirmation
History
NASA's "working definition of life" is "self-sustaining chemical system capable of Darwinian evolution".[1] As of 2025, the only NASA mission designed to look for life was the Viking program to Mars, launched in 1976. Per Neveu et al.:[1]
If taken independently, some of the measurements made during the experiments performed by Viking could be interpreted as demonstrating the presence of metabolic activity. However, taken together with a null result on a chemical measurement of life's organic building blocks, they cannot exclude the conclusion that life was absent in the Viking samples.
The Ladder is designed to "define the burden of proof that must be met to convince a majority of the scientific community of such a discovery." Neveu et al. lists eight criteria:[1]
Given the current understanding of life, a convincing life-detection measurement or suite of measurements must be sufficiently (1) sensitive, (2) contamination-free, and (3) repeatable; one or more features must be sufficiently (4) detectable, (5) preserved (survivable), (6) reliable (measurably different from expected abiotic signals), and (7) compatible with life as we know it; and (8) the biological interpretation must be the last-resort hypothesis.
Examples
The Cheyava Falls rock, found by the Perseverance rover on Mars in 2024, is an example of a step one on the CoLD scale, a detect of a possible signal.[6] If methane will be found on Mars, it would be assessed as level four of the scale.[7]
Criticism
CoLD scale was criticized as a useless tool that doesn't solve existing issues in scientific reporting: "CoLD scale is an inapt and easily abused tool that will do little to address the misleading terminology and sensational narratives that plague both public and scientific communications from the astrobiology community."[8]
