Confidence of Life Detection Scale

The 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]

Confidence of Life Detection Scale graphic

It is a seven-step scale:

1. Detect possible signal
2. Rule out contamination
3. Make sure biology is possible
4. Rule out non-biology
5. Find additional independent signal
6. Rule out other hypothesis
7. 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 of Life Detection" 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 detection of a possible signal.^[6] If scientists are able to eliminate all known non-biological explanations for the chemical composition of Chevaya Falls, that would be an example of a level 4 on the scale.

Criticism

The 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."^[7]