Lesson 6 of 6 · Test the alternatives
Organic molecules versus evidence of life
9 minute lesson · 3 checks along the way
Your goal for this lessonUse blanks, calibration, sampling context, and competing explanations to evaluate a chemical claim about life.
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Separate the claims in the headline
Consider the headline “Organic material found on another world.” It contains several possible claims that need different evidence. Was a signal detected? Was a identified? Did that compound originate in the sampled environment? Did biology produce it? Moving from one question to the next requires additional reasoning. A convincing answer to the first does not automatically settle the last, even for Earth’s biological chemicals.
Bennu’s returned samples illustrate the value of this distinction. Researchers reported a rich inventory of nitrogen-containing organic compounds, while explicitly avoiding a discovery-of-life claim. Their original paper considered isotopic information and chemical distributions when investigating formation conditions. Protecting a sample from uncontrolled exposure strengthens an origin argument, but the controls and the measurements still need examination. Careful preservation makes a result more interpretable; it does not itself make the chemistry biological.
Measure the background through the same process
A passes material without the intended extraterrestrial sample through relevant handling and analytical steps. It tests contamination or background introduced along that pathway. A containing a known target tests whether the method can recover that target under tested conditions. These controls answer different questions. A clean blank does not guarantee good recovery, and good recovery does not guarantee that a sample’s detected material originated off Earth.
In an invented comparison, a sample produces 120 signal units and a matched procedural blank produces 20. Assume the background is additive and the exposures, response, and relevant handling are comparable. The background-subtracted signal is 100 units. That is not automatically 100 or a concentration of 100 parts per million: a calibrated response and the analyzed sample quantity are still needed. Real blank variability also contributes uncertainty rather than disappearing after subtraction.
This Bennu mosaic combines 12 PolyCam observations taken by OSIRIS-REx on December 2, 2018, from about 24 kilometers away. It documents surface form and context; the later identification of specific organic compounds came from sample analysis.
Look closely
Which observations here concern the asteroid’s terrain, and which claim would require a laboratory result instead?
See what to notice
The mosaic resolves boulders, illuminated slopes, and the asteroid’s outline. It cannot identify an amino acid or determine its origin. Chemical identification, contamination controls, and competing formation models come from different evidence than the visible terrain.
Check 1 of 3
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Compare predictions, not just exciting words
Suppose two explicitly simplified chemical models predict a measured ratio of 2 and 4. An experiment reports 2.1 with an illustrative accepted comparison interval of 1.8 to 2.4. Under this stipulated rule, the first prediction fits and the second does not. This narrows those two models. It does not demonstrate that every conceivable process is excluded, or that the interval gives the probability that a hypothesis is true.
Useful follow-up evidence targets the remaining alternatives. Repeat measurements can test reproducibility; independent techniques can check molecular identity; spatial patterns and associated can test an environmental interpretation. A possible is a candidate sign of life whose nonbiological explanations still require evaluation. Evidence gains force through agreement across tests with different vulnerabilities, rather than through repeatedly renaming the same unexplained signal as biological.
Subtract the background, preserve the units
Original invented signal comparison. The difference estimates excess detector response under the stated assumptions and is not a direct measurement of molecule count.
Open full-size diagram in a new tab: Subtract the background, preserve the unitsCheck 2 of 3
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Work through a decision
State what a negative result can establish
A nondetection is also conditional. The target may be absent, below a , destroyed before analysis, or poorly recovered by the chosen method. A documented method can place a bound for that sampled material and measurement. Extending the conclusion to an entire asteroid, moon, or planet would require a sampling argument. Similarly, finding a compound at one site does not demonstrate that it is abundant everywhere.
NASA’s life-detection guidance treats detectability, contamination, repeatability, environmental context, and competing explanations as connected checks rather than a mechanical proof. A strong report therefore ends with a bounded claim: what was measured, which explanations it supports, which alternatives remain, and what observation would discriminate among them next. This approach lets unfamiliar chemistry remain an important discovery without asking it to carry a biological conclusion that the evidence cannot yet support.
Check 3 of 3
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Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
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