> On July 30, 1976, the LR returned its initial results from Mars. Amazingly, they were positive. As the experiment progressed, a total of four positive results, supported by five varied controls, streamed down from the twin Viking spacecraft landed some 4,000 miles apart. The data curves signaled the detection of microbial respiration on the Red Planet. The curves from Mars were similar to those produced by LR tests of soils on Earth. It seemed we had answered that ultimate question.

This article does a poor job of explaining the experiment in question: "Labeled Release" (LR).

According to Wikipedia:

> ... In the LR experiment, a sample of Martian soil was inoculated with a drop of very dilute aqueous nutrient solution. The nutrients (7 molecules that were Miller-Urey products) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive 14CO2 gas as evidence that microorganisms in the soil had metabolized one or more of the nutrients. Such a result was to be followed with the control part of the experiment as described for the PR below. The result was quite a surprise, considering the negative results of the first two tests, with a steady stream of radioactive gases being given off by the soil immediately following the first injection. The experiment was done by both Viking probes, the first using a sample from the surface exposed to sunlight and the second probe taking the sample from underneath a rock; both initial injections came back positive.[1] Subsequent injections a week later did not, however, elicit the same reaction, and according to a 1976 paper by Levin [author of the linked article] and Patricia Ann Straat the results were inconclusive.[12][13] In 1997, Levin, Straat and Barry DiGregorio co-authored a book on the issue, titled Mars: The Living Planet.

https://en.wikipedia.org/wiki/Viking_lander_biological_exper...

To recap, martian soil was incubated with small organic molecules bearing a radioactive label. A chemical process that converted them into CO2 would give off some radioactive gas. The gas was detected by both landers the first time the experiment was run, but not the second time.

It should be kept in mind that non-biological processes can also convert small organic molecules into CO2. Simple combustion will do this, for example.

This possibility is discussed later in the Wikipedia article (but not in the linked article):

> Despite the positive result from the Labeled Release experiment, a general assessment is that the results seen in the four experiments are best explained by oxidative chemical reactions with the Martian soil. One of the current conclusions is that the Martian soil, being continuously exposed to UV light from the Sun (Mars has no protective ozone layer), has built up a thin layer of a very strong oxidant. A sufficiently strong oxidizing molecule would react with the added water to produce oxygen and hydrogen, and with the nutrients to produce carbon dioxide (CO2).

However, this does not explain the discrepancy between the first and second test. And it leaves open the question of why both landers' experiments gave the same odd result (first test positive, second test negative).

The small organic molecules used as nutrients were those produced in a famous lab simulation of early earth. Its results showed that many of the basic building blocks of life could be formed by passing an electrical current through gases thought to be present in the early Earth atmosphere.