The fallacy of life on Mars

Mars image mosaic from the Viking 1 orbiter

In a previous post I discussed the fallacy of the invisible cat, where the cause was the confusion between a sufficient and a necessary condition, as indicated by the following table:

Correct deduction: Necessary condition	Fallacious deduction: Sufficient condition
B is true only if A is true. B is true. Therefore A is true.	B is true if A is true. B is true. Therefore A is true.

There is another very similar fallacy, which also consists of confusing necessary and sufficient conditions, but in reverse. In this case, the right and wrong syllogisms are indicated by the following table:

Correct deduction: Sufficient condition	Fallacious deduction: Necessary condition
B is true if A is true. A is true. Therefore B is true.	B is true only if A is true. A is true. Therefore B is true.

Let us look at one example of this fallacy, applicable to the existence of life in Mars:

Water is necessary for the existence of life.

There is water on Mars.

Therefore there is life on Mars.

The media tend to hype whenever there is talk about the possible existence of life on Mars. After a discovery suggesting the past existence of abundant liquid water on Mars, they automatically extrapolate to the existence of life, applying the above deduction. As we have seen, this is fallacious.

What is the situation about the current possible existence of life on Mars?

Artistic rendering of a Viking Lander on Mars

On July 20, 1976, on the seventh anniversary of the first landing on the moon, the capsule Viking 1 landed on Mars, followed a few months later by its twin, Viking 2, which landed on the other side of the planet. The two capsules were scheduled to perform three experiments in search of life. Two of them gave negative results, the third tested positive. However, NASA concluded that there is no life at present in the Martian areas analyzed. Why?

•           In the positive experiment, Martian soil was added to a terrestrial culture consisting of sugars and amino acids containing radioactive carbon. If there were microscopic life on Mars and it were able to take advantage of these substances in its metabolism, radioactive carbon dioxide should be emitted and identified by detectors. Indeed, it was detected, but NASA did not take it into account and came to a negative conclusion.
•         NASA's decision was correct. If the positive result would have brought them to the conclusion that there is life on Mars, they would have been applying the fallacy of the invisible cat with the following deduction:
If there is life on Mars, the result will be positive.
The result was positive.
Therefore there is life on Mars.
The problem is that the existence of life was a sufficient, but not necessary condition, for the culture to emit radioactive carbon dioxide. NASA acted properly and did not fall into the fallacy, because it gave priority to the negative results of the other two experiments.

•        The dubious experiment could have been designed in a different way. Instead of a culture, two would have been prepared, one provided with dextro-rotating sugars and levo-rotating amino acids; the other with levo-rotating sugars and dextro-rotating amino acids. If the amount of radioactive carbon dioxide had been different between the two cultures, this fact would have been a strong evidence for the existence of life on Mars, since most non biotic processes that could lead to the production of CO₂ would be eliminated, as only life (at least terrestrial life) distinguishes the chirality of its components. In fact, this design was proposed, but rejected because of budget problems.

It is curious that twenty years later NASA fell in the fallacy of the invisible cat, when in 1996 they announced the discovery of life on Mars after the detection of possible microscopic fossils in a meteorite originated in Mars found in Antarctica. The conclusion was reversed when it was noted that life was a sufficient, but not necessary condition for the supposed fossils.

The same post in Spanish
Thematic Thread on Philosophy and Logic: Previous Next

Manuel Alfonseca