Mars—Beyond the Red Horizon (23): Chapter 5 Did Life Exist on Mars—Detection, Definition, and Scale (1)
The question in this chapter, while appearing simple, is actually complex. The question "Did life exist on Mars?" inevitably includes the question "What is life?" Mars did not discover life. But more importantly, it is the fact that Mars questioned the very definition of life itself.
5.1 The Premise of the Martian Life Hypothesis—The Range of Earth-like Biochemistry
Let's start by organizing things.
There are two different approaches to the Martian life hypothesis. One is an approach that questions the possibility of "Earth-like life," and the other is an approach that questions the possibility of "alternative biochemistry."
The latter—alternative biochemistry—is mainly discussed in the context of Titan. Titan has liquid methane and ethane. If life uses methane instead of water as a solvent, there is a possibility that life exists on Titan. This assumes a biochemical system fundamentally different from Earth-like life.
In the case of Mars, the situation is different. Mars once had liquid water. The foundation of carbon chemistry is also common with Earth. As energy sources, volcanic activity, ultraviolet rays, and redox gradients may have existed.
Therefore, the Martian life hypothesis does not require "alternative biochemistry." It is a question of whether Earth-like life could exist in the Martian environment.
Earth's life consists of organic compounds with carbon as a skeleton. It uses water as a solvent. It obtains energy through metabolism. It self-replicates. These are the basic characteristics of Earth-like life.
Mars may have met at least some of these conditions at some point in the past.
Noachian Mars had liquid water. Carbon compounds exist universally in outer space and also reached Mars. Energy was supplied by volcanic activity. Redox gradients—for example, the boundary between an oxidative surface and a reductive subsurface—may also have existed.
In other words, the possibility that Earth-like microorganisms could have existed on Mars cannot be denied at this time.
5.2 The Viking Experiments (1976)—Experiments That Exposed the Question
The 1976 Viking program was a historic undertaking in which humanity attempted to search for life directly on another planet for the first time. But the results did not provide a simple answer.
The Viking biology experiments were structured around the following three.
Labeled Release (LR) Experiment—Nutrition and Respiration
This is an experiment in which a nutrient solution containing organic matter labeled with radioactive carbon (¹⁴C) (a mixture of formate, acetate, lactate, etc.) is added to a Martian soil sample to detect released gas. If microorganisms exist, they should metabolize the nutrients and release radioactive carbon dioxide (¹⁴CO₂).
Result: When the nutrient solution was added, radioactive gas was released rapidly. This was a response similar to a metabolic reaction. Furthermore, when the soil was sterilized by heating, this reaction was significantly reduced. This "inactivation by heating" is consistent with the characteristics of biological processes.
Gil Levin, the principal investigator of the LR experiment, continued to claim that this result was evidence of life. For decades afterward, he maintained that Viking had discovered life on Mars.
However, the majority of scientists took a different interpretation.
Gas Exchange (GEX) Experiment—Gas Changes
An experiment in which water and nutrients are added to a Martian soil sample and gas changes are measured with a gas chromatograph.
Result: A large amount of oxygen was released just by adding water vapor. This was considered a chemical reaction of a strong oxidant, not microbial metabolism.
Pyrolytic Release (PR) Experiment—Detection of Photosynthesis
An experiment in which a soil sample is placed in an atmosphere of radioactive carbon-labeled carbon dioxide and carbon monoxide, and after irradiation with light, the production of organic matter is detected. It was intended to detect processes like photosynthesis.
Result: A small amount of radioactive organic matter was detected. However, since a similar reaction was observed in the heated (sterilized) control sample, it was interpreted as being due to non-biological processes.
In addition to the three above, the following was also conducted.
GC-MS (Gas Chromatograph-Mass Spectrometer)—Direct Detection of Organic Matter
An attempt to directly detect organic matter in the soil.
Result: Almost no organic matter was detected. The organic matter that was detected (such as chloromethane) was considered to be a contaminant brought from Earth.
How should this contradiction—the positive reaction of the LR experiment and the non-detection of organic matter by the GC-MS—be interpreted?
Many scientists supported a chemical explanation. Martian soil contains strong oxidizing substances such as peroxides and perchlorates, which may have reacted with organic matter to release gas. Inactivation by heating can also be explained by the fact that the oxidant was decomposed by heat.
The presence of perchlorate was actually confirmed by the Phoenix lander in 2008. Martian soil contained a high concentration of perchlorate of about 0.5 percent.
The official view of the Viking program is that "no evidence of life was discovered." However, the results of the LR experiment have not been completely denied even today.
5.3 Exposure of the Definition of Life—What the Experiments Questioned Back
What the Viking experiments exposed is the problem of the definition of life.
The Viking biology experiments were based on the implicit premise that "that which metabolizes = life." If you give it nutrients, it should metabolize and release gas.
But on Mars, non-biological chemical reactions mimicked metabolism.
What is questioned here is the fundamental question of how to define life.
Questions surrounding the definition of life:
Is that which metabolizes life? But non-biological chemical reactions also show reactions similar to metabolism.
Is that which contains organic matter life? But organic matter is also synthesized non-biologically (for example, as the Miller-Urey experiment shows).
Is that which self-replicates life? But crystals also have self-replicating structures.
Is that which has evolutionary ability life? But it is impossible to detect this directly in a Martian experiment.
Is that which has a membrane and maintains an internal environment life? This is close to the current definition of "minimal life," but detecting a membrane is also difficult.
The Viking experimental design assumed Earth-like metabolism. But that premise itself came to be questioned.
Life detection experiments expose Earth-centric views of life more than life itself. This is one of the most important lessons left by the Viking program.
(Published March 14, 2026)