Mars—Beyond the Red Horizon (25): Chapter 5 Did Life Exist on Mars—Detection, Definition, and Scale (3)
5.5 Modern Mars Life Exploration—A Shift in Strategy
After the Viking program, the strategy for Mars life exploration changed significantly.
Viking attempted to detect metabolic activity on the current Martian surface. The current Martian surface is an environment where organic matter is easily decomposed due to strong ultraviolet radiation and highly oxidizing soil. If life exists, it is more likely to be underground rather than on the surface.
The strategy shifted from "detecting current metabolism" to "preserving traces of past life."
The current primary approaches are as follows.
Search for Biosignatures
Biosignatures are a general term for chemical and physical evidence indicating the presence of life. They include specific patterns of organic molecules, anomalies in isotope ratios (life preferentially uses certain isotopes), and specific minerals (such as those formed by biological activity).
Analysis of Isotope Ratios
Carbon exists as ¹²C (stable isotope), ¹³C (stable isotope), and ¹⁴C (radioactive isotope). Earth life tends to preferentially use ¹²C over ¹³C as a result of enzymatic reactions. If a similar bias in isotope ratios is found in Martian organic matter, it could be evidence of biological activity.
Analysis of Organic Matter Inside Rocks and Underground
For organic matter to be detected, preservation conditions are important. While the current Martian surface is prone to organic matter decomposition, it is more easily preserved inside rocks or deep underground. Curiosity and Perseverance are drilling into rocks to analyze the organic matter inside.
Analysis of Sedimentary Rock Cores
The sedimentary rock cores collected by Perseverance may contain preserved traces of ancient lake environments. If these samples are returned to Earth, more precise analysis will be possible.
Curiosity has already detected organic molecules such as thiophene, benzene, and propane from Martian rocks. These do not necessarily have to be of biological origin, but it has at least been confirmed that organic matter exists on Mars.
Perseverance is collecting rocks from the ancient delta of Jezero Crater that are highly likely to contain traces of life. The day these samples are returned to Earth is expected to be in the 2030s.
5.6 The Underground Possibility—The Invisible Biosphere
We must not forget another possibility. That is the underground biosphere.
On Earth, microorganisms have been discovered living deep underground in environments where neither light nor oxygen reaches. These are called "lithotrophs," and they live by utilizing chemical energy in rocks—such as the reaction between hydrogen and carbon dioxide.
Such microorganisms can continue to live for hundreds of millions of years in environments isolated from the surface.
It is believed that permafrost layers extend beneath the surface of Mars. And beneath that, there is a possibility that liquid water exists due to geothermal heat. If liquid water and chemical energy exist, an environment where Earth-like microorganisms could survive could be established.
Current Mars rovers can only drill to a depth of at most a few centimeters from the surface. What is happening hundreds of meters to several kilometers underground is still unknown.
The InSight lander was equipped with a seismometer to explore the internal structure of Mars, but no direct evidence of underground water was obtained. Future exploration plans include deeper drilling and the use of radar to explore underground structures.
The underground biosphere is the final frontier of Mars life exploration.
5.7 The Boundary Between Mars and the Concept of Life—Symmetrical Differences with Titan
Here, let us consider the contrast with Titan again.
Titan demands an expansion of the concept of life. If life exists on Titan, it would need to use liquid methane rather than water as a solvent and possess a biochemical system fundamentally different from that of Earth. Titan pushes the question of "what is life" outside the framework of Earth-like life.
Mars tests the boundary conditions of the concept of life. If life existed (or still exists) on Mars, it might possess a biochemical system similar to Earth-like life. Mars raises the question of "to what extent can Earth-like life be established?"
$$
\begin{array}{} \hline
\text{Item} & \text{Mars} & \text{Titan} \\ \hline
\text{Possibility of Life} & \text{Within the scope of Earth-like biochemistry} & \text{Possibility of alternative biochemistry} \\ \hline
\text{Solvent} & \text{Water (past)} & \text{Liquid methane (present)} \\ \hline
\text{Organic Matter} & \text{Scarce (oxidizing environment)} & \text{Abundant (reducing environment)} \\ \hline
\text{Time of Life} & \text{Possibility in the past} & \text{Possibility in the present} \\ \hline
\text{Exploration Challenges} & \text{Preservation and detection of traces} & \text{Definition and detection of alternative life} \\ \hline
\end{array}
$$
Titan expands the possibility of life outside the Earth-like framework.
Mars forces a re-examination of the conditions for the establishment and preservation of Earth-like life.
This is a symmetrical question. Titan asks "how alien can life be?" while Mars asks "how long can life of the same type persist?"
(Published 2026.3.16)