A human cannot breathe on Mars. The Martian environment is instantly lethal to an unprotected person, presenting a dual threat that simultaneously poisons the body and physically destroys it. The planet Mars, a cold desert world, possesses an atmosphere so thin and chemically toxic that it would cause death within seconds. Understanding the dangers requires looking beyond the simple lack of oxygen to examine the specific chemical makeup of the gases and the extreme absence of atmospheric pressure.
The Chemical Problem Martian Atmospheric Gases
The atmospheric composition of Mars is fundamentally incompatible with human respiration, primarily due to the overwhelming presence of carbon dioxide (CO2). Mars’s atmosphere consists of approximately 95.3% carbon dioxide by volume, a concentration that is toxic to humans even if sufficient oxygen were present. Earth’s atmosphere, for comparison, contains only about 0.04% CO2.
The remaining gases include about 2.7% nitrogen and 1.6% argon, leaving only trace amounts of life-sustaining oxygen. Molecular oxygen (O2), which humans rely on for cellular respiration, makes up a mere 0.13% of the Martian air. This level is far too low to support consciousness, leading to immediate hypoxia, a severe oxygen deficiency.
The high concentration of carbon dioxide acts as a potent respiratory toxin. Breathing air with just a few percent CO2 on Earth causes hypercapnia, leading to confusion and loss of coordination. At Mars’s nearly 96% concentration, the gas would rapidly displace any oxygen in the lungs and flood the bloodstream, causing rapid systemic failure.
Even if an unprotected person had an external supply of pure oxygen, the air’s chemical makeup would still be deadly. The sheer volume of CO2 would prevent the necessary exchange of gases in the lungs, making any attempt to breathe a toxic process.
The Physical Problem Atmospheric Pressure
While toxic CO2 is a chemical obstacle, the most immediate and fatal danger is the extreme lack of atmospheric pressure. On Earth at sea level, atmospheric pressure is approximately 1,013 millibars (mbar). On Mars, the average surface pressure is astonishingly low, hovering around 6 to 7 mbar, which is less than 1% of Earth’s pressure.
This near-vacuum state is far below the threshold required to keep water in a liquid state at human body temperature. The physical phenomenon known as ebullism occurs when the ambient pressure drops below 63 mbar, the vapor pressure of water at human body temperature (37°C or 98.6°F). Since the Martian surface pressure is roughly ten times lower than this threshold, ebullism is an unavoidable and instantaneous consequence of exposure.
Ebullism causes moisture in the body, specifically in low-pressure areas like the mouth, eyes, and lungs, to spontaneously vaporize, or boil. The low external pressure also allows dissolved gases in the bloodstream, primarily nitrogen, to form bubbles, similar to opening a carbonated soda. These bubbles would block blood flow and cause severe swelling, leading to a condition known as cardiac vapor lock, where the heart cannot effectively pump blood.
This rapid phase change causes the liquid in tissues to convert to gas, leading to severe swelling of the soft tissues. This physical destruction would happen regardless of the atmospheric composition, making the thin air the single most dangerous aspect of the Martian environment.
Immediate Effects on the Unprotected Human Body
The combined chemical and physical hazards initiate a rapid sequence of events for an unprotected human. A reflexive attempt to breathe the Martian air would fail to provide oxygen while introducing a massive dose of carbon dioxide. This lack of oxygen would cause the person to lose consciousness within approximately 10 to 15 seconds.
Simultaneously, the extreme low pressure triggers ebullism, causing moisture on exposed surfaces to boil and evaporate rapidly. The expansion of gases within the tissues would cause the body to swell, leading quickly to severe circulatory impairment and tissue damage. The skin is elastic enough that the body would not immediately rupture.
While the low average temperature on Mars (around -62°C or -80°F) would eventually cause freezing, this is not the primary cause of death. Rapid evaporative cooling would cause surface tissues to cool quickly, leading to ice formation on the mucosal surfaces of the eyes and mouth.
Death would occur within a few minutes, primarily from total hypoxia and systemic failure caused by ebullism. Irreversible brain damage begins about two to three minutes after unconsciousness. Survival is only possible with a fully pressurized suit that provides both a breathable atmosphere and the necessary counter-pressure.