Habitability, in the astrobiological context, refers to environments where the physical and chemical conditions are suitable for life to originate and persist. The search for extinct or extant life on the Red Planet is driven by the tantalizing evidence suggesting Mars once harbored conditions strikingly similar to early Earth. The current scientific understanding paints a picture of a planet that was once welcoming but has since become a hostile desert, forcing the search for life to shift from the surface to protected subsurface niches.
Essential Ingredients for Life
Life requires three conditions to thrive: a liquid solvent, a stable energy source, and chemical building blocks. Liquid water is the universal solvent, facilitating the movement of molecules and the complex chemical reactions that define metabolism. Organisms also need energy to drive their metabolic processes, which can be derived from sunlight through photosynthesis or from chemical reactions through chemosynthesis. On Earth, many extremophiles thrive by extracting energy from the oxidation or reduction of inorganic compounds, such as iron or sulfur. The final requirement is a consistent supply of chemical elements, often summarized by the acronym CHNOPS (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur), which form the foundational architecture for all known biological molecules.
Evidence of Past Habitability
Geological evidence indicates that Mars was once a warmer and wetter world, capable of sustaining liquid surface water billions of years ago. Missions such as NASA’s Curiosity and Perseverance rovers have found mineralogical and structural features that confirm this past environment. The Curiosity rover, exploring Gale Crater, discovered ancient lakebed sediments and large deposits of iron-rich carbonate minerals, which form when carbon dioxide and water interact with rock. The presence of these carbonates suggests that a thicker, carbon-dioxide-rich atmosphere once allowed for stable surface water.
Perseverance is exploring Jezero Crater, a location that once hosted a large lake and a prominent river delta. Analysis of rocks in this area has revealed complex evidence, including calcium sulfate veins and a mineral called sepiolite, which formed under alkaline conditions supportive of life. The rover has detected organic molecules and features in rock samples that may represent chemical energy sources for ancient microbes. These findings suggest the planet experienced a dynamic aqueous history, shifting from potentially harsh, acidic fluids to more neutral conditions conducive to life.
Current Challenges to Life
The surface of Mars today is a challenging environment where known life forms cannot persist unprotected. One of the primary barriers is the extremely thin atmosphere, averaging only about 6 to 7 millibars of pressure. This low pressure means that even if the temperature were to rise above freezing, any liquid water would instantaneously boil and sublimate into a gas, making stable surface water impossible.
Temperature is another limiting factor, with a global average of approximately -63°C (-80°F). While temperatures can briefly reach +20°C (70°F) at the equator during the day, the lack of an atmosphere to retain heat causes a rapid plunge to lows of -125°C to -153°C at night. This extreme diurnal temperature swing is detrimental to biological systems. Furthermore, the lack of a global magnetic field allows solar and cosmic radiation to bombard the planet’s surface virtually unimpeded. This intense radiation is capable of sterilizing the top few meters of the Martian soil, destroying complex organic molecules and any unprotected microbial life.
Finally, the Martian regolith contains high concentrations of toxic chemicals known as perchlorates. When exposed to the harsh ultraviolet radiation prevalent on the surface, these perchlorates become highly reactive oxidizing agents, creating a toxic cocktail that can rapidly kill microorganisms.
Where Life Could Hide Today
The search for life has focused on environments that can provide shielding from radiation and stable liquid water. The most promising locations are deep beneath the surface, where temperature and pressure conditions are more favorable. Seismic data from the InSight lander suggests the presence of a vast, deep-seated reservoir of water trapped within fractured rock layers between 7 and 20 kilometers below the surface. This massive underground resource could host chemosynthetic life forms similar to those found deep within Earth’s crust.
Closer to the surface, niche habitats may exist beneath layers of ice or soil, which provide sufficient radiation shielding. Computer modeling suggests that enough sunlight could penetrate through dusty water ice for photosynthetic microbes to survive up to three meters below the surface. Additionally, the perchlorate salts, while toxic, significantly lower the freezing point of water, potentially allowing for transient, briny liquid flows to exist near the surface. If life currently exists on Mars, it must be protected in these subsurface environments, insulated from the atmosphere, cold, and intense radiation of the surface.