Planetary habitability refers to conditions on a planet or natural satellite that could potentially support life, meaning the environment is suitable for its development and sustenance. NASA defines the main habitability criteria as the presence of extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and available energy sources to power metabolism. Mars has long been a primary focus in the search for habitable environments beyond Earth due to its relative proximity and past geological activity. Scientists examine signs on Mars to understand if it ever harbored life, or if it could today in protected niches.
The Presence of Water
Liquid water is the most fundamental requirement for life, playing a central role in biological processes. Substantial evidence points to a past on Mars where liquid water was abundant on its surface, with geological features like ancient riverbeds, deltas, and lakebeds visible across the Martian landscape. The Curiosity rover, for instance, has documented ancient freshwater lake environments within Gale Crater that could have been hospitable to microbial life. The detection of minerals like clays and sulfates, which only form in the presence of liquid water, provides strong chemical evidence for a wetter past. Recent research even suggests that liquid water persisted on Mars’s surface more recently than previously thought, perhaps as late as 2 to 2.5 billion years ago, as indicated by chloride salt deposits.
The search for current liquid water on Mars focuses on subsurface environments and transient phenomena, given the planet’s present cold and thin atmosphere. Subsurface ice deposits are widespread, and radar findings have suggested the presence of liquid water in subglacial lakes. While pure liquid water is unstable on the surface, the presence of salts like perchlorates can lower water’s freezing point, potentially allowing for transient liquid brines to form. These brines could exist in small quantities, forming at night and evaporating by sunrise, though many are too cold for known life. Recurring slope lineae (RSL), dark streaks that appear seasonally on Martian slopes, were once thought to be strong evidence of flowing briny water; however, recent studies suggest these features might be caused by dry granular flows of sand and dust.
Essential Chemical Elements and Energy Sources
Life requires specific chemical building blocks, commonly referred to by the acronym CHNOPS: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur. These elements form the backbone of organic molecules like proteins, nucleic acids (DNA and RNA), carbohydrates, and lipids, fundamental to all biological systems. Instruments on Mars rovers and orbiters analyze Martian soil and rock composition for these elements. Their detection and distribution indicate a planet’s potential to support life.
Beyond raw materials, an energy source is necessary to power metabolic processes. Several potential energy sources are being considered on Mars.
Sunlight, though weaker than on Earth, could theoretically support photosynthetic life, especially if protected under ice layers that filter harmful ultraviolet radiation while allowing visible light to penetrate. Chemical energy from water-rock reactions, particularly in subsurface environments, could support chemosynthetic organisms, occurring where water interacts with volcanic minerals. Geothermal energy from the planet’s internal heat could create localized warm zones beneath the surface. While Mars is less geothermally active than Earth, past volcanic activity and detected marsquakes suggest some internal heat sources might still exist, potentially creating pockets of habitability at depth.
A Conducive Environment
For life to thrive, a stable and protective environment is as important as water and chemical elements. A suitable temperature range is necessary for liquid water and efficient biochemical reactions. Mars is generally very cold (average -63°C), though equatorial regions can reach up to 20°C at midday. Subsurface environments offer more stable temperatures, insulated from surface fluctuations.
Atmospheric pressure also plays a significant role; it must be sufficient for liquid water to remain stable on the surface without instantly boiling or freezing. Mars’s atmosphere is extremely thin (less than one percent of Earth’s pressure), largely preventing standing liquid water on the surface. This low pressure, combined with cold temperatures, means any surface water would rapidly sublimate.
Radiation is another environmental factor. Mars lacks a global magnetic field and has a very thin atmosphere, leaving its surface exposed to high levels of harmful solar and cosmic radiation that can damage organic molecules and living cells.
The subsurface offers considerable protection from these harsh surface conditions. At depths of even a few meters, radiation levels are significantly reduced. The subsurface also provides insulation from temperature swings and allows for higher pressures that could support liquid water, especially with salts. While the Martian surface is largely inhospitable to known life forms, scientists are increasingly exploring the planet’s subsurface as a potential refuge where conditions might still be conducive to life.